Chin. Phys. B

The impact of honesty and trickery on a Bayesian quantum prisoners' dilemma game

Bo-Yang Liu(刘博阳), Xin Zhao(赵鑫), Hong-Yi Dai(戴宏毅), Ming Zhang(张明), Ying Liao(廖鹰), Xiao-Feng Guo(郭晓峰), Wei Gao(郜伟)

Chin. Phys. B, 2020, 29 (7): 070201. DOI: 10.1088/1674-1056/ab8c3c

Abstract ( 710 )

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To explore the influence of quantum information on the common social problem of honesty and trickery, we propose a Bayesian model for the quantum prisoners' dilemma game. In this model, the players' strategy formation is regarded as a negotiation of their move contract based on their types of decision policies, honesty or trickery. Although the implementation of quantum information cannot eliminate tricky players, players in our model can always end up with higher payoffs than in the classical game. For a good proportion of a credibility parameter value, a rational player will take an honest action, which is in remarkable contrast to the observation that players tend to defect in the classical prisoners' dilemma game. This research suggests that honesty will be promoted to enhance cooperation with the assistance of quantum information resources.

Improved hybrid parallel strategy for density matrix renormalization group method Suggested by editors

Fu-Zhou Chen(陈富州), Chen Cheng(程晨), Hong-Gang Luo(罗洪刚)

Chin. Phys. B, 2020, 29 (7): 070202. DOI: 10.1088/1674-1056/ab8a42

Abstract ( 675 )

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We propose a new heterogeneous parallel strategy for the density matrix renormalization group (DMRG) method in the hybrid architecture with both central processing unit (CPU) and graphics processing unit (GPU). Focusing on the two most time-consuming sections in the finite DMRG sweeps, i.e., the diagonalization of superblock and the truncation of subblock, we optimize our previous hybrid algorithm to achieve better performance. For the former, we adopt OpenMP application programming interface on CPU and use our own subroutines with higher bandwidth on GPU. For the later, we use GPU to accelerate matrix and vector operations involving the reduced density matrix. Applying the parallel scheme to the Hubbard model with next-nearest hopping on the 4-leg ladder, we compute the ground state of the system and obtain the charge stripe pattern which is usually observed in high temperature superconductors. Based on simulations with different numbers of DMRG kept states, we show significant performance improvement and computational time reduction with the optimized parallel algorithm. Our hybrid parallel strategy with superiority in solving the ground state of quasi-two dimensional lattices is also expected to be useful for other DMRG applications with large numbers of kept states, e.g., the time dependent DMRG algorithms.

Single-photon scattering controlled by an imperfect cavity

Liwei Duan(段立伟), Qing-Hu Chen(陈庆虎)

Chin. Phys. B, 2020, 29 (7): 070301. DOI: 10.1088/1674-1056/ab90ee

Abstract ( 671 )

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We study the single-photon transport in the coupled-resonator waveguide (CRW) controlled by an imperfect cavity. A Lorentzian spectrum is introduced to describe the dissipation. We find that the probability current conservation can be broken, although the imperfect cavity is a Hermitian system. The coupling strength between the imperfect cavity and the CRW has significant influences near the resonant frequency. With the increase of the coupling strength, the transmission coefficient becomes smaller. The spectral width plays a dominant role under the off-resonant condition, where the transmission coefficient is greatly suppressed with the increase of the spectral width. We also observe an abrupt jump of the transmission and reflection coefficients when the hopping amplitude is large enough. All the distinctive behaviors are closely related to the complex effective potential induced by the imperfect cavity.

Exact solution of the (1+2)-dimensional generalized Kemmer oscillator in the cosmic string background with the magnetic field

Yi Yang(杨毅), Shao-Hong Cai(蔡绍洪), Zheng-Wen Long(隆正文), Hao Chen(陈浩), Chao-Yun Long(龙超云)

Chin. Phys. B, 2020, 29 (7): 070302. DOI: 10.1088/1674-1056/ab888e

Abstract ( 775 )

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We study a two-dimensional generalized Kemmer oscillator in the cosmic string spacetime with the magnetic field to better understand the contribution from gravitational field caused by topology defects, and present the exact solutions to the generalized Kemmer equation in the cosmic string with the Morse potential and Coulomb-liked potential through using the Nikiforov-Uvarov (NU) method and biconfluent Heun equation method, respectively. Our results give the topological defect's correction for the wave function, energy spectrum and motion equation, and show that the energy levels of the generalized Kemmer oscillator rely on the angular deficit α connected with the linear mass density m of the cosmic string and characterized the metric's structure in the cosmic string spacetime.

One-decoy state reference-frame-independent quantum key distribution Suggested by editors

Xiang Li(李想), Hua-Wei Yuan(远华伟), Chun-Mei Zhang(张春梅), Qin Wang(王琴)

Chin. Phys. B, 2020, 29 (7): 070303. DOI: 10.1088/1674-1056/ab90f6

Abstract ( 754 )

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Reference-frame-independent quantum key distribution (RFI-QKD) has been proven to be very useful and practical under realistic environment. Here, we present a scheme for one-decoy state RFI-QKD based on the work of Rusca et al. [Appl. Phys. Lett. 112, 171104 (2018)], and carry out investigation on its performance under realistic experimental conditions. Numerical simulation results show that the one-decoy state RFI-QKD can achieve comparable performance in terms of secret key rate and transmission distance as the two-decoy state correspondence under practical experimental conditions. On contrast, it does not need to prepare the vacuum state in the former case, substantially reducing the experimental complexity and random number consumptions. Therefore, our present proposal seems very promising in practical implementations of RFI-QKD.

Tighter constraints of multiqubit entanglementin terms of Rényi-α entropy

Meng-Li Guo(郭梦丽), Bo Li(李波), Zhi-Xi Wang(王志玺), Shao-Ming Fei(费少明)

Chin. Phys. B, 2020, 29 (7): 070304. DOI: 10.1088/1674-1056/ab8e2e

Abstract ( 704 )

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Quantum entanglement plays essential roles in quantum information processing. The monogamy and polygamy relations characterize the entanglement distributions in the multipartite systems. We present a class of monogamy inequalities related to the μ-th power of the entanglement measure based on Rényi-α entropy, as well as polygamy relations in terms of the μ-th power of Rényi-α entanglement of assistance. These monogamy and polygamy relations are shown to be tighter than the existing ones.

Suppression of Coriolis error in weak equivalence principle test using ⁸⁵Rb-⁸⁷Rb dual-species atom interferometer

Wei-Tao Duan(段维涛), Chuan He(何川), Si-Tong Yan(闫思彤), Yu-Hang Ji(冀宇航), Lin Zhou(周林), Xi Chen(陈曦), Jin Wang(王谨), Ming-Sheng Zhan(詹明生)

Chin. Phys. B, 2020, 29 (7): 070305. DOI: 10.1088/1674-1056/ab969a

Abstract ( 653 )

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Coriolis effect is an important error source in the weak equivalence principle (WEP) test using atom interferometer. In this paper, the problem of Coriolis error in WEP test is studied theoretically and experimentally. In theoretical simulation, the Coriolis effect is analyzed by establishing an error model. The measurement errors of Eötvös coefficient (η) in WEP test related to experimental parameters, such as horizontal-velocity difference and horizontal-position difference of atomic clouds, horizontal-position difference of detectors, and rotation compensation of Raman laser's mirror are calculated. In experimental investigation, the position difference between ⁸⁵Rb and ⁸⁷Rb atomic clouds is reduced to 0.1 mm by optimizing the experimental parameters, an alternating detection method is used to suppress the error caused by detection position difference, thus the Coriolis error related to the atomic clouds and detectors is reduced to 1.1×10^-9. This Coriolis error is further corrected by compensating the rotation of Raman laser's mirror, and the total uncertainty of η measurement related to the Coriolis effect is reduced as δη=4.4×10 ^-11.

Probe of topological invariants using quantum walks of a trapped ion in coherent state space Suggested by editors

Ya Meng(蒙雅), Feng Mei(梅锋), Gang Chen(陈刚), Suo-Tang Jia(贾锁堂)

Chin. Phys. B, 2020, 29 (7): 070501. DOI: 10.1088/1674-1056/ab8893

Abstract ( 659 )

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We present a protocol to realize topological discrete-time quantum walks, which comprise a sequence of spin-dependent flipping displacement operations and quantum coin tossing operations, with a single trapped ion. It is demonstrated that the information of bulk topological invariants can be extracted by measuring the average projective phonon number when the walk takes place in coherent state space. Interestingly, the specific chiral symmetry owned by our discrete-time quantum walks simplifies the measuring process. Furthermore, we prove the robustness of such bulk topological invariants by introducing dynamical disorder and decoherence. Our work provides a simple method to measure bulk topological features in discrete-time quantum walks, which can be experimentally realized in the system of single trapped ions.

Stable soliton propagation in a coupled (2+1) dimensional Ginzburg-Landau system Suggested by editors

Li-Li Wang(王丽丽), Wen-Jun Liu(刘文军)

Chin. Phys. B, 2020, 29 (7): 070502. DOI: 10.1088/1674-1056/ab90ea

Abstract ( 645 )

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A coupled (2+1)-dimensional variable coefficient Ginzburg-Landau equation is studied. By virtue of the modified Hirota bilinear method, the bright one-soliton solution of the equation is derived. Some phenomena of soliton propagation are analyzed by setting different dispersion terms. The influences of the corresponding parameters on the solitons are also discussed. The results can enrich the soliton theory, and may be helpful in the manufacture of optical devices.

Michelson laser interferometer-based vibration noise contribution measurement method for cold atom interferometry gravimeter

Ning Zhang(张宁), Qingqing Hu(胡青青), Qian Wang(王倩), Qingchen Ji(姬清晨), Weijing Zhao(赵伟靖), Rong Wei(魏荣), Yuzhu Wang(王育竹)

Chin. Phys. B, 2020, 29 (7): 070601. DOI: 10.1088/1674-1056/ab8885

Abstract ( 692 )

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The measurement performance of the atom interferometry absolute gravimeter is strongly affected by the ground vibration noise. We propose a vibration noise evaluation scheme using a Michelson laser interferometer constructed by the intrinsic Raman laser of the atomic gravimeter. Theoretical analysis shows that the vibration phase measurement accuracy is better than 4 mrad, which corresponds to about 10^-10 g accuracy for a single shot gravity measurement. Compared with the commercial seismometer or accelerometer, this method is a simple, low cost, direct, and fully synchronized measurement of the vibration phase which should benefit the development of the atomic gravimeter. On the other side, limited by equivalence principle, the result of the laser interferometer is not absolute but relative vibration measurement. Triangular cap method could be used to evaluation the noise contribution of vibration, which is a different method from others and should benefit the development of the atomic gravimeter.

A transportable optical lattice clock at the National Time Service Center

De-Huan Kong(孔德欢), Zhi-Hui Wang(王志辉), Feng Guo(郭峰), Qiang Zhang(张强), Xiao-Tong Lu(卢晓同), Ye-Bing Wang(王叶兵), Hong Chang(常宏)

Chin. Phys. B, 2020, 29 (7): 070602. DOI: 10.1088/1674-1056/ab9290

Abstract ( 834 )

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We report a transportable one-dimensional optical lattice clock based on ⁸⁷Sr at the National Time Service Center. The transportable apparatus consists of a compact vacuum system and compact optical subsystems. The vacuum system with a size of 90 cm×20 cm×42 cm and the beam distributors are assembled on a double-layer optical breadboard. The modularized optical subsystems are integrated on independent optical breadboards. By using a 230 ms clock laser pulse, spin-polarized spectroscopy with a linewidth of 4.8 Hz is obtained which is close to the 3.9 Hz Fourier-limit linewidth. The time interleaved self-comparison frequency instability is determined to be 6.3×10^-17 at an averaging time of 2000 s.

An artificial synapse by superlattice-like phase-change material for low-power brain-inspired computing

Qing Hu(胡庆), Boyi Dong(董博义), Lun Wang(王伦), Enming Huang(黄恩铭), Hao Tong(童浩), Yuhui He(何毓辉), Ming Xu(徐明), Xiangshui Miao(缪向水)

Chin. Phys. B, 2020, 29 (7): 070701. DOI: 10.1088/1674-1056/ab892a

Abstract ( 747 )

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Phase-change material (PCM) is generating widespread interest as a new candidate for artificial synapses in bio-inspired computer systems. However, the amorphization process of PCM devices tends to be abrupt, unlike continuous synaptic depression. The relatively large power consumption and poor analog behavior of PCM devices greatly limit their applications. Here, we fabricate a GeTe/Sb₂Te₃ superlattice-like PCM device which allows a progressive RESET process. Our devices feature low-power consumption operation and potential high-density integration, which can effectively simulate biological synaptic characteristics. The programming energy can be further reduced by properly selecting the resistance range and operating method. The fabricated devices are implemented in both artificial neural networks (ANN) and convolutional neural network (CNN) simulations, demonstrating high accuracy in brain-like pattern recognition.

Study on γ-ray source from the resonant reaction ¹⁹F(p,αγ)¹⁶O at E_p=340 keV

Fu-Long Liu(刘伏龙), Wan-Sha Yang(杨婉莎), Ji-Hong Wei(魏继红), Di Wu(吴笛), Yang-Fan He(何阳帆), Yu-Chen Li(李雨尘), Tian-Li Ma(马田丽), Yang-Ping Shen(谌阳平), Qi-Wen Fan(樊启文), Chuang-Ye He(贺创业), Bing Guo(郭冰), Nai-Yan Wang(王乃彦)

Chin. Phys. B, 2020, 29 (7): 070702. DOI: 10.1088/1674-1056/ab96a0

Abstract ( 875 )

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High energy γ-ray can be used in many fields, such as nuclear resonant fluorescence, nuclear medicine imaging. One of the methods to generate high-energy γ-ray is nuclear resonant reaction. The ¹⁹F(p, αγ) ¹⁶O reaction was used to generate 6.13-MeV γ-ray in this work. The angular distribution of 6.13-MeV γ-ray was measured by six LaBr₃ detectors. The thick-target yield curve of 6.13-MeV γ-ray had been measured. The maximum yield was determined to be (1.85±0.01)×10^-8 γ/proton, which was measured by HPGe detector and LaBr₃ detector. The absolute efficiency of all the detectors was calibrated using ⁶⁰Co and ²⁷Al(p, γ) ²⁸Si reaction at E_p=992 keV. The cross section and total resonant width of the reaction were determined to be 95.1±1.0 mb (1 b=10^-24 cm²) and Γ_CM=2.21±0.22 keV, respectively.

Silicon-based optoelectronic synaptic devices

Lei Yin(尹蕾), Xiaodong Pi(皮孝东), Deren Yang(杨德仁)

Chin. Phys. B, 2020, 29 (7): 070703. DOI: 10.1088/1674-1056/ab973f

Abstract ( 771 )

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High-performance neuromorphic computing (i.e., brain-like computing) is envisioned to seriously demand optoelectronically integrated artificial neural networks (ANNs) in the future. Optoelectronic synaptic devices are critical building blocks for optoelectronically integrated ANNs. For the large-scale deployment of high-performance neuromorphic computing in the future, it would be advantageous to fabricate optoelectronic synaptic devices by using advanced silicon (Si) technologies. This calls for the development of Si-based optoelectronic synaptic devices. In this work we review the use of Si materials to make optoelectronic synaptic devices, which have either two-terminal or three-terminal structures. A series of important synaptic functionalities have been well mimicked by using these Si-based optoelectronic synaptic devices. We also present the outlook of using Si materials for optoelectronic synaptic devices.

Accurate electron affinity of atomic cerium and excited states of its anion Suggested by editors

Xiao-Xi Fu(付筱茜), Ru-Lin Tang(唐如麟), Yu-Zhu Lu(陆禹竹), Chuan-Gang Ning(宁传刚)

Chin. Phys. B, 2020, 29 (7): 073201. DOI: 10.1088/1674-1056/ab90e9

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Electron affinities (EA) of most lanthanide elements still remain unknown owing to their relatively lower EA values and the fairly complicated electronic structures. In the present work, we report the high-resolution photoelectron spectra of atomic cerium anion Ce^- using the slow electron velocity-map imaging method in combination with a cold ion trap. The electron affinity of Ce is determined to be 4840.62(21) cm^-1 or 0.600160(26) eV. Moreover, several excited states of Ce^- (⁴H_9/2, ⁴I_9/2, ²H_9/2, ²G_9/2, ²G_7/2, ⁴H_13/2, ²F_5/2, and ⁴I_13/2) are observed.

Laser-assisted XUV double ionization of helium atoms: Intensity dependence of joint angular distributions Suggested by editors

Fengzheng Zhu(朱风筝), Genliang Li(黎根亮), Aihua Liu(刘爱华)

Chin. Phys. B, 2020, 29 (7): 073202. DOI: 10.1088/1674-1056/ab90ef

Abstract ( 734 )

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We investigate the intensity effect of ultrashort assisting infrared laser pulse on the single-XUV-photon double ionization of helium atoms by solving full six-dimensional time-dependent Schrödinger equation with implement of finite element discrete variable representation. The studies of joint energy distributions and joint angular distributions of the two photoelectrons reveal the competition for ionized probabilities between the photoelectrons with odd parity and photoelectrons with even parity in single-XUV-photon double ionization process in the presence of weak infrared laser field, and such a competition can be modulated by changing the intensity of the weak assisting-IR laser pulses. The emission angles of the two photoelectrons can be adjusted by changing the laser parameters as well. We depict how the assisting-IR laser field enhances and/or enables the back-to-back and side-by-side emission of photoelectrons created in double ionization process.

Surface for methane combustion: O(³P)+CH₄→OH+CH₃

Ya Peng(彭亚), Zhong-An Jiang(蒋仲安), Ju-Shi Chen(陈举师)

Chin. Phys. B, 2020, 29 (7): 073401. DOI: 10.1088/1674-1056/ab90f5

Abstract ( 494 )

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Kinetic investigations including quasi-classical trajectory and canonical unified statistical theory method calculations are carried out on a potential energy surface for the hydrogen-abstraction reaction from methane by atom O(³P). The surface is constructed using a modified Shepard interpolation method. The ab initio calculations are performed at the CCSD(T) level. Taking account of the contribution of inner core electrons to electronic correlation interaction in ab initio electronic structure calculations, modified optimized aug-cc-pCVQZ basis sets are applied to the all-electrons calculations. On this potential energy surface, the triplet oxygen atom attacks methane in a near-collinear H-CH₃ direction to form a saddle point with barrier height of 13.55 kcal/mol, which plays a key role in the kinetics of the title reaction. For the temperature range of 298-2500 K, our calculated thermal rate constants for the O(³P)+CH₄→ OH+CH₃ reaction show good agreement with relevant experimental data. This work provides detailed mechanism of this gas-phase reaction and a theoretical guidance for methane combustion.

Tilt adjustment for a portable absolute atomic gravimeter

Hong-Tai Xie(谢宏泰), Bin Chen(陈斌), Jin-Bao Long(龙金宝), Chun Xue(薛春), Luo-Kan Chen(陈泺侃), Shuai Chen(陈帅)

Chin. Phys. B, 2020, 29 (7): 073701. DOI: 10.1088/1674-1056/ab90f9

Abstract ( 675 )

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For an atomic gravimeter, the measured value of the Earth's gravity acceleration g is the projection of the local gravity on the direction of Raman laser beams. To accurately measure the g, the Raman laser beams should be parallel to the g direction. We analyze the tilt effect of the Raman beams on g measurement and present a general method for the tilt adjustment. The systematic error caused by the tilt angle is evaluated as 0 (+0, -0.8) μGal (1 μGal=10 nm/s²) and the drift is also compensated in real time. Our method is especially suitable for the portable atomic gravimeter which focuses on the mobility and field applications.

Scattering and absorption characteristics of non-spherical cirrus cloud ice crystal particles in terahertz frequency band

Tao Xie(谢涛), Meng-Ting Chen(陈梦婷), Jian Chen(陈健), Feng Lu(陆风), Da-Wei An(安大伟)

Chin. Phys. B, 2020, 29 (7): 074102. DOI: 10.1088/1674-1056/ab84d3

Abstract ( 398 )

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We used discrete dipole approximation (DDA) to examine the scattering and absorption characteristics of spherical ice crystal particles. On this basis, we studied the scattering characteristics of spherical ice crystal particles at different frequencies and non-spherical ice crystal particles with different shapes, aspect ratios, and spatial orientations. The results indicate that the DDA and Mie methods yield almost the same results for spherical ice crystal particles, illustrating the superior calculation accuracy of the DDA method. Compared with the millimeter wave band, the terahertz band particles have richer scattering characteristics and can detect ice crystal particles more easily. Different frequencies, shapes, aspect ratios, and spatial orientations have specific effects on the scattering and absorption characteristics of ice crystal particles. The results provide an important theoretical basis for the design of terahertz cloud radars and related cirrus detection methods.

Improved spatial filtering velocimetry and its application in granular flow measurement

Ping Kong(孔平), Bi-De Wang(王必得), Peng Wang(王蓬), Zivkovic V, Jian-Qing Zhang(张建青)

Chin. Phys. B, 2020, 29 (7): 074201. DOI: 10.1088/1674-1056/ab8ac2

Abstract ( 623 )

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Spatial filtering velocimetry (SFV) has the advantages of simple structure, good stability, and wide applications. However, the traditional linear CCD-based SFV method requires an accurate angle between the direction of linear CCD and the direction of moving object, so it is not suitable for measuring a complex flow field or two-dimensional speed in a granular media. In this paper, a new extension of spatial filtering method (SFM) based on high speed array CCD camera is proposed as simple and effective technique for measuring two-dimensional speed field of granular media. In particular, we analyzed the resolution and range of array CCD-based SFV so that the reader can clarify the application scene of this method. This method has a particular advantage for using orthogonal measurement to avoid the angle measurement, which were problematic when using linear CCD to measure the movement. Finally, the end-wall effects of the granular flow in rotating drum is studied with different experimental conditions by using this improved technique.

Reversion of weak-measured quantum entanglement state

Shao-Jiang Du(杜少将), Yonggang Peng(彭勇刚), Hai-Ran Feng(冯海冉), Feng Han(韩峰), Lian-Wu Yang(杨连武), Yu-Jun Zheng(郑雨军)

Chin. Phys. B, 2020, 29 (7): 074202. DOI: 10.1088/1674-1056/ab8c3e

Abstract ( 660 )

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We theoretically study the reversible process of quantum entanglement state by means of weak measurement and corresponding reversible operation. We present a protocol of the reversion operation in two bodies based on the theory of reversion of single photon and then expend it in quantum communication channels. The theoretical results demonstrate that the protocol does not break the information transmission after a weak measurement and a reversible measurement with the subsequent process in the transmission path. It can reverse the perturbed entanglement intensity evolution to its original state. Under the condition of different weak measurement intensity the protocol can reverse the perturbed quantum entanglement system perfectly. In the process we can get the classical information described by information gain from the quantum system through weak measurement operation. On the other hand, in order to realize complete reversibility, the classical information of the quantum entanglement system must obey a limited range we present in this paper in the reverse process.

Photoelectron momentum distributions of single-photon ionization under a pair of elliptically polarized attosecond laser pulses

Hui-Fang Cui(崔会芳), Xiang-Yang Miao(苗向阳)

Chin. Phys. B, 2020, 29 (7): 074203. DOI: 10.1088/1674-1056/ab8ac0

Abstract ( 657 )

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The attosecond ionization dynamics of atoms has attracted extensive attention in these days. However, the role of the initial state is not clearly understood. To address this question, we perform simulations on the neon atom and its model atom with different initial states by numerically solving the corresponding two-dimensional time-dependent Schrödinger equations. We theoretically investigate atomic photoelectron momentum distributions (PMDs) by a pair of elliptically polarized attosecond laser pulses. We find that the PMD is sensitive not only to the ellipticities of the pulses, the relative helicity, and time delay of the pulses, but also to the symmetry of the initial electronic states. Results are analyzed by the first-order time-dependent perturbation theory (TDPT) and offer a new tool for detecting the rotation direction of the ring currents.

Optical nonreciprocity in a piezo-optomechanical system

Yu-Ming Xiao(肖玉铭), Jun-Hao Liu(刘军浩), Qin Wu(吴琴), Ya-Fei Yu(於亚飞), Zhi-Ming Zhang(张智明)

Chin. Phys. B, 2020, 29 (7): 074204. DOI: 10.1088/1674-1056/ab8abf

Abstract ( 708 )

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We theoretically study the optical nonreciprocity in a piezo-optomechanical microdisk resonator, in which the cavity modes and the mechanical mode are optically pumped and piezoelectrically driven, respectively. For asymmetric optical pumping and different piezoelectrical drivings, our system shows some nonreciprocal optical responses. We find that our system can function as an optical isolator, a nonreciprocal amplifier, or a nonreciprocal phase shifter.

575-fs passively mode-locked Yb:CaF₂ ceramic laser

Cong Wang(王聪), Qian-Qian Hao(郝倩倩), Wei-Wei Li(李威威), Hai-Jun Huang(黄海军), Shao-Zhao Wang(王绍钊), Da-Peng Jiang(姜大朋), Jie Liu(刘杰), Bing-Chu Mei(梅炳初), Liang-Bi Su(苏良碧)

Chin. Phys. B, 2020, 29 (7): 074205. DOI: 10.1088/1674-1056/ab8ac5

Abstract ( 717 )

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A Yb-doped CaF₂ transparent ceramics was successfully fabricated by the hot-pressed method and its laser characteristics were studied. A broad tuning performance and mode-locked laser operation were demonstrated in this ceramics for the first time, to our best knowledge. A 60-nm continuous-wavelength tunable laser from 1019 nm to 1079 nm was obtained with a birefringent filter. By employing a semiconductor saturable absorber mirror without additional dispersion compensation elements, a continuous-wave mode-locked laser with pulse duration as short as 575 fs was delivered, at a central wavelength of 1048.5 nm. The oscillator is operated under a repetition rate of 55 MHz. These results indicate that the Yb:CaF₂ transparent ceramics is an ideal candidate for the development of ultrafast lasers in the near-infrared regime.

Pulse generation in Yb-doped polarization-maintaining fiber laser by nonlinear polarization evolution

Cheng-Bin Liang(梁成斌), Yan-Rong Song(宋晏蓉), Zi-Kai Dong(董自凯), Yun-Feng Wu(吴云峰), Jin-Rong Tian(田金荣), Run-Qin Xu(徐润亲)

Chin. Phys. B, 2020, 29 (7): 074206. DOI: 10.1088/1674-1056/ab8ac6

Abstract ( 847 )

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We demonstrate a self-started, long-term stable polarization-maintaining mode-locked fiber laser based on the nonlinear polarization evolution technique. A polarized beam splitter is inserted into the cavity of the linear polarization-maintaining fiber laser to facilitate self-started mode-locking. Pulses with single pulse energy of 26.9 nJ and average output power of 73.9 mW are obtained at the pump power of 600 mW. The transmission characteristics of artificial saturable absorber used in this laser are analyzed theoretically, the influence of the half-wave plate state on mode-locking is discussed, and the mode-locking range is obtained, which is well consistent with the experimental results.

A two-mode squeezed light based on a double-pump phase-matching geometry

Xuan-Jian He(何烜坚), Jun Jia(贾俊), Gao-Feng Jiao(焦高锋), Li-Qing Chen(陈丽清), Chun-Hua Yuan(袁春华), Wei-Ping Zhang(张卫平)

Chin. Phys. B, 2020, 29 (7): 074207. DOI: 10.1088/1674-1056/ab8abd

Abstract ( 745 )

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We theoretically investigate the frequency-nondegenerate and frequency degenerate squeezed lights with a four-wave mixing process (4WM) driven by two pump fields crossing at a small angle. Different from a 4WM process driven by a single pump field, the refractive index of the corresponding probe field, n_p, can be converted to a value that is greater than 1 or less than 1 by an angle adjustment. In the new region with n_p < 1, the bandwidth of the gain is relatively large due to the slow change in the refractive index with the two-photon detuning. In this region with an exchange of the roles of the pump and probe beams, the frequency degenerate and spatial nondegenerate twin beams can be generated, which has potential application in quantum information and quantum metrology.

Irradiation study of liquid crystal variable retarder for Full-disk Magneto-Graph payload onboard ASO-S mission ^Hot!

Jun-Feng Hou(侯俊峰), Hai-Feng Wang(王海峰), Gang Wang(王刚), Yong-Quan Luo(骆永全), Hong-Wei Li(李宏伟), Zhen-Long Zhang(张振龙), Dong-Guang Wang(王东光), Yuan-Yong Deng(邓元勇)

Chin. Phys. B, 2020, 29 (7): 074208. DOI: 10.1088/1674-1056/ab8c40

Abstract ( 770 )

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The Advanced Space-based Solar Observatory (ASO-S) is a mission proposed by the Chinese Solar Physics Community. As one of the three payloads of ASO-S, the Full-disc Magneto-Graph (FMG) will measure the photospheric magnetic fields of the entire solar disk with high spatial and temporal resolution, and high magnetic sensitivity, where liquid crystal variable retarder (LCVR) is the key to whether FMG can achieve its scientific goal. So far, there is no space flight experience for LCVR. Therefore, irradiation study for LCVRs becomes more important and urgent in order to make sure their safety and reliability in space application. In this paper, γ irradiation, proton irradiation, and ultra-violet (UV) irradiation are tested for LCVRs respectively. The optical and chemical properties during irradiation tests are measured and analyzed. For optical properties, there is no significant change in those parameters FMG payload concerned except the retardation. Although there is no drastic degradation in the retardation versus voltage during irradiations, the amount of retardation variation is much higher than the instrument requirements. Thus, an in-flight retardation versus voltage should be added in FMG payload, reducing or even avoiding the impact of retardation change. For chemical properties, the clearing point and birefringence of the LC materials almost have no change; the ion density dose not change below 60 krad[Si], but begin to increase dramatically above 60 krad[Si].

Progress on the ⁴⁰Ca⁺ ion optical clock

Baolin Zhang(张宝林), Yao Huang(黄垚), Huaqing Zhang(张华青), Yanmei Hao(郝艳梅), Mengyan Zeng(曾孟彦), Hua Guan(管桦), Kelin Gao(高克林)

Chin. Phys. B, 2020, 29 (7): 074209. DOI: 10.1088/1674-1056/ab9432

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Progress of the ⁴⁰Ca⁺ ion optical clock based on the 4²S_1/2-3d ²D_5/2 electric quadrupole transition is reported. By setting the drive frequency to the “magic” frequency Ω₀, the frequency uncertainty caused by the scalar Stark shift and second-order Doppler shift induced by micromotion is reduced to the 10^-19 level. By precisely measuring the differential static scalar polarizability △α₀, the uncertainty due to the blackbody radiation (BBR) shift (coefficient) is reduced to the 10^-19 level. With the help of a second-order integrating servo algorithm, the uncertainty due to the servo error is reduced to the 10^-18 level. The total fractional uncertainty of the ⁴⁰Ca⁺ ion optical clock is then improved to 2.2×10^-17, whereas this value is mainly restricted by the uncertainty of the BBR shift due to temperature fluctuations. The state preparation is introduced together with improvements in the pulse sequence, and furthermore, a better signal to noise ratio (SNR) and less dead time are achieved. The clock stability of a single clock is improved to 4.8×10^-15/√τ (in seconds).

High-performance frequency stabilization of ultraviolet diode lasers by using dichroic atomic vapor spectroscopy and transfer cavity

Danna Shen(申丹娜), Liangyu Ding(丁亮宇), Qiuxin Zhang(张球新), Chenhao Zhu(朱晨昊), Yuxin Wang(王玉欣), Wei Zhang(张威), Xiang Zhang(张翔)

Chin. Phys. B, 2020, 29 (7): 074210. DOI: 10.1088/1674-1056/ab8c41

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We develop a high-performance ultraviolet (UV) frequency stabilization technique implemented directly on UV diode lasers by combining the dichroic atomic vapor laser lock and the resonant transfer cavity lock. As an example, we demonstrate a stable locking with measured frequency standard deviations of approximately 200 kHz and 300 kHz for 399 nm and 370 nm diode lasers in 20 min. We achieve a long-term frequency drift of no more than 1 MHz for the target 370 nm laser within an hour, which is further verified with fluorescence count rates of a single trapped ¹⁷¹Yb⁺ ion. We also find strong linear correlations between lock points and environmental factors such as temperature and atmospheric pressure. Our approach provides a simple and stable solution at a relatively low cost, and features flexible control, high feedback bandwidth and minimal power consumption of the target UV laser.

Simulation of microswimmer hydrodynamics with multiparticle collision dynamics

Andreas Z?ttl

Chin. Phys. B, 2020, 29 (7): 074701. DOI: 10.1088/1674-1056/ab943f

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In this review we discuss the recent progress in the simulation of soft active matter systems and in particular the hydrodynamics of microswimmers using the method of multiparticle collision dynamics, which solves the hydrodynamic flows around active objects on a coarse-grained level. We first present a brief overview of the basic simulation method and the coupling between microswimmers and fluid. We then review the current achievements in simulating flexible and rigid microswimmers using multiparticle collision dynamics, and briefly conclude and discuss possible future directions.

Temperature dependence of mode coupling effect in piezoelectric vibrator made of [001]_c-poled Mn-doped 0.24PIN-0.46PMN-0.30PT ternary single crystals with high electromechanical coupling factor

Nai-Xing Huang(黄乃兴), En-Wei Sun(孙恩伟), Rui Zhang(张锐), Bin Yang(杨彬), Jian Liu(刘俭), Tian-Quan Lü(吕天全), Wen-Wu Cao(曹文武)

Chin. Phys. B, 2020, 29 (7): 075201. DOI: 10.1088/1674-1056/ab8a3e

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The influence of temperature on mode coupling effect in piezoelectric vibrators remains unclear. In this work, we discuss the influence of temperature on two-dimensional (2D) mode coupling effect and electromechanical coupling coefficient of cylindrical [001]_c-poled Mn-doped 0.24PIN-0.46PMN-0.30PT piezoelectric single-crystal vibrator with an arbitrary configuration ratio. The electromechanical coupling coefficient k_t decreases with temperature increasing, whereas k₃₃ is largely invariant in a temperature range of 25 ℃-55 ℃. With the increase of temperature, the shift in the ‘mode dividing point’ increases the scale of the poling direction of the piezoelectric vibrator. The temperature has little effect on coupling constant Γ. At a given temperature, the coupling constant Γ of the cylindrical vibrator is slightly greater than that of the rectangular vibrator. When the temperature changes, the applicability index (M) values of the two piezoelectric vibrators are close to 1, indicating that the coupling theory can be applied to piezoelectric vibrators made of late-model piezoelectric single crystals.

Spontaneous growth of the reconnection electric field during magnetic reconnection with a guide field: A theoretical model and particle-in-cell simulations

Kai Huang(黄楷), Quan-Ming Lu(陆全明), Rong-Sheng Wang(王荣生), Shui Wang(王水)

Chin. Phys. B, 2020, 29 (7): 075202. DOI: 10.1088/1674-1056/ab8da0

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Reconnection electric field is a key element of magnetic reconnection. It quantifies the change of magnetic topology and the dissipation of magnetic energy. In this work, two-dimensional (2D) particle-in-cell (PIC) simulations are performed to study the growth of the reconnection electric field in the electron diffusion region (EDR) during magnetic reconnection with a guide field. At first, a seed electric field is produced due to the excitation of the tearing-mode instability. Then, the reconnection electric field in the EDR, which is dominated by the electron pressure tensor term, suffers a spontaneous growth stage and grows exponentially until it saturates. A theoretical model is also proposed to explain such a kind of growth. The reconnection electric field in the EDR is found to be directly proportional to the electron outflow speed. The time derivative of electron outflow speed is proportional to the reconnection electric field in the EDR because the outflow is formed after the inflow electrons are accelerated by the reconnection electric field in the EDR and then directed away along the outflow direction. This kind of reinforcing process at last leads to the exponential growth of the reconnection electric field in the EDR.

Analysis of extreme ultraviolet spectra of laser-produced Cd plasmas

Mohammedelnazier Bakhiet, Maogen Su(苏茂根), Shiquan Cao(曹世权), Qi Min(敏琦), Duixiong Sun(孙对兄), Siqi He(何思奇), Lei Wu(吴磊), Chenzhong Dong(董晨钟)

Chin. Phys. B, 2020, 29 (7): 075203. DOI: 10.1088/1674-1056/ab8894

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In order to provide detailed information about Cd structure and gain more insight regarding ionization degrees and types of transition, as well as the understanding of the temporal evolution behavior of laser produced Cd plasmas, extreme ultraviolet spectra of laser-produced cadmium (Cd) plasmas have been measured in the 8.4-12 nm region using spatio-temporally resolved laser-produced plasma spectroscopy technique. Spectral features were analyzed by the Hartree-Fock (HF) method with relativistic correlations (HFR) using the Cowan code. The results showed that the 4p-5s resonance transition arrays from Cd⁹⁺ to Cd¹³⁺ merged to form intense lines in this spectral region. A number of new spectral features from Cd⁹⁺ and Cd¹⁰⁺ ions are reported in this study. Based on the assumption of a normalized Boltzmann distribution among the excited states associated with a steady-state collisional-radiative model, the plasma parameters were obtained by comparing the experimental and simulated spectra. As a result, we succeeded in reproducing the synthetic spectra for different time delays, which yielded good agreement with the experiments. The temporal evolution behaviors of electron temperature and electron density of plasma were also analyzed.

Electrical modeling of dielectric barrier discharge considering surface charge on the plasma modified material Suggested by editors

Hong-Lu Guan(关弘路), Xiang-Rong Chen(陈向荣), Tie Jiang(江铁), Hao Du(杜浩), Ashish Paramane, Hao Zhou(周浩)

Chin. Phys. B, 2020, 29 (7): 075204. DOI: 10.1088/1674-1056/ab8a3f

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We present the variations of electrical parameters of dielectric barrier discharge (DBD) when the DBD generator is used for the material modification, whereas the relevant physical mechanism is also elaborated. An equivalent circuit model is applied for a DBD generator working in a filament discharging mode, considering the addition of epoxy resin (EP) as the plasma modified material. The electrical parameters are calculated through the circuit model. The surface conductivity, surface potential decay, trap distributions and surface charge distributions on the EP surface before and after plasma treatments were measured and calculated. It is found that the coverage area of micro-discharge channels on the EP surface is increased with the discharging time under the same applied AC voltage. The results indicate that the plasma modified material could influence the ignition of new filaments in return during the modification process. Moreover, the surface conductivity and density of shallow traps with low trap energy of the EP samples increase after the plasma treatment. The surface charge distributions indicate that the improved surface properties accelerate the movement and redistribution of charge carriers on the EP surface. The variable electrical parameters of discharge are attributed to the redistribution of deposited surface charge on the plasma modified EP sample surface.

Theoretical study on martensitic-type transformation path from rutile phase to α-PbO₂ phase of TiO₂

Wen-Xuan Wang(王文轩), Zhen-Yi Jiang(姜振益), Yan-Ming Lin(林彦明), Ji-Ming Zheng(郑继明), Zhi-Yong Zhang(张志勇)

Chin. Phys. B, 2020, 29 (7): 076101. DOI: 10.1088/1674-1056/ab8abe

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The martensitic-type phase transformation paths from the rutile to the α-PbO₂ phase of TiO₂ are studied with linear interpolation and NEB/G-SSNEB methods based on first-principles calculations. Its potential energy surface and the lowest energy path are revealed. Our results indicate that the titanium atoms of the rutile phase shuffle along the [0-11]_rut crystal direction to form the α-PbO₂ phase. During the phase transition, the oxygen atoms are dragged by the heavier titanium atoms and then reach their new equilibrium positions. The barrier of phase transition from nudged elastic band theory is about 231 meV, which is qualitatively consistent with previous theoretical calculations from the monoclinic phase to the tetragonal phase for ZrO₂ and HfO₂. Debye model can also be successfully used to predict the pressure and temperature of the phase transformation.

Tunable electronic structures of germanane/antimonene van der Waals heterostructures using an external electric field and normal strain

Xing-Yi Tan(谭兴毅), Li-Li Liu(刘利利), Da-Hua Ren(任达华)

Chin. Phys. B, 2020, 29 (7): 076102. DOI: 10.1088/1674-1056/ab8a39

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Van der Waals (vdW) heterostructures have attracted significant attention because of their widespread applications in nanoscale devices. In the present work, we investigate the electronic structures of germanane/antimonene vdW heterostructure in response to normal strain and an external electric field by using the first-principles calculations based on density functional theory (DFT). The results demonstrate that the germanane/antimonene vdW heterostructure behaves as a metal in a [-1, -0.6] V/Å range, while it is a direct semiconductor in a [-0.5, 0.2] V/Å range, and it is an indirect semiconductor in a [0.3, 1.0] V/Å range. Interestingly, the band alignment of germanane/antimonene vdW heterostructure appears as type-Ⅱ feature both in a [-0.5, 0.1] range and in a [0.3, 1] V/Å range, while it shows the type-I character at 0.2 V/Å. In addition, we find that the germanane/antimonene vdW heterostructure is an indirect semiconductor both in an in-plane biaxial strain range of [-5%, -3%] and in an in-plane biaxial strain range of [3%, 5%], while it exhibits a direct semiconductor character in an in-plane biaxial strain range of [-2%, 2%]. Furthermore, the band alignment of the germanane/antimonene vdW heterostructure changes from type-Ⅱ to type-I at an in-plane biaxial strain of -3%. The adjustable electronic structure of this germanane/antimonene vdW heterostructure will pave the way for developing the nanoscale devices.

Comparison of cavities and extended defects formed in helium-implanted 6H-SiC at room temperature and 750 ℃

Qing Liao(廖庆), Bingsheng Li(李炳生), Long Kang(康龙), Xiaogang Li(李小刚)

Chin. Phys. B, 2020, 29 (7): 076103. DOI: 10.1088/1674-1056/ab8abc

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The formation of cavities in silicon carbide is vitally useful to “smart-cut” and metal gettering in semiconductor industry. In this study, cavities and extended defects formed in helium (He) ions implanted 6H-SiC at room temperature (RT) and 750 ℃ followed by annealing at 1500 ℃ are investigated by a combination of transmission electron microscopy and high-resolution electron microscopy. The observed cavities and extended defects are related to the implantation temperature. Heterogeneously distributed cavities and extended defects are observed in the helium-implanted 6H-SiC at RT, while homogeneously distributed cavities and extended defects are formed after He-implanted 6H-SiC at 750 ℃. The possible reasons are discussed.

Structural, mechanical, and electronic properties of Zr-Te compounds from first-principles calculations

Peng Wang(王鹏), Ning-Chao Zhang(张宁超), Cheng-Lu Jiang(蒋城露), Fu-Sheng Liu(刘福生), Zheng-Tang Liu(刘正堂), Qi-Jun Liu(刘其军)

Chin. Phys. B, 2020, 29 (7): 076201. DOI: 10.1088/1674-1056/ab8da7

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The first-principles calculations based on density functional theory are used to obtain structural, mechanical, and electronic properties of Zr-Te compounds. The optimized structural parameters are consistent with the available experimental data. The calculated mechanical properties and formation energy show that the Zr-Te compounds are all mechanically and thermodynamically stable. The bulk modulus B, shear modulus G, Young's modulus E, Debye temperature Θ_D, and sound velocity v_m are listed, which are positively correlated with the increasing of atomic fraction of Zr. The behaviors of density of states of Zr-Te compounds are obtained. Furthermore, the electronic properties are discussed to clarify the bonding characteristics of compounds. The electronic characteristics demonstrate that the Zr-Te systems with different phases are both covalent and metallic.

Giant mechanocaloric materials for solid-state cooling

Junran Zhang(张俊然), Yixuan Xu(徐逸轩), Shihai An(安世海), Ying Sun(孙莹), Xiaodong Li(李晓东), Yanchun Li(李延春)

Chin. Phys. B, 2020, 29 (7): 076202. DOI: 10.1088/1674-1056/ab8a40

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This article reviews the research progress of measurement techniques and materials on the mechanocaloric effect over the past few decades. Mechanocaloric materials can be divided into elastocaloric and barocaloric materials depending on the applied uniaxial stress or hydrostatic pressure. Elastocaloric materials include non-magnetic shape memory alloys, polymers, and rare-earth compounds. Barocaloric materials include magnetic shape memory alloys, ferroelectric ceramics, superionic conductors, and oxyfluorides. The mechanocaloric effects of these classes of materials are systematically compared in terms of the isothermal entropy change and adiabatic temperature change. In addition to the thermal effects, other characteristics closely related to the application of mechanocaloric materials are also summarized. Finally, perspectives for further development of mechanocaloric materials in the solid-state cooling area are discussed.

Generating two-dimensional quantum gases with high stability Suggested by editors

Bo Xiao(肖波), Xuan-Kai Wang(王宣恺), Yong-Guang Zheng(郑永光), Yu-Meng Yang(杨雨萌), Wei-Yong Zhang(章维勇), Guo-Xian Su(苏国贤), Meng-Da Li(李梦达), Xiao Jiang(江晓), Zhen-Sheng Yuan(苑震生)

Chin. Phys. B, 2020, 29 (7): 076701. DOI: 10.1088/1674-1056/ab8ac8

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Quantum gas microscopy has enabled the study on intriguing properties of ultracold atoms in optical lattices. It provides the cutting-edge technology for manipulating quantum many-body systems. In such experiments, atoms have to be prepared into a two-dimensional (2D) system for being resolved by microscopes with limited depth of focus. Here we report an experiment on slicing a single layer of the atoms trapped in a few layers of pancake-shaped optical traps to create a 2D system. This technique is implemented with a microwave “knife”, i.e., a microwave field with a frequency defined by the resonant condition with the Zeeman-shifted atomic levels related to a gradient magnetic field. It is crucial to keep a stable preparation of the desired layer to create the 2D quantum gas for future experimental applications. To achieve this, the most important point is to provide a gradient magnetic field with low noises and slow drift in combination with a properly optimized microwave pulse. Monitoring the electric current source and the environmental magnetic field, we applied an actively stabilizing circuit and realized a field drift of 0.042(3) mG/hour. This guarantees creating the single layer of atoms with an efficiency of 99.92(3)% while atoms are hardly seen in other layers within 48 hours, satisfying future experimental demands on studying quantum many-body physics.

Tuning the alignment of pentacene on copper substrate by annealing-assistant surface functionalization

Qiao-Jun Cao(曹巧君), Shuang Wen(温爽), Hai-Peng Xie(谢海鹏), Bi-Yun Shi(施碧云), Qun Wang(王群), Cong-Rong Lu(卢从蓉), Yongli Gao(高永利), Wei-Dong Dou(窦卫东)

Chin. Phys. B, 2020, 29 (7): 076801. DOI: 10.1088/1674-1056/ab8da2

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Controlling the alignment and packing structure of organic molecules on solid substrate surfaces at molecule level is essential to develop high-performance organic thin film (OTF) devices. Pentacene, which is a typical p-type semiconductor material usually adopts lying-down geometry on metal substrates owning to π-d coupling between pentacene and metal substrates. However, in this study, we found that pentacene molecules can be adsorbed on an anneal-treated Cu (111) surface with their long axis perpendicular to substrate surface. Highly ordered single-layer pentacene film with stand-up molecular geometry was achieved on this substrate. It was found that the functionalization of Cu surface with C=O groups due to annealing treatment should be accounted for standing-up geometry of pentacene on Cu substrate. This observation shed light on the tuning of the alignment and packing structure of organic molecules.

Influence comparison of N₂ and NH₃ nitrogen sources on AlN films grown by halide vapor phase epitaxy

Jing-Jing Chen(陈晶晶), Jun Huang(黄俊), Xu-Jun Su(苏旭军), Mu-Tong Niu(牛牧童), Ke Xu(徐科)

Chin. Phys. B, 2020, 29 (7): 076802. DOI: 10.1088/1674-1056/ab90ed

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A comparison of the nitrogen sources (N₂ and NH₃) influence on AlN films grown by high-temperature halide vapor phase epitaxy (HVPE) is reported. The x-ray rocking curves (XRCs) indicate that the full width at half maximum (FWHM) of (0002) plane for AlN films using N₂ as nitrogen source is generally smaller than that using NH₃. Optical microscope and atomic force microscope (AFM) results show that it is presently still more difficult to control the crack and surface morphology of AlN films with thicknesses of 5-10 μm using N₂ as the nitrogen source compared to that using NH₃. Compared with one-step growth, two-step growth strategy has been proved more effective in stress control and reducing the density of threading dislocations for AlN epilayers using N₂ as the nitrogen source. These investigations reveal that using N₂ as nitrogen source in HVPE growth of AlN is immature at present, but exhibits great potential.

Structural evolution and magnetic properties of ScLi_n (n=2-13) clusters: A PSO and DFT investigation

Lu Li(栗潞), Xiu-Hua Cui(崔秀花), Hai-Bin Cao(曹海宾), Yi Jiang(姜轶), Hai-Ming Duan(段海明), Qun Jing(井群), Jing Liu(刘静), Qian Wang(王倩)

Chin. Phys. B, 2020, 29 (7): 077101. DOI: 10.1088/1674-1056/ab8d9e

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The stable geometries, electronic structures, and magnetic behaviors of the ScLi_n (n=2-13) clusters are investigated by using particle swarm optimization (PSO) and density functional theory (DFT). The results show that these clusters have three-dimensional (3D) structures except ScLi₂, and ScLi₁₂, and ScLi₁₃ that possess the cage-like structures. In analyses of the average binding energy, second-order difference of energy, and fragmentation energy, ScLi₁₂ cluster is identified as magnetic superatom. The magnetic moment for each of these clusters owns an oscillating curve of different cluster sizes, and their magnetic moments are further investigated using molecular orbitals and jellium model. Of ScLi_n (n=2-13) clusters, ScLi₁₂ has the largest spin magnetic moment (3 μ_B), and molecular orbitals of ScLi₁₂ can be described as 1S²1P⁶1D_α⁵D_β². Additionally, Mulliken population and AdNDP bonding analysis are discussed and the results reveal that the Sc atom and Li_n atoms make equal contribution to the total magnetic moment, and atomic charges transfer between Sc atoms and Li atoms.

Lifshitz transition in triangular lattice Kondo-Heisenberg model

Lan Zhang(张欄), Yin Zhong(钟寅), Hong-Gang Luo(罗洪刚)

Chin. Phys. B, 2020, 29 (7): 077102. DOI: 10.1088/1674-1056/ab8da4

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Motivated by recent experimental progress on triangular lattice heavy-fermion compounds, we investigate possible Lifshitz transitions and the scanning tunnel microscope (STM) spectra of the Kondo-Heisenberg model on the triangular lattice. In the heavy Fermi liquid state, the introduced Heisenberg antiferromagnetic interaction (J_H) results in the twice Lifshitz transition at the case of the nearest-neighbour electron hopping but with next-nearest-neighbour hole hopping and the case of the nearest-neighbour hole hopping but with next-nearest-neighbour electron hopping, respectively. Driven by J_H, the Lifshitz transitions on triangular lattice are all continuous in contrast to the case on square lattice. Furthermore, the STM spectra shows rich line-shape which is influenced by the Kondo coupling J_K, the Heisenberg antiferromagnetic interaction J_H, and the ratio of the tunneling amplitude of f-electron t_f versus conduction electron t_c. Our work provides a possible scenario to understand the Fermi surface topology and the quantum critical point in heavy-fermion compounds.

Point-contact spectroscopy on antiferromagnetic Kondo semiconductors CeT₂Al₁₀ (T=Ru and Os) Suggested by editors

Jie Li(李洁), Li-Qiang Che(车利强), Tian Le(乐天), Jia-Hao Zhang(张佳浩), Pei-Jie Sun(孙培杰), Toshiro Takabatake, Xin Lu(路欣)

Chin. Phys. B, 2020, 29 (7): 077103. DOI: 10.1088/1674-1056/ab8dab

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We have carried out point-contact spectroscopy (PCS) measurements on one family of antiferromagnetic Kondo semiconductor CeT₂Al₁₀ (T=Ru and Os) with a Néel temperature T_N ～27.5 and 28.5 K, respectively. Their PCS conductance curves both exhibit a characteristic coherent double-peak-structure at temperatures below T_N, signaling an AFM gap around the Fermi surface. The temperature dependent AFM gap △₁ follows a Bardeen-Cooper-Schrieffer (BCS)-like mean-field behavior with a moderate gap anisotropy for PCS along different crystal axes. Another asymmetric gap-like feature is observed for both compounds at temperatures far below T_N, which is consistent with opening of a new hybridization gap △_h inside the long-range ordered AFM state. Our results suggest a common itinerant nature of the anomalous AFM ordering, constraining theoretical models to explain the AFM origin in CeRu₂Al₁₀ and CeOs₂Al₁₀.

Exciton optical absorption in asymmetric ZnO/ZnMgO double quantum wells with mixed phases

Zhi-Qiang Han(韩智强), Li-Ying Song(宋丽颖), Yu-Hai Zan(昝宇海), Shi-Liang Ban(班士良)

Chin. Phys. B, 2020, 29 (7): 077104. DOI: 10.1088/1674-1056/ab8d9d

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The optical absorption of exciton interstate transition in Zn_1-xlMg_xlO/ZnO/Zn_1-xcMg_xcO/ZnO/Zn_1-xrMg_xrO asymmetric double quantum wells (ADQWs) with mixed phases of zinc-blende and wurtzite in Zn_1-xMg_xO for 0.37< x < 0.62 is discussed. The mixed phases are taken into account by our weight model of fitting. The states of excitons are obtained by a finite difference method and a variational procedure in consideration of built-in electric fields (BEFs) and the Hartree potential. The optical absorption coefficients (OACs) of exciton interstate transition are obtained by the density matrix method. The results show that Hartree potential bends the conduction and valence bands, whereas a BEF tilts the bands and the combined effect enforces electrons and holes to approach the opposite interfaces to decrease the Coulomb interaction effects between electrons and holes. Furthermore, the OACs indicate a transformation between direct and indirect excitons in zinc-blende ADQWs due to the quantum confinement effects. There are two kinds of peaks corresponding to wurtzite and zinc-blende structures respectively, and the OACs merge together under some special conditions. The computed result of exciton interband emission energy agrees well with a previous experiment. Our conclusions are helpful for further relative theoretical studies, experiments, and design of devices consisting of these quantum well structures.

Structural, electronic, and magnetic properties of quaternary Heusler CrZrCoZ compounds: A first-principles study

Xiao-Ping Wei(魏小平), Tie-Yi Cao(曹铁义), Xiao-Wei Sun(孙小伟), Qiang Gao(高强), Peifeng Gao(高配峰), Zhi-Lei Gao(高治磊), Xiao-Ma Tao(陶小马)

Chin. Phys. B, 2020, 29 (7): 077105. DOI: 10.1088/1674-1056/ab969b

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Using the first-principles calculations, we study the structural, electronic, and magnetic properties along with exchange interactions and Curie temperatures for CrZrCoZ (Z=Al, Ga, In, Tl, Si, Pb) quaternary Heusler alloys. The results show that the CrZrCoZ alloys are half-metallic ferrimagnets, and their total spin magnetic moments, which are mainly carried by the Cr atom, obey the Slater-Pauling rule. Analysis of local density of states confirms that the exchange splitting between e_g and t_2g states leads to the formation of half-metallic gap. According to the calculated Heisenberg exchange coupling parameters, it is found that the Cr(A)-Cr(A) and Cr(A)-Zr(B) exchanges dominate the appearance of ferrimagnetic states in CrZrCoZ (Z=Al, Ga, In, Tl, Pb) alloys, and it is the Cr(A)-Zr(B) and Zr(B)-Zr(B) exchanges for CrZrCoSi alloy. Finally, we estimate the Curie temperatures of CrZrCoZ by using mean-field approximation, it is found that the CrZrCoZ (Z=Al, Ga, In, Tl, Pb) alloys have noticeably higher Curie temperatures than room temperature. So, we expect that the CrZrCoZ alloys are promising candidates in spintronic applications in future.

Modulation of carrier lifetime in MoS₂ monolayer by uniaxial strain ^Hot!

Hao Hong(洪浩), Yang Cheng(程阳), Chunchun Wu(吴春春), Chen Huang(黄琛), Can Liu(刘灿), Wentao Yu(于文韬), Xu Zhou(周旭), Chaojie Ma(马超杰), Jinhuan Wang(王金焕), Zhihong Zhang(张智宏), Yun Zhao(赵芸), Jie Xiong(熊杰), Kaihui Liu(刘开辉)

Chin. Phys. B, 2020, 29 (7): 077201. DOI: 10.1088/1674-1056/ab99ba

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Carrier lifetime is one of the most fundamental physical parameters that characterizes the average time of carrier recombination in any material. The control of carrier lifetime is the key to optimizing the device function by tuning the electro-optical conversion quantum yield, carrier diffusion length, carrier collection process, etc. Till now, the prevailing modulation methods are mainly by defect engineering and temperature control, which have limitations in the modulation direction and amplitude of the carrier lifetime. Here, we report an effective modulation on the ultrafast dynamics of photoexcited carriers in two-dimensional (2D) MoS₂ monolayer by uniaxial tensile strain. The combination of optical ultrafast pump-probe technique and time-resolved photoluminescence (PL) spectroscopy reveals that the carrier dynamics through Auger scattering, carrier-phonon scattering, and radiative recombination keep immune to the strain. But strikingly, the uniaxial tensile strain weakens the trapping of photoexcited carriers by defects and therefore prolongs the corresponding carrier lifetime up to 440% per percent applied strain. Our results open a new avenue to enlarge the carrier lifetime of 2D MoS₂, which will facilitate its applications in high-efficient optoelectronic and photovoltaic devices.

Effect of weak disorder in multi-Weyl semimetals ^Hot!

Zhen Ning(宁震), Bo Fu(付博), Qinwei Shi(石勤伟), Xiaoping Wang(王晓平)

Chin. Phys. B, 2020, 29 (7): 077202. DOI: 10.1088/1674-1056/ab9612

Abstract ( 839 )

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We study the behaviors of three-dimensional double and triple Weyl fermions in the presence of weak random potential. By performing the Wilsonian renormalization group (RG) analysis, we reveal that the quasiparticle experiences strong renormalization which leads to the modification of the density of states and quasiparticle residue. We further utilize the RG analysis to calculate the classical conductivity and show that the diffusive transport is substantially corrected due to the novel behavior of the quasiparticle and can be directly measured by experiments.

Epitaxial fabrication of monolayer copper arsenide on Cu(111) Suggested by editors

Shuai Zhang(张帅), Yang Song(宋洋), Jin Mei Li(李金梅), Zhenyu Wang(王振宇), Chen Liu(刘晨), Jia-Ou Wang(王嘉鸥), Lei Gao(高蕾), Jian-Chen Lu(卢建臣), Yu Yang Zhang(张余洋), Xiao Lin(林晓), Jinbo Pan(潘金波), Shi Xuan Du(杜世萱), Hong-Jun Gao(高鸿钧)

Chin. Phys. B, 2020, 29 (7): 077301. DOI: 10.1088/1674-1056/ab8db3

Abstract ( 863 )

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We report the epitaxial growth of monolayer copper arsenide (CuAs) with a honeycomb lattice on Cu(111) by molecular beam epitaxy (MBE). Scanning tunneling microscopy (STM), low energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) verify the √3×√3 superlattice of monolayer CuAs on Cu(111) substrate. Angle-resolved photoemission spectroscopy (ARPES) measurements together with DFT calculations demonstrate the electronic band structures of monolayer CuAs and reveal its metallic nature. Further calculations show that charge transfer from Cu(111) substrate to monolayer CuAs lifts the Fermi level and tunes the band structure of the monolayer CuAs. This high-quality epitaxial monolayer CuAs with potential tunable band gap holds promise on the applications in nano-electronic devices.

Probing the Majorana bound states in a hybrid nanowire double-quantum-dot system by scanning tunneling microscopy Suggested by editors

Jia Liu(刘佳), Ke-Man Li(李科曼), Feng Chi(迟锋), Zhen-Guo Fu(付振国), Yue-Fei Hou(侯跃飞), Zhigang Wang(王志刚), Ping Zhang(张平)

Chin. Phys. B, 2020, 29 (7): 077302. DOI: 10.1088/1674-1056/ab8889

Abstract ( 763 )

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We propose an interferometer composing of a scanning tunneling microscope (STM), double quantum dots (DQDs), and a semiconductor nanowire carrying Majorana bound states (MBSs) at its ends induced by the proximity effect of an s-wave superconductor, to probe the existence of the MBSs in the dots. Our results show that when the energy levels of DQDs are aligned to the energy of MBSs, the zero-energy spectral functions of DQDs are always equal to 1/2, which indicates the formation of the MBSs in the DQDs and is also responsible for the zero-bias conductance peak. Our findings suggest that the spectral functions of the DQDs may be an excellent and convenient quantity for detecting the formation and stability of the spatially separated MBSs in quantum dots.

Stress and strain analysis of Si-based Ⅲ-V template fabricated by ion-slicing Suggested by editors

Shuyan Zhao(赵舒燕), Yuxin Song(宋禹忻), Hao Liang(梁好), Tingting Jin(金婷婷), Jiajie Lin(林家杰), Li Yue(岳丽), Tiangui You(游天桂), Chang Wang(王长), Xin Ou(欧欣), Shumin Wang(王庶民)

Chin. Phys. B, 2020, 29 (7): 077303. DOI: 10.1088/1674-1056/ab8a35

Abstract ( 720 )

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Strain and stress were simulated using finite element method (FEM) for three Ⅲ-V-on-Insulator (Ⅲ-VOI) structures, i.e., InP/SiO₂/Si, InP/Al₂O₃/SiO₂/Si, and GaAs/Al₂O₃/SiO₂/Si, fabricated by ion-slicing as the substrates for optoelectronic devices on Si. The thermal strain/stress imposes no risk for optoelectronic structures grown on InPOI at a normal growth temperature using molecular beam epitaxy. Structures grown on GaAsOI are more dangerous than those on InPOI due to a limited critical thickness. The intermedia Al₂O₃ layer was intended to increase the adherence while it brings in the largest risk. The simulated results reveal thermal stress on Al₂O₃ over 1 GPa, which is much higher than its critical stress for interfacial fracture. InPOI without an Al₂O₃ layer is more suitable as the substrate for optoelectronic integration on Si.

Improvement of valley splitting and valley injection efficiency for graphene/ferromagnet heterostructure ^Hot!

Longxiang Xu(徐龙翔), Wengang Lu(吕文刚), Chen Hu(胡晨), Qixun Guo(郭奇勋), Shuai Shang(尚帅), Xiulan Xu(徐秀兰), Guanghua Yu(于广华), Yu Yan(岩雨), Lihua Wang(王立华), Jiao Teng(滕蛟)

Chin. Phys. B, 2020, 29 (7): 077304. DOI: 10.1088/1674-1056/ab8db2

Abstract ( 717 )

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The valley splitting has been realized in the graphene/Ni heterostructure with the splitting value of 14 meV, and the obtained valley injecting efficiency from the heterostructure into graphene was 6.18% [Phys. Rev. B 92 115404 (2015)]. In this paper, we report a way to improve the valley splitting and the valley injecting efficiency of the graphene/Ni heterostructure. By intercalating an Au monolayer between the graphene and the Ni, the split can be increased up to 50 meV. However, the valley injecting efficiency is not improved because the splitted valley area of graphene moves away from the Fermi level. Then, we mend the deviation by covering a monolayer of Cu on the graphene. As a result, the valley injecting efficiency of the Cu/graphene/Au/Ni heterostructure reaches 10%, which is more than 60% improvement compared to the simple graphene/Ni heterostructure. Then we theoretically design a valley-injection device based on the Cu/graphene/Au/Ni heterostructure and demonstrate that the valley injection can be easily switched solely by changing the magnetization direction of Ni, which can be used to generate and control the valley-polarized current.

Anomalous spectral weight transfer in the nematic state of iron-selenide superconductor Suggested by editors

C Cai(蔡淙), T T Han(韩婷婷), Z G Wang(王政国), L Chen(陈磊), Y D Wang(王宇迪), Z M Xin(信子鸣), M W Ma(马明伟), Yuan Li(李源), Y Zhang(张焱)

Chin. Phys. B, 2020, 29 (7): 077401. DOI: 10.1088/1674-1056/ab90ec

Abstract ( 697 )

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Nematic phase intertwines closely with high-T_c superconductivity in iron-based superconductors. Its mechanism, which is closely related to the pairing mechanism of superconductivity, still remains controversial. Comprehensive characterization of the electronic state reconstruction in the nematic phase is thus crucial. However, most experiments focus only on the reconstruction of band dispersions. Another important characteristic of electronic state, the spectral weight, has not been studied in details so far. Here, we studied the spectral weight transfer in the nematic phase of FeSe_0.9S_0.1 using angle-resolved photoemission spectroscopy and in-situ detwinning technique. There are two elliptical electron pockets overlapping with each other orthogonally at the Brillouin zone corner. We found that, upon cooling, one electron pocket loses spectral weight and fades away, while the other electron pocket gains spectral weight and becomes pronounced. Our results show that the symmetry breaking of the electronic state is manifested by not only the anisotropic band dispersion but also the band-selective modulation of the spectral weight. Our observation completes our understanding of the nematic electronic state, and put strong constraints on the theoretical models. It further provides crucial clues to understand the gap anisotropy and orbital-selective pairing in iron-selenide superconductors.

Anomalous magnetoresistance in detwinned EuFe₂As₂ Suggested by editors

Zhuang Xu(徐状), Junxiang Pan(潘俊香), Zhen Tao(陶镇), Ruixian Liu(刘瑞鲜), Guotai Tan(谈国太)

Chin. Phys. B, 2020, 29 (7): 077402. DOI: 10.1088/1674-1056/ab90e4

Abstract ( 578 )

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The in-plane magnetotransport of detwinned EuFe₂As₂ single crystal has been investigated. In the antiferromagnetic phase of Eu²⁺ spins, very different magnetoresistance results are observed upon the change of the external magnetic field direction and the current direction. This could be attributed to the tunable orientation of the Eu²⁺ spins under magnetic field. Electron scattering by spin fluctuation, superzone boundary effect, and cyclotron motion of charge carriers are used to interpret the observed anomalous magnetoresistance which is measured by using a current along a direction. The remarkable features of magnetoresistance suggest that itinerant electrons strongly couple with the spin configuration of Eu²⁺, which has a huge influence on the transport properties of EuFe₂As₂.

Metamagnetic transition and reversible magnetocaloric effect in antiferromagnetic DyNiGa compound

Yan-Hong Ding(丁燕红), Fan-Zhen Meng(孟凡振), Li-Chen Wang(王利晨), Ruo-Shui Liu(刘若水), Jun Shen(沈俊)

Chin. Phys. B, 2020, 29 (7): 077501. DOI: 10.1088/1674-1056/ab90f3

Abstract ( 708 )

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Rare-earth (R)-based materials with large reversible magnetocaloric effect (MCE) are attracting much attention as the promising candidates for low temperature magnetic refrigeration. In the present work, the magnetic properties and MCE of DyNiGa compound with TiNiSi-type orthorhombic structure are studied systematically. The DyNiGa undergoes a magnetic transition from antiferromagnetic (AFM) to paramagnetic state with Néel temperature T_N = 17 K. Meanwhile, it does not show thermal and magnetic hysteresis, revealing the perfect thermal and magnetic reversibility. Moreover, the AFM state can be induced into a ferromagnetic state by a relatively low field, and thus leading to a large reversible MCE, e.g., a maximum magnetic entropy change (-ΔS_M) of 10 J/kg·K is obtained at 18 K under a magnetic field change of 5 T. Consequently, the large MCE without thermal or magnetic hysteresis makes the DyNiGa a competitive candidate for magnetic refrigeration of hydrogen liquefaction.

Spin waves and transverse domain walls driven by spin waves: Role of damping Suggested by editors

Zi-Xiang Zhao(赵梓翔), Peng-Bin He(贺鹏斌), Meng-Qiu Cai(蔡孟秋), Zai-Dong Li(李再东)

Chin. Phys. B, 2020, 29 (7): 077502. DOI: 10.1088/1674-1056/ab90e5

Abstract ( 691 )

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Based on the uniform, helical and spiral domain-wall magnetic configurations, the excited spin waves are studied with emphasis on the role of damping. We find that the damping closes the gap of dispersion, and greatly influences the dispersion in the long-wave region for the spin waves of spiral wall and helical structure. For the uniform configuration, the Dzyaloshinskii-Moriya interaction determines the modification of dispersion by the damping. Furthermore, we investigate the interaction between spin waves and a moving spiral domain wall. In the presence of damping, the amplitude of spin wave can increase after running across the wall for small wave numbers. Driving by the spin waves, the wall propagates towards the spin-wave source with an increasing velocity. Unlike the case without damping, the relation between the wall velocity and the spin-wave frequency depends on the position of wall.

Anomalous Hall effect in ferromagnetic Weyl semimetal candidate Zr_1-xV_xCo_1.6Sn Suggested by editors

Guangqiang Wang(王光强), Zhanghao Sun(孙彰昊), Xinyu Si(司鑫宇), Shuang Jia(贾爽)

Chin. Phys. B, 2020, 29 (7): 077503. DOI: 10.1088/1674-1056/ab8da8

Abstract ( 812 )

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We grew single crystals of vanadium-substituted, ferromagnetic Weyl semimetal candidate Zr_1-xV_xCo_1.6Sn from molten tin flux. These solid solutions all crystallize in a full Heusler structure (L2₁) while their Curie temperatures and magnetic moments are enhanced by V-substitution. Their resistivity gradually changes from bad-metal-like to semiconductor-like with increasing x while the anomalous Hall effect (AHE), which can be well fitted by Tian-Ye-Jin (TYJ) scaling,^[1] is also enhanced. Moreover, we find an apparent electron-electron interaction (EEI) induced quantum correction in resistivity at low temperature. The anomalous Hall conductivity (AHC) dominated by the intrinsic term is not corrected.

Thickness-dependent magnetic order and phase transition in V₅S₈ Suggested by editors

Rui-Zi Zhang(张瑞梓), Yu-Yang Zhang(张余洋), Shi-Xuan Du(杜世萱)

Chin. Phys. B, 2020, 29 (7): 077504. DOI: 10.1088/1674-1056/ab8db1

Abstract ( 769 )

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V₅S₈ is an ideal candidate to explore the magnetism at the two-dimensional (2D) limit. A recent experiment has shown that the V₅S₈ thin films exhibit an antiferromagnetic (AFM) to ferromagnetic (FM) phase transition with reducing thickness. Here, for the first time, using density functional theory calculations, we report the antiferromagnetic order of bulk V₅S₈, which is consistent with the previous experiments. The specific antiferromagnetic order is reproduced when U_eff = 2 eV is applied on the intercalated vanadium atoms within LDA. We find that the origin of the magnetic ordering is from superexchange interaction. We also investigate the thickness-dependent magnetic order in V₅S₈ thin films. It is found that there is an antiferromagnetic to ferromagnetic phase transition when V₅S₈ is thinned down to 2.2 nm. The main magnetic moments of the antiferromagnetic and ferromagnetic states of the thin films are located on the interlayered vanadium atoms, which is the same as that in the bulk. Meanwhile, the strain in the thin films also influences the AFM-FM phase transition. Our results not only reveal the magnetic order and origin in bulk V₅S₈ and thin films, but also provide a set of parameters which can be used in future calculations.

Gd impurity effect on the magnetic and electronic properties of hexagonal Sr ferrites: A case study by DFT

Masomeh Taghipour, Mohammad Yousefi, Reza Fazaeli, Masoud Darvishganji

Chin. Phys. B, 2020, 29 (7): 077505. DOI: 10.1088/1674-1056/ab8d9f

Abstract ( 513 )

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The electronic and magnetic properties of strontium hexa-ferrite (SrFe₁₂O₁₉) are studied in pure state (SrFe₁₂O₁₉) and with dopant in the positions 2 and 3 of Fe atoms (SrGdFe₁₁O₁₉-I and SrGdFe₁₁O₁₉-Ⅱ, respectively) by utilizing a variety of the density functional theory (DFT) approaches including the Perdew-Burke-Ernzerhof generalized gradient approximation (PBE-GGA) and GGA plus Hubbard U parameter (GGA+U). The pure SrFe₁₂O₁₉ is a hard magnetic half-metal with an integer magnetic moment of 64.00μ_B, while using the GGA+U functional, the magnetic intensity increases, resulting in a magnetic semiconductor with a high integer magnetic moment of 120μ_B. By doping the Gd atom in the two different positions of Fe, the magnetic moment is increased to 71.68μ_B and 68.00μ_B, respectively. The magnetic moment increases and remains an integer; hence, SrGdFe₁₁O₁₉-Ⅱ can be very useful for application in magnetic memories. Moreover, applying the Hubbard parameter turns SrGdFe₁₁O₁₉-I and SrGdFe₁₁O₁₉-Ⅱ to magnetic semiconductors with a magnetic moment of 124μ_B, and the energy gap of both doped structures at spin down is found to be less than the pure case. By studying the electronic density diagram of the atoms of the crystal, it is found that the major effect to create magnetization in the pure case is due to the Fe atom. However, in the doped case, the elements Gd and Fe have the highest moment in the crystal respectively.

High permeability and bimodal resonance structure of flaky soft magnetic composite materials

Xi Liu(刘曦), Peng Wu(吴鹏), Peng Wang(王鹏), Tao Wang(王涛), Liang Qiao(乔亮), Fa-Shen Li(李发伸)

Chin. Phys. B, 2020, 29 (7): 077506. DOI: 10.1088/1674-1056/ab8c3b

Abstract ( 629 )

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We establish a theoretical bimodal model for the complex permeability of flaky soft magnetic composite materials to explain the variability of their initial permeability. The new model is motivated by finding the two natural resonance peaks to be inconsistent with the combination of the domain wall resonance and the natural resonance. In the derivation of the model, two relationships are explored: the first one is the relationship between the number of magnetic domains and the permeability, and the second one is the relationship between the natural resonance and the domain wall resonance. This reveals that the ball milling causes the number of magnetic domains to increase and the maximum initial permeability to exist after 10 h of ball milling. An experiment is conducted to demonstrate the reliability of the proposed model. The experimental results are in good agreement with the theoretical calculations. This new model is of great significance for studying the mechanism and applications of the resonance loss for soft magnetic composite materials in high frequency fields.

Ionic liquid gating control of planar Hall effect in Ni₈₀Fe₂₀/HfO₂ heterostructures

Yang-Ping Wang(汪样平), Fu-Fu Liu(刘福福), Cai Zhou(周偲), Chang-Jun Jiang(蒋长军)

Chin. Phys. B, 2020, 29 (7): 077507. DOI: 10.1088/1674-1056/ab8da9

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We report a tunable transverse magnetoresistance of the planar Hall effect (PHE), up to 48% in the Ni₈₀Fe₂₀/HfO₂ heterostructures. This control is achieved by applying a gate voltage with an ionic liquid technique at ultra-low voltage, which exhibits a gate-dependent PHE. Moreover, in the range of 0-V to 1-V gate voltage, transverse magnetoresistance of PHE can be continuously regulated. Ferromagnetic resonance (FMR) also demonstrates the shift of the resonance field at low gate voltage. This provides a new method for the design of the electric field continuous control spintronics device with ultra-low energy consumption.

Degenerate antiferromagnetic states in spinel oxide LiV₂O₄ Suggested by editors

Ben-Chao Gong(龚本超), Huan-Cheng Yang(杨焕成), Kui Jin(金魁), Kai Liu(刘凯), Zhong-Yi Lu(卢仲毅)

Chin. Phys. B, 2020, 29 (7): 077508. DOI: 10.1088/1674-1056/ab9617

Abstract ( 699 )

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The magnetic and electronic properties of spinel oxide LiV₂O₄ have been systematically studied by using the spin-polarized first-principles electronic structure calculations. We find that a series of magnetic states, in which the ferromagnetic (FM) V₄ tetrahedra are linked together through the corner-sharing antiferromagnetic (AFM) V₄ tetrahedra, possess degenerate energies lower than those of other spin configurations. The large number of these energetically degenerated states being the magnetic ground state give rise to strong magnetic frustration as well as large magnetic entropy in LiV₂O₄. The corresponding band structure and density of states of such a typical magnetic state in this series, i.e., the ditetrahedron (DT) AFM state, demonstrate that LiV₂O₄ is in the vicinity of a metal-insulator transition. Further analysis suggests that the t_2g and e_g orbitals of the V atoms play different roles in the magnetic exchange interactions. Our calculations are consistent with previous experimental measurements and shed light on understanding the exotic magnetism and the heavy-fermion behavior of LiV₂O₄.

A theoretical study on chemical ordering of 38-atom trimetallic Pd-Ag-Pt nanoalloys

Songül Taran, Ali Kemal Garip, Haydar Arslan

Chin. Phys. B, 2020, 29 (7): 077801. DOI: 10.1088/1674-1056/ab99b4

Abstract ( 568 )

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In this study, truncated octahedron (TO) structure is selected for further analysis and we focus on 38-atom Pd-Pt-Ag trimetallic nanoalloys. The best chemical ordering structures of Pd_nAg_32-nPt₆ trimetallic nanoalloys are obtained at Gupta level. The structures with the lowest energy at Gupta level are then re-optimized by density functional theory (DFT) relaxations and DFT results confirm the Gupta level calculations with small shifts on bond lengths indicating TO structure is favorable for 38-atom of Pd_nAg_32-nPt₆ trimetallic nanoalloys. The DFT excess energy analysis shows that Pd₈Ag₂₄Pt₆ composition has the lowest excess energy value in common with excess energy analysis at Gupta level. In Pd₈Ag₂₄Pt₆ composition, eight Pd atoms are central sites of 8 (111) hexagonal facets of TO, 24 Ag atoms locate on surface, and 6 Pt atoms locate at the core of the structure. It is also obtained that all of the compositions except Pd₁₈Ag₁₄Pt₆ and Pd₂₀Ag₁₂Pt₆ exhibit a octahedral Pt core. Besides, it is observed that there is a clear tendency for Ag atoms to segregate to the surface and also Pt atoms prefer to locate at core due to order parameter (R) variations.

Broadband strong optical dichroism in topological Dirac semimetals with Fermi velocity anisotropy

J Lim(林镇杰), K J A Ooi(黄健安), C Zhang(涨潮), L K Ang(洪礼祺), Yee Sin Ang(洪逸欣)

Chin. Phys. B, 2020, 29 (7): 077802. DOI: 10.1088/1674-1056/ab928e

Abstract ( 669 )

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Prototypical three-dimensional (3D) topological Dirac semimetals (DSMs), such as Cd₃As₂ and Na₃Bi, contain electrons that obey a linear momentum-energy dispersion with different Fermi velocities along the three orthogonal momentum dimensions. Despite being extensively studied in recent years, the inherent Fermi velocity anisotropy has often been neglected in the theoretical and numerical studies of 3D DSMs. Although this omission does not qualitatively alter the physics of light-driven massless quasiparticles in 3D DSMs, it does quantitatively change the optical coefficients which can lead to nontrivial implications in terms of nanophotonics and plasmonics applications. Here we study the linear optical response of 3D DSMs for general Fermi velocity values along each direction. Although the signature conductivity-frequency scaling, σ(ω)∝ω, of 3D Dirac fermion is well-protected from the Fermi velocity anisotropy, the linear optical response exhibits strong linear dichroism as captured by the universal extinction ratio scaling law, Λ_ij = (v_i/v_j)² (where i≠j denotes the three spatial coordinates x,y,z, and v_i is the i-direction Fermi velocity), which is independent of frequency, temperature, doping, and carrier scattering lifetime. For Cd₃As₂ and Na₃Bi₃, an exceptionally strong extinction ratio larger than 15 and covering a broad terahertz window is revealed. Our findings shed new light on the role of Fermi velocity anisotropy in the optical response of Dirac semimetals and open up novel polarization-sensitive functionalities, such as photodetection and light modulation.

Recent progress in graphene terahertz modulators

Xieyu Chen(陈勰宇), Zhen Tian(田震), Quan Li(李泉), Shaoxian Li(李绍限), Xueqian Zhang(张学迁), Chunmei Ouyang(欧阳春梅), Jianqiang Gu(谷建强), Jiaguang Han(韩家广), Weili Zhang(张伟力)

Chin. Phys. B, 2020, 29 (7): 077803. DOI: 10.1088/1674-1056/ab9433

Abstract ( 681 )

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Graphene has been recognized as a promising candidate in developing tunable terahertz (THz) functional devices due to its excellent optical and electronic properties, such as high carrier mobility and tunable conductivity. Here, we review graphene-based THz modulators we have recently developed. First, the optical properties of graphene are discussed. Then, graphene THz modulators realized by different methods, such as gate voltage, optical pump, and nonlinear response of graphene are presented. Finally, challenges and prospective of graphene THz modulators are also discussed.

Temperature dependent terahertz giant anisotropy and cycloidal spin wave modes in BiFeO₃ single crystal

Fan Liu(刘凡), Zuanming Jin(金钻明), Xiumei Liu(刘秀梅), Yuqing Fang(方雨青), Jiajia Guo(国家嘉), Yan Peng(彭滟), Zhenxiang Cheng(程振祥), Guohong Ma(马国宏), Yiming Zhu(朱亦鸣)

Chin. Phys. B, 2020, 29 (7): 077804. DOI: 10.1088/1674-1056/ab8dae

Abstract ( 738 )

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THz time-domain spectroscopy (THz-TDS) is used to study the THz-optical properties of a single crystal bismuth ferrite BiFeO₃ (BFO). It can be found that the anisotropy of BiFeO₃ is strongly dependent on the temperature. A giant birefringence up to around 3.6 is observed at 1 THz. The presence of a spatially modulated cycloidal antiferromagnetic structure leads to spin cycloid resonances (SCR) ψ and Φ, corresponding to the out-of-plane and in-plane modes of the spin cycloid, respectively. We distinguish the SCR with respect to their response to orthogonal polarizations of the electric fields of the incident THz beam. In addition, we observe a resonance appearing below 140 K, which might be interpreted as an electromagnon mode and related to a spin reorientation transition. Our present observations present that the temperature and polarization, as the external control parameters, can be used to modulate the THz optical properties of BFO single crystal.

Regulation mechanism of catalyst structure on diamond crystal morphology under HPHT process

Ya-Dong Li(李亚东), Yong-Shan Cheng(程永珊), Meng-Jie Su(宿梦洁), Qi-Fu Ran(冉启甫), Chun-Xiao Wang(王春晓), Hong-An Ma(马红安), Chao Fang(房超), Liang-Chao Chen(陈良超)

Chin. Phys. B, 2020, 29 (7): 078101. DOI: 10.1088/1674-1056/ab90e8

Abstract ( 856 )

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To elucidate the regulation mechanism of catalyst geometry structure to diamond growth, we establish three catalyst modes with different structures. The simulation results show that with the decrease of the protruding height of the catalyst, the low-temperature region gradually moves toward the center of the catalyst, which causes the distribution characteristics of the temperature and convection field in the catalyst to change. The temperature difference in vertical direction of the catalyst decreases gradually and increases in the horizontal direction, while the catalyst convection velocity has the same variation regularity in the corresponding directions. The variation of temperature difference and convection velocity lead the crystal growth rate in different crystal orientations to change, which directly affects the crystal morphology of the synthetic diamond. The simulation results are consistent with the experimental results, which shows the correctness of the theoretical rational analysis. This work is expected to be able to facilitate the understanding of catalyst structure regulation mechanism on diamond morphology and the providing of an important theoretical basis for the controllable growth of special crystal shape diamond under HPHT process.

Construction of monolayer IrTe₂ and the structural transition under low temperatures Suggested by editors

Aiwei Wang(王爱伟), Ziyuan Liu(刘子媛), Jinbo Pan(潘金波), Qiaochu Li(李乔楚), Geng Li(李更), Qing Huan(郇庆), Shixuan Du(杜世萱), Hong-Jun Gao(高鸿钧)

Chin. Phys. B, 2020, 29 (7): 078102. DOI: 10.1088/1674-1056/ab90eb

Abstract ( 766 )

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Bulk iridium ditelluride (IrTe₂) is a layered material and is known for its interesting electronic and structural properties, such as large spin-orbit coupling, charge ordering, and superconductivity. However, so far there is no experimental study about the fabrication of monolayer IrTe₂. Here we report the formation of IrTe₂ monolayer on Ir(111) substrate by direct tellurization method. Scanning tunneling microscope (STM) images show the coexistence of 1/5 phase and 1/6 phase structures of IrTe₂ at room temperature. We also obtained STM images showing distorted stripe feature under low temperatures. This stripe feature is possibly induced by the strain between the IrTe₂ monolayer and the metal substrate. Density functional theory (DFT) calculations show that the IrTe₂ monolayer has strong interaction with the underlying Ir(111) substrate.

Dependence of mechanical properties on the site occupancy of ternary alloying elements in γ'-Ni₃Al: Ab initio description for shear and tensile deformation ^Hot!

Minru Wen(文敏儒), Xing Xie(谢兴), Huafeng Dong(董华锋), Fugen Wu(吴福根), Chong-Yu Wang(王崇愚)

Chin. Phys. B, 2020, 29 (7): 078103. DOI: 10.1088/1674-1056/ab8a38

Abstract ( 564 )

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The site occupancy behavior of ternary alloying elements in γ'-Ni₃Al (a key strengthening phase of commercial Ni-based single-crystal superalloys) can change with temperature and alloy composition owing to the effect of entropy. Using a total-energy method based on density functional theory, the dependence of tensile and shear behaviors on the site preference of alloying elements in γ'-Ni₃Al were investigated in detail. Our results demonstrate that Fe, Ru, and Ir can significantly improve the ideal tensile and shear strength of the γ' phase when occupying the Al site, with Ru resulting in the strongest enhancement. In contrast, elements with fully filled d orbitals (i.e., Cu, Zn, Ag, and Cd) are expected to reduce the ideal tensile and shear strength. The calculated stress-strain relationships of Ni₃Al alloys indicate that none of the alloying elements can simultaneously increase the ideal strength of the γ' phase for both Ni1-site and Ni2-site substitutions. In addition, the charge redistribution and the bond length of the alloying elements and host atoms during the tensile and shear processes are analyzed to unveil the underlying electronic mechanisms.

Modeling of microporosity formation and hydrogen concentration evolution during solidification of an Al-Si alloy

Qingyu Zhang(张庆宇), Dongke Sun(孙东科), Shunhu Zhang(章顺虎), Hui Wang(王辉), Mingfang Zhu(朱鸣芳)

Chin. Phys. B, 2020, 29 (7): 078104. DOI: 10.1088/1674-1056/ab8abb

Abstract ( 586 )

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We simulate the evolution of hydrogen concentration and gas pore formation as equiaxed dendrites grow during solidification of a hypoeutectic aluminum-silicon (Al-Si) alloy. The applied lattice Boltzmann-cellular automaton-finite difference model incorporates the physical mechanisms of solute and hydrogen partitioning on the solid/liquid interface, as well as the transports of solute and hydrogen. After the quantitative validation by the simulation of capillary intrusion, the model is utilized to investigate the growth of the equiaxed dendrites and hydrogen porosity formation for an Al-(5 wt.%)Si alloy under different solidification conditions. The simulation data reveal that the gas pores favorably nucleate in the corners surrounded by the nearby dendrite arms. Then, the gas pores grow in a competitive mode. With the cooling rate increasing, the competition among different growing gas pores is found to be hindered, which accordingly increases the pore number density in the final solidification microstructure. In the late solidification stage, even though the solid fraction is increasing, the mean concentration of hydrogen in the residue melt tends to be constant, corresponding to a dynamic equilibrium state of hydrogen concentration in liquid. As the cooling rate increases or the initial hydrogen concentration decreases, the temperature of gas pore nucleation, the porosity fraction, and the mean porosity size decrease, whilst the mean hydrogen concentration in liquid increases in the late solidification stage. The simulated data present identical trends with the experimental results reported in literature.

Regulation of microtubule array inits self-organized dense active crowds

Xin-Chen Jiang(蒋新晨), Yu-Qiang Ma(马余强), Xiaqing Shi(施夏清)

Chin. Phys. B, 2020, 29 (7): 078201. DOI: 10.1088/1674-1056/ab9430

Abstract ( 710 )

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Microtubule self-organization under mechanical and chemical regulations plays a central role in cytokinesis and cellular transportations. In plant-cells, the patterns or phases of cortical microtubules organizations are the direct indicators of cell-phases. The dense nematic pattern of cortical microtubule array relies on the regulation of single microtubule dynamics with mechanical coupling to steric interaction among the self-organized microtubule crowds. Building upon previous minimal models, we investigate the effective microtubule width, microtubule catastrophe rate, and zippering angle as factors that regulate the self-organization of the dense nematic phase. We find that by incorporating the effective microtubule width, the transition from isotropic to the highly ordered nematic phase (N_I phase) with extremely long microtubules will be gapped by another nematic phase which consists of relative short microtubules (N_Ⅱ phase). The N_Ⅱ phase in the gap grows wider with the increase of the microtubule width. We further illustrate that in the dense phase, the collision-induced catastrophe rate and an optimal zippering angle play an important role in controlling the order-disorder transition, as a result of the coupling between the collision events and ordering. Our study shows that the transition to dense microtubule array requires the cross-talk between single microtubule growth and mechanical interactions among microtubules in the active crowds.

SiO₂ nanoparticle-regulated crystallization of lead halide perovskite and improved efficiency of carbon-electrode-based low-temperature planar perovskite solar cells

Zerong Liang(梁泽荣), Bingchu Yang(杨兵初), Anyi Mei(梅安意), Siyuan Lin(林思远), Hongwei Han(韩宏伟), Yongbo Yuan(袁永波), Haipeng Xie(谢海鹏), Yongli Gao(高永立), Conghua Zhou(周聪华)

Chin. Phys. B, 2020, 29 (7): 078401. DOI: 10.1088/1674-1056/ab8da5

Abstract ( 558 )

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SiO₂ nanoparticles were used to regulate the crystallizing process of lead halide perovskite films prepared by the sequential deposition method, which was used in the low-temperature-processed, carbon-electrode-basing, hole-conductor-free planar perovskite solar cells. It was observed that, after adding small amount of SiO₂ precursor (1 vol%) into the lead iodide solution, performance parameters of open-circuit voltage, short-circuit current and fill factor were all upgraded, which helped to increase the power conversion efficiency (reverse scan) from 11.44(±1.83)% (optimized at 12.42%) to 14.01(±2.14)% (optimized at 15.28%, AM 1.5G, 100 mW/cm²). Transient photocurrent decay curve measurements showed that, after the incorporation of SiO₂ nanoparticles, charge extraction was accelerated, while transient photovoltage decay and dark current curve tests both showed that recombination was retarded. The improvement is due to the improved crystallinity of the perovskite film. X-ray diffraction and scanning electron microscopy studies observed that, with incorporation of amorphous SiO₂ nanoparticles, smaller crystallites were obtained in lead iodide films, while larger crystallites were achieved in the final perovskite film. This study implies that amorphous SiO₂ nanoparticles could regulate the coarsening process of the perovskite film, which provides an effective method in obtaining high quality perovskite film.

Carbon nanotube-based nanoelectromechanical resonatoras mass biosensor

Ahmed M. Elseddawy, Adel H. Phillips, Ahmed S Bayoumi

Chin. Phys. B, 2020, 29 (7): 078501. DOI: 10.1088/1674-1056/ab888f

Abstract ( 492 )

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The use of single walled carbon nanotube-based nanoelectromechanical system (NEMS) resonator to sense the biomolecules' mass is investigated under the influence of an external ac-field. A single walled carbon nanotube (SWCNT) cantilever has been proposed and studied if the mass is attached at the tip or various intermediate positions. The shift of the resonant frequency and the quality factor have been investigated and show high sensitivity to the attached mass of biomolecule and its position. The proposed SWCNT-based NEMS resonator is a good candidate for sensing and tracing biomolecules' mass as concentration of acetone in human exhale, resulting in a painless, correct, and simple diabetics' diagnosis.

Recent progress in optoelectronic neuromorphic devices

Yan-Bo Guo(郭延博), Li-Qiang Zhu(竺立强)

Chin. Phys. B, 2020, 29 (7): 078502. DOI: 10.1088/1674-1056/ab99b6

Abstract ( 580 )

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Rapid developments in artificial intelligence trigger demands for perception and learning of external environments through visual perception systems. Neuromorphic devices and integrated system with photosensing and response functions can be constructed to mimic complex biological visual sensing behaviors. Here, recent progresses on optoelectronic neuromorphic memristors and optoelectronic neuromorphic transistors are briefly reviewed. A variety of visual synaptic functions stimulated on optoelectronic neuromorphic devices are discussed, including light-triggered short-term plasticities, long-term plasticities, and neural facilitation. These optoelectronic neuromorphic devices can also mimic human visual perception, information processing, and cognition. The optoelectronic neuromorphic devices that simulate biological visual perception functions will have potential application prospects in areas such as bionic neurological optoelectronic systems and intelligent robots.

Photoresponsive characteristics of thin film transistors with perovskite quantum dots embedded amorphous InGaZnO channels Suggested by editors

Mei-Na Zhang(张美娜), Yan Shao(邵龑), Xiao-Lin Wang(王晓琳), Xiaohan Wu(吴小晗), Wen-Jun Liu(刘文军), Shi-Jin Ding(丁士进)

Chin. Phys. B, 2020, 29 (7): 078503. DOI: 10.1088/1674-1056/ab9738

Abstract ( 637 )

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Photodetectors based on amorphous InGaZnO (a-IGZO) thin film transistor (TFT) and halide perovskites have attracted attention in recent years. However, such a stack assembly of a halide perovskite layer/an a-IGZO channel, even with an organic semiconductor film inserted between them, easily has a very limited photoresponsivity. In this article, we investigate photoresponsive characteristics of TFTs by using CsPbX₃ (X=Br or I) quantum dots (QDs) embedded into the a-IGZO channel, and attain a high photoresponsivity over 10³A·W^-1, an excellent detectivity in the order of 10¹⁶ Jones, and a light-to-dark current ratio up to 10⁵ under visible lights. This should be mainly attributed to the improved transfer efficiency of photoelectrons from the QDs to the a-IGZO channel. Moreover, spectrally selective photodetection is demonstrated by introducing halide perovskite QDs with different bandgaps. Thus, this work provides a novel strategy of device structure optimization for significantly improving the photoresponsive characteristics of TFT photodetectors.

In-memory computing to break the memory wall

Xiaohe Huang(黄晓合), Chunsen Liu(刘春森), Yu-Gang Jiang(姜育刚), Peng Zhou(周鹏)

Chin. Phys. B, 2020, 29 (7): 078504. DOI: 10.1088/1674-1056/ab90e7

Abstract ( 1189 )

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Facing the computing demands of Internet of things (IoT) and artificial intelligence (AI), the cost induced by moving the data between the central processing unit (CPU) and memory is the key problem and a chip featured with flexible structural unit, ultra-low power consumption, and huge parallelism will be needed. In-memory computing, a non-von Neumann architecture fusing memory units and computing units, can eliminate the data transfer time and energy consumption while performing massive parallel computations. Prototype in-memory computing schemes modified from different memory technologies have shown orders of magnitude improvement in computing efficiency, making it be regarded as the ultimate computing paradigm. Here we review the state-of-the-art memory device technologies potential for in-memory computing, summarize their versatile applications in neural network, stochastic generation, and hybrid precision digital computing, with promising solutions for unprecedented computing tasks, and also discuss the challenges of stability and integration for general in-memory computing.

Perpendicular magnetization switching by large spin—orbit torques from sputtered Bi₂Te₃ ^Hot!

Zhenyi Zheng(郑臻益), Yue Zhang(张悦), Daoqian Zhu(朱道乾), Kun Zhang(张昆), Xueqiang Feng(冯学强), Yu He(何宇), Lei Chen(陈磊), Zhizhong Zhang(张志仲), Dijun Liu(刘迪军), Youguang Zhang(张有光), Pedram Khalili Amiri, Weisheng Zhao(赵巍胜)

Chin. Phys. B, 2020, 29 (7): 078505. DOI: 10.1088/1674-1056/ab9439

Abstract ( 1189 )

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Spin-orbit torque (SOT) effect is considered as an efficient way to switch the magnetization and can inspire various high-performance spintronic devices. Recently, topological insulators (TIs) have gained extensive attention, as they are demonstrated to maintain a large effective spin Hall angle (θ_SH^eff), even at room temperature. However, molecular beam epitaxy (MBE), as a precise deposition method, is required to guarantee favorable surface states of TIs, which hinders the prospect of TIs towards industrial application. In this paper, we demonstrate that Bi₂Te₃ films grown by magnetron sputtering can provide a notable SOT effect in the heterostructure with perpendicular magnetic anisotropy CoTb ferrimagnetic alloy. By harmonic Hall measurement, a high SOT efficiency (8.7±0.9 Oe/(10⁹ A/m²)) and a large θ_SH^eff (3.3±0.3) are obtained at room temperature. Besides, we also observe an ultra-low critical switching current density (9.7×10⁹ A/m²). Moreover, the low-power characteristic of the sputtered Bi₂Te₃ film is investigated by drawing a comparison with different sputtered SOT sources. Our work may provide an alternative to leverage chalcogenides as a realistic and efficient SOT source in future spintronic devices.

Different potential of mean force of two-state protein GB1 and downhill protein gpW revealed by molecular dynamics simulation

Xiaofeng Zhang(张晓峰), Zilong Guo(郭子龙), Ping Yu(余平), Qiushi Li(李秋实), Xin Zhou(周昕), Hu Chen(陈虎)

Chin. Phys. B, 2020, 29 (7): 078701. DOI: 10.1088/1674-1056/ab8daf

Abstract ( 54 )

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Two-state folding and down-hill folding are two kinds of protein folding dynamics for small single domain proteins. Here we apply molecular dynamics (MD) simulation to the two-state protein GB1 and down-hill folding protein gpW to reveal the relationship of their free energy landscape and folding/unfolding dynamics. Results from the steered MD simulations show that gpW is much less mechanical resistant than GB1, and the unfolding process of gpW has more variability than that of GB1 according to their force-extension curves. The potential of mean force (PMF) of GB1 and gpW obtained by the umbrella sampling simulations shows apparent difference: PMF of GB1 along the coordinate of extension exhibits a kink transition point where the slope of PMF drops suddenly, while PMF of gpW increases with extension smoothly, which are consistent with two-state folding dynamics of GB1 and downhill folding dynamics of gpW, respectively. Our results provide insight to understand the fundamental mechanism of different folding dynamics of two-state proteins and downhill folding proteins.

Improving RNA secondary structure prediction using direct coupling analysis

Xiaoling He(何小玲), Jun Wang(王军), Jian Wang(王剑), Yi Xiao(肖奕)

Chin. Phys. B, 2020, 29 (7): 078702. DOI: 10.1088/1674-1056/ab889d

Abstract ( 44 )

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Secondary structures of RNAs are the basis of understanding their tertiary structures and functions and so their predictions are widely needed due to increasing discovery of noncoding RNAs. In the last decades, a lot of methods have been proposed to predict RNA secondary structures but their accuracies encountered bottleneck. Here we present a method for RNA secondary structure prediction using direct coupling analysis and a remove-and-expand algorithm that shows better performance than four existing popular multiple-sequence methods. We further show that the results can also be used to improve the prediction accuracy of the single-sequence methods.

Adjustable polarization-independent wide-incident-angle broadband far-infrared absorber

Jiu-Sheng Li(李九生), Xu-Sheng Chen(陈旭生)

Chin. Phys. B, 2020, 29 (7): 078703. DOI: 10.1088/1674-1056/ab8892

Abstract ( 680 )

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To promote the application of far-infrared technology, functional far-infrared devices with high performance are needed. Here, we propose a design scheme to develop a wide-incident-angle far-infrared absorber, which consists of a periodically semicircle-patterned graphene sheet, a lossless inter-dielectric spacer and a gold reflecting film. Under normal incidence for both TE- and TM-polarization modes, the bandwidth of 90% absorption of the proposed far-infrared absorber is ranging from 6.76 THz to 11.05 THz. The absorption remains more than 90% over a 4.29-THz broadband range when the incident angle is up to 50° for both TE- and TM-polarization modes. The peak absorbance of the absorber can be flexibly tuned from 10% to 100% by changing the chemical potential from 0 eV to 0.6 eV. The tunable broadband far-infrared absorber has promising applications in sensing, detection, and stealth objects.

A new nonlinear photoconductive terahertz radiation source based on photon-activated charge domain quenched mode

Wei Shi(施卫), Rujun Liu(刘如军), Chengang Dong(董陈岗), Cheng Ma(马成)

Chin. Phys. B, 2020, 29 (7): 078704. DOI: 10.1088/1674-1056/ab8c42

Abstract ( 791 )

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We present a high-performance terahertz (THz) radiation source based on the photon-activated charge domain (PACD) quenched mode of GaAs photoconductive antennas (GaAs PCA). The THz radiation characteristics of the GaAs PCA under different operating modes are studied. Compared with the linear mode, the intensity of THz wave radiated by the GaAs PCA can be greatly enhanced due to the avalanche multiplication effect of carriers in the PACD quenched mode. The results show that when the carrier multiplication ratio is 16.92, the peak-to-peak value of THz field radiated in the PACD quenched mode increases by as much as about 4.19 times compared to the maximum values in the linear mode.

Broadband terahertz time-domain spectroscopy and fast FMCW imaging: Principle and applications

Yao-Chun Shen(沈耀春), Xing-Yu Yang(杨星宇), Zi-Jian Zhang(张子健)

Chin. Phys. B, 2020, 29 (7): 078705. DOI: 10.1088/1674-1056/ab9296

Abstract ( 665 )

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We report a broadband terahertz time-domain spectroscopy (THz-TDS) which enables twenty vibrational modes of adenosine nucleoside to be resolved in a wide frequency range of 1-20 THz. The observed spectroscopic features of adenosine are in good agreement with the published spectra obtained using Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy. This much extended bandwidth leads to enhanced material characterization capability as it provides spectroscopic information on both intra- and inter-molecular vibrations. In addition, we also report a low-cost frequency modulation continuous wave (FMCW) imaging system which has a fast measurement speed of 40000 waveforms per second. Cross-sectional imaging capability through cardboard has also been demonstrated using its excellent penetration capability at a frequency range of 76-81 GHz. We anticipate that the integration of these two complementary imaging technologies would be highly desirable for many real-world applications because it provides both spectroscopic discrimination and penetration capabilities in a single instrument.

Polarization conversion metasurface in terahertz region

Chen Zhou(周晨), Jiu-Sheng Li(李九生)

Chin. Phys. B, 2020, 29 (7): 078706. DOI: 10.1088/1674-1056/ab9295

Abstract ( 877 )

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Polarization conversion is a very important electromagnetic wave manipulation method. In this paper, we investigate a high-efficiency linear-to-circular polarization and cross-polarization converter by utilizing coding metasurface. The coding particle consists of top layer metal pattern and bottom metal plate sandwiched with square F4B dielectric, which can manipulate the linear-to-circular polarization and cross-polarization converter of the reflected wave simultaneously. In the terahertz frequency range of 1.0 THz-2.0 THz, the reflection magnitudes reach approximately 90% and the axial ratio is less than 3 dB. The proposed polarization converter may lead to advances in a variety of applications such as security, microscopy, information processing, stealth technology, and data storage.

Terahertz polarization conversion and sensing with double-layer chiral metasurface

Zi-Yang Zhang(张子扬), Fei Fan(范飞), Teng-Fei Li(李腾飞), Yun-Yun Ji(冀允允), Sheng-Jiang Chang(常胜江)

Chin. Phys. B, 2020, 29 (7): 078707. DOI: 10.1088/1674-1056/ab9294

Abstract ( 762 )

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The terahertz (THz) resonance, chirality, and polarization conversion properties of a double-layer chiral metasurface have been experimentally investigated by THz time domain spectroscopy system and polarization detection method. The special symmetric geometry of each unit cell with its adjacent cells makes a strong chiral electromagnetic response in this metasurface, which leads to a strong polarization conversion effect. Moreover, compared with the traditional THz transmission resonance sensing for film thickness, the polarization sensing characterized by polarization elliptical angle (PEA) and polarization rotation angle (PRA) shows a better Q factor and figure of merit (FoM). The results show that the Q factors of the PEA and PRA reach 43.8 and 49.1 when the interval film is 20 μm, while the Q factor of THz resonance sensing is only 10.6. And these PEA and PRA can play a complementary role to obtain a double-parameter sensing method with a higher FoM, over 4 times than that of resonance sensing. This chiral metasurface and its polarization sensing method provide new ideas for the development of high-efficiency THz polarization manipulation, and open a window to the high sensitive sensing by using THz polarization spectroscopy.

Highly efficient bifacial semitransparent perovskite solar cells based on molecular doping of CuSCN hole transport layer

Shixin Hou(侯世欣), Biao Shi(石标), Pengyang Wang(王鹏阳), Yucheng Li(李玉成), Jie Zhang(张杰), Peirun Chen(陈沛润), Bingbing Chen(陈兵兵), Fuhua Hou(侯福华), Qian Huang(黄茜), Yi Ding(丁毅), Yuelong Li(李跃龙), Dekun Zhang(张德坤), Shengzhi Xu(许盛之), Ying Zhao(赵颖), Xiaodan Zhang(张晓丹)

Chin. Phys. B, 2020, 29 (7): 078801. DOI: 10.1088/1674-1056/ab99ae

Abstract ( 687 )

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Coper thiocyanate (CuSCN) is generally considered as a very hopeful inorganic hole transport material (HTM) in semitransparent perovskite solar cells (ST-PSCs) because of its low parasitic absorption, high inherent stability, and low cost. However, the poor electrical conductivity and low work function of CuSCN lead to the insufficient hole extraction and large open-circuit voltage loss. Here, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) is employed to improve conductivity of CuSCN and band alignment at the CuSCN/perovskite (PVK) interface. As a result, the average power conversion efficiency (PCE) of PSCs is boosted by ≈ 11%. In addition, benefiting from the superior transparency of p-type CuSCN HTMs, the prepared bifacial semitransparent n-i-p planar PSCs demonstrate a maximum efficiency of 14.8% and 12.5% by the illumination from the front side and back side, respectively. We believe that this developed CuSCN-based ST-PSCs will promote practical applications in building integrated photovoltaics and tandem solar cells.

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