二维有机拓扑绝缘体的研究进展

新材料的发现促进了科学与技术的进步.拓扑绝缘体是近期材料领域新的研究热点,相关研究的进一步深入,不仅加深了人们对材料物理性质的理解,也为其在自旋电子学和量子计算机等领域的潜在应用提供了有价值的参考.近年来,理论工作预测了一系列由金属和有机物构筑的二维有机拓扑绝缘体,本文主要介绍六角对称的金属有机晶格与Kagome金属有机晶格两类典型的二维有机拓扑绝缘体的研究进展,其中重点介绍了理论预测的氰基配位二维本征有机拓扑绝缘体.除了理论计算方面的工作,还简要介绍了关于二维有机拓扑绝缘体材料合成方面的实验工作.二维有机拓扑绝缘体的理论与实验研究不仅拓展了拓扑绝缘体的研究体系,还为寻找新的拓扑绝缘体材料提供了思路.     

On the photon-drag effect of photocurrent of surface states of topological insulators

The photocurrent of surface states of topological insulator due to photon-drag effect is computed, being based on pure Dirac model of surface states. The scattering by disorder is taken into account to provide a relaxation mechanism for the photocurrent. The Keldysh–Schwinger formalism has been employed for the systematic calculation of photocurrent. The helicity dependent photocurrent of sizable magnitude transverse to the in-plane photon momentum is found, which is consistent with experimental data. Other helicity independent photocurrents with various polarization states are also calculated.     

Non‐magnetic defects in the bulk of two‐dimensional topological insulators

We found that non‐magnetic defects in two‐dimensional topological insulators induce bound states of two kinds for each spin orientation: electron‐ and hole‐like states. Depending on the sign of the defect potential these states can be also of two kinds with different distribution of the electron density. The density has a maximum or minimum in the center. A surprising effect caused by the topological order is a singular dependence of the bound‐state energy on the defect potential.

     

Effective spin-2 quasi-particles at linear dispersive five-fold degenerate points with tunable topological Chern numbers

In this work, which is based on spin-2 vectors and traceless spin-2 tensors, an effective Hamiltonian is constructed with a linearly dispersive five-fold degenerate point with spin-2 vector-momentum couplings. For the model without spin-2 vector-tensor coupling, the topological Chern numbers of five bands are calculated as 4, 2, 0, ?2, ?4. After including spin-2 vector-tensor coupling, separate topological Chern numbers are obtained for different couplings. A cubic lattice of atoms with five internal states is designed to realize two five-fold degenerate points. The Chern numbers of the bands can be changed by tuning the coupling coefficient. In this work we propose a theoretical design to obtain spin-2 quasi-particles.     

Observation of topological valley transport of elastic waves in bilayer phononic crystal slabs

In this Letter, we propose a unique bilayer design of phononic crystal slabs that are constructed by two layers of snowflake phononic crystal plates connected by a honeycomb array of cylinders. By tuning orientations of snowflake-shaped holes in both layers, we achieve two kinds of valley-projected topological elastic insulators distinguished by conventional and layer-polarized topological valley Hall phases. Then, between different conventional and layer-polarized topological valley Hall phases, two kinds of edge modes, layer-mixed and layer-polarized edge modes, are obtained and explored. In finite-size samples, the interesting topological transport properties, which the elastic wave can propagate alternatively between both layers and only in a single layer, are realized by exciting layer-mixed and layer-polarized edge states. In addition, we design an interlayer converter to realize conversion of the elastic wave propagation between both layers. Our research promotes the development of topological elastic insulators and provides a route for various practical applications.     

High winding number of topological phase in non-unitary periodic quantum walk

Topological phases and their associated multiple edge states are studied by constructing a one-dimensional non-unitary multi-period quantum walk with parity-time symmetry. It is shown that large topological numbers can be obtained when choosing an appropriate time frame. The maximum value of the winding number can reach the number of periods in the one-step evolution operator. The validity of the bulk–edge correspondence is confirmed, while for an odd-period quantum walk and an even-period quantum walk, they have different configurations of the 0-energy edge state and π-energy edge state. On the boundary, two kinds of edge states always coexist in equal amount for the odd-period quantum walk, however three cases including equal amount, unequal amount or even only one type may occur for the even-period quantum walk.     

Change-over switch for quantum states transfer with topological channels in a circuit-QED lattice

We propose schemes to realize robust quantum states transfer between distant resonators using the topological edge states of a one-dimensional circuit quantum electrodynamics(QED)lattice.Analyses show that the distribution of edge states can be regulated accordingly with the on-site defects added on the resonators.And we can achieve different types of quantum state transfer without adjusting the number of lattices.Numerical simulations demonstrate that the on-site defects can be used as a change-over switch for high-fidelity single-qubit and two-qubit quantum states transfer.This work provides a viable prospect for flexible quantum state transfer in solid-state topological quantum system.     

Optical properties in a two-dimensional quantum ring: Confinement potential and Aharonov-Bohm effect

Optical properties of a two-dimensional quantum ring with pseudopotential in the presence of an external magnetic field and magnetic flux have been theoretically investigated. Our results show that both of the pseudopotential and magnetic field can affect the third non-linear susceptibility and oscillator strength. In addition, we found that the oscillator strength and the absolute value of the resonant peak of the linear, non-linear and total absorption coefficient demonstrates the Aharonov-Bohm oscillation with magnetic flux, moreover, changes in confinement potential can influence the Aharonov-Bohm oscillation in peak while the resonant peak value of the linear, non-linear and total refractive index changes decreases as magnetic flux increases.     

Unambiguous determining the Curie point in perovskite manganite with second-order phase transition by scaling method

The accurate determination of the Curie temperature (T_C) is particularly important in describing the magnetic behavior close to the paramagnetic-ferromagnetic (PM-FM) phase transition. In this paper, we studied the magnetic phase transition and accurately predicted the Curie point of perovskite manganite La_0.825Sr_0.175MnO₃. We find the compound shows a second-order PM-FM transition and has a large magnetic entropy change (MEC) in vicinity of phase transition region. Based on the scaling law and the correlation between magnetic field and MEC, the precise and magnetic-independent Curie temperature was determined to be 281.7 K, obviously lower than 285.4 K decided from the magnetization versus temperature. The reliability of new Curie temperature can be well confirmed by the modified Arrott plot together with the critical exponents. Our results not only open up a new pathway for precise definition of Curie point but also facilitate the efficient exploitation of spontaneous magnetic bubbles in perovskite manganite.     

Zone folding induced tunable topological interface states in one-dimensional phononic crystal plates

In previous literature, the realization of topological interface state in one-dimensional periodic system is strongly relied on the tedious parameter adjustment to search for the Dirac cone. In this paper, based on a strategy of zone folding, multiple topological interface modes for the shear horizontal guided waves in one dimensional phononic crystal plate are investigated by using finite element method and eigenmode matching theory, in which the Dirac points are formed by simply making the unit cell double. Significantly, by simply contracting or expanding the stubs can bring the topological phase transition. Furthermore, the topological phase transition is further achieved by varying the height of the stubs. The proposed designs will be more convenient to be applied in real engineering.     

Dynamical precession of spin in the two-dimensional spin-orbit coupled systems

We investigate the spin dynamics in the two-dimensional spin-orbit coupled system subject to an in-plane (x-y plane) constant electric field, which is assumed to be turned on at the moment $t=0$. The equation of spin precession in linear response to the switch-on of the electric field is derived in terms of Heisenberg's equation by the perturbation method up to the first order of the electric field. The dissipative effect, which is responsible for bringing the dynamical response to an asymptotic result, is phenomenologically implemented à la the Landau-Lifshitz-Gilbert equation by introducing damping terms upon the equation of spin dynamics. Mediated by the dissipative effect, the resulting spin dynamics asymptotes to a stationary state, where the spin and the momentum-dependent effective magnetic field are aligned again and have nonzero components in the out-of-plane (z) direction. In the linear response regime, the asymptotic response obtained by the dynamical treatment is in full agreement with the stationary response as calculated in the Kubo formula, which is a time-independent approach treating the applied electric field as completely time-independent. Our method provides a new perspective on the connection between the dynamical and stationary responses.     

Energy band modulation of GaAs/Al0.26Ga0.74As quantum well in 3D self-assembled nanomembranes

In this study, we investigate the modulation of energy band in 3D self-assembled nanomembranes containing GaAs/Al_0.26Ga_0.74As quantum wells (QWs). Photoluminescence (PL) characterizations demonstrate that the self-assembled structures have different optical transition properties and the modulation of the energy band is thus realized. Detailed spectral analyses disclose that the small strain change in structures with different curvatures cannot cause remarkable change in energy bands in Al_0.26Ga_0.74As layer. On the other hand, the optical transitions of GaAs QW layer is influenced by the strain evolution in term of light emission intensity. We also find the first order Stark effect in rolled-up nanomembrane with diameter of 150 μm, which is closely connected with the coupling effect between the deformation potential and the piezoelectric potential. Our work may pave a way for the fabrication of high performance rolled-QW infrared photo-detectors.     

Discrete and generalized phase space techniques in critical quantum spin chains

We apply the Wigner function formalism from quantum optics via two approaches, Wootters' discrete Wigner function and the generalized Wigner function, to detect quantum phase transitions in critical spin-$\frac{1}{2}$ systems. We develop a general formula relating the phase space techniques and the thermodynamical quantities of spin models, which we apply to single, bipartite and multi-partite systems governed by the XY and the XXZ models. Our approach allows us to introduce a novel way to represent, detect, and distinguish first-, second- and infinite-order quantum phase transitions. Furthermore, we show that the factorization phenomenon of the XY model is only directly detectable by quantities based on the square root of the bipartite reduced density matrix. We establish that phase space techniques provide a simple, experimentally promising tool in the study of many-body systems and we discuss their relation with measures of quantum correlations and quantum coherence.     

Magnetic response of an electron gas in a quantum ring of non-zero width

An investigation of the magnetic moment of an electron gas in a quantum ring of non-zero width is made. Analytic expressions are obtained for the magnetic moment. For the magnetic moment of the system, the dependence on temperature and parameters of the ring are found and investigated in detail. De Haas–van Alphen and Aharonov–Bohm oscillations are investigated.     

原子核物理中的协变密度泛函理论

文章介绍了原子核协变密度泛函理论的历史发展、理论框架、对原子核基态和激发态的描述以及在一些交叉学科领域的应用。首先,通过回顾原子核物理研究中的几个重要里程碑并结合二十一世纪原子核物理面临的机遇和挑战,对当前核物理的研究热点和重要课题进行了介绍。随后系统介绍了原子核协变密度泛函理论,内容包括协变密度泛函理论的历史发展、一般理论公式、介子交换模型、点耦合模型、交换项、张量相互作用、物理观测量的计算公式等。协变密度泛函理论的应用包括原子核基态性质和激发态性质的描述以及在核天体物理与标准模型检验中的应用。其中,基态性质包括原子核结合能、半径、单粒子能级、共振态、磁矩、晕现象等。激发态性质包括原子核磁转动、低激发态性质、集体转动、量子相变、集体振动等。在核天体物理与标准模型检验的应用中,主要以核纪年法测算宇宙年龄和Cabibbo-Kobayashi-Maskawa矩阵的幺正性检验等为例,介绍协变密度泛函理论在交叉学科领域的应用。     

Surface chemistry and catalysis of oxide model catalysts from single crystals to nanocrystals

Fundamental understandings of surface chemistry and catalysis of solid catalysts are of great importance for the developments of efficient catalysts and corresponding catalytic processes, but have been remaining as a challenge due to the complex nature of heterogeneous catalysis. Model catalysts approach based on catalytic materials with uniform and well-defined surface structures is an effective strategy. Single crystals-based model catalysts have been successfully used for surface chemistry studies of solid catalysts, but encounter the so-called “materials gap” and “pressure gap” when applied for catalysis studies of solid catalysts. Recently catalytic nanocrystals with uniform and well-defined surface structures have emerged as a novel type of model catalysts whose surface chemistry and catalysis can be studied under the same operational reaction condition as working powder catalysts, and they are recognized as a novel type of model catalysts that can bridge the “materials gap” and “pressure gap” between single crystals-based model catalysts and powder catalysts. Herein we review recent progress of surface chemistry and catalysis of important oxide catalysts including CeO₂, TiO₂ and Cu₂O acquired by model catalysts from single crystals to nanocrystals with an aim at summarizing the commonalities and discussing the differences among model catalysts with complexities at different levels. Firstly, the complex nature of surface chemistry and catalysis of solid catalysts is briefly introduced. In the following sections, the model catalysts approach is described and surface chemistry and catalysis of CeO₂, TiO₂ and Cu₂O single crystal and nanocrystal model catalysts are reviewed. Finally, concluding remarks and future prospects are given on a comprehensive approach of model catalysts from single crystals to nanocrystals for the investigations of surface chemistry and catalysis of powder catalysts approaching the working conditions as closely as possible.