Apr 2019, Volume 14 Issue 2

Cover illustration

  • The optomechanical system (OMS) can be used to couple the mechanical, optical, and electric degree of freedom. Although single-cavity OMS is much interesting, the integrated optomechanical systems provide a platform for on-chip optical architectures with added versatility, which may be useful in optical information processing and quantum communication. Therefore, we need some OptoMechanics Technology Computer-Aided Design (OMTCAD) toolbox, which is the main function of OMPY ( [Detail] ...

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    Yu-Fei Wang, De-Liang Yao, Han-Qing Zheng

    Low-energy partial-wave πN scattering data is reexamined with the help of the production representation of partial-wave S matrix, where branch cuts and poles are thoroughly under consideration. The left-hand cut contribution to the phase shift is determined, with controlled systematic error estimates, by using the results of O(p3) chiral perturbative amplitudes obtained in the extended-onmass- shell scheme. In S11 and P11 channels, severe discrepancies are observed between the phase shift data and the sum of all known contributions. Statistically satisfactory fits to the data can only be achieved by adding extra poles in the two channels. We find that a S11 resonance pole locates at zr = (0.895±0.081)−(0.164±0.023)i GeV, on the complex s-plane. On the other hand, a P11 virtual pole, as an accompanying partner of the nucleon bound-state pole, locates at zv = (0.966±0.018) GeV, slightly above the nucleon pole on the real axis below threshold. Physical origin of the two newly established poles is explored to the best of our knowledge. It is emphasized that the O(p3) calculation greatly improves the fit quality comparing with the previous O(p2) one.

    Ping Yang, Li-Xin Zhang

    Step-edge-induced nucleation plays a key role in controlling the growth of novel nanostructures and low-dimensional materials. However, it is difficult to experimentally determine the step edge structures of complex metal oxides. In this work, we present a detailed theoretical study of the stability of stoichiometric steps on sapphire(0001). Based on first-principles calculations and excess charge computation by Finnis’ approach, a pair of non-polar step edges are determined to be the most stable. By studying the adsorption characteristics of ZnO and combining previous works, we successfully explained how growth temperature and deposition rate affect the in-plane orientation of ZnO grown on sapphire(0001). The knowledge on the step edge structures and nucleation patterns would benefit the study on step-edge-guided nanostructure growth.

    Guo-Feng Zhang, Yong-Gang Peng, Hai-Qing Xie, Bin Li, Zhi-Jie Li, Chang-Gang Yang, Wen-Li Guo, Cheng-Bing Qin, Rui-Yun Chen, Yan Gao, Yu-Jun Zheng, Lian-Tuan Xiao, Suo-Tang Jia

    Understanding of charge/energy exchange processes and interfacial interactions that occur between quantum dots (QDs) and the metal oxides is of critical importance to these QD-based optoelectronic devices. This work reports on linear dipole behavior of single near-infrared emitting CdSeTe/ZnS core/shell QDs which are encased in indium tin oxide (ITO) semiconductor nanoparticles films. A strong polarization anisotropy in photoluminescence emission is observed by defocused wide-field imaging and polarization measurement techniques, and the average polarization degree is up to 0.45. A possible mechanism for the observation is presented in which the electrons, locating at single QD surface from ITO by electron transfer due to the equilibration of the Fermi levels, result in a significant Stark distortion of the QD electron/hole wavefunctions. The Stark distortion results in the linear polarization property of the single QDs. The investigation of linear dipole behavior for single QDs encased in ITO films would be helpful for further improving QD-based device performance.

    Thomas Pope, Werner Hofer

    An extended electron model fully recovers many of the experimental results of quantum mechanics while it avoids many of the pitfalls and remains generally free of paradoxes. The formulation of the manybody electronic problem here resembles the Kohn–Sham formulation of standard density functional theory. However, rather than referring electronic properties to a large set of single electron orbitals, the extended electron model uses only mass density and field components, leading to a substantial increase in computational efficiency. To date, the Hohenberg–Kohn theorems have not been proved for a model of this type, nor has a universal energy functional been presented. In this paper, we address these problems and show that the Hohenberg–Kohn theorems do also hold for a density model of this type. We then present a proof-of-concept practical implementation of this method and show that it reproduces the accuracy of more widely used methods on a test-set of small atomic systems, thus paving the way for the development of fast, efficient and accurate codes on this basis.

    Ying-Xiang Zhen, Ming Yang, Rui-Ning Wang

    Thermoelectricity is a thermorelated property that is of great importance in single-molecule junctions. The electrical conductance (σ), electron-derived thermal conductance (κel) and Seebeck coefficient (S) of B80-based single-molecule junctions are investigated by using density functional theory in combination with non-equilibrium Green’s function. When the distance between the left/right electrodes is 11.4 Å, the relationship between σ and κel obeys the Wiedemann–Franz law very well because of the strong hybridization between B80 molecular orbitals and the surface states of Au electrodes. Furthermore, the calculated Lorenz number is close to the famous value in metal or degenerate semiconductors. In addition, S is only –19.09 μV/K at 300 K, thus leading to the smaller electron’s thermoelectric figure of merit (ZelT = S2σT/κel). Interestingly, the strain and chemical potential can modulate B80-based single-molecule junctions from n-type to p-type when the compressive strain reaches –0.6 Å or the chemical potential shifts to –0.16 eV. This might be attributed that S reflects the asymmetry in the electrical conductance with respect to the chemical potential and is proportional to the slopes of the transmission spectrum.

    Yin Zhong, Qin Wang, Yu Liu, Hai-Feng Song, Ke Liu, Hong-Gang Luo

    In recent years, interacting topological insulators have emerged as new frontiers in condensed matter physics, and the hotly studied topological Kondo insulator (TKI) is one of such prototypes. Although its zero-temperature ground-state has been widely investigated, the finite temperature physics on TKI is largely unknown. Here, we explore the finite temperature properties in a simplified model for TKI, namely the one-dimensional p-wave periodic Anderson model, with numerically exact determinant quantum Monte Carlo simulation. It is found that the topological Haldane phase established for groundstate is still stable against small thermal fluctuation and its characteristic edge magnetization develops at low temperature. Such facts emphasize the robustness of (symmetry-protected) topological order against temperature effect, which always exists at real physical world. Moreover, we use the saturated low-T spin structure factor and the 1T-law of susceptibility to detect the free edge spin moment, interestingly the low-temperature upturn behavior of the latter one is similar to experimental finding in SmB6 at T<50 K. It implies that similar physical mechanism may work both for idealized models and realistic correlated electron materials. We have also identified an emergent energy scale Tcr, which signals a crossover into interesting low-T regime and seems to be the expected Ruderman–Kittel–Kasuya–Yosida coupling. Finally, the collective Kondo screening effect has been examined and it is heavily reduced at boundary, which may give a fruitful playground for novel physics beyond the wellestablished Haldane phase and topological band insulators.

    T. Chatterji, S. Rols, U. D. Wdowik

    Results of inelastic neutron scattering experiments and ab initio molecular dynamics simulations for GeTe – the parent compound of phase-change materials are reported. The inelastic neutron spectra of GeTe powder samples have been determined in the temperature range extending from 300 to 700 K. The phonon peaks undergo thermal shifts resulting from anharmonic effects being weaker for acoustic than optic modes. A small concentration of free charge carries arising from the presence of Ge-vacancies was found not to affect significantly the neutron weighted phonon densities of states of GeTe. The spectral pattern changes qualitatively across the structural phase transition, but the local structure of GeTe remains hardly affected, as confirmed by the analysis of temperature dependence of the pairdistribution function obtained from ab initio molecular dynamics investigations. The present theoretical studies support in a wide extent our experimental observations and also those provided by local probe methods.

    Kaijian Huang, Yuanshuai Sun, Shanshan Sun, Xiao Zhang, Hechang Lei

    We have synthesized and investigated physical properties of two new quaternary compounds Gd2CoAl4T2 (T= Si, Ge) single crystals, which are isostructural to Tb2NiAl4Ge2 and Er2CoAl4Ge2. The most important structural feature of these materials is the anti-CaF2-type CoAl4T2 slabs. These materials show metallic behavior below 300 K and there is a long-range antiferromagnetic (AFM) transition appearing at 20 and 27 K for Gd2CoAl4Ge2 and Gd2CoAl4Si2, respectively. Resistivity and heat capacity measurements also confirm these bulk AFM transitions. Further analysis indicates that this long-range antiferromagnetism should result from the magnetic interaction between local moments of Gd3+ ions.

    Ke Wang, Tao Hou, Yafei Ren, Zhenhua Qiao

    We systematically studied the influence of magnetic field on zero-line modes (ZLMs) in graphene and demonstrated the physical origin of their enhanced robustness by employing nonequilibrium Green’s functions and the Landauer–Büttiker formula. We found that a perpendicular magnetic field can separate the wavefunctions of the counter-propagating kink states into opposite directions. Specifically, the separation vanishes at the charge neutrality point and increases as the Fermi level deviates from the charge neutrality point and can reach a magnitude comparable to the wavefunction spread at a moderate field strength. Such spatial separation of oppositely propagating ZLMs effectively suppresses backscattering and is more significant under zigzag boundary condition than under armchair boundary condition. Moreover, the presence of magnetic field enlarges the bulk gap and suppresses the bound states, thereby further reducing the scattering. These mechanisms effectively increase the mean free paths of the ZLMs to approximately 1 μm in the presence of a disorder.

    D. J. J. Farnell
    Ji-Liang Zhang, Guang-Cun Shan
    Huichao Wang, Yanzhao Liu, Haiwen Liu, Jian Wang
  • Research article
    Miao-Miao Zhao, Zhuo Qian, Bang-Pin Hou, Yong Liu, Yong-Hong Zhao

    The absorption of single-cavity and double-cavity optomechanical systems and periodic optomechanical lattices has previously been investigated extensively. In this paper, we present the absorption of a nonperiodic cavity chain, where the absorption value on the resonance point shows switchable dips or peaks, according to whether the optomechanical interaction is at an odd or even-numbered position in the chain. Meanwhile, the value of absorption due to the optomechanical interaction varies with the number of the bare cavities. The calculated results may have some novel applications, such as detecting the position of the movable mirror in a long cavity chain, which would be useful in quantum information processing based on optomechanical systems.

    Qian Dong, Ariadna J. Torres-Arenas, Guo-Hua Sun, Wen-Chao Qiang, Shi-Hai Dong

    In this work we analyze the characteristics of quantum entanglement of the Dirac field in noninertial reference frames in the context of a new type pseudo-pure state, which is composed of the Bell states. This will help us to understand the relationship between the relativity and quantum information theory. Some states will be changed from entangled states into separable ones around the critical value F = 1/4, but there is no such a critical value for the variable y related to acceleration a. We find that the negativity NABI (ρTAABI) increases with F but decreases with the variable y, while the variation of the negativity NBIBII(ρTAABI) is opposite to that of the negativity NABI (ρTAABI). We also study the von Neumann entropies S(ρABI) and S(ρBIBII). We find that the S(ρABI) increases with variable y but S(ρBIBII) is independent of it. However, both S(ρABI) and S(ρBIBII) first decreases with F and then increases with it. The concurrences C(ρABI) and C(ρBIBII) are also discussed. We find that the former decreases with y while the latter increases with y but both of them first increase with F and then decrease with it.

    You-Ji Fan, Zhen-Fei Zheng, Yu Zhang, Dao-Ming Lu, Chui-Ping Yang

    We propose a single-step implementation of a muti-target-qubit controlled phase gate with one catstate qubit (cqubit) simultaneously controlling n–1 target cqubits. The two logic states of a cqubit are represented by two orthogonal cat states of a single cavity mode. In this proposal, the gate is implemented with n microwave cavities coupled to a superconducting transmon qutrit. Because the qutrit remains in the ground state during the gate operation, decoherence caused due to the qutrit’s energy relaxation and dephasing is greatly suppressed. The gate implementation is quite simple because only a single-step operation is needed and neither classical pulse nor measurement is required. Numerical simulations demonstrate that high-fidelity realization of a controlled phase gate with one cqubit simultaneously controlling two target cqubits is feasible with present circuit QED technology. This proposal can be extended to a wide range of physical systems to realize the proposed gate, such as multiple microwave or optical cavities coupled to a natural or artificial three-level atom.

    Jiu Liu, Lan Zhou, Wei Zhong, Yu-Bo Sheng

    Logic qubit plays an important role in current quantum communication. In this paper, we propose an efficient entanglement concentration protocol (ECP) for a new kind of logic Bell state, where the logic qubit is the concatenated Greenber–Horne–Zeilinger (C-GHZ) state. Our ECP relies on the nondemolition polarization parity check (PPC) gates constructed with cross-Kerr nonlinearity, and can distill one pair of maximally entangled logic Bell state from two same pairs of less-entangled logic Bell states. Benefit from the nondemolition PPC gates, the concentrated maximally entangled logic Bell state can be remained for further application. Moreover, our ECP can be repeated to further concentrate the less-entangled logic Bell state. By repeating the ECP, the total success probability can be effectively increased. Based on above features, this ECP may be useful in future long-distance quantum communication.