Chin. Phys. B

Compact finite difference schemes for the backward fractional Feynman-Kac equation with fractional substantial derivative

Jiahui Hu(胡嘉卉), Jungang Wang(王俊刚), Yufeng Nie(聂玉峰), Yanwei Luo(罗艳伟)

Chin. Phys. B, 2019, 28 (10): 100201. DOI: 10.1088/1674-1056/ab3af3

Abstract ( 668 )

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The fractional Feynman-Kac equations describe the distributions of functionals of non-Brownian motion, or anomalous diffusion, including two types called the forward and backward fractional Feynman-Kac equations, where the non-local time-space coupled fractional substantial derivative is involved. This paper focuses on the more widely used backward version. Based on the newly proposed approximation operators for fractional substantial derivative, we establish compact finite difference schemes for the backward fractional Feynman-Kac equation. The proposed difference schemes have the q-th (q=1,2,3,4) order accuracy in temporal direction and fourth order accuracy in spatial direction, respectively. The numerical stability and convergence in the maximum norm are proved for the first order time discretization scheme by the discrete energy method, where an inner product in complex space is introduced. Finally, extensive numerical experiments are carried out to verify the availability and superiority of the algorithms. Also, simulations of the backward fractional Feynman-Kac equation with Dirac delta function as the initial condition are performed to further confirm the effectiveness of the proposed methods.

Exact solutions of a (2+1)-dimensional extended shallow water wave equation Suggested by editors

Feng Yuan(袁丰), Jing-Song He(贺劲松), Yi Cheng(程艺)

Chin. Phys. B, 2019, 28 (10): 100202. DOI: 10.1088/1674-1056/ab3e65

Abstract ( 779 )

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We give the bilinear form and n-soliton solutions of a (2+1)-dimensional[(2+1)-D] extended shallow water wave (eSWW) equation associated with two functions v and r by using Hirota bilinear method. We provide solitons, breathers, and hybrid solutions of them. Four cases of a crucial φ(y), which is an arbitrary real continuous function appeared in f of bilinear form, are selected by using Jacobi elliptic functions, which yield a periodic solution and three kinds of doubly localized dormion-type solution. The first order Jacobi-type solution travels parallelly along the x axis with the velocity (3k₁²+α, 0) on (x, y)-plane. If φ(y)=sn(y, 3/10), it is a periodic solution. If φ(y)=cn(y, 1), it is a dormion-type-I solutions which has a maximum (3/4)k₁p₁ and a minimum -(3/4)k₁p₁. The width of the contour line is ln[(2+√6+√2+√3)/(2+√6-√2-√3)]. If φ(y)=sn(y, 1), we get a dormion-type-Ⅱ solution (26) which has only one extreme value -(3/2)k₁p₁. The width of the contour line is ln[(√2+1)/(√2-1)]. If φ(y)=sn(y, 1/2)/(1+y²), we get a dormion-type-Ⅲ solution (21) which shows very strong doubly localized feature on (x,y) plane. Moreover, several interesting patterns of the mixture of periodic and localized solutions are also given in graphic way.

Lump-type solutions of a generalized Kadomtsev-Petviashvili equation in (3+1)-dimensions

Xue-Ping Cheng(程雪苹), Wen-Xiu Ma(马文秀), Yun-Qing Yang(杨云青)

Chin. Phys. B, 2019, 28 (10): 100203. DOI: 10.1088/1674-1056/ab3f20

Abstract ( 684 )

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Through the Hirota bilinear formulation and the symbolic computation software Maple, we construct lump-type solutions for a generalized (3+1)-dimensional Kadomtsev-Petviashvili (KP) equation in three cases of the coefficients in the equation. Then the sufficient and necessary conditions to guarantee the analyticity of the resulting lump-type solutions (or the positivity of the corresponding quadratic solutions to the associated bilinear equation) are discussed. To illustrate the generality of the obtained solutions, two concrete lump-type solutions are explicitly presented, and to analyze the dynamic behaviors of the solutions specifically, the three-dimensional plots and contour profiles of these two lump-type solutions with particular choices of the involved free parameters are well displayed.

Time evolution of angular momentum coherent state derived by virtue of entangled state representation and a new binomial theorem

Ji-Suo Wang(王继锁), Xiang-Guo Meng(孟祥国), Hong-Yi Fan(范洪义)

Chin. Phys. B, 2019, 28 (10): 100301. DOI: 10.1088/1674-1056/ab3a90

Abstract ( 722 )

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We study how can an angular momentum coherent state |τ> keeps its form-invariant during time evolution governed by the Hamiltonian H=f(t)J₊+f^* (t)J_+ g(t)J_z. We discuss this topic in the context of boson realization of |τ>. By employing the entangled state representation |ζ> and deriving a new binomial theorem involving two-subscript Hermite polynomials, we derive the wave function <ζ|τ>, which turns out to be a single-subscript Hermite polynomial. Based on this result the maintenance of angular momentum coherent state during time evolution is examined, and the value of τ (t) is totally determined by the parameters involved in the Hamiltonian.

Boundary states for entanglement robustness under dephasing and bit flip channels Suggested by editors

Hong-Mei Li(李红梅), Miao-Di Guo(郭苗迪), Rui Zhang(张锐), Xue-Mei Su(苏雪梅)

Chin. Phys. B, 2019, 28 (10): 100302. DOI: 10.1088/1674-1056/ab3e67

Abstract ( 753 )

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We investigate the robustness of entanglement for a multiqubit system under dephasing and bit flip channels. We exhibit the difference between the entanglement evolution of the two forms of special states, which are locally unitarily equivalent to each other and therefore possess precisely the same entanglement properties, and demonstrate that the difference increases with the number of qubits n. Moreover, those two forms of states are either the most robust genuine entangled states or the most fragile ones, which confirm that local unitary (LU) operations can greatly enhance the entanglement robustness of n-qubit states.

Manipulating transition of a two-component Bose-Einstein condensate with a weak δ-shaped laser

Bo Li(李博), Xiao-Jun Jiang(蒋小军), Xiao-Lin Li(李晓林), Wen-Hua Hai(海文华), Yu-Zhu Wang(王育竹)

Chin. Phys. B, 2019, 28 (10): 100303. DOI: 10.1088/1674-1056/ab3e64

Abstract ( 723 )

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We theoretically study the transition dynamics of a two-component Bose-Einstein condensate driven by a train of weak δ-shaped laser pulses. We find that the atomic system can experience peculiar resonant transition even under weak optical excitations and derive the resonance condition by the perturbation method. Employing this mechanism, we propose a scheme to obtain an atomic ensemble with desired odd/even atom number and also a scheme to prepare a nonclassical state of the many-body system with fixed atom number.

Topological phases of a non-Hermitian coupled SSH ladder

J S Liu(刘建森), Y Z Han(韩炎桢), C S Liu(刘承师)

Chin. Phys. B, 2019, 28 (10): 100304. DOI: 10.1088/1674-1056/ab3f94

Abstract ( 941 )

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We study topological phases of a non-Hermitian coupled Su-Schrieffer-Heeger (SSH) ladder. The model originates from the brick-wall lattices in the two-row limit. The Hamiltonian can be brought into block off-diagonal form and the winding number can be defined with the determine of the block off-diagonal matrix. We find the determine of the off-diagonal matrix has nothing to do with the interleg hopping of the ladder. So the topological phases of the model are the same as those of the chains. Further numerical simulations verify the analysis.

Optimal estimation of the amplitude of signal with known frequency in the presence of thermal noise Suggested by editors

Jie Luo(罗杰), Jun Ke(柯俊), Yi-Chuan Liu(柳一川), Xiang-Li Zhang(张祥莉), Wei-Ming Yin(殷蔚明), Cheng-Gang Shao(邵成刚)

Chin. Phys. B, 2019, 28 (10): 100401. DOI: 10.1088/1674-1056/ab3a8c

Abstract ( 580 )

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In the torsion pendulum experiments, the thermal noise sets the most fundamental limit to the accurate estimation of the amplitude of the signal with known frequency. The variance of the conventional method can meet the limit only when the measurement time is much longer than the relaxation time of the pendulum. By using the maximum likelihood estimation and the equation-of-motion filter operator, we propose an optimal (minimum variance, unbiased) amplitude estimation method without limitation of the measurement time, where thermal fluctuation is the leading noise. While processing the experimental data tests of the Newtonian gravitational inverse square law, the variance of our method has been improved than before and the measurement time of determining the amplitude with this method has been reduced about half than before for the same uncertainty. These results are significant for the torsion experiment when the measurement time is limited.

Enhancing von Neumann entropy by chaos in spin-orbit entanglement

Chen-Rong Liu(刘郴荣), Pei Yu(喻佩), Xian-Zhang Chen(陈宪章), Hong-Ya Xu(徐洪亚), Liang Huang(黄亮), Ying-Cheng Lai(来颖诚)

Chin. Phys. B, 2019, 28 (10): 100501. DOI: 10.1088/1674-1056/ab3dff

Abstract ( 749 )

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For a quantum system with multiple degrees of freedom or subspaces, loss of coherence in a certain subspace is intimately related to the enhancement of entanglement between this subspace and another one. We investigate intra-particle entanglement in two-dimensional mesoscopic systems, where an electron has both spin and orbital degrees of freedom and the interaction between them is enabled by Rashba type of spin-orbit coupling. The geometric shape of the scattering region can be adjusted to produce a continuous spectrum of classical dynamics with different degree of chaos. Focusing on the spin degree of freedom in the weak spin-orbit coupling regime, we find that classical chaos can significantly enhance spin-orbit entanglement at the expense of spin coherence. Our finding that classical chaos can be beneficial to intra-particle entanglement may have potential applications such as enhancing the bandwidth of quantum communications.

Experimental investigation of the fluctuations in nonchaotic scattering in microwave billiards

Runzu Zhang(张润祖), Weihua Zhang(张为华), Barbara Dietz, Guozhi Chai(柴国志), Liang Huang(黄亮)

Chin. Phys. B, 2019, 28 (10): 100502. DOI: 10.1088/1674-1056/ab3f96

Abstract ( 640 )

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We report on the experimental investigation of the properties of the eigenvalues and wavefunctions and the fluctuation properties of the scattering matrix of closed and open billiards, respectively, of which the classical dynamics undergoes a transition from integrable via almost integrable to fully chaotic. To realize such a system, we chose a billiard with a 60° sector shape of which the classical dynamics is integrable, and introduced circular scatterers of varying number, size, and position. The spectral properties of generic quantum systems of which the classical counterpart is either integrable or chaotic are universal and well understood. If, however, the classical dynamics is pseudo-integrable or almost-integrable, they exhibit a non-universal intermediate statistics, for which analytical results are known only in a few cases, e.g., if it corresponds to semi-Poisson statistics. Since the latter is, above all, clearly distinguishable from those of integrable and chaotic systems, our aim was to design a billiard with these features which indeed is achievable by adding just one scatterer of appropriate size and position to the sector billiard. We demonstrated that, while the spectral properties of almost-integrable billiards are sensitive to the classical dynamics, this is not the case for the distribution of the wavefunction components, which was analyzed in terms of the strength distribution, and the fluctuation properties of the scattering matrix which coincide with those of typical, fully chaotic systems.

Photoactivation experiment of ¹⁹⁷Au(γ, n) performed with 9.17-MeV γ-ray from ¹³C(p, γ)¹⁴N

Yong-Le Dang(党永乐), Fu-Long Liu(刘伏龙), Guang-Yong Fu(付光永), Di Wu(吴笛), Nai-Yan Wang(王乃彦)

Chin. Phys. B, 2019, 28 (10): 100701. DOI: 10.1088/1674-1056/ab3a8d

Abstract ( 634 )

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High energy γ-ray can be used for nuclear waste transmutation by using the giant dipole resonance (GDR). The photonuclear reaction ¹⁹⁷Au(γ, n) is known as a standard for studies on photoactivation experiments. The previous experiments on ¹⁹⁷Au(γ, n) have been performed with bremsstrahlung, positron annihilation in flight or laser Compton scattering γ-ray. In this work, a new mono-energetic γ-ray source based on ¹³C(p, γ)¹⁴N reaction is used to measure the cross section of ¹⁹⁷Au(γ, n) and the measured value is compared with the results obtained with other ways.

Interface properties and electronic structures of aromatic molecules with anhydride and thio-functional groups on Ag (111) and Au (111) substrates

Wei-Qi Yu(余维琪), Hong-Jun Xiao(肖红君), Ge-Ming Wang(王戈明)

Chin. Phys. B, 2019, 28 (10): 103101. DOI: 10.1088/1674-1056/ab4276

Abstract ( 663 )

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First-principles calculations for several aromatic molecules with anhydride and thio groups on Ag (111) and Au (111) reveal that the self-assembly structures and the interface properties are mainly determined by the functional groups of aromatic molecules. Detailed investigations of the electronic structures show that the electrons in molecular backbone are redistributed and charge transfer occurs through the bond between the metal and the functional groups after these molecules have been deposited on a metal substrate. The interaction between Ag (111) (or Au (111)) and aromatic molecules with anhydride functional groups strengthens the π bonds in the molecular backbone, while that between Ag (111) (or Au (111)) and aromatic molecules with sulfur weakens the π bonds. However, the intrinsic electronic structures of the molecules are mostly conserved. The large-sized aromatic backbone has less influence on the nature of electronic structures than the small-sized one, either at the interface or at the molecules. These results are useful to build the good metal-molecule contact in molecule-based devices.

Elastic properties of anatase titanium dioxide nanotubes: A molecular dynamics study

Kang Yang(杨康), Liang Yang(杨亮), Chang-Zhi Ai(艾长智), Zhao Wang(王赵), Shi-Wei Lin(林仕伟)

Chin. Phys. B, 2019, 28 (10): 103102. DOI: 10.1088/1674-1056/ab3da2

Abstract ( 661 )

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The elastic properties of anatase nanotubes are investigated by molecular dynamics (MD) simulations. Young's modulus, Poisson ratio, and shear modulus are calculated by transversely isotropic structure model. The calculated elastic constants of bulk rutile, anatase, and Young's modulus of nanotube are in good agreement with experimental values, respectively, demonstrating that the Matsui and Akaogi (MA) potential function used in the simulation can accurately present the elastic properties of anatase titanium dioxide nanotubes. For single wall anatase titanium dioxide nanotube, the elastic moduli are shown to be sensitive to structural details such as the chirality and radius. For different chirality nanotubes with the same radius, the elastic constants are not proportional to the chiral angle. The elastic properties of the nanotubes with the chiral angle of 0° are worse than those of other chiral nanotubes. For nanotubes with the same chirality but different radii, the elastic constant, Young's modulus, and shear modulus decrease as the radius increases. But there exist maximal values in a radius range of 10 nm-15 nm. Such information can not only provide a deep understanding of the influence of geometrical structure on nanotubes mechanical properties, but also present important guidance to optimize the composite behavior by using nanotubes as the addition.

Theoretical investigation of the pressure broadening D₁ and D₂ lines of cesium atoms colliding with ground-state helium atoms

Moussaoui Abdelaziz, Alioua Kamel, Allouche Abdul-rahman, Bouledroua Moncef

Chin. Phys. B, 2019, 28 (10): 103103. DOI: 10.1088/1674-1056/ab4043

Abstract ( 569 )

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Full quantum mechanical calculations are performed to determine the broadening in the far wings of the cesium D₁ and D₂ line shapes arising from elastic collisions of Cs atom with inert helium atoms. The potential energy curves of the low-lying CsHe molecular states, as well as the related transition dipole moments, are carefully computed from ab initio methods based on state-averaged complete active space self-consistent field-multireference configuration interaction (SA-CASSCF-MRCI) calculations, involving the spin-orbit effect, and taking into account the Davidson and BSSE corrections. The absorption and emission reduced coefficients are determined in the temperature and wavelength ranges of 323-3000 K and 800-1000 nm, respectively. Both profiles of the absorption and the emission are dominated by the free-free transitions, and exhibit a satellite peak in the blue wing near the wavelength 825 nm, attributed to B²Σ_1/2⁺→X²Σ_1/2⁺ transitions. The results are in good agreement with previous experimental and theoretical works.

Cluster structure prediction via CALYPSO method

Yonghong Tian(田永红), Weiguo Sun(孙伟国), Bole Chen(陈伯乐), Yuanyuan Jin(金圆圆), Cheng Lu(卢成)

Chin. Phys. B, 2019, 28 (10): 103104. DOI: 10.1088/1674-1056/ab4274

Abstract ( 907 )

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Cluster science as a bridge linking atomic molecular physics and condensed matter inspired the nanomaterials development in the past decades, ranging from the single-atom catalysis to ligand-protected noble metal clusters. The corresponding studies not only have been restricted to the search for the geometrical structures of clusters, but also have promoted the development of cluster-assembled materials as the building blocks. The CALYPSO cluster prediction method combined with other computational techniques have significantly stimulated the development of the cluster-based nanomaterials. In this review, we will summarize some good cases of cluster structure by CALYPSO method, which have also been successfully identified by the photoelectron spectra experiments. Beginning with the alkali-metal clusters, which serve as benchmarks, a series of studies are performed on the size-dependent elemental clusters which possess relatively high stability and interesting chemical physical properties. Special attentions are paid to the boron-based clusters because of their promising applications. The NbSi₁₂ and BeB₁₆ clusters, for example, are two classic representatives of the silicon- and boron-based clusters, which can be viewed as building blocks of nanotubes and borophene. This review offers a detailed description of the structural evolutions and electronic properties of medium-sized pure and doped clusters, which will advance fundamental knowledge of cluster-based nanomaterials and provide valuable information for further theoretical and experimental studies.

Ab initio investigation of excited state dual hydrogen bonding interactions and proton transfer mechanism for novel oxazoline compound

Yu-Sheng Wang(王玉生), Min Jia(贾敏), Qiao-Li Zhang(张巧丽), Xiao-Yan Song(宋晓燕), Da-Peng Yang(杨大鹏)

Chin. Phys. B, 2019, 28 (10): 103105. DOI: 10.1088/1674-1056/ab4042

Abstract ( 724 )

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Owing to the importance of excited state dynamical relaxation, the excited state intramolecular proton transfer (ESIPT) mechanism for a novel compound containing dual hydrogen bond (abbreviated as “1-enol”) is studied in this work. Using density functional theory (DFT) and time-dependent density functional theory (TDDFT) method, the experimental electronic spectra can be reproduced for 1-enol compound. We first verify the formation of dual intramolecular hydrogen bonds, and then confirm that the dual hydrogen bond should be strengthened in the first excited state. The photo-excitation process is analyzed by using frontier molecular orbital (HOMO and LUMO) for 1-enol compound. The obvious intramolecular charge transfer (ICT) provides the driving force to effectively facilitate the ESIPT process in the S₁ state. Exploration of the constructed S₀-state and S₁-state potential energy surface (PES) reveals that only the excited state intramolecular single proton transfer occurs for 1-enol system, which makes up for the deficiencies in previous experiment.

Asymmetric structure of atomic above-threshold ionization spectrum in two-color elliptically polarized laser fields

Xu-Cong Zhou(周旭聪), Shang Shi(石尚), Fei Li(李飞), Yu-Jun Yang(杨玉军), Jing Chen(陈京), Qing-Tian Meng(孟庆田), Bing-Bing Wang(王兵兵)

Chin. Phys. B, 2019, 28 (10): 103201. DOI: 10.1088/1674-1056/ab3c2a

Abstract ( 636 )

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According to the frequency-domain theory, we investigate the asymmetric structure of above-threshold ionization (ATI) spectrum of an atom in two-color elliptically polarized (EP) laser fields. When both laser fields are linearly polarized (LP), the spectrum shows that the multi-plateau structure is symmetric about the emitted angle of electron at π/2, while the spectrum becomes asymmetric and shifts rightwards with the increase of the EP degree of the IR laser field. Since the total ATI process is regarded as including direct ATI and the rescattering ATI, we analyze the spectrum structure of direct ATI and rescattering ATI separately. Using the saddle-point approximation, we find that for direct ATI, the fringes on the spectrum are mainly attributed to the fact that the ionization probability becomes very small when the direction of emitted electrons is perpendicular to the direction of the XUV laser polarization; while for the rescattering ATI, the interference fringes on the spectrum mainly come from the superposition of the waist structures on the spectra of all sub-channels.

Role of the dressed and bound states on below-threshold harmonic generation of He atom

Jian-Wei Zhou(周建伟), Zhi-Hong Jiao(焦志宏), Peng-Cheng Li(李鹏程), Xiao-Xin Zhou(周效信)

Chin. Phys. B, 2019, 28 (10): 103202. DOI: 10.1088/1674-1056/ab3bd4

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High-order harmonic generation below ionization threshold of He atom in the laser field is investigated by solving the three-dimensional time-dependent Schrödinger equation. An angular momentum-dependent model potential of He atom was used for getting the accurate energy levels of singlet states. The satellite-peak structures of the below-threshold harmonic generation (BTHG) of He are observed. We analyze the emission properties of the BTHG by employing a synchrosqueezing transform technique. We find that the satellite-peak structures have two types related to two kinds of transitions. One is the transition of the dressed states of the excited states, the other is the transition between the excited states and the ground state in the field-free case. Furthermore, our results show that the maximum Stark shift of the 2p state is about 0.9U_p (penderomotive energy), and that of the 4p state is about 1.0U_p. It indicates that the energy difference between some satellite- and main-peaks of the BTHG can be used to measure the maximum Stark shift of the excited states of He atom in the laser field.

Selection of right-circular-polarized harmonics from p orbital of neon atom by two-color bicircular laser fields

Chang-Long Xia(夏昌龙), Yue-Yue Lan(兰悦跃), Qian-Qian Li(李倩倩), Xiang-Yang Miao(苗向阳)

Chin. Phys. B, 2019, 28 (10): 103203. DOI: 10.1088/1674-1056/ab4278

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The polarization properties of high-order harmonic generation (HHG) in the two-color circularly polarized laser fields are investigated by numerically solving the two-dimensional time-dependent Schrödinger equation. By adding a wavelength of 1600-nm right-circular-polarized field to an 800-nm left-circular-polarized field, HHG is simulated from a real model of neon atom with p orbital, but not from a hydrogen-like atom model with s orbital. The orders of 3n+1 can be selected while the orders of 3n+2 are suppressed by adjusting the intensities of the two pulses. The physical mechanism is analyzed by time-frequency analysis and semiclassical model.

Helicity of harmonic generation and attosecond polarization with bichromatic circularly polarized laser fields Suggested by editors

Jun Zhang(张军), Tong Qi(齐桐), Xue-Fei Pan(潘雪飞), Jing Guo(郭静), Kai-Guang Zhu(朱凯光), Xue-Shen Liu(刘学深)

Chin. Phys. B, 2019, 28 (10): 103204. DOI: 10.1088/1674-1056/ab4176

Abstract ( 737 )

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We theoretically investigate the high-order harmonic generation (HHG) of helium atom driven by bichromatic counter-rotating circularly polarized laser fields. By changing the intensity ratio of the two driving laser fields, the spectral chirality of the HHG can be controlled. As the intensity ratio increases, the spectral chirality will change from positive- to negative-value around a large intensity ratio of the two driving fields when the total laser intensity keeps unchanged. However, the sign of the spectral chirality can be changed from positive to negative around a small intensity ratio of the two driving fields when the total laser intensity changes. At this time, we can effectively control the helicity of the harmonic spectrum and the polarization of the resulting attosecond pulses by adjusting the intensity ratio of the two driving laser fields. As the intensity ratio and the total intensity of the driving laser fields increase, the relative intensity of either the left-circularly or right-circularly polarized harmonic can be enhanced. The attosecond pulses can evolve from being elliptical to near linear correspondingly.

Dynamic and inner-dressing control of four-wave mixing in periodically-driven atomic system

Yuan-Yuan Li(李院院), Li Li(李莉), Yun-Zhe Zhang(张云哲), Lei Zhang(张雷)

Chin. Phys. B, 2019, 28 (10): 104201. DOI: 10.1088/1674-1056/ab3b52

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Four-wave-mixing (FWM) process is examined by using density matrix formalism in a periodically-driven atomic medium. Numerical result shows that FWM signals can be controlled by selecting different dynamic parameters of the probe field and strengths of the inner-dressing fields. It is also shown that the controllable FWM process is dominantly influenced by the evolution of atomic population difference and two-photon coherence. This dynamic and inner-dressing control of FWM is probably used for optimizing the optical nonlinear process and information processing.

Unconventional phase transition of phase-change-memory materials for optical data storage

Nian-Ke Chen(陈念科), Xian-Bin Li(李贤斌)

Chin. Phys. B, 2019, 28 (10): 104202. DOI: 10.1088/1674-1056/ab3cc3

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Recent years, optically controlled phase-change memory draws intensive attention owing to some advanced applications including integrated all-optical nonvolatile memory, in-memory computing, and neuromorphic computing. The light-induced phase transition is the key for this technology. Traditional understanding on the role of light is the heating effect. Generally, the RESET operation of phase-change memory is believed to be a melt-quenching-amorphization process. However, some recent experimental and theoretical investigations have revealed that ultrafast laser can manipulate the structures of phase-change materials by non-thermal effects and induces unconventional phase transitions including solid-to-solid amorphization and order-to-order phase transitions. Compared with the conventional thermal amorphization, these transitions have potential superiors such as faster speed, better endurance, and low power consumption. This article summarizes some recent progress of experimental observations and theoretical analyses on these unconventional phase transitions. The discussions mainly focus on the physical mechanism at atomic scale to provide guidance to control the phase transitions for optical storage. Outlook on some possible applications of the non-thermal phase transition is also presented to develop new types of devices.

Proof-of-principle experimental demonstration of quantum secure imaging based on quantum key distribution Suggested by editors

Yi-Bo Zhao(赵义博), Wan-Li Zhang(张万里), Dong Wang(王东), Xiao-Tian Song(宋萧天), Liang-Jiang Zhou(周良将), Chi-Biao Ding(丁赤飚)

Chin. Phys. B, 2019, 28 (10): 104203. DOI: 10.1088/1674-1056/ab3e66

Abstract ( 731 )

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We present a quantum secure imaging (QSI) scheme based on the phase encoding and weak+vacuum decoy-state BB84 protocol of quantum key distribution (QKD). It allows us to implement a computational ghost imaging (CGI) system with more simplified equipment and reconstructed algorithm by using a digital micro-mirror device (DMD) to preset the specific spatial distribution of the light intensity. What is more, the quantum bit error rate (QBER) and the secure key rate analytical functions of QKD are used to see through the intercept-resend jamming attacks and ensure the authenticity of the imaging information. In the experiment, we obtained the image of the object quickly and efficiently by measuring the signal photon counts with a single-photon detector (SPD), and achieved a secure key rate of 571.0 bps and a secure QBER of 3.99%, which is well below the lower bound of QBER of 14.51%. Besides, our imaging system uses a laser with invisible wavelength of 1550 nm, whose intensity is as low as single-photon, that can realize weak-light imaging and is immune to the stray light or air turbulence, thus it will become a better choice for quantum security radar against intercept-resend jamming attacks.

Numerical investigation on coherent mid-infrared supercontinuum generation in chalcogenide PCFs with near-zero flattened all-normal dispersion profiles

Jie Han(韩杰), Sheng-Dong Chang(常圣东), Yan-Jia Lyu(吕彦佳), Yong Liu(刘永)

Chin. Phys. B, 2019, 28 (10): 104204. DOI: 10.1088/1674-1056/ab3f25

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We design a novel all-normal flat near-zero dispersion chalcogenide photonic crystal fiber (PCF) for generating mid-infrared (MIR) supercontinuum (SC). The proposed PCF with a core made of As₂Se₃ glass and uniform air holes in the cladding is selectively filled with As₂S₅ glass. By carefully engineering the PCF with an all-normal flat near-zero dispersion profile, the anomalous-dispersion soliton effect is reduced, thus enabling broadband highly coherent SC to be generated. We also investigate the influence of the pulse parameters on the SC generation. Broadband SC covering 1.4 μm-10 μm with perfect coherence is achieved by pumping the proposed 3-cm-long PCF with 3-μm 100-fs pulses. The results provide a potential all-fiber realization of the broadband coherent MIR-SC.

Second-order interference of two independent photons with different spectra

Yu Zhou(周宇), Jian-Bin Liu(刘建彬), Huai-Bin Zheng(郑淮斌), Hui Chen(陈辉), Fu-Li Li(李福利), Zhuo Xu(徐卓)

Chin. Phys. B, 2019, 28 (10): 104205. DOI: 10.1088/1674-1056/ab3f23

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The second-order interference of two independent photons with different spectra in a Shih-Alley/Hong-Ou-Mandel interferometer is studied in Feynman's path integral theory. There is a second-order interference pattern for photons with different spectra if the photons are indistinguishable for the employed detection system. The conditions to observe the second-order temporal beating with photons of different spectra are analyzed. The influence of the response time of the detection system on the observed second-order interference pattern is also discussed. It is a direct result of that measurement in quantum mechanics is dependent on the employed measuring apparatus. The results are helpful to understand the physics of two-photon interference in different schemes.

Polymer/silica hybrid waveguide Y-branch power splitter with loss compensation based on NaYF₄: Er³⁺, Yb³⁺ nanocrystals Suggested by editors

Yue-Wu Fu(符越吾), Tong-He Sun(孙潼鹤), Mei-Ling Zhang(张美玲), Xu-Cheng Zhang(张绪成), Fei Wang(王菲), Da-Ming Zhang(张大明)

Chin. Phys. B, 2019, 28 (10): 104206. DOI: 10.1088/1674-1056/ab3f24

Abstract ( 747 )

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A polymer waveguide Y-branch power splitter with loss compensation is proposed based on NaYF₄:Er³⁺, Yb³⁺ nanocrystals prepared by a high temperature thermal decomposition method. The Y-branch power splitter is designed as a structure of embedded waveguide, and its core material is nanocrystals-doped SU-8. The insertion loss of the device is~15 dB. For an input signal power of 0.05 mW and a pump power of 267.7 mW, the two branches with 5.81-dB and 5.41-dB loss compensations at 1530 nm are achieved respectively. A polymer waveguide Y-branch power splitter with loss compensation has an important research significance.

Properties of multi-Gaussian Schell-model beams carrying an edge dislocation propagating in oceanic turbulence

Da-Jun Liu(刘大军), Yao-Chuan Wang(王耀川), Gui-Qiu Wang(王桂秋), Hong-Ming Yin(尹鸿鸣), Hai-Yang Zhong(仲海洋)

Chin. Phys. B, 2019, 28 (10): 104207. DOI: 10.1088/1674-1056/ab3f21

Abstract ( 656 )

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Based on the theory of coherence, the model of multi-Gaussian Schell-model (MGSM) beams carrying an edge dislocation generated by the MGSM source is introduced. The analytical cross-spectral density of MGSM beams carrying an edge dislocation propagating in oceanic turbulence is derived, and used to study the evolution properties of the MGSM beams carrying an edge dislocation. The results indicate that the MGSM beam carrying an edge dislocation propagating in oceanic turbulence will evolve from the profile with two intensity peaks into a flat-topped beam caused by the MGSM source, and the beam will evolve into the Gaussian-like beam due to the influences of oceanic turbulence in the far field. As the propagation distance increases, the MGSM beam carrying an edge dislocation propagating in oceanic turbulence with the larger rate of dissipation of mean-squared temperature (χ_T) and ratio of temperature to salinity contribution to the refractive index spectrum (ε) or the smaller rate of dissipation of kinetic energy per unit mass of fluid (ξ) evolves into the flat-topped beam or a Gaussian beam faster.

Extraordinary transmission and reflection in PT-symmetric two-segment-connected triangular optical waveguide networks with perfect and broken integer waveguide length ratios

Jia-Ye Wu(吴嘉野), Xu-Hang Wu(吴栩航), Xiang-Bo Yang(杨湘波), Hai-Ying Li(李海盈)

Chin. Phys. B, 2019, 28 (10): 104208. DOI: 10.1088/1674-1056/ab3f92

Abstract ( 608 )

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By adjusting the waveguide length ratio, we study the extraordinary characteristics of electromagnetic waves propagating in one-dimensional (1D) parity-time-symmetric (PT-symmetric) two-segment-connected triangular optical waveguide networks with perfect and broken integer waveguide length ratios respectively. It is found that the number and the corresponding frequencies of the extremum spontaneous PT-symmetric breaking points are dependent on the waveguide length ratio. Near the extremum breaking points, ultrastrong extraordinary transmissions are created and the maximal can arrive at, respectively, 2.4079×10¹⁴ and 4.3555×10¹³ in both kinds of networks. However, bidirectional invisibility can only be produced by the networks with broken integer waveguide length ratio, whose mechanism is explained in detail from the perspective of photonic band structure. The findings of this work can be useful optical characteristic control in the fabrication of PT-symmetric optical waveguide networks, which possesses great potential in designing optical amplifiers, optical energy saver devices, and special optical filters.

Low insertion loss silicon-based spatial light modulator with high reflective materials outside Fabry-Perot cavity Suggested by editors

Li-Fei Tian(田立飞), Ying-Xin Kuang(匡迎新), Zhong-Chao Fan(樊中朝), Zhi-Yong Li(李智勇)

Chin. Phys. B, 2019, 28 (10): 104209. DOI: 10.1088/1674-1056/ab427c

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The extinction ratio and insertion loss of spatial light modulator are subject to the material problem, thus limiting its applications. One reflection-type silicon-based spatial light modulator with high reflective materials outside the Fabry-Perot cavity is demonstrated in this paper. The reflectivity values of the outside-cavity materials with different film layer numbers are simulated. The reflectivity values of 6-pair Ta₂O₅/SiO₂ films at 1550 nm are experimentally verified to be as high as 99.9%. The surfaces of 6-pair Ta₂O₅/SiO₂ films are smooth:their root-mean-square roughness values are as small as 0.53 nm. The insertion loss of the device at 1550 nm is only 1.2 dB. The high extinction ratio of the device at 1550 nm and 11 V is achieved to be 29.7 dB. The spatial light modulator has a high extinction ratio and low insertion loss for applications.

Multi-objective strategy to optimize dithering technique for high-quality three-dimensional shape measurement

Ning Cai(蔡宁), Zhe-Bo Chen(陈浙泊), Xiang-Qun Cao(曹向群), Bin Lin(林斌)

Chin. Phys. B, 2019, 28 (10): 104210. DOI: 10.1088/1674-1056/ab427b

Abstract ( 706 )

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Dithering optimization techniques can be divided into the phase-optimized technique and the intensity-optimized technique. The problem with the former is the poor sensitivity to various defocusing amounts, and the problem with the latter is that it cannot enhance phase quality directly nor efficiently. In this paper, we present a multi-objective optimization framework for three-dimensional (3D) measurement by utilizing binary defocusing technique. Moreover, a binary patch optimization technique is used to solve the time-consuming issue of genetic algorithm. It is demonstrated that the presented technique consistently obtains significant phase performance improvement under various defocusing amounts.

Characterize and optimize the four-wave mixing in dual-interferometer coupled silicon microrings ^Hot!

Chao Wu(吴超), Yingwen Liu(刘英文), Xiaowen Gu(顾晓文), Shichuan Xue(薛诗川), Xinxin Yu(郁鑫鑫), Yuechan Kong(孔月婵), Xiaogang Qiang(强晓刚), Junjie Wu(吴俊杰), Zhihong Zhu(朱志宏), Ping Xu(徐平)

Chin. Phys. B, 2019, 28 (10): 104211. DOI: 10.1088/1674-1056/ab3f9b

Abstract ( 1028 )

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By designing and fabricating a series of dual-interferometer coupled silicon microrings, the coupling condition of the pump, signal, and idler beams can be engineered independently and then we carried out both the continuous-wave and pulse pumped four-wave mixing experiments to verify the dependence of conversion efficiency on the coupling conditions of the four interacting beams, respectively. Under the continuous-wave pump, the four-wave mixing efficiency gets maximized when both the pump and signal/idler beams are closely operated at the critical coupling point, while for the pulse pump case, the efficiency can be enhanced greatly when the pump and converted idler beams are all overcoupled. These experiment results agree well with our theoretical calculations. Our design provides a platform for explicitly characterizing the four-wave mixing under different pumping conditions, and offers a method to optimize the four-wave mixing, which will facilitate the development of on-chip all-optical signal processing with a higher efficiency or reduced pump power.

Single event upset on static random access memory devices due to spallation, reactor, and monoenergetic neutrons Suggested by editors

Xiao-Ming Jin(金晓明), Wei Chen(陈伟), Jun-Lin Li(李俊霖), Chao Qi(齐超), Xiao-Qiang Guo(郭晓强), Rui-Bin Li(李瑞宾), Yan Liu(刘岩)

Chin. Phys. B, 2019, 28 (10): 104212. DOI: 10.1088/1674-1056/ab4175

Abstract ( 599 )

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This paper presents new neutron-induced single event upset (SEU) data on the SRAM devices with the technology nodes from 40 nm to 500 nm due to spallation, reactor, and monoenergetic neutrons. The SEU effect is investigated as a function of incident neutron energy spectrum, technology node, byte pattern and neutron fluence rate. The experimental data show that the SEU effect mainly depends on the incident neutron spectrum and the technology node, and the SEU sensitivity induced by low-energy neutrons significantly increases with the technology downscaling. Monte-Carlo simulations of nuclear interactions with device architecture are utilized for comparing with the experimental results. This simulation approach allows us to investigate the key parameters of the SEU sensitivity, which are determined by the technology node and supply voltage. The simulation shows that the high-energy neutrons have more nuclear reaction channels to generate more secondary particles which lead to the significant enhancement of the SEU cross-sections, and thus revealing the physical mechanism for SEU sensitivity to the incident neutron spectrum.

Theoretical framework for geoacoustic inversion by adjoint method

Yang Wang(汪洋), Xiao-Feng Zhao(赵小峰)

Chin. Phys. B, 2019, 28 (10): 104301. DOI: 10.1088/1674-1056/ab3f93

Abstract ( 824 )

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Traditional geoacoustic inversions are generally solved by matched-field processing in combination with meta-heuristic global searching algorithms which usually need massive computations. This paper proposes a new physical framework for geoacoustic retrievals. A parabolic approximation of wave equation with non-local boundary condition is used as the forward propagation model. The expressions of the corresponding tangent linear model and the adjoint operator are derived, respectively, by variational method. The analytical expressions for the gradient of the cost function with respect to the control variables can be formulated by the adjoint operator, which in turn can be used for optimization by the gradient-based method.

Evolution of real contact area during stick-slip movement observed by total reflection method

Zhijun Luo(罗治军), Baojiang Song(宋保江), Jingyu Han(韩靖宇), Shaoze Yan(阎绍泽)

Chin. Phys. B, 2019, 28 (10): 104601. DOI: 10.1088/1674-1056/ab3f1f

Abstract ( 650 )

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We build an experiment system based on total reflection (TR) method to observe the evolution of real contact area of polymethyl methacrylate (PMMA) in the continual stick-slip movement. The bilateral friction is adopted to overcome the bending moment in the lateral friction movement. Besides some classical phenomena of stick-slip movement such as periodical slow increase of frictional force in sticking phase and a sudden drop when slipping, a special phenomenon that the contact area increases with the tangential force is observed, which was called junction growth by Tabor in 1959. Image processing methods are developed to observe the variation of the junction area. The results show that the center of the strongest contact region will keep sticking under the tangential force until the whole slipping, the strongest point undergoes three stages in one cycle, which are named as sticking stage, fretting stage, and cracking stage, respectively. The combined analysis reveals a physical process of stick-slip movement:the tangential force causes the increase of the real contact area, which reduces the pressure between the contact spots and finally leads to the slipping. Once slipping occurs, the real contact area drops to the original level resulting in the pressure increase to the original level, which makes the sticking happen again.

Stabilized seventh-order dissipative compact scheme for two-dimensional Euler equations

Jia-Xian Qin(秦嘉贤), Ya-Ming Chen(陈亚铭), Xiao-Gang Deng(邓小刚)

Chin. Phys. B, 2019, 28 (10): 104701. DOI: 10.1088/1674-1056/ab3f26

Abstract ( 726 )

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We derive in this paper a time stable seventh-order dissipative compact finite difference scheme with simultaneous approximation terms (SATs) for solving two-dimensional Euler equations. To stabilize the scheme, the choice of penalty coefficients for SATs is studied in detail. It is demonstrated that the derived scheme is quite suitable for multi-block problems with different spacial steps. The implementation of the scheme for the case with curvilinear grids is also discussed. Numerical experiments show that the proposed scheme is stable and achieves the design seventh-order convergence rate.

Numerical simulation on dynamic behaviors of bubbles flowing through bifurcate T-junction in microfluidic device

Liang-Yu Wu(吴梁玉), Ling-Bo Liu(刘凌波), Xiao-Tian Han(韩笑天), Qian-Wen Li(李倩文), Wei-Bo Yang(杨卫波)

Chin. Phys. B, 2019, 28 (10): 104702. DOI: 10.1088/1674-1056/ab3f27

Abstract ( 730 )

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Based on the volume of fluid (VOF) method, a numerical model of bubbles splitting in a microfluidic device with T-junction is developed and solved numerically. Various flow patterns are distinguished and the effects of bubble length, capillary number, and diameter ratio between the mother channel and branch are discussed. The break-up mechanism is explored in particular. The results indicate that the behaviors of the bubbles can be classified into two categories:break-up and non-break. Under the condition of slug flowing, the branches are obstructed by the bubbles that the pressure difference drives the bubbles into break-up state, while the bubbles that retain non-break state flow into an arbitrary branch under bubbling flow condition. The break-up of the short bubbles only occurs when the viscous force from the continuous phase overcomes the interfacial tension. The behavior of the bubbles transits from non-break to break-up with the increase of capillary number. In addition, the increasing of the diameter ratio is beneficial to the symmetrical break-up of the bubbles.

Construction of an H_α diagnostic system and its application to determine neutral hydrogen densities on the Keda Torus eXperiment Suggested by editors

Junfeng Zhu(朱军锋), Tao Lan(兰涛), Ge Zhuang(庄革), Tijian Deng(邓体建), Jie Wu(吴捷), Hangqi Xu(许航齐), Chen Chen(陈晨), Sen Zhang(张森), Jiaren Wu(邬佳仁), Yiming Zu(祖一鸣), Hong Li(李弘), Jinlin Xie(谢锦林), Ahdi Liu(刘阿娣), Zixi Liu(刘子奚), Zhengwei Wu(吴征威), Hai Wang(汪海), Xiaohui Wen(温晓辉), Haiyang Zhou(周海洋), Chijin Xiao(肖持进), Weixing Ding(丁卫星), Wandong Liu(刘万东)

Chin. Phys. B, 2019, 28 (10): 105201. DOI: 10.1088/1674-1056/ab427a

Abstract ( 680 )

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A 10-channel H_α diagnostic system has been designed with the rapid response rate of 300 kHz, spatial resolution of about 40 mm, and overlap between adjacent channels of about 3%, and it has been implemented successfully on Keda Torus eXperiment (KTX), a newly constructed, reversed field pinch (RFP) experimental device at the University of Science and Technology of China (USTC). This diagnostic system is a very important tool for the initial KTX operations. It is compact, with an aperture slit replacing the traditional optical lens system. A flexural interference filter is designed to prevent the center wavelength from shifting too much as the increase of angle from vertical incidence. To eliminate the stray light, the interior of the system is covered with the black aluminum foil having a very high absorptivity. Using the H_α emission data, together with the profiles of electron temperature and density obtained from the Langmuir probe, the neutral density profiles have been calculated for KTX plasmas. The rapid response rate and good spatial resolution of this H_α diagnostic system will be beneficial for many studies in RFP plasma physics.

The inverse Bremsstrahlung absorption in the presence of Maxwellian and non-Maxwellian electrons

Mehdi Sharifian, Fatemeh Ghoveisi, Leila Gholamzadeh, Narges Firouzi Farrashbandi

Chin. Phys. B, 2019, 28 (10): 105202. DOI: 10.1088/1674-1056/ab4173

Abstract ( 532 )

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Inverse Bremsstrahlung absorption (IBA) of an intense laser field in plasma containing Maxwellian and non-Maxwellian (with Kappa and q-nonextensive distribution functions) electrons is studied analytically. Our results show that IBA decreases with an increase in temperature at high intensities and a decrease in plasma density for all kinds of distribution functions. Another striking result is that IBA is independent of the laser intensity at low intensity but is dependent on it when the intensity is going to rise. Also, it could be find that the behavior of the absorption as the function of laser intensity for the Kappa distribution with κ=10 at low intensity is close to that for the Maxwellian distribution, but at high intensity it is close to that in the presence of q-nonextensive electrons with q=0.9. These results provide insights into the inverse Bremsstrahlung absorption in the laser-plasma interactions.

Defects and electrical properties in Al-implanted 4H-SiC after activation annealing

Yi-Dan Tang(汤益丹), Xin-Yu Liu(刘新宇), Zheng-Dong Zhou(周正东), Yun Bai(白云), Cheng-Zhan Li(李诚瞻)

Chin. Phys. B, 2019, 28 (10): 106101. DOI: 10.1088/1674-1056/ab3cc2

Abstract ( 733 )

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The defects and electrical properties in Al-implanted 4H-SiC after activation annealing (1600 ℃-1800 ℃) are investigated. High temperature annealing can reduce the ion implantation-induced damage effectively, but it may induce extended defects as well, which are investigated by using Rutherford backscattering spectroscopy (RBS/C), secondary ion mass spectroscopy (SIMS), and transmission electron microscopy (TEM) analyses. According to the ratio of the channeled intensity to the random intensity in the region just below the surface scattering peak (X_min) and RBS/C analysis results, the ion implantation-induced surface damages can be effectively reduced by annealing at temperatures higher than 1700 ℃, while the defects near the bottom of the ion-implanted layer cannot be completely annealed out by high temperature and long time annealing process, which is also demonstrated by SIMS and TEM analyses. Referring to the defect model and TEM analyses, an optimized annealing condition can be achieved through balancing the generation and elimination of carbon vacancies in the ion implanted layers. Furthermore, the electrical and surface properties are also analyzed, and the hole concentration, mobility, and resistivity are obtained through the Hall effect. The optimized activation annealing conditions of 1800 ℃/5 min are achieved, under which the lower defects and acceptable electrical properties are obtained.

First principles study of interactions of oxygen-carbon-vacancy in bcc Fe

Yuan You(由园), Mu-Fu Yan(闫牧夫), Ji-Hong Yan(闫纪红), Gang Sun(孙刚), Chao Wang(王超)

Chin. Phys. B, 2019, 28 (10): 106102. DOI: 10.1088/1674-1056/ab3a8f

Abstract ( 534 )

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Behaviors of C or O in bcc Fe and interactions of C-O and oxygen-carbon-vacancy (O-C-□) are investigated by first principles calculations. Octahedral interstitial site is the most stable position for an O atom in bcc Fe. The migration energy of an O atom in bcc Fe is 0.46 eV. The strength of O-Fe (1nn) bond (0.32) is slightly greater than that of Fe-Fe metallic bond (0.26). Repulsive interactions of C-C, O-O, and C-O exist in bcc Fe. When the concentration of FIA (FIA refers to C or O) is relatively high, a vacancy can attract four FIAs and form stable FIAs-□ complex.

Structural and electronic properties of transition-metal chalcogenides Mo₅S₄ nanowires ^Hot!

Ming-Shuai Qiu(邱明帅), Huai-Hong Guo(郭怀红), Ye Zhang(张也), Bao-Juan Dong(董宝娟), Sajjad Ali(阿里.萨贾德), Teng Yang(杨腾)

Chin. Phys. B, 2019, 28 (10): 106103. DOI: 10.1088/1674-1056/ab3f9a

Abstract ( 725 )

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Transition-metal chalcogenide nanowires (TMCN) as a viable candidate for nanoscale applications have been attracting much attention for the last few decades. Starting from the rigid building block of M₆ octahedra (M=transition metal), depending on the way of connection between M₆ and decoration by chalcogenide atoms, multiple types of extended TMCN nanowires can be constructed based on some basic rules of backbone construction proposed here. Note that the well-known Chevrel-phase based M₆X₆ and M₆X₉ (X=chalcogenide atom) nanowires, which are among our proposed structures, have been successfully synthesized by experiment and well studied. More interestingly, based on the construction principles, we predict three new structural phases (the cap, edge, and C&E phases) of Mo₅S₄, one of which (the edge phase) has been obtained by top-down electron beam lithography on two-dimensional MoS₂, and the C&E phase is yet to be synthesized but appears more stable than the edge phase. The stability of the new phases of Mo₅S₄ is further substantiated by crystal orbital overlapping population (COOP), phonon dispersion relation, and thermodynamic calculation. The barrier of the structural transition between different phases of Mo₅S₄ shows that it is very likely to realize an conversion from the experimentally achieved structure to the most stable C&E phase. The calculated electronic structure shows an interesting band nesting between valence and conduction bands of the C&E Mo₅S₄ phase, suggesting that such a nanowire structure can be well suitable for optoelectronic sensor applications.

Discovery of superhard materials via CALYPSO methodology

Shuangshuang Zhang(张爽爽), Julong He(何巨龙), Zhisheng Zhao(赵智胜), Dongli Yu(于栋利), Yongjun Tian(田永君)

Chin. Phys. B, 2019, 28 (10): 106104. DOI: 10.1088/1674-1056/ab4179

Abstract ( 1043 )

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The study of superhard materials plays a critical role in modern industrial applications due to their widespread applications as cutting tools, abrasives, exploitation drills, and coatings. The search for new superhard materials with superior performance remains a hot topic and is mainly considered as two classes of materials:(i) the light-element compounds in the B-C-N-O(-Si) system with strong and short covalent bonds, and (ii) the transition-element light-element compounds with strong covalent bonds frameworks and high valence electron density. In this paper, we review the recent achievements in the prediction of superhard materials mostly using the advanced CALYPSO methodology. A number of novel, superhard crystals of light-element compounds and transition-metal borides, carbides, and nitrides have been theoretically identified and some of them account well for the experimentally mysterious phases. To design superhard materials via CALYPSO methodology is independent of any known structural and experimental data, resulting in many remarkable structures accelerating the development of new superhard materials.

The CALYPSO methodology for structure prediction

Qunchao Tong(童群超), Jian Lv(吕健), Pengyue Gao(高朋越), Yanchao Wang(王彦超)

Chin. Phys. B, 2019, 28 (10): 106105. DOI: 10.1088/1674-1056/ab4174

Abstract ( 1008 )

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Structure prediction methods have been widely used as a state-of-the-art tool for structure searches and materials discovery, leading to many theory-driven breakthroughs on discoveries of new materials. These methods generally involve the exploration of the potential energy surfaces of materials through various structure sampling techniques and optimization algorithms in conjunction with quantum mechanical calculations. By taking advantage of the general feature of materials potential energy surface and swarm-intelligence-based global optimization algorithms, we have developed the CALYPSO method for structure prediction, which has been widely used in fields as diverse as computational physics, chemistry, and materials science. In this review, we provide the basic theory of the CALYPSO method, placing particular emphasis on the principles of its various structure dealing methods. We also survey the current challenges faced by structure prediction methods and include an outlook on the future developments of CALYPSO in the conclusions.

Pressure-induced new chemistry

Jianyan Lin(蔺健妍), Xin Du(杜鑫), Guochun Yang(杨国春)

Chin. Phys. B, 2019, 28 (10): 106106. DOI: 10.1088/1674-1056/ab3f91

Abstract ( 658 )

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It has long been recognized that the valence electrons of an atom dominate the chemical properties, while the inner-shell electrons or outer empty orbital do not participate in chemical reactions. Pressure, as a fundamental thermodynamic variable, plays an important role in the preparation of new materials. More recently, pressure stabilized a series of unconventional stoichiometric compounds with new oxidation states, in which the inner-shell electrons or outer empty orbital become chemically active. Here, we mainly focus on the recent advances in high-pressure new chemistry including novel chemical bonding and new oxidation state, identified by first-principles swarm intelligence structural search calculations. The aim of this review is to provide an up-to-date research progress on the chemical bonding with inner-shell electrons or outer empty orbital, abnormal interatomic charge transfer, hypervalent compounds, and chemical reactivity of noble gases. Personal outlook on the challenge and opportunity in this field are proposed in the conclusion.

Geoscience material structures prediction via CALYPSO methodology

Andreas Hermann

Chin. Phys. B, 2019, 28 (10): 106107. DOI: 10.1088/1674-1056/ab43bc

Abstract ( 702 )

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Many properties of planets such as their interior structure and thermal evolution depend on the high-pressure properties of their constituent materials. This paper reviews how crystal structure prediction methodology can help shed light on the transformations materials undergo at the extreme conditions inside planets. The discussion focuses on three areas:(i) the propensity of iron to form compounds with volatile elements at planetary core conditions (important to understand the chemical makeup of Earth's inner core), (ii) the chemistry of mixtures of planetary ices (relevant for the mantle regions of giant icy planets), and (iii) examples of mantle minerals. In all cases the abilities and current limitations of crystal structure prediction are discussed across a range of example studies.

High-pressure electrides: From design to synthesis

Biao Wan(万彪), Jingwu Zhang(张静武), Lailei Wu(吴来磊), Huiyang Gou(缑慧阳)

Chin. Phys. B, 2019, 28 (10): 106201. DOI: 10.1088/1674-1056/ab3f95

Abstract ( 1016 )

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Electrides are unique ionic compounds that electrons serve as the anions. Many electrides with fascinating physical and chemical properties have been discovered at ambient condition. Under pressure, electrides are also revealed to be ubiquitous crystal morphology, enriching the geometrical topologies and electronic properties of electrides. In this Review, we overview the formation mechanism of high-pressure electrides (HPEs) and outline a scheme for exploring new HPEs from pre-design, CALYPSO assisted structural searches, indicators for electrides, to experimental synthesis. Moreover, the evolution of electronic dimensionality under compression is also discussed to better understand the dimensional distribution of anionic electrons in HPEs.

Physical properties of ternary thallium chalcogenes Tl₂MQ₃ (M=Zr, Hf; Q=S, Se, Te) via ab-initio calculations

Engin Ateser, Oguzhan Okvuran, Yasemin Oztekin Ciftci, Haci Ozisik, Engin Deligoz

Chin. Phys. B, 2019, 28 (10): 106301. DOI: 10.1088/1674-1056/ab427d

Abstract ( 618 )

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We have reported a first principles study of structural, mechanical, electronic, and thermoelectric properties of the monoclinic ternary thallium chalcogenes Tl₂MQ₃ (M=Zr, Hf; Q=S, Se, Te). The electronic band structure calculations confirm that all compounds exhibit semiconductor character. Especially, Tl₂ZrTe₃ and Tl₂HfTe₃ can be good candidates for thermoelectric materials, having narrow band gaps of 0.169 eV and 0.21 eV, respectively. All of the compounds are soft and brittle according to the second-order elastic constant calculations. Low Debye temperatures also support the softness. We have obtained the transport properties of the compounds by using rigid band and constant relaxation time approximations in the context of Boltzmann transport theory. The results show that the compounds could be considered for room temperature thermoelectric applications (ZT~0.9).

Phosphine-free synthesis of FeTe₂ nanoparticles and self-assembly into tree-like nanoarchitectures

Hongyu Wang(王红宇), Min Wu(武敏), Yixuan Wang(王艺璇), Hao Wang(王浩), Xiaoli Huang(黄晓丽), Xinyi Yang(杨新一)

Chin. Phys. B, 2019, 28 (10): 106401. DOI: 10.1088/1674-1056/ab3b51

Abstract ( 647 )

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Manipulating the self-assembly of transition metal telluride nanocrystals (NCs) creates opportunities for exploring new properties and device applications. Iron ditelluride (FeTe₂) has recently emerged as a new class of magnetic semiconductor with three-dimensional (3D) magnetic ordering and narrow band gap structure, yet the self-assembly of FeTe₂ NCs has not been achieved. Herein, the tree-like FeTe₂ nanoarchitectures with orthorhombic crystal structure have been successfully synthesized by hot-injection solvent thermal approach using phosphine-free Te precursor. The morphology, size, and crystal structure have been investigated using transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and powder x-ray diffraction (XRD). We study the formation process of tree-like FeTe₂ NCs according to trace the change of the sample morphology with the reaction time. It was found that the FeTe₂ nanoparticles show oriented aggregation and self-assembly behavior with the increase of reaction time, which is attributed to size-dependent magnetism properties of the samples. The magnetic interaction is thought to be the driving force of nanoparticle self-organization.

Expansion dynamics of a spherical Bose-Einstein condensate Suggested by editors

Rui-Zong Li(李睿宗), Tian-You Gao(高天佑), Dong-Fang Zhang(张东方), Shi-Guo Peng(彭世国), Ling-Ran Kong(孔令冉), Xing Shen(沈星), Kai-Jun Jiang(江开军)

Chin. Phys. B, 2019, 28 (10): 106701. DOI: 10.1088/1674-1056/ab4177

Abstract ( 744 )

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We experimentally and theoretically observe the expansion behaviors of a spherical Bose-Einstein condensate. A rubidium condensate is produced in an isotropic optical dipole trap with an asphericity of 0.037. We measure the variation of the condensate size in the expansion process after switching off the trap. The free expansion of the condensate is isotropic, which is different from that of the condensate usually produced in the anisotropic trap. We derive an analytic solution of the expansion behavior based on the spherical symmetry, allowing a quantitative comparison with the experimental measurement. The interaction energy of the condensate is gradually converted into the kinetic energy during the expansion and after a long time the kinetic energy saturates at a constant value. We obtain the interaction energy of the condensate in the trap by probing the long-time expansion velocity, which agrees with the theoretical calculation. This work paves a way to explore novel quantum states of ultracold gases with the spherical symmetry.

Highly reliable and selective ethanol sensor based on α-Fe₂O₃ nanorhombs working in realistic environments Suggested by editors

Wenjun Yan(闫文君), Xiaomin Zeng(曾小敏), Huan Liu(刘欢), Chunwei Guo(郭春伟), Min Ling(凌敏), Houpan Zhou(周后盘)

Chin. Phys. B, 2019, 28 (10): 106801. DOI: 10.1088/1674-1056/ab3af1

Abstract ( 511 )

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A highly reliable and selective ethanol gas sensor working in realistic environments based on alpha-Fe₂O₃ (α-Fe₂O₃) nanorhombs is developed. The sensor is fabricated by integrating α-Fe₂O₃ nanorhombs onto a low power microheater based on micro-electro-mechanical systems (MEMS) technology. The α-Fe₂O₃ nanorhombs, prepared via a solvothermal method, is characterized by transmission electron microscopy (TEM), Raman spectroscopy, x-ray diffraction (XRD), and x-ray photoelectron spectroscopy (XPS). The sensing performances of the α-Fe₂O₃ sensor to various toxic gases are investigated. The optimum sensing temperature is found to be about 280 ℃. The sensor shows excellent selectivity to ethanol. For various ethanol concentrations (1 ppm-20 ppm), the response and recovery times are around 3 s and 15 s at the working temperature of 280 ℃, respectively. Specifically, the α-Fe₂O₃ sensor exhibits a response shift less than 6% to ethanol at 280 ℃ when the relative humidity (RH) increases from 30% to 70%. The good tolerance to humidity variation makes the sensor suitable for reliable applications in Internet of Things (IoT) in realistic environments. In addition, the sensor shows great long-term repeatability and stability towards ethanol. A possible gas sensing mechanism is proposed.

Charge trapping memory device based on the Ga₂O₃ films as trapping and blocking layer

Bing Bai(白冰), Hong Wang(王宏), Yan Li(李岩), Yunxia Hao(郝云霞), Bo Zhang(张博), Boping Wang(王博平), Zihang Wang(王子航), Hongqi Yang(杨红旗), Qihang Gao(高启航), Chao Lü(吕超), Qingshun Zhang(张庆顺), Xiaobing Yan(闫小兵)

Chin. Phys. B, 2019, 28 (10): 106802. DOI: 10.1088/1674-1056/ab3e62

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We present a new charge trapping memory (CTM) device with the Au/Ga₂O₃/SiO₂/Si structure, which is fabricated by using the magnetron sputtering, high-temperature annealing, and vacuum evaporation techniques. Transmission electron microscopy diagrams show that the thickness of the SiO₂ tunneling layer can be controlled by the annealing temperature. When the devices are annealed at 760 ℃, the measured C-V hysteresis curves exhibit a maximum 6 V memory window under a ±13 V sweeping voltage. In addition, a slight degradation of the device voltage and capacitance indicates the robust retention properties of flat-band voltage and high/low state capacitance. These distinctive advantages are attributed to oxygen vacancies and inter-diffusion layers, which play a critical role in the charge trapping process.

Electronic properties of size-dependent MoTe₂/WTe₂ heterostructure Suggested by editors

Jing Liu(刘婧), Ya-Qiang Ma(马亚强), Ya-Wei Dai(戴雅薇), Yang Chen(陈炀), Yi Li(李依), Ya-Nan Tang(唐亚楠), Xian-Qi Dai(戴宪起)

Chin. Phys. B, 2019, 28 (10): 107101. DOI: 10.1088/1674-1056/ab3b53

Abstract ( 710 )

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Lateral two-dimensional (2D) heterostructures have opened up unprecedented opportunities in modern electronic device and material science. In this work, electronic properties of size-dependent MoTe₂/WTe₂ lateral heterostructures (LHSs) are investigated through the first-principles density functional calculations. The constructed periodic multi-interfaces patterns can also be defined as superlattice structures. Consequently, the direct band gap character remains in all considered LHSs without any external modulation, while the gap size changes within little difference range with the building blocks increasing due to the perfect lattice matching. The location of the conduction band minimum (CBM) and the valence band maximum (VBM) will change from P-point to Γ-point when m plus n is a multiple of 3 for A-mn LHSs as a result of Brillouin zone folding. The bandgap located at high symmetry Γ-point is favourable to electron transition, which might be useful to optoelectronic device and could be achieved by band engineering. Type-Ⅱ band alignment occurs in the MoTe₂/WTe₂ LHSs, for electrons and holes are separated on the opposite domains, which would reduce the recombination rate of the charge carriers and facilitate the quantum efficiency. Moreover, external biaxial strain leads to efficient bandgap engineering. MoTe₂/WTe₂ LHSs could serve as potential candidate materials for next-generation electronic devices.

Hubbard model on an anisotropic checkerboard lattice at finite temperatures: Magnetic and metal-insulator transitions Suggested by editors

Hai-Di Liu(刘海迪)

Chin. Phys. B, 2019, 28 (10): 107102. DOI: 10.1088/1674-1056/ab4279

Abstract ( 576 )

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We study magnetic and Mott transitions of the Hubbard model on the geometrically frustrated anisotropic checkerboard lattice at half filling using cellular dynamical mean-field theory. Phase diagrams over a wide area of the parameter space are obtained by varying the interparticle interaction strength, geometric frustration strength, and temperature. Our results show that frustration and thermal fluctuations play a competing role against the interactions and in general favor a metallic phase without antiferromagnetic order. Due to their interplay, the system exhibits competition between antiferromagnetic insulator, antiferromagnetic metal, paramagnetic insulator, and paramagnetic metal phases in the intermediate-interaction regime. In the strong-interaction limit, which reduces to the Heisenberg model, our result is consistent with previous studies.

Benchmarking the simplest slave-particle theory with Hubbard dimer

Wei-Wei Yang(杨薇薇), Hong-Gang Luo(罗洪刚), Yin Zhong(钟寅)

Chin. Phys. B, 2019, 28 (10): 107103. DOI: 10.1088/1674-1056/ab3dfe

Abstract ( 667 )

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Slave-particle method is a powerful tool to tackle the correlation effect in quantum many-body physics. Although it has been successfully used to comprehend various intriguing problems, such as Mott metal-insulator transition and Kondo effect, there is still no convincing theory so far on the availability and limitation of this method. The abuse of slave-particle method may lead to wrong physics. As the simplest slave-particle method, Z₂ slave spin, which is widely applied to many strongly correlated problems, is highly accessible and researchable. In this work, we will uncover the nature of the Z₂ slave-spin method by studying a two-site Hubbard model. After exploring aspects of properties of this toy model, we make a comparative analysis of the results obtained by three methods:(i) slave-spin method on mean-field level, (ii) slave-spin method with gauge constraint, and (iii) the exact solution as a benchmark. We find that, protected by the particle-hole symmetry, the slave-spin mean-field method can recover the static properties of ground state exactly at half filling. Furthermore, in the parameter space where both U and T are small enough, the slave-spin mean-field method is also reliable in calculating the dynamic and thermal dynamic properties. However, when U or T is considerably large, the mean-field approximation gives ill-defined behaviors, which result from the unphysical states in the enlarged Hilbert space. These findings lead to our conclusion that the accuracy of slave particle can be guaranteed if we can exclude all unphysical states by enforcing gauge constraints. Our work demonstrates the promising prospect of slave-particle method in studying complex strongly correlated models with specific symmetry or in certain parameter space.

The role of CALYPSO in the discovery of high-T_c hydrogen-rich superconductors

Wenwen Cui(崔文文), Yinwei Li(李印威)

Chin. Phys. B, 2019, 28 (10): 107104. DOI: 10.1088/1674-1056/ab4253

Abstract ( 790 )

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Hydrogen-rich compounds are promising candidates for high-T_c or even room-temperature superconductors. The search for high-T_c hydrides poses a major experimental challenge because there are many known hydrides and even more unknown hydrides with unusual stoichiometries under high pressure. The combination of crystal structure prediction and first-principles calculations has played an important role in the search for high-T_c hydrides, especially in guiding experimental synthesis. Crystal structure AnaLYsis by Particle Swarm Optimization (CALYPSO) is one of the most efficient methods for predicting stable or metastable structures from the chemical composition alone. This review summarizes the superconducting hydrides predicted using CALYPSO. We focus on two breakthroughs toward room-temperature superconductors initiated by CALYPSO:the prediction of high-T_c superconductivity in compressed hydrogen sulfide and lanthanum hydrides, both of which have been confirmed experimentally and have set new record T_c values. We also address the challenges and outlook in this field.

A review of current research on spin currents and spin-orbit torques

Xiao-Yu Feng(冯晓玉), Qi-Han Zhang(张琪涵), Han-Wen Zhang(张瀚文), Yi Zhang(张祎), Rui Zhong(钟瑞), Bo-Wen Lu(卢博文), Jiang-Wei Cao(曹江伟), Xiao-Long Fan(范小龙)

Chin. Phys. B, 2019, 28 (10): 107105. DOI: 10.1088/1674-1056/ab425e

Abstract ( 798 )

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Spintronics is a new discipline focusing on the research and application of electronic spin properties. After the discovery of the giant magnetoresistance effect in 1988, spintronics has had a huge impact on scientific progress and related applications in the development of information technology. In recent decades, the main motivation in spintronics has been efficiently controlling local magnetization using electron flow or voltage rather than controlling the electron flow using magnetization. Using spin-orbit coupling in a material can convert a charge current into a pure spin current (a flow of spin momenta without a charge flow) and generate a spin-orbit torque on the adjacent ferromagnets. The ability of spintronic devices to utilize spin-orbit torques to manipulate the magnetization has resulted in large-scale developments such as magnetic random-access memories and has boosted the spintronic research area. Here in, we review the theoretical and experimental results that have established this subfield of spintronics. We introduce the concept of a pure spin current and spin-orbit torques within the experimental framework, and we review transport-, magnetization-dynamics-, and optical-based measurements and link then to both phenomenological and microscopic theories of the effect. The focus is on the related progress reported from Chinese universities and institutes, and we specifically highlight the contributions made by Chinese researchers.

Negative transconductance effect in p-GaN gate AlGaN/GaN HEMTs by traps in unintentionally doped GaN buffer layer

Mei Ge(葛梅), Qing Cai(蔡青), Bao-Hua Zhang(张保花), Dun-Jun Chen(陈敦军), Li-Qun Hu(胡立群), Jun-Jun Xue(薛俊俊), Hai Lu(陆海), Rong Zhang(张荣), You-Dou Zheng(郑有炓)

Chin. Phys. B, 2019, 28 (10): 107301. DOI: 10.1088/1674-1056/ab3e00

Abstract ( 944 )

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We investigate the negative transconductance effect in p-GaN gate AlGaN/GaN high-electron-mobility transistor (HEMT) associated with traps in the unintentionally doped GaN buffer layer. We find that a negative transconductance effect occurs with increasing the trap concentration and capture cross section when calculating transfer characteristics. The electron tunneling through AlGaN barrier and the reduced electric field discrepancy between drain side and gate side induced by traps are reasonably explained by analyzing the band diagrams, output characteristics, and the electric field strength of the channel of the devices under different trap concentrations and capture cross sections.

Optical response of an inverted InAs/GaSb quantum well in an in-plane magnetic field Suggested by editors

Xiaoguang Wu(吴晓光)

Chin. Phys. B, 2019, 28 (10): 107302. DOI: 10.1088/1674-1056/ab3c29

Abstract ( 543 )

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The optical response of an inverted InAs/GaSb quantum well is studied theoretically. The influence of an in-plane magnetic field that is applied parallel to the quantum well is considered. This in-plane magnetic field will induce a dynamical polarization even when the electric field component of the external optical field is parallel to the quantum well. The electron-electron interaction in the quantum well system will lead to the de-polarization effect. This effect is found to be important and is taken into account in the calculation of the optical response. It is found that the main feature in the frequency dependence of the velocity-velocity correlation function remains when the velocity considered is parallel to the in-plane magnetic field. When the direction of the velocity is perpendicular to the in-plane magnetic field, the de-polarization effect will suppress the oscillatory behavior in the corresponding velocity-velocity correlation function. The in-plane magnetic field can change the band structure of the quantum well drastically from a gapped semiconductor to a no-gapped semi-metal, but it is found that the distribution of the velocity matrix elements or the optical transition matrix elements in the wave vector space has the same two-tadpole topology.

Observation of hopping transitions for delocalized electrons by temperature-dependent conductance in siliconjunctionless nanowire transistors Suggested by editors

Yang-Yan Guo(郭仰岩), Wei-Hua Han(韩伟华), Xiao-Song Zhao(赵晓松), Ya-Mei Dou(窦亚梅), Xiao-Di Zhang(张晓迪), Xin-Yu Wu(吴歆宇), Fu-Hua Yang(杨富华)

Chin. Phys. B, 2019, 28 (10): 107303. DOI: 10.1088/1674-1056/ab3e68

Abstract ( 640 )

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We demonstrate transitions of hopping behaviors for delocalized electrons through the discrete dopant-induced quantum dots in n-doped silicon junctionless nanowire transistors by the temperature-dependent conductance characteristics. There are two obvious transition platforms within the critical temperature regimes for the experimental conductance data, which are extracted from the unified transfer characteristics for different temperatures at the gate voltage positions of the initial transconductance g_m peak in V_g1 and valley in V_g2. The crossover temperatures of the electron hopping behaviors are analytically determined by the temperature-dependent conductance at the gate voltages V_g1 and V_g2. This finding provides essential evidence for the hopping electron behaviors under the influence of thermal activation and long-range Coulomb interaction.

Emerging properties of two-dimensional twisted bilayer materials

Yang Cheng(程阳), Chen Huang(黄琛), Hao Hong(洪浩), Zixun Zhao(赵子荀), Kaihui Liu(刘开辉)

Chin. Phys. B, 2019, 28 (10): 107304. DOI: 10.1088/1674-1056/ab3e46

Abstract ( 813 )

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Recent studies in van der Waals coupled two-dimensional (2D) bilayer materials have demonstrated a new freedom for material engineering by the formation of moiré pattern. By tuning the twist angle between two layers, one can modulate their electronic band structures and therefore the associated electrical transport and optical properties, which are distinct from the original ones of each individual layer. These new properties excite great passion in the exploration of new quantum states and possible applications of 2D bilayers. In this article, we will mainly review the prevailing fabrication methods and emerging physical properties of twisted bilayer materials and lastly give out a perspective of this topic.

Enhanced spin-dependent thermopower in a double-quantum-dot sandwiched between two-dimensional electron gases

Feng Chi(迟锋), Zhen-Guo Fu(付振国), Liming Liu(刘黎明), Ping Zhang(张平)

Chin. Phys. B, 2019, 28 (10): 107305. DOI: 10.1088/1674-1056/ab3f98

Abstract ( 593 )

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We study the spin-dependent thermopower in a double-quantum-dot (DQD) embedded between the left and right two-dimensional electron gases (2DEGs) in doped quantum wells under an in-plane magnetic field. When the separation between the DQD is smaller than the Fermi wavelength in the 2DEGs, the asymmetry in the dots' energy levels leads to pronounced quantum interference effects characterized by the Dicke line-shape of the conductance, which are sensitive to the properties of the 2DEGs. The magnitude of the thermopower, which denotes the generated voltage in response to an infinitesimal temperature difference between the two 2DEGs under vanishing charge current, will be obviously enhanced by the Dicke effect. The application of the in-plane magnetic field results in the polarization of the spin-up and spin-down conductances and thermopowers, and enables an efficient spin-filter device in addition to a tunable pure spin thermopower in the absence of its charge counterpart.

Recent progress on the prediction of two-dimensional materials using CALYPSO

Cheng Tang(唐程), Gurpreet Kour, Aijun Du(杜爱军)

Chin. Phys. B, 2019, 28 (10): 107306. DOI: 10.1088/1674-1056/ab41ea

Abstract ( 1015 )

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In recent years, structure design and predictions based on global optimization approach as implemented in CALYPSO software have gained great success in accelerating the discovery of novel two-dimensional (2D) materials. Here we highlight some most recent research progress on the prediction of novel 2D structures, involving elements, metal-free and metal-containing compounds using CALYPSO package. Particular emphasis will be given to those 2D materials that exhibit unique electronic and magnetic properties with great potentials for applications in novel electronics, optoelectronics, magnetronics, spintronics, and photovoltaics. Finally, we also comment on the challenges and perspectives for future discovery of multi-functional 2D materials.

Electronic structure of molecular beam epitaxy grown 1T'-MoTe₂ film and strain effect ^Hot!

Xue Zhou(周雪), Zeyu Jiang(姜泽禹), Kenan Zhang(张柯楠), Wei Yao(姚维), Mingzhe Yan(颜明哲), Hongyun Zhang(张红云), Wenhui Duan(段文晖), Shuyun Zhou(周树云)

Chin. Phys. B, 2019, 28 (10): 107307. DOI: 10.1088/1674-1056/ab43ba

Abstract ( 979 )

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Atomically thin transition metal dichalcogenide films with distorted trigonal (1T') phase have been predicted to be candidates for realizing quantum spin Hall effect. Growth of 1T' film and experimental investigation of its electronic structure are critical. Here we report the electronic structure of 1T'-MoTe₂ films grown by molecular beam epitaxy (MBE). Growth of the 1T'-MoTe₂ film depends critically on the substrate temperature, and successful growth of the film is indicated by streaky stripes in the reflection high energy electron diffraction (RHEED) and sharp diffraction spots in the low energy electron diffraction (LEED). Angle-resolved photoemission spectroscopy (ARPES) measurements reveal a metallic behavior in the as-grown film with an overlap between the conduction and valence bands. First principles calculation suggests that a suitable tensile strain along the a-axis direction is needed to induce a gap to make it an insulator. Our work not only reports the electronic structure of MBE grown 1T'-MoTe₂ films, but also provides insights for strain engineering to make it possible for quantum spin Hall effect.

Robust two-gap strong coupling superconductivity associated with low-lying phonon modes in pressurized Nb₅Ir₃O superconductors ^Hot!

Bosen Wang(王铂森), Yaoqing Zhang(张尧卿), Shuxiang Xu(徐淑香), Kento Ishigaki, Kazuyuki Matsubayashi, Jin-Guang Cheng(程金光), Hideo Hosono, Yoshiya Uwatoko

Chin. Phys. B, 2019, 28 (10): 107401. DOI: 10.1088/1674-1056/ab4047

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We report robust superconducting state and gap symmetry of Nb₅Ir₃O via electrical transport and specific heat measurements. The analysis of specific heat manifests that Nb₅Ir₃O is a strongly coupled superconductor with ΔC/γ_nT_c~1.91 and double s-wave superconducting gaps of 2△_L(0)/k_BT_c~6.56 and 2△_S(0)/k_BT_c~2.36 accounting for 90% and 10%, respectively. The (C_p-γ_nT)/T³ vs. T plot shows a broad peak at~23 K, indicating phonon softening and the appearance of low-lying phonon mode associated with the interstitial oxygen. This behavior explains the monotonic increase of T_c in Nb₅Ir₃O_(1-δ) by strengthening the electron-phonon coupling and enlarging the density of states at Fermi level. The Hall coefficient is temperature independent below 200 K, and changes its sign from positive to negative above 250 K, suggesting that carrier is across the hole- to electron-dominant regions and the multi-band electronic structures. On warming, the resistivity shows a gradual crossover from T²- to T³-dependence at a critical temperature T^*, and a broad peak at a temperature T_p. The reduced T_c under pressure is linearly correlated with lattice parameters c/a ratio and T_p, suggesting the important phonon contributions in Nb₅Ir₃O as a phonon-medicated superconductor. Possible physical mechanisms are proposed.

Magnetoresistance hysteresis in topological Kondo insulator SmB₆ nanowire

Ling-Jian Kong(孔令剑), Yong Zhou(周勇), Hua-Ding Song(宋化鼎), Da-Peng Yu(俞大鹏), Zhi-Min Liao(廖志敏)

Chin. Phys. B, 2019, 28 (10): 107501. DOI: 10.1088/1674-1056/ab3a89

Abstract ( 780 )

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SmB₆, a topological Kondo insulator, with a gapped bulk state and metallic surface state has aroused great research interest. Here, we report an exotic hysteresis behavior of magnetoresistance in individual SmB₆ nanowire in a temperature range in which both surface and bulk states contribute to the total conductance. Under a magnetic field parallel to the SmB₆ nanowire, the resistance suddenly increases at the turning point from up-sweep to down-sweep of the magnetic field. The magnetoresistance hysteresis loops are well consistent with the magnetocaloric effect. Our results suggest that the SmB₆ nanowires possess potential applications in the magnetic cooling technology.

Electrical transport and optical properties of Cd₃As₂ thin films

Yun-Kun Yang(杨运坤), Fa-Xian Xiu(修发贤), Feng-Qiu Wang(王枫秋), Jun Wang(王军), Yi Shi(施毅)

Chin. Phys. B, 2019, 28 (10): 107502. DOI: 10.1088/1674-1056/ab3a91

Abstract ( 839 )

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Cd₃As₂, as a three-dimensional (3D) topological Dirac semimetal, has attracted wide attention due to its unique physical properties originating from the 3D massless Dirac fermions. While many efforts have been devoted to the exploration of novel physical phenomena such as chiral anomaly and phase transitions by using bulk crystals, the development of high-quality and large-scale thin films becomes necessary for practical electronic and optical applications. Here, we report our recent progress in developing single-crystalline thin films with improved quality and their optical devices including Cd₃As₂-based heterojunctions and ultrafast optical switches. We find that a post-annealing process can significantly enhance the crystallinity of Cd₃As₂ in both intrinsic and Zn-doped thin films. With excellent characteristics of high mobility and linear band dispersion, Cd₃As₂ exhibits a good optical response in the visible-to-mid-infrared range due to an advantageous optical absorption, which is reminiscent of 3D graphene. It also behaves as an excellent saturable absorber in the mid-infrared regime. Through the delicate doping process in this material system, it may further open up the long-sought parameter space crucial for the development of compact and high-performance mid-infrared ultrafast sources.

Magnetic vortex gyration mediated by point-contact position

Hua-Nan Li(李化南), Zi-Wei Fan(笵紫薇), Jia-Xin Li(李佳欣), Yue Hu(胡月), Hui-Lian Liu(刘惠莲)

Chin. Phys. B, 2019, 28 (10): 107503. DOI: 10.1088/1674-1056/ab4277

Abstract ( 569 )

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Micromagnetic simulation is employed to study the gyration motion of magnetic vortices in distinct permalloy nanodisks driven by a spin-polarized current. The critical current density for magnetic vortex gyration, eigenfrequency, trajectory, velocity and the time for a magnetic vortex to obtain the steady gyration are analyzed. Simulation results reveal that the magnetic vortices in larger and thinner nanodisks can achieve a lower-frequency gyration at a lower current density in a shorter time. However, the magnetic vortices in thicker nanodisks need a higher current density and longer time to attain steady gyration but with a higher eigenfrequency. We also find that the point-contact position exerts different influences on these parameters in different nanodisks, which contributes to the control of the magnetic vortex gyration. The conclusions of this paper can serve as a theoretical basis for designing nano-oscillators and microwave frequency modulators.

Two-dimensional XSe₂ (X=Mn, V) based magnetic tunneling junctions with high Curie temperature

Longfei Pan(潘龙飞), Hongyu Wen(文宏玉), Le Huang(黄乐), Long Chen(陈龙), Hui-Xiong Deng(邓惠雄), Jian-Bai Xia(夏建白), Zhongming Wei(魏钟鸣)

Chin. Phys. B, 2019, 28 (10): 107504. DOI: 10.1088/1674-1056/ab3e45

Abstract ( 903 )

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Two-dimensional (2D) magnetic crystals have attracted great attention due to their emerging new physical phenomena. They provide ideal platforms to study the fundamental physics of magnetism in low dimensions. In this research, magnetic tunneling junctions (MTJs) based on XSe₂ (X=Mn, V) with room-temperature ferromagnetism were studied using first-principles calculations. A large tunneling magnetoresistance (TMR) of 725.07% was obtained in the MTJs based on monolayer MnSe₂. Several schemes were proposed to improve the TMR of these devices. Moreover, the results of our non-equilibrium transport calculations showed that the large TMR was maintained in these devices under a finite bias. The transmission spectrum was analyzed according to the orbital components and the electronic structure of the monolayer XSe₂ (X=Mn, V). The results in this paper demonstrated that the MTJs based on a 2D ferromagnet with room-temperature ferromagnetism exhibited reliable performance. Therefore, such devices show the possibility for potential applications in spintronics.

Optoelectronic properties analysis of silicon light-emitting diode monolithically integrated in standard CMOS IC

Yanxu Chen(陈彦旭), Dongliang Xu(许栋梁), Kaikai Xu(徐开凯), Ning Zhang(张宁), Siyang Liu(刘斯扬), Jianming Zhao(赵建明), Qian Luo(罗谦), Lukas W. Snyman, Jacobus W. Swart

Chin. Phys. B, 2019, 28 (10): 107801. DOI: 10.1088/1674-1056/ab3e44

Abstract ( 1071 )

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Si p⁺n junction diodes operating in the mode of avalanche breakdown are capable of emitting light in the visible range of 400-900 nm. In this study, to realize the switching speed in the GHz range, we present a transient model to shorten the carrier lifetime in the high electric field region by accumulating carriers in both p and n type regions. We also verify the optoelectronic characteristics by disclosing the related physical mechanisms behind the light emission phenomena. The emission of visible light by a monolithically integrated Si diode under the reverse bias is also discussed. The light is emitted as spatial sources by the defects located at the p-n junction of the reverse-biased diode. The influence of the defects on the electrical behavior is manifested as a current-dependent electroluminescence.

Cascaded plasmonic nanorod antenna for large broadband local electric field enhancement Suggested by editors

Dou Zhang(张豆), Zhong-Jian Yang(杨中见), Jun He(何军)

Chin. Phys. B, 2019, 28 (10): 107802. DOI: 10.1088/1674-1056/ab3f99

Abstract ( 657 )

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We propose a cascaded plasmonic nanorod antenna for large broadband electric near-field enhancement. The structure has one big gold nanorod on each side of a small two-wire antenna which consists of two small gold nanorods. For each small nanorod, the enhanced and broadened optical response can be obtained due to the efficient energy transfer from its adjacent big nanorod through strong plasmonic near-field coupling. Thus, the electric field intensity of the cascaded antenna is significantly larger and broader than that of the individual small two-wire antenna. The resonant position, field intensity enhancement, and spectral width of the cascaded antenna are highly tunable by varying the geometry of the system. The quantum efficiency of the cascaded antenna is also greatly enhanced compared with that of the small antenna. Our results are important for the applications in field-enhanced spectroscopy.

Photoluminescence properties of blue and green multiple InGaN/GaN quantum wells

Chang-Fu Li(李长富), Kai-Ju Shi(时凯居), Ming-Sheng Xu(徐明升), Xian-Gang Xu(徐现刚), Zi-Wu Ji(冀子武)

Chin. Phys. B, 2019, 28 (10): 107803. DOI: 10.1088/1674-1056/ab4046

Abstract ( 807 )

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The photoluminescence (PL) properties of blue multiple InGaN/GaN quantum well (BMQW) and green multiple InGaN/GaN quantum well (GMQW) formed on a single sapphire substrate are investigated. The results indicate that the peak energy of GMQW-related emission (P_G) exhibits more significant “S-shaped” dependence on temperature than that of BMQW-related emission (P_B), and the excitation power-dependent carrier-scattering effect is observed only in the P_G emission; the excitation power-dependent total blue-shift (narrowing) of peak position (line-width) for the P_G emission is more significant than that for the P_B emission; the GMQW shows a lower internal quantum efficiency than the BMQW. All of these results can be attributed to the fact that the GMQW has higher indium content than the BMQW due to its lower growth temperature and late growth, and the higher indium content in the GMQW induces a more significant compositional fluctuation, a stronger quantum confined Stark effect, and more non-radiative centers.

Effect of AlN coating on hydrogen permeability and surface structure of VT6 alloy by vacuum arc ion plating

Zi-Yi Ding(丁子祎)

Chin. Phys. B, 2019, 28 (10): 108101. DOI: 10.1088/1674-1056/ab3a8e

Abstract ( 725 )

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We study the absorption of hydrogen of metal by the permeability method. With the help of the gas reaction controller (GRC), the absorptive capacity of hydrogen, which is a function of time, temperature and pressure, can be recorded. The effect of the performance of the hydrogen permeability of AlN coating on the titanium alloy surface structure is studied. In the research, the AlN is selected to be added to the titanium alloy sample VT6, and the properties of the titanium alloy are investigated, and the hydrogen absorption rate of the coating is calculated by performing the hydrogen saturation of the test sample. The results show that under 600 ℃ the AlN film reduces the hydrogen absorption rate of titanium alloy and improves the surface properties of VT6 alloy.

Effect of sintering temperature on luminescence properties of borosilicate matrix blue-green emitting color conversion glass ceramics

Qiao-Yu Zheng(郑巧瑜), Yang Li(李杨), Wen-Juan Wu(吴文娟), Ming-Ming Shi(石明明), Bo-Bo Yang(杨波波), Jun Zou(邹军)

Chin. Phys. B, 2019, 28 (10): 108102. DOI: 10.1088/1674-1056/ab3f97

Abstract ( 750 )

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The color conversion glass ceramics which were made of borosilicate matrix co-doped (SrBaSm)Si₂O₂N₂:(Eu³⁺Ce³⁺) blue-green phosphors were prepared by two-step method in co-sintering. The change in luminescence properties and the drift of chromaticity coordinates (CIE) of the (SrBaSm)Si₂O₂N₂:(Eu³⁺Ce³⁺) blue-green phosphors and the color conversion glass ceramics were studied in the sintering temperature range from 600 ℃ to 800 ℃. The luminous intensity and internal quantum yield (QY) of the blue-green phosphors and glass ceramics decreased with the sintering temperature increasing. When the sintering temperature increased beyond 750 ℃, the phosphors and the color conversion glass ceramics almost had no peak in photoluminescence (PL) and excitation (PLE) spectra. The results showed that the blue-green phosphors had poor thermal stability at higher temperature. The lattice structure of the phosphors was destroyed by the glass matrix and the Ce³⁺ in the phosphors was oxidized to Ce⁴⁺, which further caused a decrease in luminescent properties of the color conversion glass ceramics.

Flexible rGO/Fe₃O₄ NPs/polyurethane film with excellent electromagnetic properties

Wei-Qi Yu(余维琪), Yi-Chen Qiu(邱怡宸), Hong-Jun Xiao(肖红君), Hai-Tao Yang(杨海涛), Ge-Ming Wang(王戈明)

Chin. Phys. B, 2019, 28 (10): 108103. DOI: 10.1088/1674-1056/ab4041

Abstract ( 599 )

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Large-area and flexible reduced graphene oxide (rGO)/Fe₃O₄ NPs/polyurethane (PU) composite films are fabricated by a facile solution-processable method. The monolayer assembly of Fe₃O₄ nanoparticles with a high particle-stacking density on the graphene oxide (GO) sheets is achieved by mixing two immiscible solutions of Fe₃O₄ nanoparticles in hexane and GO in dimethylformide (DMF) by a mild sonication. The x-ray diffraction and Raman spectrum confirm the reduced process of rGO by a simple thermal treatment. The permittivity value of the composite in a frequency range of 0.1 GHz-18 GHz increases with annealing temperature of GO increasing. For 5-wt% rGO/Fe₃O₄ NPs/PU, the maximum refection loss (RL) of over -35 dB appears at 4.5 GHz when the thickness of film increases to 5 mm. The rGO/Fe₃O₄ NPs/PU film, exhibiting good electromagnetic properties over GHz frequency range, could be a potential candidate as one of microwave absorption materials in flexible electronic devices.

Parameter identification and state-of-charge estimation approach for enhanced lithium-ion battery equivalent circuit model considering influence of ambient temperatures

Hui Pang(庞辉), Lian-Jing Mou(牟联晶), Long Guo(郭龙)

Chin. Phys. B, 2019, 28 (10): 108201. DOI: 10.1088/1674-1056/ab3af5

Abstract ( 724 )

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It is widely accepted that the variation of ambient temperature has great influence on the battery model parameters and state-of-charge (SOC) estimation, and the accurate SOC estimation is a significant issue for developing the battery management system in electric vehicles. To address this problem, in this paper we propose an enhanced equivalent circuit model (ECM) considering the influence of different ambient temperatures on the open-circuit voltage for a lithium-ion battery. Based on this model, the exponential-function fitting method is adopted to identify the battery parameters according to the test data collected from the experimental platform. And then, the extended Kalman filter (EKF) algorithm is employed to estimate the battery SOC of this battery ECM. The performance of the proposed ECM is verified by using the test profiles of hybrid pulse power characterization (HPPC) and the standard US06 driving cycles (US06) at various ambient temperatures, and by comparing with the common ECM with a second-order resistance capacitor. The simulation and experimental results show that the enhanced battery ECM can improve the battery SOC estimation accuracy under different operating conditions.

Analysis of non-uniform hetero-gate-dielectric dual-material control gate TFET for suppressing ambipolar nature and improving radio-frequency performance

Hui-Fang Xu(许会芳), Jian Cui(崔健), Wen Sun(孙雯), Xin-Feng Han(韩新风)

Chin. Phys. B, 2019, 28 (10): 108501. DOI: 10.1088/1674-1056/ab3a8b

Abstract ( 930 )

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A tunnel field-effect transistor (TFET) is proposed by combining various advantages together, such as non-uniform gate-oxide layer, hetero-gate-dielectric (HGD), and dual-material control-gate (DMCG) technology. The effects of the length of non-uniform gate-oxide layer and dual-material control-gate on the on-state, off-state, and ambipolar currents are investigated. In addition, radio-frequency performance is studied in terms of gain bandwidth product, cut-off frequency, transit time, and transconductance frequency product. Moreover, the length of non-uniform gate-oxide layer and dual-material control-gate are optimized to improve the on-off current ratio and radio-frequency performances as well as the suppression of ambipolar current. All results demonstrate that the proposed device not only suppresses ambipolar current but also improves radio-frequency performance compared with the conventional DMCG TFET, which makes the proposed device a better application prospect in the advanced integrated circuits.

Opto-electromechanically induced transparency in a hybrid opto-electromechanical system

Hui Liu(刘慧), Li-Guo Qin(秦立国), Li-Jun Tian(田立君), Hong-Yang Ma(马鸿洋)

Chin. Phys. B, 2019, 28 (10): 108502. DOI: 10.1088/1674-1056/ab3af2

Abstract ( 654 )

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We study opto-electromechanically induced transparency in a hybrid opto-electromechanical system made up of an optical cavity tunneling-coupled to an opto-mechanical cavity, which is capacitively coupled to a charged mechanical oscillator by a charged and moveable mechanical cavity mirror as an interface. By studying the effects of the different parameters on the output field, we propose a scheme to modulate the opto-electromechanically induced transparency (OEMIT). Our results show that the OEMIT with the transparency windows from single to double to triple can be modulated by changing the tunneling, opto-mechanical and electrical couplings. In addition, the explanation of the OEMIT with multi-windows is given by the energy level diagram based on quantum interference. Our investigation will provide an optimal platform to manipulate the transmission of optical field via microfabricated opto-electromechanical device.

Designing of spin filter devices based on zigzag zinc oxide nanoribbon modified by edge defect

Bao-Rui Huang(黄保瑞), Fu-Chun Zhang(张富春), Yan-Ning Yang(杨延宁), Zhi-Yong Zhang(张志勇), Wei-Guo Wang(王卫国)

Chin. Phys. B, 2019, 28 (10): 108503. DOI: 10.1088/1674-1056/ab3b50

Abstract ( 682 )

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The spin-dependent electronic transport properties of a zigzag zinc oxide (ZnO) nanoribbon are studied by using density functional theory with non-equilibrium Green's functions. We calculate the spin-polarized band structure, projected density of states, Bloch states, and transmission spectrum of the ZnO nanoribbon. It is determined that all Bloch states are located at the edge of the ZnO nanoribbon. The spin-up transmission eigenchannels are contributed from Zn 4s orbital, whereas the spin-down transmission eigenchannels are contributed from Zn 4s and O 2p orbitals. By analyzing the current-voltage curves for the opposite spins of the ZnO nanoribbon device, negative differential resistance (NDR) and spin filter effect are observed. Moreover, by constructing the ZnO nanoribbon modified by the Zn-edge defect, the spin-up current is severely suppressed because of the destruction of the spin-up transmission eigenchannels. However, the spin-down current is preserved, thus resulting in the perfect spin filter effect. Our results indicate that the ZnO nanoribbon modulated by the edge defect is a practical design for a spin filter.

Quantitative heterogeneity and subgroup classification based on motility of breast cancer cells Suggested by editors

Ling Xiong(熊玲), Yanping Liu(刘艳平), Ruchuan Liu(刘如川), Wei Yuan(袁伟), Gao Wang(王高), Yi He(何益), Jianwei Shuai(帅建伟), Yang Jiao(焦阳), Xixiang Zhang(张溪祥), Weijing Han(韩伟静), Junle Qu(屈军乐), Liyu Liu(刘雳宇)

Chin. Phys. B, 2019, 28 (10): 108701. DOI: 10.1088/1674-1056/ab3af4

Abstract ( 748 )

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Cancer cell motility and its heterogeneity play an important role in metastasis, which is responsible for death of 90% of cancer patients. Here, in combination with a microfluidic technique, single-cell tracking, and systematic motility analysis, we present a rapid and quantitative approach to judge the motility heterogeneity of breast cancer cells MDA-MB-231 and MCF-7 in a well-defined three-dimensional (3D) microenvironment with controllable conditions. Following this approach, identification of highly mobile active cells in a medium with epithelial growth factor will provide a practical tool for cell invasion and metastasis investigation of multiple cancer cell types, including primary cells. Further, this approach could potentially become a speedy (~hours) and efficient tool for basic and clinical diagnosis.

Theory and method of dual-energy x-ray grating phase-contrast imaging

Feng Rong(荣锋), Yan Gao(高艳), Cui-Juan Guo(郭翠娟), Wei Xu(徐微), Wei Xu(徐伟)

Chin. Phys. B, 2019, 28 (10): 108702. DOI: 10.1088/1674-1056/ab3f1d

Abstract ( 772 )

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The principle of dual-energy x-ray grating phase-contrast imaging (DEPCI) is clarified by using the theory of x-ray interference and Fresnel diffraction. A new method of retrieving phase from the two interferograms is proposed for DEPCI, and its feasibility is verified via simulation. Finally, the proposed method applied to DEPCI experiment demonstrates the effectiveness of the method. This paper lays the theoretical foundation for performance optimization of DEPCI and the further integration of DEPCI and computed tomography.

Benefit community promotes evolution of cooperation in prisoners' dilemma game

Jianwei Wang(王建伟), Jialu He(何佳陆), Fengyuan Yu(于逢源), Wei Chen(陈伟), Rong Wang(王蓉), Ke Yu(于可)

Chin. Phys. B, 2019, 28 (10): 108703. DOI: 10.1088/1674-1056/ab3f1e

Abstract ( 727 )

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Exploring the emergence and maintenance of cooperation in social dilemma is valuable and it arises considerable concerns of many researchers. In this paper, we propose a mechanism to promote cooperation, called benefit community, in which cooperators linking together form a common benefit community and all their payoffs obtained from game are divided coequally. The robustness of conclusions is tested for the PDG (prisoners' dilemma game) on square lattice and WS small world network. We find that cooperation can be promoted by this typical mechanism, especially, it can diffuse and prevail more easily and rapidly on the WS small world network than it on the square lattice, even if a big temptation to defect b. Our research provides a feasible direction to resolve the social dilemma.

Influence of matrigel on the shape and dynamics of cancer cells ^Hot! Suggested by editors

Teng Ye(叶腾), Feng Qiu(邱峰)

Chin. Phys. B, 2019, 28 (10): 108704. DOI: 10.1088/1674-1056/ab4275

Abstract ( 720 )

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The interaction between extracellular matrices and cancer cells plays an important role in regulating cancer cell behaviors. In this article, we use matrigel to mimic extracellular matrices and investigate experimentally how matrigel influences the shape and dynamics of breast cancer cells (MDA-MB-231-GFP cells). We find that matrigel facilitates cancer cells' migration and shape deformation. The influences of the matrigel concentration are also reported.

Theoretical analyses of stock correlations affected by subprime crisis and total assets: Network properties and corresponding physical mechanisms

Shi-Zhao Zhu(朱世钊), Yu-Qing Wang(王玉青), Bing-Hong Wang(汪秉宏)

Chin. Phys. B, 2019, 28 (10): 108901. DOI: 10.1088/1674-1056/ab3f22

Abstract ( 761 )

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In the field of statistical mechanics and system science, it is acknowledged that the financial crisis has a profound influence on stock market. However, the influence of total asset of enterprise on stock quote was not considered in the previous studies. In this work, a modified cross-correlation matrix that focuses on the influence of total asset on stock quote is introduced into the analysis of the stocks collected from Asian and American stock markets, which is different from the previous studies. The key results are obtained as follows. Firstly, stock is more greatly correlated with big asset than with small asset. Secondly, the higher the correlation coefficient among stocks, the larger the eigenvector is. Thirdly, in different periods, like the pre-subprime crisis period and the peak of subprime crisis period, Asian stock quotes show that the component of the third eigenvector of the cross-correlation matrix decreases with the asset of the enterprise decreasing. Fourthly, by simulating the threshold network, the small network constructed by 10 stocks with large assets can show the large network state constructed by 30 stocks. In this research we intend to fully explain the physical mechanism for understanding the historical correlation between stocks and provide risk control strategies in the future.

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