Chin. Phys. B

Novel energy dissipative method on the adaptive spatial discretization for the Allen-Cahn equation

Jing-Wei Sun(孙竟巍), Xu Qian(钱旭), Hong Zhang(张弘), and Song-He Song(宋松和)

Chin. Phys. B, 2021, 30 (7): 070201. DOI: 10.1088/1674-1056/abe37b

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We propose a novel energy dissipative method for the Allen-Cahn equation on nonuniform grids. For spatial discretization, the classical central difference method is utilized, while the average vector field method is applied for time discretization. Compared with the average vector field method on the uniform mesh, the proposed method can involve fewer grid points and achieve better numerical performance over long time simulation. This is due to the moving mesh method, which can concentrate the grid points more densely where the solution changes drastically. Numerical experiments are provided to illustrate the advantages of the proposed concrete adaptive energy dissipative scheme under large time and space steps over a long time.

Approximate analytical solutions and mean energies of stationary Schrödinger equation for general molecular potential

Eyube E S, Rawen B O, and Ibrahim N

Chin. Phys. B, 2021, 30 (7): 070301. DOI: 10.1088/1674-1056/abe371

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The Schrödinger equation is solved with general molecular potential via the improved quantization rule. Expression for bound state energy eigenvalues, radial eigenfunctions, mean kinetic energy, and potential energy are obtained in compact form. In modeling the centrifugal term of the effective potential, a Pekeris-like approximation scheme is applied. Also, we use the Hellmann-Feynman theorem to derive the relation for expectation values. Bound state energy eigenvalues, wave functions and meanenergies of Woods-Saxon potential, Morse potential, Möbius squared and Tietz-Hua oscillators are deduced from the general molecular potential. In addition, we use our equations to compute the bound state energy eigenvalues and expectation values for four diatomic molecules viz. H₂, CO, HF, and O₂. Results obtained are in perfect agreement with the data available from the literature for the potentials and molecules. Studies also show that as the vibrational quantum number increases, the mean kinetic energy for the system in a Tietz-Hua potential increases slowly to a threshold value and then decreases. But in a Morse potential, the mean kinetic energy increases linearly with vibrational quantum number increasing.

Thermodynamic properties of massless Dirac-Weyl fermions under the generalized uncertainty principle

Guang-Hua Xiong(熊光华), Chao-Yun Long(龙超云), and He Su(苏贺)

Chin. Phys. B, 2021, 30 (7): 070302. DOI: 10.1088/1674-1056/abe1aa

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The Dirac-Weyl equation characterized quasi-particles in the T3 lattice are studied under external magnetic field using the generalized uncertainty principle (GUP). The energy spectrum of the quasi-particles is found by the Nikiforov-Uvarov method. Based on the energy spectrum obtained, the thermodynamic properties are given, and the influence of the GUP on the statistical properties of systems is discussed. The results show that the energy and thermodynamic functions of massless Dirac-Weyl fermions in the T3 lattice depend on the variation of the GUP parameter.

Steered coherence and entanglement in the Heisenberg XX chain under twisted boundary conditions

Yu-Hang Sun(孙宇航) and Yu-Xia Xie(谢玉霞)

Chin. Phys. B, 2021, 30 (7): 070303. DOI: 10.1088/1674-1056/abe1a9

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We study steered coherence (SC) and entanglement in a three-spin Heisenberg XX model under twisted boundary conditions and show that their strengths can be significantly enhanced by tuning the twist angle. The optimal twist angle θ_opt for achieving the maximum l₁ norm of SC is π in the region of weak field B and decreases gradually from π to 0 when B increases after a critical value, while for the relative entropy of SC, θ_opt equals π in the weak field region and 0 otherwise. The entanglement and the critical temperature above which the entanglement vanishes can also be significantly enhanced by tuning the twist angle from 0 to π.

Improving the purity of heralded single-photon sources through spontaneous parametric down-conversion process ^Hot!

Jing Wang(王静), Chun-Hui Zhang(张春辉), Jing-Yang Liu(刘靖阳), Xue-Rui Qian(钱雪瑞), Jian Li(李剑), and Qin Wang(王琴)

Chin. Phys. B, 2021, 30 (7): 070304. DOI: 10.1088/1674-1056/abfb5c

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The high-purity single-photon source plays an important role in the field of quantum information. Usually, it is generated through spontaneous parametric down-conversion process. In this paper, we investigate and summarize a few approaches on obtaining single-photon sources with a high purity using either PPKTP or PPLN nonlinear crystals. Moreover, we present improved schemes to increase the purity based on existing work, corresponding applicable conditions and procedures are discussed and analyzed. Besides, we carry out numerical simulations and show that nearly perfect purity can be reached even without using any filters. Therefore, this work might provide valuable references for the generation and application of high purity single-photon sources.

Suppression of servo error uncertainty to 10^-18 level using double integrator algorithm in ion optical clock Suggested by editors

Jin-Bo Yuan(袁金波), Jian Cao(曹健), Kai-Feng Cui(崔凯枫), Dao-Xin Liu(刘道信), Yi Yuan(袁易), Si-Jia Chao(晁思嘉), Hua-Lin Shu(舒华林), and Xue-Ren Huang(黄学人)

Chin. Phys. B, 2021, 30 (7): 070305. DOI: 10.1088/1674-1056/abf918

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A universal locking model for single ion optical clocks was built based on a simple integrator and a double integrator. Different integrator algorithm parameters have been analyzed in both numerical simulations and experiments. The frequency variation measured by the comparison of two optical clocks coincides well with the simulation results for different second integrator parameters. According to the experimental results, the sensitivity of the servo error influenced by laser frequency drift with the addition of a double integrator was suppressed by a factor of 107. In a week-long comparison of optical clocks, the relative uncertainty of the servo error is determined to be 1.9×10^-18, which is meaningful for the systematic uncertainty of the transportable single ⁴⁰Ca⁺ ion optical clock entering the 10^-18 level.

Creation and annihilation phenomena of electron and positron pairs in an oscillating field

M Jiang(江淼), D D Su(苏丹丹), N S Lin(林南省), and Y J Li(李英骏)

Chin. Phys. B, 2021, 30 (7): 070306. DOI: 10.1088/1674-1056/abe378

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The combination of an oscillating and a static field is used to study the creation and annihilation phenomena during the pair creation process. The time evolution, spatial density and momentum distribution of the created particles for a fermionic system are presented, which demonstrate that with the increasing static field intensity, the number of the created particles experiences a distinguishable decrease in every period of the oscillating field, which is caused by the annihilation phenomena between the created electrons and positrons.

Nonlocal advantage of quantum coherence and entanglement of two spins under intrinsic decoherence

Bao-Min Li(李保民), Ming-Liang Hu(胡明亮), and Heng Fan(范桁)

Chin. Phys. B, 2021, 30 (7): 070307. DOI: 10.1088/1674-1056/abff2a

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We investigate the nonlocal advantage of quantum coherence (NAQC) and entanglement for two spins coupled via the Heisenberg interaction and under the intrinsic decoherence. Solutions of this decoherence model for the initial spin-1/2 and spin-1 maximally entangled states are obtained, based on which we calculate the NAQC and entanglement. In the weak region of magnetic field, the NAQC behaves as a damped oscillation with the time evolves, while the entanglement decays exponentially (behaves as a damped oscillation) for the spin-1/2 (spin-1) case. Moreover, the decay of both the NAQC and entanglement can be suppressed significantly by tuning the magnetic field and anisotropy of the spin interaction to some decoherence-rate-determined optimal values.

Universal quantum control based on parametric modulation in superconducting circuits

Dan-Yu Li(李丹宇), Ji Chu(储继), Wen Zheng(郑文), Dong Lan(兰栋), Jie Zhao(赵杰), Shao-Xiong Li(李邵雄), Xin-Sheng Tan(谭新生), and Yang Yu(于扬)

Chin. Phys. B, 2021, 30 (7): 070308. DOI: 10.1088/1674-1056/ac01c3

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As superconducting quantum circuits are scaling up rapidly towards the noisy intermediate-scale quantum (NISQ) era, the demand for electronic control equipment has increased significantly. To fully control a quantum chip of N qubits, the common method based on up-conversion technology costs at least 2×N digital-to-analog converters (DACs) and N IQ mixers. The expenses and complicate mixer calibration have become a hinderance for intermediate-scale quantum control. Here we propose a universal control scheme for superconducting circuits, fully based on parametric modulation. To control N qubits on a chip, our scheme only requires N DACs and no IQ mixer, which significantly reduces the expenses. One key idea in the control scheme is to introduce a global pump signal for single-qubit gates. We theoretically explain how the universal gates are constructed using parametric modulation. The fidelity analysis shows that parametric single-qubit (two-qubit) gates in the proposed scheme can achieve low error rates of 10^-4, with a gate time of about 60 ns (100 ns).

Universal quantum circuit evaluation on encrypted data using probabilistic quantum homomorphic encryption scheme

Jing-Wen Zhang(张静文), Xiu-Bo Chen(陈秀波), Gang Xu(徐刚), and Yi-Xian Yang(杨义先)

Chin. Phys. B, 2021, 30 (7): 070309. DOI: 10.1088/1674-1056/ac003b

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Homomorphic encryption has giant advantages in the protection of privacy information. In this paper, we present a new kind of probabilistic quantum homomorphic encryption scheme for the universal quantum circuit evaluation. Firstly, the pre-shared non-maximally entangled states are utilized as auxiliary resources, which lower the requirements of the quantum channel, to correct the errors in non-Clifford gate evaluation. By using the set synthesized by Clifford gates and T gates, it is feasible to perform the arbitrary quantum computation on the encrypted data. Secondly, our scheme is different from the previous scheme described by the quantum homomorphic encryption algorithm. From the perspective of application, a two-party probabilistic quantum homomorphic encryption scheme is proposed. It is clear what the computation and operation that the client and the server need to perform respectively, as well as the permission to access the data. Finally, the security of probabilistic quantum homomorphic encryption scheme is analyzed in detail. It demonstrates that the scheme has favorable security in three aspects, including privacy data, evaluated data and encryption and decryption keys.

Fast qubit initialization in a superconducting circuit

Tianqi Huang(黄天棋), Wen Zheng(郑文), Shuqing Song(宋树清), Yuqian Dong(董煜倩), Xiaopei Yang(杨晓沛), Zhikun Han(韩志坤), Dong Lan(兰栋), and Xinsheng Tan(谭新生)

Chin. Phys. B, 2021, 30 (7): 070310. DOI: 10.1088/1674-1056/abff2b

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We demonstrate an active reset protocol in a superconducting quantum circuit. The thermal population on the excited state of a transmon qubit is reduced through driving the transitions between the qubit and an ancillary qubit. Furthermore, we investigate the efficiency of this approach at different temperatures. The result shows that population in the first excited state can be dropped from 7% to 2.55% in 27 ns at 30 mK. The efficiency improves as the temperature increases. Compared to other schemes, our proposal alleviates the requirements for measurement procedure and equipment. With the increase of qubit integration, the fast reset technique holds the promise of improving the fidelity of quantum control.

Projective representation of D₆ group in twisted bilayer graphene

Noah F. Q. Yuan

Chin. Phys. B, 2021, 30 (7): 070311. DOI: 10.1088/1674-1056/ac00a3

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Within the framework of continuum model, we study the projective representation of emergent D₆ point group in twisted bilayer graphene. We then construct tight-binding models of the lowest bands without and with external electromagnetic fields, based on the projective representation.

Applying a global pulse disturbance to eliminate spiral waves in models of cardiac muscle Suggested by editors

Jian Gao(高见), Changgui Gu(顾长贵), and Huijie Yang(杨会杰)

Chin. Phys. B, 2021, 30 (7): 070501. DOI: 10.1088/1674-1056/abf553

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Removal of spiral waves in cardiac muscle is necessary because of their threat to life. Common methods for this removal are to apply a local disturbance to the media, such as a periodic forcing. However, most of these methods accelerate the beating of the cardiac muscle, resulting in the aggravation of the ventricular tachycardia, which directly threatens life. In the present study, in order to clear off spiral waves, a global pulse-disturbance is applied to the media based on three models of cardiac muscle. It is found that the spiral waves are eliminated and the frequency of the cardiac muscle is decreased in a short time, and finally, the state of the medium reaches the normal oscillation, which supports a target waves. Our method sheds light on the removal of spiral waves in cardiac muscle and can prevent the ventricular tachycardia as well as the ventricular fibrillation.

Evaluation of second-order Zeeman frequency shift in NTSC-F2

Jun-Ru Shi(施俊如), Xin-Liang Wang(王心亮), Yang Bai(白杨), Fan Yang(杨帆), Yong Guan(管勇), Dan-Dan Liu(刘丹丹), Jun Ruan(阮军), and Shou-Gang Zhang(张首刚)

Chin. Phys. B, 2021, 30 (7): 070601. DOI: 10.1088/1674-1056/abe375

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Caesium atomic fountain clock is a primary frequency standard, which realizes the duration of second. Its performance is mostly dominated by the frequency accuracy, and the C-field induced second-order Zeeman frequency shift is the major effect, which limits the accuracy improvement. By applying a high-precision current supply and high-performance magnetic shieldings, the C-field stability has been improved significantly. In order to achieve a uniform C-field, this paper proposes a doubly wound C-field solenoid, which compensates the radial magnetic field along the atomic flight region generated by the lead-out single wire and improves the accuracy evaluation of second-order Zeeman frequency shift. Based on the stable and uniform C-field, we launch the selected atoms to different heights and record the magnetically sensitive Ramsey transition $|F = 3, m_{F} = -1\rangle \to |F = 4, m_{F} = -1\rangle$ central frequency, obtaining this frequency shift as 131.03$\times $10$^{-15}$ and constructing the C-field profile ($\sigma = 0.15$ nT). Meanwhile, during normal operation, we lock NTSC-F2 to the central frequency of the magnetically sensitive Ramsey transition $|F = 3, m_{F} = -1\rangle \to |F = 4, m_{F} = -1\rangle$ fringe for ten consecutive days and record this frequency fluctuation in time domain. The first evaluation of second-order Zeeman frequency shift uncertainty is 0.10$\times $10$^{-15}$. The total deviation of the frequency fluctuation on the clock transition induced by the C-field instability is less than 2.6$\times $10$^{-17}$. Compared with NTSC-F1, NTSC-F2, there appears a significant improvement.

Consensus problems on networks with free protocol

Xiaodong Liu(柳晓东) and Lipo Mo(莫立坡)

Chin. Phys. B, 2021, 30 (7): 070701. DOI: 10.1088/1674-1056/abe113

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In the real world, rule makers can only restrict, not completely control the behavior of the governed, while the governed can only choose their behavior patterns under these restrictions. In this paper, we design a new control protocol called free protocol to describe this situation. First, we calculate consensus probabilities based on the information of the interaction networks. Then, sufficient conditions are obtained for all agents converging to a same value with probability one. Finally, numerical simulation results are given to verify the above results.

A comparative study on radiation reliability of composite channel InP high electron mobility transistors

Jia-Jia Zhang(张佳佳), Peng Ding(丁芃), Ya-Nan Jin(靳雅楠), Sheng-Hao Meng(孟圣皓), Xiang-Qian Zhao(赵向前), Yan-Fei Hu(胡彦飞), Ying-Hui Zhong(钟英辉), and Zhi Jin(金智)

Chin. Phys. B, 2021, 30 (7): 070702. DOI: 10.1088/1674-1056/abe2fd

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This paper proposes a reasonable radiation-resistant composite channel structure for InP HEMTs. The simulation results show that the composite channel structure has excellent electrical properties due to increased modulation doping efficiency and carrier confinement. Moreover, the direct current (DC) and radio frequency (RF) characteristics and their reliability between the single channel structure and the composite channel structure after 75-keV proton irradiation are compared in detail. The results show that the composite channel structure has excellent radiation tolerance. Mechanism analysis demonstrates that the composite channel structure weakens the carrier removal effect. This phenomenon can account for the increase of native carrier and the decrease of defect capture rate.

A strategy to improve the electrochemical performance of Ni-rich positive electrodes: Na/F-co-doped LiNi_0.6Mn_0.2Co_0.2O₂ Suggested by editors

Hui Wan(万惠), Zhixiao Liu(刘智骁), Guangdong Liu(刘广东), Shuaiyu Yi(易帅玉), Fei Gao(高飞), Huiqiu Deng(邓辉球), Dingwang Yuan(袁定旺), and Wangyu Hu(胡望宇)

Chin. Phys. B, 2021, 30 (7): 073101. DOI: 10.1088/1674-1056/abeeeb

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Ni-rich layered lithium transition metal oxides LiNi_xMn_yCo_zO₂ (1 - y-z ≥ 0.6) are promising candidates for cathode materials, but their practical applications are hindered by high-voltage instability and fast capacity fading. Using density functional theory calculations, we demonstrate that Na-, F-doping, and Na/F-co-doping can stabilize the structure and result into a higher open circuit voltage than pristine LiNi_0.6Mn_0.2Co_0.2O₂ (NMC622) during the charging process, which may attain greater discharge capacity. F doping may inhibit the diffusion of Li ions at the beginning and end of charging; Na doping may improve Li ion diffusion due to the increase in Li layer spacing, consistent with prior experiments. Na/F-co-doping into NMC622 promotes rate performance and reduces irreversible phase transitions for two reasons: (i) a synergistic effect between Na and F can effectively restrain the Ni/Li mixing and then enhances the mobility of Li ions and (ii) Ni/Li mixing hinders the Ni ions to migrate into Li layers and thus, stabilizes the structure. This study proposes that a layer cathode material with high electrochemical performance can be achieved via rational dopant modification, which is a promising strategy for designing efficient Li ion batteries.

State-to-state dynamics of reactions H+DH'(v = 0,j = 0) → HH'(v',j')+D/HD(v',j')+H' with time-dependent quantum wave packet method Suggested by editors

Juan Zhao(赵娟), Da-Guang Yue(岳大光), Lu-Lu Zhang(张路路), Shang Gao(高尚), Zhong-Bo Liu(刘中波), and Qing-Tian Meng(孟庆田)

Chin. Phys. B, 2021, 30 (7): 073102. DOI: 10.1088/1674-1056/abf559

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State-to-state time-dependent quantum dynamics calculations have been carried out to study ${\rm H}+{\rm DH}^{\prime} \rightarrow {\rm HH}^{\prime}+{\rm D/HD}+{\rm H}^{\prime}$ reactions on BKMP2 surface. The total integral cross sections of both reactions are in good agreement with earlier theoretical and experimental results, moreover the rotational state-resolved reaction cross sections of ${\rm H}+{\rm DH}^{\prime} \rightarrow {\rm HH}^{\prime}+{\rm D}$ at collision energy $E_{\rm C} =0.5$ eV are closer to the experimental values than the ones calculated by Chao et al. [J. Chem. Phys. 117 8341 (2002)], which proves the higher precision of the quantum calculation in this work. In addition, the state-to-state dynamics of ${\rm H}+{\rm DH}^{\prime} \rightarrow {\rm HD}^{\prime}+{\rm H}$ reaction channel have been discussed in detail, and the differences of the micro-mechanism of the two reaction channels have been revealed and analyzed clearly.

Plasticity and melting characteristics of metal Al with Ti-cluster under shock loading

Dong-Lin Luan(栾栋林), Ya-Bin Wang(王亚斌), Guo-Meng Li(李果蒙), Lei Yuan(袁磊), and Jun Chen(陈军)

Chin. Phys. B, 2021, 30 (7): 073103. DOI: 10.1088/1674-1056/abe1a0

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Impurity agglomeration has a significant influence on shock response of metal materials. In this paper, the mechanism of Ti-clusters in metal Al under shock loading is investigated by non-equilibrium molecular dynamics simulations. Our results show that the Ti-cluster has obvious effects on the dislocation initiation and melting of bulk Al. First, the Ti clusters induces the strain concentrate and leads the dislocations to be initiated from the interface of Ti cluster. Second, dislocation distribution from the Ti-cluster model results in a formation of a grid-like structure, while the dislocation density is reduced compared with that from the perfect Al model. Third, the critical shock velocity of dislocation from the Ti-cluster model is lower than from perfect Al model. Furthermore, it is also found that the temperature near the interface of Ti-cluster is 100 K-150 K higher than in the other areas, which means that Ti-cluster interface melts earlier than the bulk area.

Highly accurate theoretical study on spectroscopic properties of SH including spin-orbit coupling

Shu-Tao Zhao(赵书涛), Xin-Peng Liu(刘鑫鹏), Rui Li(李瑞), Hui-Jie Guo(国慧杰), and Bing Yan(闫冰)

Chin. Phys. B, 2021, 30 (7): 073104. DOI: 10.1088/1674-1056/ac003f

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The multi-reference configuration interaction method plus Davidson correction (MRCI$+$Q) are adopted to study the low-lying states of SH with consideration of scalar relativistic effect, core-valence (CV) electron correlation, and spin-orbit coupling (SOC) effect. The SOC effect on the low-lying states is considered by utilizing the full Breit-Pauli operator. The potential energy curves (PECs) of 10 $\Lambda$-S states and 18 $\Omega$ states are calculated. The dipole moments of 10 $\Lambda$-S states are calculated, and the variation along the internuclear distance is explained by the electronic configurations. With the help of calculated SO matrix elements, the possible predissociation channels of A$^{2}\Sigma^{+}$, c$^{4}\Sigma^{-}$ and F$^{2}\Sigma^{-}$ are discussed. The Franck-Condon factors of A$^{2}\Sigma^{+}$-X$^{2}\Pi $, F$^{2}\Sigma^{-}$-X$^{2}\Pi $ and E$^{2}\Sigma^{+}$-X$^{2}\Pi$ transitions are determined, and the radiative lifetimes of A$^{2}\Sigma^{+}$ and F$^{2}\Sigma^{-}$ states are evaluated, which are in good agreement with previous experimental results.

HeTDSE: A GPU based program to solve the full-dimensional time-dependent Schrödinger equation for two-electron helium subjected to strong laser fields

Xi Zhao(赵曦), Gangtai Zhang(张刚台), Tingting Bai(白婷婷), Jun Wang(王俊), and Wei-Wei Yu(于伟威)

Chin. Phys. B, 2021, 30 (7): 073201. DOI: 10.1088/1674-1056/abe1a4

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Electron-electron correlation plays an important role in the underlying dynamics in physics and chemistry. Helium is the simplest and most fundamental two-electron system. The dynamic process of helium in a strong laser field is still a challenging issue because of the large calculation cost. In this study, a graphic processing unit (GPU) openACC based ab initio numerical simulations package HeTDSE is developed to solve the full-dimensional time-dependent Schrödinger equation of helium subjected to a strong laser pulse. HeTDSE uses B-spline basis sets expansion method to construct the radial part of the wavefunction, and the spherical harmonic functions is used to express for the angular part. Adams algorithm is employed for the time propagation. Our example shows that HeTDSE running on an NVIDIA Kepler K20 GPU can outperform the one on an Intel E5-2640 single CPU core by a factor of 147. HeTDSE code package can be obtained from the author or from the author's personal website (doi: 10.13140/RG.2.2.15334.45128) directly under the GPL license, so HeTDSE can be downloaded, used and modified freely.

Simulation and experiment of the cooling effect of trapped ion by pulsed laser Suggested by editors

Chang-Da-Ren Fang(方长达人), Yao Huang(黄垚), Hua Guan(管桦), Yuan Qian(钱源), and Ke-Lin Gao(高克林)

Chin. Phys. B, 2021, 30 (7): 073701. DOI: 10.1088/1674-1056/abf91f

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We investigate the process of pulsed laser cooling using a self-constructed molecular dynamics simulation (MD-Simulation) program. We simulate the Doppler cooling process and pulsed laser Doppler cooling process of a single ⁴⁰Ca⁺ ion, and the comparison with the experimental results shows that this self-constructed MD-Simulation program works well in the weak laser intensity situation. Furthermore, we analyze the pulsed laser Doppler cooling process of a single ²⁷Al⁺ ion. This program can be used to analyze the molecular dynamic process of various situations of Doppler cooling in an ion trap, which could give predictions and experimental guidance.

Numerical analysis of motional mode coupling of sympathetically cooled two-ion crystals

Li-Jun Du(杜丽军), Yan-Song Meng(蒙艳松), Yu-Ling He(贺玉玲), and Jun Xie(谢军)

Chin. Phys. B, 2021, 30 (7): 073702. DOI: 10.1088/1674-1056/abfc3e

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A two-ion pair in a linear Paul trap is extensively used in the research of the simplest quantum-logic system; however, there are few quantitative and comprehensive studies on the motional mode coupling of two-ion systems yet. This study proposes a method to investigate the motional mode coupling of sympathetically cooled two-ion crystals by quantifying three-dimensional (3D) secular spectra of trapped ions using molecular dynamics simulations. The 3D resonance peaks of the ⁴⁰Ca⁺-²⁷Al⁺ pair obtained by using this method were in good agreement with the 3D in- and out-of-phase modes predicted by the mode coupling theory for two ions in equilibrium and the frequency matching errors were lower than 2%. The obtained and predicted amplitudes of these modes were also qualitatively similar. It was observed that the strength of the sympathetic interaction of the ⁴⁰Ca⁺-²⁷Al⁺ pair was primarily determined by its axial in-phase coupling. In addition, the frequencies and amplitudes of the ion pair's resonance modes (in all dimensions) were sensitive to the relative masses of the ion pair, and a decrease in the mass mismatch enhanced the sympathetic cooling rates. The sympathetic interactions of the ⁴⁰Ca⁺-²⁷Al⁺ pair were slightly weaker than those of the ²⁴Mg⁺-²⁷Al⁺ pair, but significantly stronger than those of ⁹Be⁺-²⁷Al⁺. However, the Doppler cooling limit temperature of ⁴⁰Ca⁺ is comparable to that of ⁹Be⁺ but lower than approximately half of that of ²⁴Mg⁺. Furthermore, laser cooling systems for ⁴⁰Ca⁺ are more reliable than those for ²⁴Mg⁺ and ⁹Be⁺. Therefore, ⁴⁰Ca⁺ is probably the best laser-cooled ion for sympathetic cooling and quantum-logic operations of ²⁷Al⁺ and has particularly more notable comprehensive advantages in the development of high reliability, compact, and transportable ²⁷Al⁺ optical clocks. This methodology may be extended to multi-ion systems, and it will greatly aid efforts to control the dynamic behaviors of sympathetic cooling as well as the development of low-heating-rate quantum logic clocks.

Single pixel imaging based on semi-continuous wavelet transform

Chao Gao(高超), Xiaoqian Wang(王晓茜), Shuang Wang(王爽), Lidan Gou(苟立丹), Yuling Feng(冯玉玲), Guangyong Jin(金光勇), and Zhihai Yao(姚治海)

Chin. Phys. B, 2021, 30 (7): 074201. DOI: 10.1088/1674-1056/abe373

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Single pixel imaging is a novel imaging technique, and it becomes a focus of research in recent years due to its advantages such as high lateral resolution and high robustness to noise. Imaging speed is one of the critical shortcomings, which limits the further development and applications of this technique. In this paper, we focus on the issues of imaging efficiency of a single pixel imaging system. We propose semi-continuous wavelet transform (SCWT) protocol and introduce the protocol into the single pixel imaging system. The proposed protocol is something between continuous wavelet transform and discrete wavelet transform, which allows the usage of those smooth (usually non-orthogonal, and they have advantages in representing smooth signals compressively, which can improve the imaging speed of single pixel imaging) wavelets and with limited numbers of measurements. The proposed imaging scheme is studied, and verified by simulations and experiments. Furthermore, a comparison between our proposed scheme and existing imaging schemes are given. According to the results, the proposed SCWT scheme is proved to be effective in reconstructing a image compressively.

Super-sensitivity measurement of tiny Doppler frequency shifts based on parametric amplification and squeezed vacuum state

Zhi-Yuan Wang(王志远), Zi-Jing Zhang(张子静), and Yuan Zhao(赵远)

Chin. Phys. B, 2021, 30 (7): 074202. DOI: 10.1088/1674-1056/abe3e5

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The precision measurement of Doppler frequency shifts is of great significance for improving the precision of speed measurement. This paper proposes a precision measurement scheme of tiny Doppler shifts by a parametric amplification process and squeezed vacuum state. This scheme takes a parametric amplification process and squeezed vacuum state into a detection system, so that the measurement precision of tiny Doppler shifts can exceed the Cramér-Rao bound of coherent light. Simultaneously, a simulation study is carried out on the theoretical basis, and the following results are obtained: for the signal light of Gaussian mode, when the amplification factor g=1 and the squeezed factor r=0.5, the measurement error of Doppler frequency shifts is 14.4% of the Cramér-Rao bound of the coherent light in our system. At the same time, when the local light mode and squeezed vacuum state mode are optimized, the measurement precision of this scheme can be further improved by $\sqrt {{\left( {2n + 1} \right)} / {\left( {n + 1} \right)}}$ times, where n is the mode-order of the signal light.

Spectral filtering of dual lasers with a high-finesse length-tunable cavity for rubidium atom Rydberg excitation Suggested by editors

Yang-Yang Liu(刘杨洋), Zhuo Fu(付卓), Peng Xu(许鹏), Xiao-Dong He(何晓东), Jin Wang(王谨), and Ming-Sheng Zhan(詹明生)

Chin. Phys. B, 2021, 30 (7): 074203. DOI: 10.1088/1674-1056/abf91b

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We propose and demonstrate an alternative method for spectral filtering and frequency stabilization of both 780-nm and 960-nm lasers using a high-finesse length-tunable cavity (HFLTC). Firstly, the length of HFLTC is stabilized to a commercial frequency reference. Then, the two lasers are locked to this HFLTC using the Pound-Drever-Hall (PDH) method which can narrow the linewidths and stabilize the frequencies of both lasers simultaneously. Finally, the transmitted lasers of HFLTC with each power up to about 100 μW, which act as seed lasers, are amplified using the injection locking method for single-atom Rydberg excitation. The linewidths of obtained lasers are narrowed to be less than 1 kHz, meanwhile the obtained lasers' phase noise around 750 kHz are suppressed about 30 dB. With the spectrally filtered lasers, we demonstrate a Rabi oscillation between the ground state and Rydberg state of single-atoms in an optical trap tweezer with a decay time of (67±37) μs, which is almost not affected by laser phase noise. We found that the maximum short-term laser frequency fluctuation of a single excitation lasers is at ～3.3 kHz and the maximum long-term laser frequency drift of a single laser is ～46 kHz during one month. Our work develops a stable and repeatable method to provide multiple laser sources of ultra-low phase noise, narrow linewidth, and excellent frequency stability, which is essential for high precision atomic experiments, such as neutral atom quantum computing, quantum simulation, quantum metrology, and so on.

Orientation dependence in high harmonics of ZnO with polarization corrections to counteract the birefringent effect Suggested by editors

Yin-Fu Zhang(张银福), Teng-Fei Huang(黄腾飞), Jia-Peng Li(李佳鹏), Ke Yang(杨可), Liang Li(李亮), Xiao-Song Zhu(祝晓松), Peng-Fei Lan(兰鹏飞), and Pei-Xiang Lu(陆培祥)

Chin. Phys. B, 2021, 30 (7): 074204. DOI: 10.1088/1674-1056/abf3b9

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We investigate the influence of the birefringence on the high-order harmonics in an a-cut ZnO crystal with mid-infrared laser pulses. The high harmonics exhibit strong dependence on the alignment of the crystal with respect to the laser polarization. We introduce the Jones calculus to counteract the birefringent effect and obtain the harmonics with polarization corrections in ZnO. We show that the birefringent effect plays an important role in the orientation dependence of HHG.

Preparation and properties of GAGG:Ce/glass composite scintillation material

Wei-Jie Zhang(张伟杰), Qin-Hua Wei(魏钦华), Xiao Shen(沈潇), Gao Tang(唐高), Zhen-Hua Chen(陈振华), Lai-Shun Qin(秦来顺), and Hong-Sheng Shi(史宏声)

Chin. Phys. B, 2021, 30 (7): 074205. DOI: 10.1088/1674-1056/abe3ea

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The translucent GGAG:Ce/glass composites are prepared successfully by ball-milling, tableting, and pressureless sintering. The thickness of composites is about 400 μm. The x-ray diffraction (XRD), differential scanning calorimetry (DSC), density of composite materials are measured and discussed systematically. The scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) elemental mapping are employed to analyze the particle size, the shape of powders, and the distribution of GGAG:Ce particles in the glass matrix, respectively. The decay time, ultraviolet, (UV), x-ray excitation luminescence spectra, and temperature spectra are studied. The results show that the composite materials have high light output, good thermostability, and short decay time. The method adopted in this work is an effective method to reduce the preparation time and cost of the sample. The ultralow afterglow indicates that the composite materials have an opportunity to be used for x-ray detection and imaging.

Monte Carlo simulations of electromagnetically induced transparency in a square lattice of Rydberg atoms

Shang-Yu Zhai(翟尚宇) and Jin-Hui Wu(吴金辉)

Chin. Phys. B, 2021, 30 (7): 074206. DOI: 10.1088/1674-1056/abd75a

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We study the steady optical response of a square lattice in which all trapped atoms are driven by a probe and a coupling fields into the ladder configuration of electromagnetically induced transparency (EIT). It turns out to be a many-body problem in the presence of van der Waals (vdW) interaction among atoms in the upmost Rydberg state, so Monte Carlo (MC) calculation based on density matrix equations have been done after introducing a sufficiently large cut-off radius. It is found that the absorption and dispersion of EIT spectra depends critically on a few key parameters like lattice dimension, unitary vdW shift, probe Rabi frequency, and coupling detuning. Through modulating these parameters, it is viable to change symmetries of the absorption and dispersion spectra and control on demand depth and position of the transparency window. Our MC calculation is expected to be instructive in understanding many-body quantum coherence effects and in manipulating non-equilibrium quantum phenomena by utilizing vdW interactions of Rydberg atoms.

Fe-doped ZnS film fabricated by electron beam evaporation and its application as saturable absorber for Er:ZBLAN fiber laser

Jiu-Lin Yang(杨久林), Guo-Ying Feng(冯国英), Du-Xin Qing(卿杜鑫), Ya-Jie Wu(吴雅婕), Yun Luo(罗韵), and Jian-Jun Wang(王建军)

Chin. Phys. B, 2021, 30 (7): 074207. DOI: 10.1088/1674-1056/abf343

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High-quality Fe-doped ZnS films have been fabricated by electron beam evaporation. After the doping, the fabricated films still maintain the preferential crystalline orientation and phase purity of the host ZnS. According to the observation of surface morphology, the root mean-square roughness of the samples increases slightly with the increase of doping content. All of the prepared samples are in cubic zinc blende structure of ZnS. Transmission spectrum confirms a more obvious dip near 3 μm with higher dopant concentration and it can be attributed to the typical ⁵E→⁵T₂ transition of Fe²⁺. Fe-doped ZnS film is also successfully used for Q-switched Er:ZBLAN fiber laser.

Modeling of cascaded high isolation bidirectional amplification in long-distance fiber-optic time and frequency synchronization system

Kuan-Lin Mu(穆宽林), Xing Chen(陈星), Zheng-Kang Wang(王正康), Yao-Jun Qiao(乔耀军), and Song Yu(喻松)

Chin. Phys. B, 2021, 30 (7): 074208. DOI: 10.1088/1674-1056/abf105

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We propose a physical model of estimating noise and asymmetry brought by high isolation Bi-directional erbium-doped fiber amplifiers (Bi-EDFAs), no spontaneous lasing even with high gain, in longdistance fiber-optic time and frequency (T/F) synchronization system. It is found that the Rayleigh scattering noise can be suppressed due to the high isolation design, but the amplified spontaneous emission (ASE) noise generated by the high isolation Bi-EDFA and the bidirectional asymmetry of the transmission link caused by the high isolation Bi-EDFA will deteriorate the stability of the system. The calculated results show that under the influence of ASE noise, the frequency instability of a 1200 km system composed of 15 high isolation Bi-EDFAs is 1.773×10^-13/1 s. And the instability caused by asymmetry is 2.6064×10^-16/30000-35000 s if the total asymmetric length of the bidirectional link length is 30 m. The intensity noises originating from the laser and detector, the transfer delay fluctuations caused by the variation in ambient temperature and the jitter in laser output wavelength are also studied. The experiment composed of three high isolation Bi-EDFAs is done to confirm the theoretical analysis. In summary, the paper shows that the short-term instability of the T/F synchronization system composed of high isolation Bi-EDFAs is limited by the accumulation of ASE noise of amplifiers and the laser frequency drift, while the long-term instability is limited by the periodic variation in ambient temperature and the asymmetry of the amplifiers. The research results are useful for pointing out the direction to improve the stability of the fiber-optic T/F synchronization system.

An ultrasonic multi-wave focusing and imaging method for linear phased arrays

Yu-Xiang Dai(戴宇翔), Shou-Guo Yan(阎守国), and Bi-Xing Zhang(张碧星)

Chin. Phys. B, 2021, 30 (7): 074301. DOI: 10.1088/1674-1056/abf91d

Abstract ( 329 )

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To overcome the inherent limits of traditional single wave imaging for nondestructive testing, the multi-wave focusing and imaging method is thoroughly studied. This method makes the compressional waves and shear waves focused in both emission and reception processes, which strengthens the focusing energy and improves the signal-to-noise ratio of received signals. A numerical model is developed to study the characteristics of a multi-wave focusing field. It is shown that the element width approaching 0.8 wavelengths of shear waves can keep a balance between the radiation energy of two waves, which can achieve a desirable multi-wave focusing performance. And an experiment using different imaging methods for a linear phased array is performed. It can be concluded that due to the combination of the propagation and reflection characteristics of two waves, the multi-wave focusing and imaging method can significantly improve the imaging distinguishability of defects and expand the available sweeping range to a sector of -65° to 65°.

Investigation of hypersonic flows through a cavity with sweepback angle in near space using the DSMC method

Guangming Guo(郭广明), Hao Chen(陈浩), Lin Zhu(朱林), and Yixiang Bian(边义祥)

Chin. Phys. B, 2021, 30 (7): 074701. DOI: 10.1088/1674-1056/abf7ad

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Near space has been paid more and more attentionin recent years due to its military application value. However, flow characteristics of some fundamental configurations (e.g., the cavity) in near space have rarely been investigated due to rarefied gas effects, which make the numerical simulation methods based on continuous flow hypothesis lose validity. In this work, the direct simulation Monte Carlo (DSMC), one of the most successful particle simulation methods in treating rarefied gas dynamics, is employed to explore flow characteristics of a hypersonic cavity with sweepback angle in near space by considering a variety of cases, such as the cavity at a wide range of altitudes 20-60 km, the cavity at freestream Mach numbers of 6-20, and the cavity with a sweepback angle of 30°-90°. By analyzing the simulation results, flow characteristics are obtained and meanwhile some interesting phenomena are also found. The primary recirculation region, which occupies the most area of the cavity, causes pressure and temperature stratification due to rotational motion of fluid inside it, whereas the pressure and temperature in the secondary recirculation region, which is a small vortex and locates at the lower left corner of the cavity, change slightly due to low-speed movement of fluid inside it. With the increase of altitude, both the primary and secondary recirculation regions contract greatly and it causes them to separate. A notable finding is that rotation direction of the secondary recirculation region would be reversed at a higher altitude. The overall effect of increasing the Mach number is that the velocity, pressure, and temperature within the cavity increase uniformly. The maximum pressure nearby the trailing edge of the cavity decreases rapidly as the sweepback angle increases, whereas the influence of sweepback angle on velocity distribution and maximum temperature within the cavity is slight.

Effect of the particle temperature on lift force of nanoparticle in a shear rarefied flow Suggested by editors

Jun-Jie Su(苏俊杰), Jun Wang(王军), and Guo-Dong Xia(夏国栋)

Chin. Phys. B, 2021, 30 (7): 075101. DOI: 10.1088/1674-1056/abf351

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The nanoparticles suspended in a shear flow are subjected to a shear lift force, which is of great importance for the nanoparticle transport. In previous theoretical analysis on the shear lift, it is usually assumed that the particle temperature is equal to the temperature of the surrounding gas media. However, in some particular applications, the particle temperature can significantly differ from the gas temperature. In the present study, the effect of particle temperature on the shear lift of nanoparticles is investigated and the corresponding formulas of shear lift force are derived based on the gas kinetic theory. For extremely small nanoparticles (with radius R<2 nm) or large nanoparticles (R>20 nm), the influence of the particle temperature can be neglected. For the intermediate particle size, the relative error induced by the equal gas-particle temperature can be significant. Our findings can bring an insight into accurate evaluation of the nanoparticle transport properties.

Structure and magnetic properties of RAlSi (R=light rare earth)

Tai Wang(王泰), Yongquan Guo(郭永权), and Cong Wang(王聪)

Chin. Phys. B, 2021, 30 (7): 075102. DOI: 10.1088/1674-1056/abe3e8

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We prepared the semimetals RAlSi (R,=,light rare earth), and systematically study their crystal structures and magnetic properties. X-ray diffractions confirm the coexistence of the site-disordered phase with group space of $I4_1/amd$ and the noncentrosymmetrically ordered phase with space group of $I4_1md$ in RAlSi alloy. The ordered phase is the main phase in RAlSi alloy. RAlSi alloys show nonmagnetic character for ${\rm R}={\rm La}$, low temperature ferromagnetic order for ${\rm R}={\rm Ce}$, Pr, and paramagnetic character for ${\rm R}={\rm Nd}$, respectively. SmAlSi shows metamagnetic transition at 10 K and ferromagnetic order at 143 K, respectively. SmAlSi follows the van Vleck paramagnetic model in its paramagnetic region. The magnetization curves of RAlSi (${\rm R}={\rm Ce}$, Pr, Sm) follow the mixed model of ferromagnetism and paramagnetism, and the fitted saturation moment $M_{\rm S}$ depends on the moment of trivalent rare earth. The paramagnetic susceptibility $\chi $ of RAlSi is going up with increasing the atomic order numbers of rare earth elements. This reveals that the magnetic property of RAlSi originates from the rare earth.

Synthesis of SiC/graphene nanosheet composites by helicon wave plasma

Jia-Li Chen(陈佳丽), Pei-Yu Ji(季佩宇), Cheng-Gang Jin(金成刚), Lan-Jian Zhuge(诸葛兰剑), and Xue-Mei Wu(吴雪梅)

Chin. Phys. B, 2021, 30 (7): 075201. DOI: 10.1088/1674-1056/abe1a1

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We report an approach to the rapid, one-step, preparation of a variety of wide-bandgap silicon carbide/graphene nanosheet (SiC/GNSs) composites by using a high-density helicon wave plasma (HWP) source. The microstructure and morphology of the SiC/GNSs are characterized by using scanning electron microscopy (SEM), Raman spectroscopy, x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), and fluorescence (PL). The nucleation mechanism and the growth model are discussed. The existence of SiC and graphene structure are confirmed by XRD and Raman spectra. The electron excitation temperature is calculated by the intensity ratio method of optical emission spectroscopy. The main peak in the PL test is observed at 420 nm, with a corresponding bandgap of 2.95 eV that indicates the potential for broad application in blue light emission and ultraviolet light emission, field electron emission, and display devices.

Neoclassical tearing mode stabilization by electron cyclotron current drive for HL-2M tokamak Suggested by editors

Jing-Chun Li(李景春), Jia-Qi Dong(董家齐), Xiao-Quan Ji(季小全), and You-Jun Hu(胡友俊)

Chin. Phys. B, 2021, 30 (7): 075203. DOI: 10.1088/1674-1056/abfb52

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Investigation of neoclassical tearing mode and its suppression by electron cyclotron current drive (ECCD) has been carried out in HL-2M tokamak. The current driving capability of the electron cyclotron wave is evaluated. It is found that the deposition location can be effectively controlled by changing the poloidal angle. The validation of electron cyclotron wave heating and current driving has been demonstrated for the upper launcher port. We show that 3.0 MW and 2.5 MW modulated ECCD can completely stabilize (2,1) and (3,2) NTMs, respectively. The non-modulated ECCD, radial misalignment as well as current profile broadening have deleterious effect on the NTM stabilization. The time required for suppression of (3,2) mode is shorter than that required for the suppression of (2,1) mode. Moreover, the time needed for complete stabilization at different initial island width has been quantitatively presented and analyzed.

Reduction of impurity confinement time by combined heating of LHW and ECRH in EAST

Zong Xu(许棕), Zhen-Wei Wu(吴振伟), Ling Zhang(张凌), Yue-Heng Huang(黄跃恒), Wei Gao(高伟), Yun-Xin Cheng(程云鑫), Xiao-Dong Lin(林晓东), Xiang Gao(高翔), Ying-Jie Chen(陈颖杰), Lei Li(黎嫘), Yin-Xian Jie(揭银先), Qing Zang(臧庆), Hai-Qing Liu(刘海庆), and EAST team

Chin. Phys. B, 2021, 30 (7): 075205. DOI: 10.1088/1674-1056/abe1a6

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The core impurity confinement properties are experimentally investigated in the Experimental Advanced Superconducting Tokamak (EAST) plasma heated by lower hybrid wave (LHW) and electron cyclotron resonance heating (ECRH) (LHW+ECRH). It is shown that the impurity confinement time (τ_imp) in the L-mode plasma jointly heated by LHW and ECRH is weakly dependent on electron density but strongly dependent on the heating power, thus it is shorter than that in LHW-only heated L-mode plasma with the similar plasma parameters. The combined heating of LHW and ECRH can reduce the collisionality and indicates a more effective heating method for core τ_imp reduction and normalized poloidal beta (β_P) improvement. It should be emphasized that in this high β_P operation window the small ELM regime can be accessed, and an L-mode level τ_imp (40 ms-80 ms) and high β_N (～1.7) can be obtained simultaneously. It means that this typical small ELMy H-mode regime has an advantage in avoiding the serious tungsten accumulation, and will be competitive in future long-pulse steady-state and high-performance operation with high-Z material plasma-facing components.

Heredity of clusters in the rapidly cooling processes of Al-doped Zr₅₀Cu₅₀ melts and its correlation with the glass-forming ability

Dadong Wen(文大东), Yonghe Deng(邓永和), Ming Gao(高明), and Zean Tian(田泽安)

Chin. Phys. B, 2021, 30 (7): 076101. DOI: 10.1088/1674-1056/abea93

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The heredity of clusters in rapidly cooled (Zr₅₀Cu₅₀)_100-xAl_x melts and its correlation with glass-forming ability (GFA) are studied via molecular dynamics simulations. Pair distribution function and the largest standard cluster (LSC) are adopted to characterize the local atomic structures in the (Zr₅₀Cu₅₀)_100-xAl_x systems. The [12/555] icosahedra and their medium-range order (IMRO) play an important role in forming (Zr₅₀Cu₅₀)_100-xAl_x metallic glasses (MGs). The fraction of [12/555], the number of IMRO, and the maximum size of IMRO in MGs increase significantly with increasing x. A tracking study further reveals that the configuration heredity of icosahedral clusters starts from supercooled liquids. No direct correlation exists between the GFA and the onset temperature of continuous or stated heredity. Instead, a larger hereditary supercooled degree of icosahedra matches with better GFA of Al-doped Zr₅₀Cu₅₀ alloys.

Non-monotonic temperature evolution of nonlocal structure-dynamics correlation in CuZr glass-forming liquids Suggested by editors

W J Jiang(江文杰) and M Z Li(李茂枝)

Chin. Phys. B, 2021, 30 (7): 076102. DOI: 10.1088/1674-1056/abf113

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The structure-dynamics correlations in a nonlocal manner were investigated in CuZr metallic glass-forming liquids via classical molecular dynamics simulations. A spatial coarse-graining approach was employed to incorporate the nonlocal structural information of given structural order parameters in the structure-dynamics relationship. It is found that the correlation between structure order parameters and dynamics increases with increasing coarse-graining length and has a characteristic length scale. Moreover, the characteristic correlation length exhibits a non-monotonic temperature evolution as temperature approaches glass transition temperature, which is not sensitive to the considered structure order parameters. Our results unveil a striking change in the structure-dynamics correlation, which involves no fitting theoretical interpretation. These findings provide new insight into the structure-dynamics correlation in glass transition.

Small activation entropy bestows high-stability of nanoconfined D-mannitol Suggested by editors

Lin Cao(曹琳), Li-Jian Song(宋丽建), Ya-Ru Cao(曹亚茹), Wei Xu(许巍), Jun-Tao Huo(霍军涛), Yun-Zhuo Lv(吕云卓), and Jun-Qiang Wang(王军强)

Chin. Phys. B, 2021, 30 (7): 076103. DOI: 10.1088/1674-1056/abf919

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It has been a long-standing puzzling problem that some glasses exhibit higher glass transition temperatures (denoting high stability) but lower activation energy for relaxations (denoting low stability). In this paper, the relaxation kinetics of the nanoconfined D-mannitol (DM) glass was studied systematically using a high-precision and high-rate nanocalorimeter. The nanoconfined DM exhibits enhanced thermal stability compared to the free DM. For example, the critical cooling rate for glass formation decreases from 200 K/s to below 1 K/s; the T_g increases by about 20 K-50 K. The relaxation kinetics is analyzed based on the absolute reaction rate theory. It is found that, even though the activation energy E^* decreases, the activation entropy S^* decreases much more for the nanoconfined glass that yields a large activation free energy G^* and higher thermal stability. These results suggest that the activation entropy may provide new insights in understanding the abnormal kinetics of nanoconfined glassy systems.

A novel two-dimensional SiO sheet with high-stability, strain tunable electronic structure, and excellent mechanical properties Suggested by editors

Shijie Liu(刘世杰) and Hui Du(杜慧)

Chin. Phys. B, 2021, 30 (7): 076104. DOI: 10.1088/1674-1056/abf34d

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Using the structure search of particle swarm optimization (PSO) algorithm combined with density functional theory (DFT), we conduct a systematic two-dimensional (2D) material research on the SiO and discover a P2 monolayer structure. The phonon spectrum shows that the 2D P2 is dynamic-stable under ambient pressure. Molecular dynamics simulations show that 2D P2 can still exist stably at a high temperature of 1000 K, indicating that 2D P2 has application potential in high-temperature environments. The intrinsic 2D P2 structure has a quasi-direct band gap of 3.2 eV. The 2D P2 structure can be transformed into a direct band gap semiconductor by appropriate strain, and the band gap can be adjusted to the ideal band gap of 1.2 eV-1.6 eV for photovoltaic materials. These unique properties of the 2D P2 structure make it expected to have potential applications in nanomechanics and nanoelectronics.

Magnetoelectric coupling effect of polarization regulation in BiFeO₃/LaTiO₃ heterostructures

Chao Jin(金超), Feng-Zhu Ren(任凤竹), Wei Sun(孙伟), Jing-Yu Li(李静玉), Bing Wang(王冰), and Qin-Fen Gu(顾勤奋)

Chin. Phys. B, 2021, 30 (7): 076105. DOI: 10.1088/1674-1056/abf924

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An effective regulation of the magnetism and interface of ferromagnetic materials is not only of great scientific significance, but also has an urgent need in modern industry. In this work, by using the first-principles calculations, we demonstrate an effective approach to achieve non-volatile electrical control of ferromagnets, which proves this idea in multiferroic heterostructures of ferromagnetic LaTiO₃ and ferroelectric BiFeO₃. The results show that the magnetic properties and two-dimensional electron gas concentrations of LaTiO₃ films can be controlled by changing the polarization directions of BiFeO₃. The destroyed symmetry being introduced by ferroelectric polarization of the system leads to the transfer and reconstruction of the Ti-3d electrons, which is the fundamental reason for the changing of magnetic properties. This multiferroic heterostructures will pave the way for non-volatile electrical control of ferromagnets and have potential applications.

Structural modulation and physical properties of cobalt-doped layered La₂M₅As₃O₂ (M= Cu, Ni) compounds Suggested by editors

Lei Yang(杨蕾), Yan-Peng Song(宋艳鹏), Jun-Jie Wang(王俊杰), Xu Chen(陈旭), Hui-Jing Du(杜会静), and Jian-Gang Guo(郭建刚)

Chin. Phys. B, 2021, 30 (7): 076106. DOI: 10.1088/1674-1056/abf349

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We investigate the structural variation and physical properties of layered La₂M₅As₃O₂ (M=Cu, Ni) compound upon Co doping. It is found that the substitution of Co ion just induces the monotonous change of lattice constants without observing the anomalous kink in superconducting La₂(Cu_1-xNi_x)₅As₃O₂ solid-solutions. Meanwhile, this doping barely changes As-As bond length in [M₅As₃]^2- subunit (±2%), being significantly smaller than 7% shrinkage of that in La₂(Cu_1-xNi_x)₅As₃O₂. Therefore, the doping dependence of crystal structure exhibits similar trend with Ba_1-xK_xFe₂As₂ without the interference of As1-As2 bonding, implying that the Co substitution for Cu/Ni is hole-doped. In terms of physical property, La₂(Cu_1-xCo_x)₅As₃O₂ turns into itinerant ferromagnetic metal, while La₂(Ni_1-xCo_x)₅As₃O₂ shows paramagnetism and suppressed structural phase transition upon Co-doping. The distinct structural variation and absence of superconductivity provide important clues to understand the effect of As-As bond in [M₅As₃]^2- subunit.

Prediction of scandium tetraboride from first-principles calculations: Crystal structures, phase stability, mechanical properties,and hardness

Bin-Hua Chu(初斌华) and Yuan Zhao(赵元)

Chin. Phys. B, 2021, 30 (7): 076107. DOI: 10.1088/1674-1056/abe116

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Using the evolutionary methodology for crystal structure prediction, we have predicted the orthorhombic Cmcm and Pnma phases for ScB₄. The earlier proposed CrB₄-, FeB₄-, MnB₄-, and ReP₄-type structures for ScB₄ are excluded. It is first discovered that the Cmcm phase transforms to the Pnma phase at about 18 GPa. Moreover, both phases are dynamically and mechanically stable. The large bulk modulus, shear modulus, and Young's modulus of the two phases make it an optimistic low compressible material. Moreover, the strong covalent bonding nature of ScB₄ is confirmed by the ELF analysis. The strong covalent bonding contributes greatly to its stability.

Structural and electrical transport properties of charge density wave material LaAgSb₂ under high pressure Suggested by editors

Bowen Zhang(张博文), Chao An(安超), Xuliang Chen(陈绪亮), Ying Zhou(周颖), Yonghui Zhou(周永惠), Yifang Yuan(袁亦方), Chunhua Chen(陈春华), Lili Zhang(张丽丽), Xiaoping Yang(杨晓萍), and Zhaorong Yang(杨昭荣)

Chin. Phys. B, 2021, 30 (7): 076201. DOI: 10.1088/1674-1056/abf643

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Layered lanthanum silver antimonide LaAgSb₂ exhibits both charge density wave (CDW) order and Dirac-cone-like band structure at ambient pressure. Here, we systematically investigate the pressure evolution of structural and electronic properties of LaAgSb₂ single crystal. We show that the CDW order is destabilized under compression, as evidenced by the gradual suppression of magnetoresistance. At P_C～ 22 GPa, synchrotron x-ray diffraction and Raman scattering measurements reveal a structural modification at room-temperature. Meanwhile, the sign change of the Hall coefficient is observed at 5 K. Our results demonstrate the tunability of CDW order in the pressurized LaAgSb₂ single crystal, which can be helpful for its potential applications in the next-generation devices.

Inverted V-shaped evolution of superconducting temperature in SrBC under pressure Suggested by editors

Ru-Yi Zhao(赵如意), Xun-Wang Yan(闫循旺), and Miao Gao(高淼)

Chin. Phys. B, 2021, 30 (7): 076301. DOI: 10.1088/1674-1056/abfbcc

Abstract ( 418 )

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Based on density functional first-principles calculations and anisotropic Eliashberg equations, we have investigated the electronic structure, lattice dynamics, and phonon-mediated superconductivity in newly synthesized layered compound SrBC under pressure. Different from LiBC and MgB₂, our calculations surprisingly reveal that SrBC is isotropic in compressibility, due to the accumulation of substantial electrons in the interstitial region. We find that the Sr phonons strongly couple with B-2p_z orbital and the interstitial states, giving rise to a two-gap superconductivity in SrBC, whose transition temperature shows an inverted V-shaped dependence on pressure. The maximal transition temperature is about 22 K at 50 GPa. On both sides of 50 GPa, the transition temperature exhibits quasi-linear variation with positive and negative slopes, respectively. Such a variation of transition temperature is infrequent among phonon-mediated superconductors. The competition between enhanced electron-phonon matrix element and hardened phonons plays an essential role in governing the behavior of the critical temperature.

Anomalous bond-length behaviors of solid halogens under pressure

Min Wu(吴旻), Ye-Feng Wu(吴烨峰), and Yi Ma(马毅)

Chin. Phys. B, 2021, 30 (7): 076401. DOI: 10.1088/1674-1056/abf4bd

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The three halogen solids (Cl₂, Br₂, and I₂) have the isostructural diatomic molecular phase I with a space group of Cmca at ambient pressure. At high pressure, they all go through an intermediate phase V with incommensurate structures before eventually dissociating into the monatomic phase Ⅱ. However, a new structural transition between phase I and V with anomalous bond-length behavior was observed in bromine under pressure, which, so far, has not been confirmed in iodine and chlorine. Here, we perform first-principles calculations for iodine and chlorine. The new structural transition was predicted to be common to all three halogens under pressure. The transition pressures might be systematically underestimated by the imperfect van der Waals correction method, but they follow the order Cl₂ > Br₂ > I₂, which is consistent with other pressure-induced structural transitions such as metallization and the molecular-to-monatomic transition.

Strain-tunable electronic and optical properties of h-BN/BC₃ heterostructure with enhanced electron mobility

Zhao-Yong Jiao(焦照勇), Yi-Ran Wang(王怡然), Yong-Liang Guo(郭永亮), and Shu-Hong Ma(马淑红)

Chin. Phys. B, 2021, 30 (7): 076801. DOI: 10.1088/1674-1056/abe29d

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By using first-principles calculation, we study the properties of h-BN/BC₃ heterostructure and the effects of external electric fields and strains on its electronic and optical properties. It is found that the semiconducting h-BN/BC₃ has good dynamical stability and ultrahigh stiffness, enhanced electron mobility, and well-preserved electronic band structure as the BC₃ monolayer. Meanwhile, its electronic band structure is slightly modified by an external electric field. In contrast, applying an external strain can mildly modulate the electronic band structure of h-BN/BC₃ and the optical property exhibits an apparent redshift under a compressive strain relative to the pristine one. These findings show that the h-BN/BC₃ hybrid can be designed as optoelectronic device with moderately strain-tunable electronic and optical properties.

Fabrication of GaAs/SiO₂/Si and GaAs/Si heterointerfaces by surface-activated chemical bonding at room temperature Suggested by editors

Rui Huang(黄瑞), Tian Lan(兰天), Chong Li(李冲), Jing Li(李景), and Zhiyong Wang(王智勇)

Chin. Phys. B, 2021, 30 (7): 076802. DOI: 10.1088/1674-1056/abf917

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The room-temperature (RT) bonding mechanisms of GaAs/SiO₂/Si and GaAs/Si heterointerfaces fabricated by surface-activated bonding (SAB) are investigated using a focused ion beam (FIB) system, cross-sectional scanning transmission electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDX) and scanning acoustic microscopy (SAM). According to the element distribution detected by TEM and EDX, it is found that an intermixing process occurs among different atoms at the heterointerface during the RT bonding process following the surface-activation treatment. The diffusion of atoms at the interface is enhanced by the point defects introduced by the process of surface activation. We can confirm that through the point defects, a strong heterointerface can be created at RT. The measured bonding energies of GaAs/SiO₂/Si and GaAs/Si wafers are 0.7 J/m² and 0.6 J/m². The surface-activation process can not only remove surface oxides and generate dangling bonds, but also enhance the atomic diffusivity at the interface.

Non-Hermitian Kitaev chain with complex periodic and quasiperiodic potentials Suggested by editors

Xiang-Ping Jiang(蒋相平), Yi Qiao(乔艺), and Junpeng Cao(曹俊鹏)

Chin. Phys. B, 2021, 30 (7): 077101. DOI: 10.1088/1674-1056/abfa08

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We study the topological properties of the one-dimensional non-Hermitian Kitaev model with complex either periodic or quasiperiodic potentials. We obtain the energy spectrum and the phase diagrams of the system by using the transfer matrix method as well as the topological invariant. The phase transition points are given analytically. The Majorana zero modes in the topological nontrivial regimes are obtained. Focusing on the quasiperiodic potential, we obtain the phase transition from the topological superconducting phase to the Anderson localization, which is accompanied with the Anderson localization-delocalization transition in this non-Hermitian system. We also find that the topological regime can be reduced by increasing the non-Hermiticity.

Electronic structures of vacancies in Co₃Sn₂S₂ Suggested by editors

Yuxiang Gao(高于翔), Xin Jin(金鑫), Yixuan Gao(高艺璇), Yu-Yang Zhang(张余洋), and Shixuan Du(杜世萱)

Chin. Phys. B, 2021, 30 (7): 077102. DOI: 10.1088/1674-1056/abfa05

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Co₃Sn₂S₂ has attracted a lot of attention for its multiple novel physical properties, including topological nontrivial surface states, anomalous Hall effect, and anomalous Nernst effect. Vacancies, which play important roles in functional materials, have attracted increasing research attention. In this paper, by using density functional theory calculations, we first obtain band structures and magnetic moments of Co₃Sn₂S₂ with exchange-correlation functionals at different levels. It is found that the generalized gradient approximation gives the positions of Weyl points consistent with experiments in bulk Co₃Sn₂S₂. We then investigate the electronic structures of defects on surfaces with S and Sn terminations which have been observed in experiments. The results show that the single sulfur vacancy on the S-terminated surface introduces localized bond states inside the bandgap near the Fermi level. For di- and tri-sulfur vacancies, the localized defect states hybridize with neighboring ones, forming bonding states as well as anti-bonding states. The Sn vacancy on the Sn-terminated surface also introduces localized bond states, which are merged with the valence bands. These results provide a reference for future experimental investigations of vacancies in Co₃Sn₂S₂.

Tunable bandgaps and flat bands in twisted bilayer biphenylene carbon Suggested by editors

Ya-Bin Ma(马亚斌), Tao Ouyang(欧阳滔), Yuan-Ping Chen(陈元平), and Yue-E Xie(谢月娥)

Chin. Phys. B, 2021, 30 (7): 077103. DOI: 10.1088/1674-1056/ac009e

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Owing to the interaction between the layers, the twisted bilayer two-dimensional (2D) materials exhibit numerous unique optical and electronic properties different from the monolayer counterpart, and have attracted tremendous interests in current physical research community. By means of first-principles and tight-binding model calculations, the electronic properties of twisted bilayer biphenylene carbon (BPC) are systematically investigated in this paper. The results indicate that the effect of twist will not only leads to a phase transition from semiconductor to metal, but also an adjustable band gap in BPC (0 meV to 120 meV depending on the twist angle). Moreover, unlike the twisted bilayer graphene (TBG), the flat bands in twisted BPC are no longer restricted by “magic angles” i.e., abnormal flat bands could be appeared as well at several specific large angles in addition to the small angles. The charge density of these flat bands possesses different local modes, indicating that they might be derived from different stacked modes and host different properties. The exotic physical properties presented in this work foreshow twisted BPC a promising material for the application of terahertz and infrared photodetectors and the exploration of strong correlation.

Bilayer twisting as a mean to isolate connected flat bands in a kagome lattice through Wigner crystallization

Jing Wu(吴静), Yue-E Xie(谢月娥), Ming-Xing Chen(陈明星), Jia-Ren Yuan(袁加仁), Xiao-Hong Yan(颜晓红), Sheng-Bai Zhang(张绳百), and Yuan-Ping Chen(陈元平)

Chin. Phys. B, 2021, 30 (7): 077104. DOI: 10.1088/1674-1056/abd7d6

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The physics of flat band is novel and rich but difficult to access. In this regard, recently twisting of bilayer van der Waals (vdW)-bounded two-dimensional (2D) materials has attracted much attention, because the reduction of Brillouin zone will eventually lead to a diminishing kinetic energy. Alternatively, one may start with a 2D kagome lattice, which already possesses flat bands at the Fermi level, but unfortunately these bands connect quadratically to other (dispersive) bands, leading to undesirable effects. Here, we propose, by first-principles calculation and tight-binding modeling, that the same bilayer twisting approach can be used to isolate the kagome flat bands. As the starting kinetic energy is already vanishingly small, the interlayer vdW potential is always sufficiently large irrespective of the twisting angle. As such the electronic states in the (connected) flat bands become unstable against a spontaneous Wigner crystallization, which is expected to have interesting interplays with other flat-band phenomena such as novel superconductivity and anomalous quantum Hall effect.

Quantifying plasmon resonance and interband transition contributions in photocatalysis of gold nanoparticle

Liang Dong(董亮), Chengyun Zhang(张成云), Lei Yan(严蕾), Baobao Zhang(张宝宝), Huan Chen(陈环), Xiaohu Mi(弥小虎), Zhengkun Fu(付正坤), Zhenglong Zhang(张正龙), and Hairong Zheng(郑海荣)

Chin. Phys. B, 2021, 30 (7): 077301. DOI: 10.1088/1674-1056/abfa0c

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Localized surface plasmon has been extensively studied and used for the photocatalysis of various chemical reactions. However, the different contributions between plasmon resonance and interband transition in photocatalysis has not been well understood. Here, we study the photothermal and hot electrons effects for crystal transformation by combining controlled experiments with numerical simulations. By photo-excitation of NaYF₄:Eu³⁺@Au composite structure, it is found that the plasmonic catalysis is much superior to that of interband transition in the experiments, owing to the hot electrons generated by plasmon decay more energetic to facilitate the reaction. We emphasize that the energy level of hot electrons plays an essential role for improving the photocatalytic activity. The results provide guidelines for improving the efficiency of plasmonic catalysis in future experimental design.

Thermodynamic criterion for searching high mobility two-dimensional electron gas at KTaO₃ interface ^Hot!

Wen-Xiao Shi(时文潇), Hui Zhang(张慧), Shao-Jin Qi(齐少锦), Jin-E Zhang(张金娥), Hai-Lin Huang(黄海林), Bao-Gen Shen(沈保根), Yuan-Sha Chen(陈沅沙), and Ji-Rong Sun(孙继荣)

Chin. Phys. B, 2021, 30 (7): 077302. DOI: 10.1088/1674-1056/ac078c

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Two-dimensional electron gases (2DEGs) formed at the interface between two oxide insulators present a promising platform for the exploration of emergent phenomena. While most of the previous works focused on SrTiO$_{3}$-based 2DEGs, here we took the amorphous-ABO$_{3}$/KTaO$_{3}$ system as the research object to study the relationship between the interface conductivity and the redox property of B-site metal in the amorphous film. The criterion of oxide-oxide interface redox reactions for the B-site metals, Zr, Al, Ti, Ta, and Nb in conductive interfaces was revealed: the formation heat of metal oxide, ${\Delta H}_{\rm f}^{\rm o}$, is lower than $-350 $ kJ/(mol O) and the work function of the metal $\varPhi $ is in the range of 3.75 eV$ <\varPhi <4.4$ eV. Furthermore, we found that the smaller absolute value of ${\Delta H}_{\rm f}^{\rm o}$ and the larger value of $\varPhi $ of the B-site metal would result in higher mobility of the two-dimensional electron gas that formed at the corresponding amorphous-ABO$_{3}$/KTaO$_{3}$ interface. This finding paves the way for the design of high-mobility all-oxide electronic devices.

Impact of O₂ post oxidation annealing on the reliability of SiC/SiO₂ MOS capacitors

Peng Liu(刘鹏), Ji-Long Hao(郝继龙), Sheng-Kai Wang(王盛凯), Nan-Nan You(尤楠楠), Qin-Yu Hu(胡钦宇), Qian Zhang(张倩), Yun Bai(白云), and Xin-Yu Liu(刘新宇)

Chin. Phys. B, 2021, 30 (7): 077303. DOI: 10.1088/1674-1056/abf644

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The effects of dry O₂ post oxidation annealing (POA) at different temperatures on SiC/SiO₂ stacks are comparatively studied in this paper. The results show interface trap density (D_it) of SiC/SiO₂ stacks, leakage current density (J_g), and time-dependent dielectric breakdown (TDDB) characteristics of the oxide, are affected by POA temperature and are closely correlated. Specifically, D_it, J_g, and inverse median lifetime of TDDB have the same trend against POA temperature, which is instructive for SiC/SiO₂ interface quality improvement. Moreover, area dependence of TDDB characteristics for gate oxide on SiC shows different electrode areas lead to same slope of TDDB Weibull curves.

High-performance self-powered photodetector based on organic/inorganic hybrid van der Waals heterojunction of rubrene/silicon Suggested by editors

Yancai Xu(徐彦彩), Rong Zhou(周荣), Qin Yin(尹钦), Jiao Li(李娇), Guoxiang Si(佀国翔), and Hongbin Zhang(张洪宾)

Chin. Phys. B, 2021, 30 (7): 077304. DOI: 10.1088/1674-1056/abf345

Abstract ( 343 )

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Organic/inorganic hybrid van der Waals heterostructure with an atomically abrupt interface has attracted great research interests within the field of multifunctional electronic and optoelectronic devices. The integration of organic rubrene films with inorganic Si semiconductors can avoid the atomic mutual-diffusion at the interface, and provide the possibility of forming two-dimensional van der Waals heterojunction accompanied with the type-Ⅱ energy band alignment, due to the transfer behaviors of majority carriers at the interface. In this study, the high-quality rubrene/Si van der Waals heterostructure with an electronically abrupt junction was prepared, and a self-powered photodetector was then constructed based on this hybrid heterojunction. The photodetector demonstrated an excellent switching response to the 1064 nm monochromatic light with large on/off current ratio of 7.0×10³, the maximum photocurrent of 14.62 mA, the maximum responsivity of 2.07 A/W, the maximum detectivity of 2.9×10¹¹ Jones, and a fast response time of 13.0 μs. This study offers important guidance for preparing high-quality rubrene/Si hybrid van der Waals heterostructure with desirable band alignment, and the designed heterojunction photodetector has an important application prospect in the field of multifunctional optoelectronics.

Degradation of gate-recessed MOS-HEMTs and conventional HEMTs under DC electrical stress

Yi-Dong Yuan(原义栋), Dong-Yan Zhao(赵东艳), Yan-Rong Cao(曹艳荣), Yu-Bo Wang(王于波), Jin Shao(邵瑾), Yan-Ning Chen(陈燕宁), Wen-Long He(何文龙), Jian Du(杜剑), Min Wang(王敏), Ye-Ling Peng(彭业凌), Hong-Tao Zhang(张宏涛), Zhen Fu(付振), Chen Ren(任晨), Fang Liu(刘芳), Long-Tao Zhang(张龙涛), Yang Zhao(赵扬), Ling Lv(吕玲), Yi-Qiang Zhao(赵毅强), Xue-Feng Zheng(郑雪峰), Zhi-Mei Zhou(周芝梅), Yong Wan(万勇), and Xiao-Hua Ma(马晓华)

Chin. Phys. B, 2021, 30 (7): 077305. DOI: 10.1088/1674-1056/abe117

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The performance degradation of gate-recessed metal-oxide-semiconductor high electron mobility transistor (MOS-HEMT) is compared with that of conventional high electron mobility transistor (HEMT) under direct current (DC) stress, and the degradation mechanism is studied. Under the channel hot electron injection stress, the degradation of gate-recessed MOS-HEMT is more serious than that of conventional HEMT devices due to the combined effect of traps in the barrier layer, and that under the gate dielectric of the device. The threshold voltage of conventional HEMT shows a reduction under the gate electron injection stress, which is caused by the barrier layer traps trapping the injected electrons and releasing them into the channel. However, because of defects under gate dielectrics which can trap the electrons injected from gate and deplete part of the channel, the threshold voltage of gate-recessed MOS-HEMT first increases and then decreases as the conventional HEMT. The saturation phenomenon of threshold voltage degradation under high field stress verifies the existence of threshold voltage reduction effect caused by gate electron injection.

Fabrication of sulfur-doped cove-edged graphene nanoribbons on Au(111) ^Hot!

Huan Yang(杨欢), Yixuan Gao(高艺璇), Wenhui Niu(牛雯慧), Xiao Chang(常霄), Li Huang(黄立), Junzhi Liu(刘俊治), Yiyong Mai(麦亦勇), Xinliang Feng(冯新亮), Shixuan Du(杜世萱), and Hong-Jun Gao(高鸿钧)

Chin. Phys. B, 2021, 30 (7): 077306. DOI: 10.1088/1674-1056/abfbd0

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The on-surface synthesis from predesigned organic precursors can yield graphene nanoribbons (GNRs) with atomically precise widths, edge terminations and dopants, which facilitate the tunning of their electronic structures. Here, we report the synthesis of novel sulfur-doped cove-edged GNRs (S-CGNRs) on Au(111) from a specifically designed precursor containing thiophene rings. Scanning tunneling microscopy and non-contact atomic force microscopy measurements elucidate the formation of S-CGNRs through subsequent polymerization and cyclodehydrogenation, which further result in crosslinked branched structures. Scanning tunneling spectroscopy results reveal the conduction band minimum of the S-CGNR locates at 1.2 eV. First-principles calculations show that the S-CGNR possesses an energy bandgap of 1.17 eV, which is evidently smaller than that of an undoped cove-edged GNR (1.7 eV), suggesting effective tuning of the bandgap by introducing sulfur atoms. Further increasing the coverage of precursors close to a monolayer results in the formation of linear-shaped S-CGNRs. The fabrication of S-CGNRs provides one more candidate in the GNR toolbox and promotes the future applications of heteroatom-doped graphene nanostructures.

Suppression of leakage effect of Majorana bound states in the T-shaped quantum-dot structure

Wei-Jiang Gong(公卫江), Yu-Hang Xue(薛宇航), Xiao-Qi Wang(王晓琦), Lian-Lian Zhang(张莲莲), and Guang-Yu Yi(易光宇)

Chin. Phys. B, 2021, 30 (7): 077307. DOI: 10.1088/1674-1056/abf4f8

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We theoretically study the transport properties in the T-shaped double-quantum-dot structure, by considering the dot in the main channel to be coupled to the Majorana bound state (MBS) at one end of the topological superconducting nanowire. It is found that the side-coupled dot governs the effect of the MBS on the transport behavior. When its level is consistent with the energy zero point, the MBS contributes little to the conductance spectrum. Otherwise, the linear conductance exhibits notable changes according to the inter-MBS coupling manners. In the absence of inter-MBS coupling, the linear conductance value keeps equal to e²/2h when the level of the side-coupled dot departs from the energy zero point. However, the linear conductance is always analogous to the MBS-absent case once the inter-MBS coupling comes into play. These findings provide new information about the leakage effect of MBSs in quantum-dot structures.

Collective excitations and quantum size effects on the surfaces of Pb(111) films: An experimental study Suggested by editors

Yade Wang(王亚德), Zijian Lin(林子荐), Siwei Xue(薛思玮), Jiade Li(李佳德), Yi Li(李毅), Xuetao Zhu(朱学涛), and Jiandong Guo(郭建东)

Chin. Phys. B, 2021, 30 (7): 077308. DOI: 10.1088/1674-1056/abfbcd

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Pb(111) film is a special system that exhibits strong quantum size effects in many electronic properties. The collective excitations, i.e., plasmons, in Pb(111) films are also expected to show signatures of the quantum size effect. Here, using high-resolution electron energy loss spectroscopy, we measured the plasmons on the surface of Pb(111) films with different film thicknesses and analyzed the plasmon dispersions. One surface plasmon branch exhibits prominent damping in the small momentum range, which can be attributed to the interaction between the top and bottom interfaces of the Pb(111) films. With the film thickness increasing, the critical momentum characterizing the damping in Pb(111) films decays not only much slower in Pb(111) films than in other metal films, and even in films with the thickness up to 40 monolayers the damping still exists. The slow decay of the surface plasmon damping, manifesting the strong quantum size effect in Pb(111) films, might be related to the strong nesting of the Fermi surface along the (111) direction.

Substitution effect on the superconductivity in Mo_3-xRe_xAl₂C with β-Mn structure prepared by microwave method

Jun-Nan Sun(孙俊男), Bin-Bin Ruan(阮彬彬), Meng-Hu Zhou(周孟虎), Yin Chen(陈银), Qing-Song Yang(杨清松), Lei Shan(单磊), Ming-Wei Ma(马明伟), Gen-Fu Chen(陈根富), and Zhi-An Ren(任治安)

Chin. Phys. B, 2021, 30 (7): 077401. DOI: 10.1088/1674-1056/abfb5a

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We report the microwave synthesis and the doping effect of Mo_3-xRe_xAl₂C (0 ≤ x ≤ 0.3) superconductor. Re doping into Mo₃Al₂C results in a regular shrinkage of the lattice, marked by the linear decrease of lattice parameter a from 6.868(1) Å (for Mo₃Al₂C) to 6.846(2) Å (for Mo_2.7Re_0.3Al₂C). Upon Re doping, T_c of Mo_3-xRe_xAl₂C first increases and then decreases, with the maximum T_c = 9.14 K at the optimal doping level of x = 0.09. Our report provides a convenient method to synthesize Mo_3-xRe_xAl₂C within minutes, and also marks the first Re doping study with enhanced superconductivity on the non-centrosymmetric superconductor Mo₃Al₂C.

Fabrication and characterization of Al-Mn superconducting films for applications in TES bolometers

Qing Yu(余晴), Yi-Fei Zhang(张翼飞), Chang-Hao Zhao(赵昌昊), Kai-Yong He(何楷泳), Ru-Tian Huang(黄汝田), Yong-Cheng He(何永成), Xin-Yu Wu(吴歆宇), Jian-She Liu(刘建设), and Wei Chen(陈炜)

Chin. Phys. B, 2021, 30 (7): 077402. DOI: 10.1088/1674-1056/abe0c6

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Superconducting transition edge sensor (TES) bolometers require superconducting films to have controllable transition temperatures T_c in different practical applications. The value of T_c strongly affects thermal conductivity and thermal noise performance of TES detectors. Al films doped with Mn (Al-Mn) of different concentrations can accomplish tunable T_c. A magnetron sputtering machine is used to deposit the Al-Mn films in this study. Fabrication parameters including sputtering pressure and annealing process are studied and their influences on T_c and superconducting transition width ΔT_c are optimized. The Al-Mn films with Δ T_c below 1.0 mK for T_c in a range of 520 mK-580 mK are successfully fabricated.

Superconductivity in an intermetallic oxide Hf₃Pt₄Ge₂O ^Hot!

Chengchao Xu(徐程超), Hong Wang(王鸿), Huanfang Tian(田焕芳), Youguo Shi(石友国), Zi-An Li(李子安), Ruijuan Xiao(肖睿娟), Honglong Shi(施洪龙), Huaixin Yang(杨槐馨), and Jianqi Li(李建奇)

Chin. Phys. B, 2021, 30 (7): 077403. DOI: 10.1088/1674-1056/abfb53

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Discovery of a new superconductor with distinct crystal structure and chemistry often provides great opportunity for further expanding superconductor material base, and also leads to better understanding of superconductivity mechanisms. Here, we report the discovery of superconductivity in a new intermetallic oxide Hf₃Pt₄Ge₂O synthesized through a solid-state reaction. The Hf₃Pt₄Ge₂O crystallizes in a cubic structure (space group Fm-3m) with a lattice constant of a = 1.241 nm, whose stoichiometry and atomic structure are determined by electron microscopy and x-ray diffraction techniques. The superconductivity at 4.1 K and type-Ⅱ superconducting nature are evidenced by the electrical resistivity, magnetic susceptibility, and specific heat measurements. The intermetallic oxide Hf₃Pt₄Ge₂O system demonstrates an intriguing structural feature that foreign oxygen atoms can be accommodated in the interstitial sites of the ternary intermetallic framework. We also successfully synthesized a series of Hf₃Pt₄Ge₂O_1+δ (-0.25 ≤ δ ≤ 0.5), and found the δ-dependent superconducting transition temperature T_c. The atomic structure and the electronic structure are also substantiated by first-principles calculations. Our results present an entirely new family of superconductors with distinct structural and chemical characteristics, and could attract research interest in further finding new superconductors and exploring novel physics pertaining to the 5d-electron in these intermetallic compound systems.

Lateral magnetic stiffness under different parameters in a high-temperature superconductor levitation system

Yong Yang(杨勇) and Yun-Yi Wu(吴云翼)

Chin. Phys. B, 2021, 30 (7): 077404. DOI: 10.1088/1674-1056/abe0c2

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Magnetic stiffness determines the stability of a high-temperature superconductor (HTS) magnetic levitation system. The quantitative properties of the physical and geometrical parameters that affect the stiffness of HTS levitation systems should be identified for improving the stiffness by some effective methods. The magnetic stiffness is directly related to the first-order derivative of the magnetic force with respect to the corresponding displacement, which indicates that the effects of the parameters on the stiffness should be different from the relationships between the forces and the same parameters. In this paper, we study the influences of some physical and geometrical parameters, including the strength of the external magnetic field (B₀) produced by a rectangular permanent magnet (PM), critical current density (J_c), the PM-to-HTS area ratio (α), and thickness ratio (β), on the lateral stiffness by using a numerical approach under zero-field cooling (ZFC) and field cooling (FC) conditions. In the first and second passes of the PM, the lateral stiffness at most of lateral positions essentially increases with B₀ increasing and decreases with β increasing in ZFC and FC. The largest lateral stiffness at every lateral position is almost produced by the minimum value of J_c, which is obviously different from the lateral force-J_c relation. The α-dependent lateral stiffness changes with some parameters, which include the cooling conditions of the bulk HTS, lateral displacement, and movement history of the PM. These findings can provide some suggestions for improving the lateral stiffness of the HTS levitation system.

Two-dimensional square-Au₂S monolayer: A promising thermoelectric material with ultralow lattice thermal conductivity and high power factor

Wei Zhang(张伟), Xiao-Qiang Zhang(张晓强), Lei Liu(刘蕾), Zhao-Qi Wang(王朝棋), and Zhi-Guo Li(李治国)

Chin. Phys. B, 2021, 30 (7): 077405. DOI: 10.1088/1674-1056/abe115

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The search for new two-dimensional (2D) harvesting materials that directly convert (waste) heat into electricity has received increasing attention. In this work, thermoelectric (TE) properties of monolayer square-Au₂S are accurately predicted using a parameter-free ab initio Boltzmann transport formalism with fully considering the spin-orbit coupling (SOC), electron-phonon interactions (EPIs), and phonon-phonon scattering. It is found that the square-Au₂S monolayer is a promising room-temperature TE material with an n-type (p-type) figure of merit ZT = 2.2 (1.5) and an unexpected high n-type ZT = 3.8 can be obtained at 600 K. The excellent TE performance of monolayer square-Au₂S can be attributed to the ultralow lattice thermal conductivity originating from the strong anharmonic phonon scattering and high power factor due to the highly dispersive band edges around the Fermi level. Additionally, our analyses demonstrate that the explicit treatments of EPIs and SOC are highly important in predicting the TE properties of monolayer square-Au₂S. The present findings will stimulate further the experimental fabrication of monolayer square-Au₂S-based TE materials and offer an in-depth insight into the effect of SOC and EPIs on TE transport properties.

Josephson current in an irradiated Weyl semimetal junction Suggested by editors

Han Wang(王含) and Rui Shen(沈瑞)

Chin. Phys. B, 2021, 30 (7): 077406. DOI: 10.1088/1674-1056/ac0348

Abstract ( 372 )

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The influence of the off-resonant circularly polarized light on the Josephson current in the time-reversal broken superconducting Weyl semimetal junctions is investigated by using the Bogoliubov-de Gennes equation and the transfer matrix approach. Both the zero momentum BCS pairing states and the finite momentum Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) pairing states are considered for the Weyl superconductors. When a circularly polarized light is applied, it is shown that the current phase relation remains unchanged for the BCS pairing with the increasing of incident radiation intensity A₀. For FFLO pairing, the Josephson current exhibits the 0-π transition and periodic oscillation as a function of A₀. The dependence of free energy and critical current on A₀ are also investigated.

Magnetism and giant magnetocaloric effect in rare-earth-based compounds R₃BWO₉ (R = Gd, Dy, Ho) ^Hot!

Lu-Ling Li(李炉领), Xiao-Yu Yue(岳小宇), Wen-Jing Zhang(张文静), Hu Bao(鲍虎), Dan-Dan Wu(吴丹丹), Hui Liang(梁慧), Yi-Yan Wang(王义炎), Yan Sun(孙燕), Qiu-Ju Li(李秋菊), and Xue-Feng Sun(孙学峰)

Chin. Phys. B, 2021, 30 (7): 077501. DOI: 10.1088/1674-1056/abf916

Abstract ( 748 )

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The magnetism and magnetocaloric effect (MCE) of rare-earth-based tungstate compounds $R_{3}$BWO$_{9 }$ ($R = {\rm Gd}$, Dy, Ho) have been studied by magnetic susceptibility, isothermal magnetization, and specific heat measurements. No obvious long-range magnetic ordering can be found down to 2 K. The Curie-Weiss fitting and magnetic susceptibilities under different applied fields reveal the existence of weak short-range antiferromagnetic couplings at low temperature in these systems. The calculations of isothermal magnetization exhibit a giant MCE with the maximum changes of magnetic entropy being 54.80 J/kg$\cdot$K at 2 K for Gd$_{3}$BWO$_{9}$, 28.5 J/kg$\cdot$K at 6 K for Dy$_{3}$BWO$_{9}$, and 29.76 J/kg$\cdot$K at 4 K for Ho$_{3}$BWO$_{9}$, respectively, under a field change of 0-7 T. Especially for Gd$_{3}$BWO$_{9}$, the maximum value of magnetic entropy change ($-\Delta S_{M}^{\max}$) and adiabatic temperature change ($ - \Delta T_{\rm ad}^{\max}$) are 36.75 J/kg$\cdot$K and 5.56 K for a low field change of 0-3 T, indicating a promising application for low temperature magnetic refrigeration.

Magnetocrystalline anisotropy and dynamic spin reorientation of half-doped Nd_0.5Pr_0.5FeO₃ single crystal

Haotian Zhai(翟浩天), Tian Gao(高湉), Xu Zheng(郑旭), Jiali Li(李佳丽), Bin Chen(陈斌), Hongliang Dong(董洪亮), Zhiqiang Chen(陈志强), Gang Zhao(赵钢), Shixun Cao(曹世勋), Chuanbing Cai(蔡传兵), and Vyacheslav V. Marchenkov

Chin. Phys. B, 2021, 30 (7): 077502. DOI: 10.1088/1674-1056/ac0786

Abstract ( 419 )

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The single crystals of Nd$_{0.5}$Pr$_{0.5}$FeO$_{3}$ were successfully grown by optical floating zone method. Room temperature x-ray diffraction and Laue photograph declared the homogeneity and high quality of the crystal. The significant magnetic anisotropy and multiple magnetic transitions illustrate the complex magnetic structure. At high temperatures ($T > 66 $ K), it shows the typical characteristics of $\varGamma _{4}$(G$_{x}$, A$_{y}$, F$_{z}$) state. With the decrease of the temperature, it undergoes a first-order spin reorientation transition from $\varGamma _{4}$(G$_{x}$, A$_{y}$, F$_{z}$) to $\varGamma _{2}$(F$_{x}$, C$_{y}$, G$_{z}$) state in the temperature window of 45-66 K under an applied magnetic field of 0.01 T. As the temperature drops to $\sim 17 $ K, a new magnetic interaction mechanism works, which results in a further enhancement of magnetization. The ${T}$-$H$ phase diagram of Nd$_{0.5}$Pr$_{0.5}$FeO$_{3}$ single crystal was finally constructed.

Zero-field skyrmions in FeGe thin films stabilized through attaching a perpendicularly magnetized single-domain Ni layer Suggested by editors

Zi-Bo Zhang(张子博) and Yong Hu(胡勇)

Chin. Phys. B, 2021, 30 (7): 077503. DOI: 10.1088/1674-1056/abf4bc

Abstract ( 419 )

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A numerical study reports that the zero-field skyrmions in FeGe thin films are stabilized when a FeGe layer is exchange coupled to a single-domain Ni layer, which has been magnetized perpendicularly. Due to the small thickness, an easy-plane anisotropy in the FeGe layer is taken into account, and the skyrmion-crystal state is favored to appear for low anisotropies and intermediate FeGe/Ni interlayer exchange couplings, and finally transformed from a labyrinth-like and into an out-of-plane uniform state for the large couplings or into an in-plane state for the high anisotropies. Furthermore, the maximum skyrmion charge number is bigger for the periodic and fixed boundary conditions with an out-of-plane magnetization; on the contrary, the Bloch-type skyrmions can be frozen and stabilized for the larger couplings on the fixed boundary with an in-plane magnetization, similar to the experimental results of the magnetic-field-induced skyrmions. Finally, the skyrmion charge number and diameter both decrease if the nonmagnetic defects exist, and the skyrmion centers are prone to being captured by defect sites. This work evidences that the ensembles of homochiral skyrmions stabilized in the multilayers fabricated by well-established technologies present a roadmap to design new classes of the materials that can host skyrmions.

Magnetic properties and resistivity of a 2:17-type SmCo magnet doped with ZrO₂

Qi-Qi Yang(杨棋棋), Zhuang Liu(刘壮), Chao-Yue Zhang(张超越), Hai-Chen Wu(吴海辰), Xiao-Lei Gao(高晓磊), Yi-Long Ma(马毅龙), Ren-Jie Chen(陈仁杰), and A-Ru Yan(闫阿儒)

Chin. Phys. B, 2021, 30 (7): 077504. DOI: 10.1088/1674-1056/abf7a9

Abstract ( 425 )

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In order to counteract the demagnetization caused by eddy current loss, widespread attention has been devoted to increasing the resistivity of permanent magnets. We prepared 2:17-type SmCo magnets doped with different ZrO₂ contents and investigated the influence of the ZrO₂ content on the magnetic properties and resistive anisotropism. The results showed that not only was the resistivity of the magnet improved, but, in addition, the coercivity of the magnet was significantly increased. The microstructure was studied with TEM, which showed that ZrO₂ doping was able to cause a decrease in the lamellar phase density and the growth of cellular structures. The increased grain boundaries and Sm₂O₃ phases were favorable to the improvement of resistivity. The decrease of the lamellar phases caused a narrowing of the resistive anisotropism. The additional Cu in the center of the cellular boundaries was the main reason for the enhancement of H_cj. However, an excessive amount caused an increase of the Zr₆(FeCo)₂₃ phase and a deterioration of the cellular structure, thereby leading to a decrease in coercivity.

Magnon bands in twisted bilayer honeycomb quantum magnets

Xingchuan Zhu(朱兴川), Huaiming Guo(郭怀明), and Shiping Feng(冯世平)

Chin. Phys. B, 2021, 30 (7): 077505. DOI: 10.1088/1674-1056/abeee5

Abstract ( 438 )

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We study the magnon bands of twisted bilayer honeycomb quantum magnets using linear spin wave theory. Although the interlayer coupling can be ferromagnetic or antiferromagnetic, we keep the intralayer one ferromagnetic to avoid possible frustration. For the interlayer ferromagnetic case, we find the magnon bands have similar features with the corresponding electronic energy spectrums. Although the linear dispersions near the Dirac points are preserved in the magnon bands of twisted bilayer magnets, their slopes are reduced with the decrease of the twist angles. On the other hand, the interlayer antiferromagnetic couplings generate quite different magnon spectra. The two single-layered magnon spectra are usually decoupled due to the opposite orientations of the spins in the two layers. We also develop a low-energy continuous theory for very small twist angles, which has been verified to fit well with the exact tight-binding calculations. Our results may be experimentally observed due to the rapid progress in two-dimensional magnetic materials.

Spectral polarization-encoding of broadband laser pulses by optical rotatory dispersion and its applications in spectral manipulation Suggested by editors

Xiaowei Lu(陆小微), Congying Wang(王聪颖), Xuanke Zeng(曾选科), Jiahe Lin(林家和), Yi Cai(蔡懿), Qinggang Lin(林庆钢), Huangcheng Shangguan(上官煌城), Zhenkuan Chen(陈振宽), Shixiang Xu(徐世祥), and Jingzhen Li(李景镇)

Chin. Phys. B, 2021, 30 (7): 077801. DOI: 10.1088/1674-1056/abf0fb

Abstract ( 355 )

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We propose a kind of spectral polarization-encoding (SPE) for broadband light pulses, which is realized by inducing optical rotatory dispersion (ORD), and decoded by compensating ORD. Combining with polarization-sensitive devices, SPE can not only work to control polarization-dependent transmission for central wavelength or bandwidth-tunable filtering, but also can be used for broadband regenerative or multi-pass amplification with a polarization-dependent gain medium to improve output bandwidth. SPE is entirely passive thus very simple to be designed and aligned. By using an ORD crystal with a good transmission beyond 3-μm mid-infrared region, e.g., AgGaS₂, SPE promises to be applied for the wavelength tuning lasers in mid-infrared region, where the tunning devices are rather under developed compared with those in visible and near-infrared region.

Linear and nonlinear optical response of g-C₃N₄-based quantum dots Suggested by editors

Jing-Zhi Zhang(张竞之) and Hong Zhang(张红)

Chin. Phys. B, 2021, 30 (7): 077802. DOI: 10.1088/1674-1056/abec2f

Abstract ( 325 )

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Graphite carbon nitride (g-C₃N₄) attracts wide-ranging research interest due to its extraordinary physicochemical properties and promising applications ranging from heterogeneous catalysis to fuel cells. In this work, we design different g-C₃N₄-based quantum dots (gCNQDs), carry out a systematic study of optical properties, and elucidate the shape selectivity, composite nanostructure, and outfield effect. In particular, composites of gCNQDs and metal nanochains present excellent optical response, making it applicable to bioimaging, nano-plasma devices, and metalloenzyme in infrared light related fields. Besides, QDs which original bridging nitrogen atoms are replaced by amino (-NH₂), hydroxyl (-OH), and methyl (-CH₃) functional groups respectively, have excellent spectral selectivity in the deep ultraviolet region. More interestingly, in the study of the laser interaction with materials, the gCNQDs exhibit extremely high stability and light corrosion resistance. Phase transition from insulation to metal is observed under the critical condition of about 5 eV intensity or 337 nm wavelength. All provided theoretical support for designs and applications in g-C₃N₄ quantum devices.

Faraday rotations, ellipticity, and circular dichroism in magneto-optical spectrum of moiré superlattices

J A Crosse and Pilkyung Moon

Chin. Phys. B, 2021, 30 (7): 077803. DOI: 10.1088/1674-1056/ac051f

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We study the magneto-optical conductivity of a number of van der Waals heterostructures, namely, twisted bilayer graphene, AB-AB and AB-BA stacked twisted double bilayer graphene and monolayer graphene and AB-stacked bilayer graphene on hexagonal boron nitride. As the magnetic field increases, the absorption spectrum exhibits a self-similar recursive pattern reflecting the fractal nature of the energy spectrum. Whilst twisted bilayer graphene displays only weak circular dichroism, the other four structures display strong circular dichroism with monolayer graphene and AB-stacked bilayer graphene on hexagonal boron nitride being particularly pronounced owing to strong inversion symmetry breaking properties of the hexagonal boron nitride layer. As the left and right circularly polarized light interact with these structures differently, plane-polarized incident light undergoes a Faraday rotation and gains an ellipticity when transmitted. The size of the respective angles is on the order of a degree.

Super deformability and thermoelectricity of bulk γ-InSe single crystals Suggested by editors

Bin Zhang(张斌), Hong Wu(吴宏), Kunling Peng(彭坤岭), Xingchen Shen(沈星辰), Xiangnan Gong(公祥南), Sikang Zheng(郑思康), Xu Lu(卢旭), Guoyu Wang(王国玉), and Xiaoyuan Zhou(周小元)

Chin. Phys. B, 2021, 30 (7): 078101. DOI: 10.1088/1674-1056/abf133

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Indium selenide, a Ⅲ-V group semiconductor with layered structure, attracts intense attention in various photoelectric applications, due to its outstanding properties. Here, we report super deformability and thermoelectricity of γ-InSe single crystals grown by modified Bridgeman method. The crystal structure of InSe is studied systematically by transmission electron microscopy methods combined with x-ray diffraction and Raman spectroscopy. The predominate phase of γ-InSe with dense stacking faults and local multiphases is directly demonstrated at atomic scale. The bulk γ-InSe crystals demonstrate surprisingly high intrinsic super deformative ability which is highly pliable with bending strains exceeding 12.5% and 264% extension by rolling. At the meantime, InSe also possesses graphite-like features which is printable, writable, and erasable. Finally, the thermoelectric properties of γ-InSe bulk single crystals are preliminary studied and thermal conductivity can be further reduced via bending-induced defects. These findings will enrich the knowledge of structural and mechanical properties' flexibility of InSe and shed lights on the intrinsic and unique mechanical properties of InSe polytypes.

Growth of high-crystallinity uniform GaAs nanowire arrays by molecular beam epitaxy Suggested by editors

Yu-Bin Kang(亢玉彬), Feng-Yuan Lin(林逢源), Ke-Xue Li(李科学), Ji-Long Tang(唐吉龙), Xiao-Bing Hou(侯效兵), Deng-Kui Wang(王登魁), Xuan Fang(方铉), Dan Fang(房丹), Xin-Wei Wang(王新伟), and Zhi-Peng Wei(魏志鹏)

Chin. Phys. B, 2021, 30 (7): 078102. DOI: 10.1088/1674-1056/abf920

Abstract ( 449 )

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The self-catalyzed growth of GaAs nanowires (NWs) on silicon (Si) is an effective way to achieve integration between group Ⅲ-V elements and Si. High-crystallinity uniform GaAs NW arrays were grown by solid-source molecular beam epitaxy (MBE). In this paper, we describe systematic experiments which indicate that the substrate treatment is crucial to the highly crystalline and uniform growth of one-dimensional nanomaterials. The influence of natural oxidation time on the crystallinity and uniformity of GaAs NW arrays was investigated and is discussed in detail. The GaAs NW crystallinity and uniformity are maximized after 20 days of natural oxidation time. This work provides a new solution for producing high-crystallinity uniform Ⅲ-V nanowire arrays on wafer-scale Si substrates. The highly crystalline uniform NW arrays are expected to be useful for NW-based optical interconnects and Si platform optoelectronic devices.

Crystal growth, structure and optical properties of Pr³⁺-doped yttria-stabilized zirconia single crystals

Dai-Ni Wang(王黛妮), Shou-Lei Xu(徐守磊), Xiang-Yu Wang(王翔宇), Si-Yao Li(李思瑶), Xing Hong(洪杏), Bernard A. Goodman, and Wen Deng(邓文)

Chin. Phys. B, 2021, 30 (7): 078103. DOI: 10.1088/1674-1056/abfb55

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The development of blue semiconductor light-emitting diodes (LEDs) has produced potential applications for Pr-doped materials that can absorb blue light, especially crystals, and we now report structure and optical properties for high-quality Pr-doped single crystals of yttria-stabilized zirconia (YSZ) grown by the optical floating zone (FZ) method. X-ray diffraction (XRD) and Raman spectroscopy showed that all of the single crystal samples were in the cubic phase, whereas the corresponding ceramic samples contained a mixture of monoclinic and cubic phases. X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) spectroscopy showed that Pr was present as the Pr³⁺ ion in ceramic rods and single crystals after heating to high temperatures. The absorption and photoluminescence excitation (PLE) spectra of the Pr-doped YSZ crystals measured at room temperature showed strong absorption of blue light, while their photoluminescence (PL) spectra showed five emission peaks at 565 nm, 588 nm, 614 nm, 638 nm, and 716 nm under 450 nm excitation. The optimum luminescence properties were obtained with the crystal prepared using 0.15 mol% Pr₆O₁₁, and those with higher concentrations showed evidence of quenching of the luminescence properties. In addition, the color purity of Pr-doped YSZ single crystal reached 98.9% in the orange-red region.

Vertical MBE growth of Si fins on sub-10 nm patterned substrate for high-performance FinFET technology

Shuang Sun(孙爽), Jian-Huan Wang(王建桓), Bao-Tong Zhang(张宝通), Xiao-Kang Li(李小康), Qi-Feng Cai(蔡其峰), Xia An(安霞), Xiao-Yan Xu(许晓燕), Jian-Jun Zhang(张建军), and Ming Li(黎明)

Chin. Phys. B, 2021, 30 (7): 078104. DOI: 10.1088/1674-1056/abf63f

Abstract ( 426 )

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A high quality epitaxial Si layer by molecular beam epitaxy (MBE) on Si (001) substrates was demonstrated to fabricate a channel with low density defects for high-performance FinFET technology. In order to study the effects of fin width and crystallography orientation on the MBE behavior, a 30 nm thick Si layer was deposited on the top of an etched Si fin with different widths from 10 nm to 50 nm and orientations of <100> and <110>. The result shows that a defect-free Si film was obtained on the fin by MBE, since the etching damage was confined in the bottom of the epitaxial layer. In addition, the vertical growth of the epitaxial Si layer was observed on sub-10 nm <100> Si fins, and this was explained by a kinetic mechanism.

Equilibrium folding and unfolding dynamics to reveal detailed free energy landscape of src SH3 protein by magnetic tweezers Suggested by editors

Huanhuan Su(苏环环), Hao Sun(孙皓), Haiyan Hong(洪海燕), Zilong Guo(郭子龙), Ping Yu(余平), and Hu Chen(陈虎)

Chin. Phys. B, 2021, 30 (7): 078201. DOI: 10.1088/1674-1056/abfb56

Abstract ( 442 )

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Src SH3 protein domain is a typical two-state protein which has been confirmed by research of denaturant-induced unfolding dynamics. Force spectroscopy experiments by optical tweezers and atomic force microscopy have measured the force-dependent unfolding rates with different kinds of pulling geometry. However, the equilibrium folding and unfolding dynamics at constant forces has not been reported. Here, using stable magnetic tweezers, we performed equilibrium folding and unfolding dynamic measurement and force-jump measurement of src SH3 domain with tethering points at its N- and C-termini. From the obtained force-dependent transition rates, a detailed two-state free energy landscape of src SH3 protein is constructed with quantitative information of folding free energy, transition state barrier height and position, which exemplifies the capability of magnetic tweezers to study protein folding and unfolding dynamics.

Electron density distribution of LiMn₂O₄ cathode investigated by synchrotron powder x-ray diffraction Suggested by editors

Tongtong Shang(尚彤彤), Dongdong Xiao(肖东东), Qinghua Zhang(张庆华), Xuefeng Wang(王雪锋), Dong Su(苏东), and Lin Gu(谷林)

Chin. Phys. B, 2021, 30 (7): 078202. DOI: 10.1088/1674-1056/abf347

Abstract ( 476 )

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Electron density plays an important role in determining the properties of functional materials. Revealing the electron density distribution experimentally in real space can help to tune the properties of materials. Spinel LiMn₂O₄ is one of the most promising cathode candidates because of its high voltage, low cost, and non-toxicity, but suffers severe capacity fading during electrochemical cycling due to the Mn dissolution. Real-space measurement of electron distribution of LiMn₂O₄ experimentally can provide direct evaluation on the strength of Mn-O bond and give an explanation of the structure stability. Here, through high energy synchrotron powder x-ray diffraction (SPXRD), accurate electron density distribution in spinel LiMn₂O₄ has been investigated based on the multipole model. The electron accumulation between Mn and O atoms in deformation density map indicates the shared interaction of Mn-O bond. The quantitative topological analysis at bond critical points shows that the Mn-O bond is relatively weak covalent interaction due to the oxygen loss. These findings suggest that oxygen stoichiometry is the key factor for preventing the Mn dissolution and capacity fading.

Artificial synaptic behavior of the SBT-memristor

Gang Dou(窦刚), Ming-Long Dou(窦明龙), Ren-Yuan Liu(刘任远), and Mei Guo(郭梅)

Chin. Phys. B, 2021, 30 (7): 078401. DOI: 10.1088/1674-1056/ac05a3

Abstract ( 383 )

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The synapse of human brain neurons is not only the transmission channel of information, but also the basic unit of human brain learning and information storing. The artificial synapse is constructed based on the Sr_0.97Ba_0.03TiO_3-x (SBT) memristor, which realizes the short-term and long-term plasticity of the synapse. The experiential learning and non-associative learning behavior in accordance with human cognitive rules are realized by using the SBT-memristor-based synapse. The process of synaptic habituation and sensitization is analyzed. This study provides insightful guidance for realization of artificial synapse and the development of artificial neural network.

A radar-infrared compatible broadband absorbing surface: Design and analysis

Qing-Tao Yu(余庆陶), Yuan-Song Zeng(曾元松), and Guo-Jia Ma(马国佳)

Chin. Phys. B, 2021, 30 (7): 078402. DOI: 10.1088/1674-1056/abea7f

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A radar-infrared compatible stealth surface is designed and analyzed. Without modifying the radar absorbing material (RAM), the design can theoretically achieve radar-infrared compatibility and broadband radar absorption through surface patterns and structures. A transmission-line-based model (TLM) is developed to analyze the radar absorbing performance of the surface. Optimization of the structure geometries is conducted aiming to maximize the -10 dB absorption bandwidth in 2-18 GHz. Surface with optimized structure geometries exhibits a superior absorption bandwidth, more than twice the bandwidth of the original 1.5 mm RAM slab, while maintaining a relatively low infrared emissivity.

Integrated superconducting circuit for qubit and resonator protection

Xiao-Pei Yang(杨晓沛), Zhi-Kun Han(韩志坤), Shu-Qing Song(宋树清), Wen Zheng(郑文), Dong Lan(兰栋), Xin-Sheng Tan(谭新生), and Yang Yu(于扬)

Chin. Phys. B, 2021, 30 (7): 078403. DOI: 10.1088/1674-1056/ac003d

Abstract ( 455 )

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A semi-infinite waveguide acts as a mirror and helps protect the qubit in front of it from decoherence. Here, we investigate the interference effect in an open waveguide consisting of resonators with different decay rates. We find that a lossy resonator works as a mirror and changes the effective decay rate of the other. The spontaneous radiation of qubit is related to its environment, and we can control it by arranging the lossy resonator's position or frequency. Our approach helps improving the qubit performance, as well as the quantum gate fidelities.

Trigger mechanism of PDSOI NMOS devices for ESD protection operating under elevated temperatures

Jia-Xin Wang(王加鑫), Xiao-Jing Li(李晓静), Fa-Zhan Zhao(赵发展), Chuan-Bin Zeng(曾传滨), Duo-Li Li(李多力), Lin-Chun Gao(高林春), Jiang-Jiang Li(李江江), Bo Li(李博), Zheng-Sheng Han(韩郑生), and Jia-Jun Luo(罗家俊)

Chin. Phys. B, 2021, 30 (7): 078501. DOI: 10.1088/1674-1056/abe2fe

Abstract ( 482 )

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Trigger characteristics of electrostatic discharge (ESD) protecting devices operating under various ambient temperatures ranging from 30 ℃ to 195 ℃ are investigated. The studied ESD protecting devices are the H-gate NMOS transistors fabricated with a 0.18-μm partially depleted silicon-on-insulator (PDSOI) technology. The measurements are conducted by using a transmission line pulse (TLP) test system. The different temperature-dependent trigger characteristics of grounded-gate (GGNMOS) mode and the gate-triggered (GTNMOS) mode are analyzed in detail. The underlying physical mechanisms related to the effect of temperature on the first breakdown voltage V_T1 are investigated through the assist of technology computer-aided design (TCAD) simulation.

Design and investigation of novel ultra-high-voltage junction field-effect transistor embedded with NPN

Xi-Kun Feng(冯希昆), Xiao-Feng Gu(顾晓峰), Qin-Ling Ma(马琴玲), Yan-Ni Yang(杨燕妮), and Hai-Lian Liang(梁海莲)

Chin. Phys. B, 2021, 30 (7): 078502. DOI: 10.1088/1674-1056/abe2fb

Abstract ( 366 )

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Ultra-high-voltage (UHV) junction field-effect transistors (JFETs) embedded separately with the lateral NPN (JFET-LNPN), and the lateral and vertical NPN (JFET-LVNPN), are demonstrated experimentally for improving the electrostatic discharge (ESD) robustness. The ESD characteristics show that both JFET-LNPN and JFET-LVNPN can pass the 5.5-kV human body model (HBM) test. The JFETs embedded with different NPNs have 3.75 times stronger in ESD robustness than the conventional JFET. The failure analysis of the devices is performed with scanning electron microscopy, and the obtained delayer images illustrate that the JFETs embedded with NPN transistors have good voltage endurance capabilities. Finally, the internal physical mechanism of the JFETs embedded with different NPNs is investigated with emission microscopy and Sentaurus simulation, and the results confirm that the JFET-LVNPN has stronger ESD robustness than the JFET-LNPN, because the vertical NPN has a better electron collecting capacity. The JFET-LVNPN is helpful in providing a strong ESD protection and functions for a power device.

Extended-source broken gate tunnel FET for improving direct current and analog/radio-frequency performance

Hui-Fang Xu(许会芳), Wen Sun(孙雯), and Na Wang(王娜)

Chin. Phys. B, 2021, 30 (7): 078503. DOI: 10.1088/1674-1056/abe372

Abstract ( 473 )

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The various advantages of extended-source (ES), broken gate (BG), and hetero-gate-dielectric (HGD) technology are blended together for the proposed tunnel field-effect transistor (ESBG TFET) in order to enhance the direct-current and analog/radio-frequency performance. The source of the ESBG TFET is extended into channel for the purpose of increasing the point and line tunneling in the device at the tunneling junction, and then, the on-state current for the ESBG TFET increases. The influence of the source region length on the direct-current and radio-frequency performance parameters of the ESBG TFET is analyzed in detail. The results show that the proposed TFET exhibits a high on-state current to off-state current ratio of 10¹³, large transconductance of 1200 μS/μm, high cut-off frequency of 72.8 GHz, and high gain bandwidth product of 14.3 GHz. Apart from these parameters, the ESBG TFET also demonstrates high linearity distortion parameters in terms of the second- and third-order voltage intercept points, the third-order input interception point, and the third-order intermodulation distortion. Therefore, the ESBG TFET greatly promotes the application potential of conventional TFETs.

Deep-ultraviolet and visible dual-band photodetectors by integrating Chlorin e6 with Ga₂O₃

Yue Zhao(赵越), Jin-Hao Zang(臧金浩), Xun Yang(杨珣), Xue-Xia Chen(陈雪霞), Yan-Cheng Chen(陈彦成), Kai-Yong Li(李凯永), Lin Dong(董林), and Chong-Xin Shan(单崇新)

Chin. Phys. B, 2021, 30 (7): 078504. DOI: 10.1088/1674-1056/abf91c

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Gallium oxide (Ga₂O₃) is a promising material for deep-ultraviolet (DUV) detection. In this work, Chlorin e6 (Ce6) has been integrated with Ga₂O₃ to achieve a DUV and visible dual-band photodetector, which can achieve multiple target information and improve the recognition rate. The photodetector shows two separate response bands at 268 nm and 456 nm. The DUV response band has a responsivity of 9.63 A/W with a full width at half maximum (FWHM) of 54.5 nm; the visible response band has a responsivity of 1.17 A/W with an FWHM of 45.3 nm. This work may provide a simple way to design and fabricate photodetectors with dual-band response.

Phase- and spin-dependent manipulation of leakage of Majorana mode into double quantum dot

Fu-Bin Yang(羊富彬), Gan Ren(任淦), and Lin-Guo Xie(谢林果)

Chin. Phys. B, 2021, 30 (7): 078505. DOI: 10.1088/1674-1056/abe1a8

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We present a phase- and spin-dependent manipulation of leakage of a Majorana mode into a double quantum dot. We study the density of states (DOS) to show the effect of phase change factor on the Majorana leakage into (out) of a double quantum dot. The DOS is derived from the Green's function of the quantum dot by the equation of motion method, and exhibits a formant structure when φ=0,2π and a resonance shape when φ=0.5π and 1.5π. Also, it changes more strongly under the spin-polarized coefficient than the non-polarized lead. Such a theoretical model can be modified to explore the spin-dependent effect in the hybrid Majorana quantum dot system.

Dual-wavelength ultraviolet photodetector based on vertical (Al,Ga)N nanowires and graphene Suggested by editors

Min Zhou(周敏), Yukun Zhao(赵宇坤), Lifeng Bian(边历峰), Jianya Zhang(张建亚), Wenxian Yang(杨文献), Yuanyuan Wu(吴渊渊), Zhiwei Xing(邢志伟), Min Jiang(蒋敏), and Shulong Lu(陆书龙)

Chin. Phys. B, 2021, 30 (7): 078506. DOI: 10.1088/1674-1056/abff23

Abstract ( 336 )

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Due to the wide application of UV-A (320 nm-400 nm) and UV-C (200 nm-280 nm) photodetectors, dual-wavelength (UV-A/UV-C) photodetectors are promising for future markets. A dual-wavelength UV photodetector based on vertical (Al,Ga)N nanowires and graphene has been demonstrated successfully, in which graphene is used as a transparent electrode. Both UV-A and UV-C responses can be clearly detected by the device, and the rejection ratio (R_{254 nm}/R_{450 nm}) exceeds 35 times at an applied bias of -2 V. The short response time of the device is less than 20 ms. Furthermore, the underlying mechanism of double ultraviolet responses has also been analyzed systematically. The dual-wavelength detections could mainly result from the appropriate ratio of the thicknesses and the enough energy band difference of (Al,Ga)N and GaN sections.

Modeling hydrogen exchange of proteins by a multiscale method ^Hot!

Wentao Zhu(祝文涛), Wenfei Li(李文飞), and Wei Wang(王炜)

Chin. Phys. B, 2021, 30 (7): 078701. DOI: 10.1088/1674-1056/abe377

Abstract ( 406 )

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We proposed a practical way for mapping the results of coarse-grained molecular simulations to the observables in hydrogen change experiments. By combining an atomic-interaction based coarse-grained model with an all-atom structure reconstruction algorithm, we reproduced the experimental hydrogen exchange data with reasonable accuracy using molecular dynamics simulations. We also showed that the coarse-grained model can be further improved by imposing experimental restraints from hydrogen exchange data via an iterative optimization strategy. These results suggest that it is feasible to develop an integrative molecular simulation scheme by incorporating the hydrogen exchange data into the coarse-grained molecular dynamics simulations and therefore help to overcome the accuracy bottleneck of coarse-grained models.

Structure-based simulations complemented by conventional all-atom simulations to provide new insights into the folding dynamics of human telomeric G-quadruplex

Yun-Qiang Bian(边运强), Feng Song(宋峰), Zan-Xia Cao(曹赞霞), Jia-Feng Yu(于家峰), and Ji-Hua Wang(王吉华)

Chin. Phys. B, 2021, 30 (7): 078702. DOI: 10.1088/1674-1056/abe1a7

Abstract ( 415 )

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The hybrid atomistic structure-based model has been validated to be effective in investigation of G-quadruplex folding. In this study, we performed large-scale conventional all-atom simulations to complement the folding mechanism of human telomeric sequence Htel24 revealed by a multi-basin hybrid atomistic structure-based model. Firstly, the real time-scale of folding rate, which cannot be obtained from the structure-based simulations, was estimated directly by constructing a Markov state model. The results show that Htel24 may fold as fast as on the order of milliseconds when only considering the competition between the hybrid-1 and hybrid-2 G-quadruplex conformations. Secondly, in comparison with the results of structure-based simulations, more metastable states were identified to participate in the formation of hybrid-1 and hybrid-2 conformations. These findings suggest that coupling the hybrid atomistic structure-based model and the conventional all-atom model can provide more insights into the folding dynamics of DNA G-quadruplex. As a result, the multiscale computational framework adopted in this study may be useful to study complex processes of biomolecules involving large conformational changes.

Magnetic-resonance image segmentation based on improved variable weight multi-resolution Markov random field in undecimated complex wavelet domain

Hong Fan(范虹), Yiman Sun(孙一曼), Xiaojuan Zhang(张效娟), Chengcheng Zhang(张程程), Xiangjun Li(李向军), and Yi Wang(王乙)

Chin. Phys. B, 2021, 30 (7): 078703. DOI: 10.1088/1674-1056/abe1a2

Abstract ( 465 )

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To solve the problem that the magnetic resonance (MR) image has weak boundaries, large amount of information, and low signal-to-noise ratio, we propose an image segmentation method based on the multi-resolution Markov random field (MRMRF) model. The algorithm uses undecimated dual-tree complex wavelet transformation to transform the image into multiple scales. The transformed low-frequency scale histogram is used to improve the initial clustering center of the K-means algorithm, and then other cluster centers are selected according to the maximum distance rule to obtain the coarse-scale segmentation. The results are then segmented by the improved MRMRF model. In order to solve the problem of fuzzy edge segmentation caused by the gray level inhomogeneity of MR image segmentation under the MRMRF model, it is proposed to introduce variable weight parameters in the segmentation process of each scale. Furthermore, the final segmentation results are optimized. We name this algorithm the variable-weight multi-resolution Markov random field (VWMRMRF). The simulation and clinical MR image segmentation verification show that the VWMRMRF algorithm has high segmentation accuracy and robustness, and can accurately and stably achieve low signal-to-noise ratio, weak boundary MR image segmentation.

Numerical simulations of partial elements excitation for hemispherical high-intensity focused ultrasound phased transducer

Yanqiu Zhang(张艳秋), Hao Zhang(张浩), Tianyu Sun(孙天宇), Ting Pan(潘婷), Peiguo Wang(王佩国), and Xiqi Jian(菅喜岐)

Chin. Phys. B, 2021, 30 (7): 078704. DOI: 10.1088/1674-1056/ac05a2

Abstract ( 365 )

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The hemispherical phased transducer maximizes the coverage of the skull and the ultrasonic energy per unit area of the skull is minimized, thereby reducing the risk of skull burns, but the transducer has a small focal area adjustment range, increasing the focal length of treatment is an urgent question for this type of transducer. In this paper, a three-dimensional high-intensity focused ultrasound (HIFU) transcranial propagation model is established based on the human head structure. The finite difference time domain (FDTD) is combined with the Westervelt acoustic wave nonlinear propagation equation and Penne's biological heat conduction equation for numerical simulation of the sound pressure field and temperature field. Forming a treatable focal area in a small-opening hemispherical transducer with a small amount of numerical simulation calculation focusing at a set position to determine the minimum partial excitation area ratio of focusing. And then, applying these preliminary results to a large-opening diameter hemispherical transducer and the temperature field formed by it or full excitation is studied. The results show that the focus area with the excitation area ratio of less than 22% moves forward to the transducer side when the excitation sound is formed. When the excitation area ratio is greater than or equal to 23%, it focuses at the set position. In the case of partial incentives, using 23% of the partial array, the adjustable range of the treatable focal area formed in the three-dimensional space is larger than that of the full excitation.

Highly flexible and excellent performance continuous carbon nanotube fibrous thermoelectric modules for diversified applications ^Hot!

Xiao-Gang Xia(夏晓刚), Qiang Zhang(张强), Wen-Bin Zhou(周文斌), Zhuo-Jian Xiao(肖卓建), Wei Xi(席薇), Yan-Chun Wang(王艳春), and Wei-Ya Zhou(周维亚)

Chin. Phys. B, 2021, 30 (7): 078801. DOI: 10.1088/1674-1056/abff33

Abstract ( 423 )

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A highly flexible and continuous fibrous thermoelectric (TE) module with high-performance has been fabricated based on an ultra-long single-walled carbon nanotube fiber, which effectively avoids the drawbacks of traditional inorganic TE based modules. The maximum output power density of a 1-cm long fibrous TE module with 8 p-n pairs can reach to 3436 μW·cm^-2, the power per unit weight to 2034 μW·g^-1, at a steady-state temperature difference of 50 K. The continuous fibrous TE module is used to detect temperature change of a single point, which exhibits a good responsiveness and excellent stability. Because of its adjustability in length, the flexible fibrous TE module can satisfy the transformation of the temperature difference between two distant heat sources into electrical energy. Based on the signal of the as-fabricated TE module, a multi-region recognizer has been designed and demonstrated. The highly flexible and continuous fibrous TE module with excellent performance shows a great potential in diversified applications of TE generation, temperature detection, and position identification.

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