spin-Peierls系统低能量激发谱的理论研究

用玻色化技术和量子自洽方法研究了spin-Peierls系统的低能量激发谱,计算了二聚化相的基态、单粒子激发态和双粒子束缚态的能量、阻挫对其低能量行为的影响及其各自的自旋-自旋关联函数.结果表明,随着阻挫的增大,spin-Peierls系统中的基态能会逐渐减小,单粒子激发态能隙和双粒子束缚态能隙却会增大.双粒子束缚态和基态的关联函数具有类似的短程关联,而单粒子激发态的关联函数具有长程关联.因此导出,单粒子激发态为自旋三重态,双粒子束缚态与基态类似为自旋单态,它存在于双粒子连续激发态的下边.该结果与Ain等 关键词： spin-Peierls系统 束缚态 关联函数     

双能态自旋-晶格声子耦合量子隧道系统的非经典能态和量子相干耗散

采用关联表象变分波函数方案, 介入三个非经典关联效应, 求解有限温度双能态自旋-晶格声子耦合量子隧道系统的非经典态, 着重研究化解由于粒子自旋-单声子相互作用引起的量子涨落导致双能态系统的退相干性量子耗散. 这三个非经典关联效应是: 1) 声子位移-粒子自旋 (σ_z)间强非绝热关联; 2) 声子压缩态效应及其伴随发生的单声子相干态-声子压缩态两过程相干效应; 3) 由关联表象导致的声子位移(U_D)与声子压缩(U_S)的表象关联非绝热修正. 结果表明: 由于引入粒子自旋-双声子相互作用, 大幅度地增强了声子场压缩态, 特别是更进一步极大幅度地增强了非经典压缩-相干态效应. 因此, 由粒子自旋-单声子相互作用产生的Debye-Walle相干弹性散射效应导致量子隧道项(-Δ₀σ_x)的强烈指数衰减及其伴随严重的量子相干损失的极大幅度的抑制, 并且自旋-晶格声子耦合量子隧道系统的非经典态能量大幅度降低. 关键词： 非经典能态 量子隧穿相干损失 自旋-双声子相互作用 压缩相干态效应     

基态简并和非简并高分子的光吸收和量子晶格涨落

通过一个简单的孤子反孤子对模型来模拟一维有机高分子中的量子晶格涨落,高分子中电子能隙内的光吸收是由孤子反孤子对激发的两个分别来源于导带的最低未占据和价带的最高占据离散能级之间的跃迁所产生的.这样的光吸收没有能隙.结果明显表明,基态非简并将抑制量子晶格涨落,从而减少电子能隙内的光吸收,使得基态非简并高分子的光吸收有一相对明显的吸收边,这与实验是一致的. 关键词：     

准周期对spin-Peierls模型中晶格畸变和能隙的影响

采用严格对角化方法和自洽方法，研究准周期相互作用对spin-Peierls模型中晶格畸变和能隙的影响.研究表明，引入准周期作用后，spin-Peierls模型中晶格畸变和能谱表现出准周期结构的自相似特性.另外，在弹性系数K取不同值时，随着准周期作用强度的增强，晶格畸变的平均值可能会增大、减小或先增大后减小，其对应的能隙也有相似的行为. 关键词： 准周期相互作用 spin-Peierls模型     

纵向横向晶体场中自旋为1的量子伊辛二聚化链的基态

研究纵向横向晶体场中自旋为1的量子伊辛自旋二聚化链的基态,发现模型存在一种隐藏的守恒量.采用Jordan-Wigner变换可将其严格映射到自旋为1/2的横场伊辛模型,得到基态能谱的严格解析表达式,给出最小费米激发能隙、最小空穴激发能隙、横向磁矩、横向统计磁化率、最近邻纵向自旋关联函数及基态相图.结果表明:系统的基态强烈依赖于系统的参数,当晶体场二聚化强度变化时系统会呈现一系列量子相变现象.     

含有Dzyaloshinskii-Moriya相互作用的自旋1键交替海森伯模型的量子相变和拓扑序标度

利用张量网络表示的无限矩阵乘积态算法研究了含有Dzyaloshinskii-Moriya (DM)相互作用的键交替海森伯模型的量子相变和临界标度行为.基于矩阵乘积态的基态波函数计算了系统的量子纠缠熵及非局域拓扑序.数据表明,随着键交替强度变化,系统从拓扑有序的Haldane相转变为局域有序的二聚化相.同时DM相互作用抑制了系统的二聚化,并最终打破系统的完全二聚化.另外,通过对相变点附近二聚化序的一阶导数和长程弦序的数值拟合,分别得到了此模型相变的特征临界指数a和b的值.结果表明,随着DM相互作用强度的增强, a逐渐减小,同时b逐渐增大. DM相互作用强度影响着此模型的临界行为.针对此模型的临界性质的研究,揭示了量子自旋相互作用的彼此竞争机制,对今后研究含有DM相互作用的自旋多体系统中拓扑量子相变临界行为提供一定的借鉴与参考.     

链间超交换相互作用下spin-Peierls梯子模型的基态行为

采用Lanczos数值计算方法研究了具有链间海森堡相互作用和链间Dzyaloshinskii-Moriya(DM)相互作用spin-Peierls梯子模型的基态行为.分析了链间海森堡相互作用和链间DM相互作用对系统二聚化的影响.结果发现，这两种链间作用都不利于系统的二聚化，会造成系统基态二聚化的减小，这说明随着系统维度的增大，系统的各向相互作用会限制系统的二聚化，甚至会破坏系统的二聚化相. 关键词： spin-Peierls模型 海森堡相互作用 DM相互作用 二聚化     

反铁磁链中Dzyaloshinskii–Moriya机制驱动矢量自旋手征束缚态的研究

Dzyaloshinskii-Moriya(DM)相互作用驱动的矢量自旋手征态,能和声子发生耦合,具备非常丰富的物理现象.本论文以一维反铁磁链中自旋手征-声子耦合模型为基础,研究不同声子环境下,耦合强度对自旋手征动力学演化过程的影响规律.结果表明,对自旋S=1/2的系统,在不同的声子浴(sub-Ohmic(0 1))中,均会在非相干到相干自旋涨落过程中产生无能隙一级相变,其来源是自旋手征束缚态的形成.相变发生的临界自旋-声子耦合强度正比于自旋涨落大小,反比于系统德拜频率.当自旋-声子耦合强度超过其临界值时,自旋手征束缚态的产生将极大地抑制非相干自旋涨落.     

Rashba自旋轨道耦合下square-octagon晶格的拓扑相变

本文研究了各向同性square-octagon晶格在内禀自旋轨道耦合、Rashba自旋轨道耦合和交换场作用下的拓扑相变,同时引入陈数和自旋陈数对系统进行拓扑分类.系统在自旋轨道耦合和交换场的影响下会出现许多拓扑非平庸态,包括时间反演对称破缺的量子自旋霍尔态和量子反常霍尔态.特别的是,在时间反演对称破缺的量子自旋霍尔效应中,无能隙螺旋边缘态依然能够完好存在.调节交换场或者填充因子的大小会导致系统发生从时间反演对称破缺的量子自旋霍尔态到自旋过滤的量子反常霍尔态的拓扑相变.边缘态能谱和自旋谱的性质与陈数和自旋陈数的拓扑刻画完全一致.这些研究成果为自旋量子操控提供了一个有趣的途径.     

原子质心运动与暗态原子

本文利用高阶量子绝热近似方法讨论了量子微腔驻波场中三能级暗态原子与光场相互作用中原子质心运动对电子,光场系统动力学的影响。文章分析了绝热近似成立的条件以及非绝热因子对动力学演化的影响。文中还计算了原子在腔中的非绝热跃迁几率及各部分能量分布的平均值。     

Dynamic spin Jahn-Teller effect in small magnetic clusters

We study the effect of spin-phonon coupling in small magnetic clusters, concentrating on a S=1/2 ring of 4 spins coupled antiferromagnetically. If the phonons are treated as classical variables, there is a critical value of the spin-phonon coupling above which a static distortion occurs. This is a good approximation if the zero point energy is small compared to the energy gain due to the distortion, which is true for large exchange interactions compared to the phonon energy (J≫ħω). In the opposite limit, one can integrate out the phonon degrees of freedom and get an effective spin Hamiltonian. Using exact diagonalizations to include the quantum nature of both spins and phonons, we obtain the spectrum in the whole range of parameters and explicit the crossover between the classical and quantum regimes. We then establish quantitatively the limits of validity of two widely used approaches (one in the quantum and one in the classical limits) and show that they are quite poor for small magnetic clusters. We also show that upon reducing ħω/J the first excitation of a 4-site cluster becomes a singlet, a result that could be relevant for Cu₂Te₂O₅Br₂.     

Coupled Magnetoelastic Waves and the Effective Spin-Phonon Coupling Parameter in High-Temperature Superconductors

The present paper is devoted to finding the spectrum of spin-phonon interaction and elucidating the mechanism of critical temperature increase in high-temperature superconductors. The spin-wave dynamic equations and the spectrum of coupled spin-phonon oscillations are obtained. The dependence of the spin wave spectrum on the spin-phonon coupling parameter is investigated. It is demonstrated that the superconducting transition temperature is directly proportional to the spin-phonon interaction force.     

Nuclear spin driven quantum relaxation in LiY0.998Ho0.002F4

Staircaselike hysteresis loops of the magnetization of a LiY0.998Ho0.002F4 single crystal are observed at subkelvin temperatures and low field sweep rates. This behavior results from quantum dynamics at avoided level crossings of the energy spectrum of single Ho3+ ions in the presence of hyperfine interactions. Enhanced quantum relaxation in constant transverse fields allows the study of the relative magnitude of tunnel splittings. At faster sweep rates, nonequilibrated spin-phonon and spin-spin transitions, mediated by weak dipolar interactions, lead to magnetization oscillations and additional steps.     

Nonadiabatic interaction of acoustic phonons with spins <Emphasis Type="Italic">S</Emphasis>=1/2 in the two-dimensional Heisenberg model

The ground state of a two-dimensional antiferromagnet having spins S=1/2 and interacting with acoustic phonons is investigated in the nonadiabatic approximation using the quantum-mechanical Monte Carlo method. The critical parameters of the spin-phonon coupling, corresponding to the formation of bound spin-phonon excitations, crystal symmetry lowering, and the emergence of a gap in the spin excitation spectrum, as well as the antiferromagnet-quantum spin liquid transition, are determined. The orthorhombicity parameter, the sublattice magnetization, the violation of the spherical symmetry of spin-spin correlation functions, and the magnetic moment in Gd₂CuO₄ and Eu₂CuO₄ are calculated.     

Two-spinon and orbital excitations of the spin-Peierls system TiOCl

We combine high-resolution resonant inelastic x-ray scattering with cluster calculations utilizing a recently derived effective magnetic scattering operator to analyze the polarization, excitation energy, and momentum-dependent excitation spectrum of the low-dimensional quantum magnet TiOCl in the range expected for orbital and magnetic excitations (0-2.5?eV). Ti 3d orbital excitations yield complete information on the temperature-dependent crystal-field splitting. In the spin-Peierls phase we observe a dispersive two-spinon excitation and estimate the inter- and intradimer magnetic exchange coupling from a comparison to cluster calculations.     

Quantum magnetostriction oscillations in a two-dimensional antiferromagnet with spin-phonon interaction in a magnetic field

The ground state of a two-dimensional antiferromagnet with S=1/2 interacting with acoustic phonons in a magnetic field was studied by the quantum Monte Carlo method in the nonadiabatic approximation. Oscillations of the amplitude of the root-mean-square displacement of ions and the average phonon occupation number in a magnetic field were found. Local maxima were revealed in the distribution functions of site magnetic moments and ion displacements. The saturation magnetization was calculated as a function of the spin-phonon coupling constant.     

Spin-flip relaxation in a two-electron quantum dot with spin-phonon coupling

We study the spin-flip process from the first excited state to the ground state due to the spin-phonon coupling in a two-electron quantum dot in the presence of a magnetic field. We give several possible relaxation channels before and after the crossing of the Zeeman sublevels. Our results show that the Coulomb interactions between the electrons of different channels play quite different roles and thus inducing different spin relaxation behaviors.     

Quantum Phase Transition in Two Species of Cold Bosonic Atoms

In this letter,the superfluid-Mott-insulator phase transition of two-species cold bosonic atoms in an optical lattices is studied.The Hamiltonian of this model is diagonalized by means of Bogliubov transformations and by the inversion symmetry of the optical lattice,the energy spectrum of this system is obtained.From the energy gap of the excitation spectrum,the quantum phase transition condition is obtained and it is determined by the competition between the interatomic repulsions and the tunnel coupling.It is found that there exists an ordinary fluid phase when taking the zero wave-vector limit.     

Spin-phonon coupling induced frustration in the exactly solved spin-1/2 Ising model on a decorated planar lattice

Spin-1/2 Ising model with a spin-phonon coupling on decorated planar lattices partially amenable to lattice vibrations is examined using the decoration-iteration transformation and harmonic approximation. It is shown that the magnetoelastic coupling gives rise to an effective antiferromagnetic next-nearest-neighbour interaction, which competes with the nearest-neighbour interaction and is responsible for a frustration of decorating spins. A strong enough spin-phonon coupling consequently leads to an appearance of striking partially ordered and partially disordered phase, where a perfect antiferromagnetic alignment of nodal spins is accompanied with a complete disorder of decorating spins. Thermal dependences of the specific heat are explored in detail.     

Density-matrix renormalization group study of the spin-Peierls instability in the antiferromagnetic Heisenberg ladder

The columnar dimerized antiferromagnetic S?=?1/2 spin ladder is numerically studied by the density-matrix renormalization-group (DMRG) method. The elastic lattice with spin-phonon coupling ?? and lattice elastic force k is introduced into the system. Thus the S?=?1?/?2 Heisenberg spin chain is unstable towards dimerization (the spin-Peierls transition). However, the dimerization should be suppressed if the rung coupling J _?? is sufficiently large, and a Columnar dimer to Rung singlet phase transition takes place. After a self-consistent calculation of the dimerization, we determine the quantum phase diagram by numerically computing the singlet-triplet gap, the dimerization amplitude, the order parameters, the rung spin correlation and quantum entropies. Our results show that the phase boundary between the Columnar dimer phase and Rung singlet phase is approximately of the form J _?? ~ \hbox{$(\frac{k}{\alpha^{2}})^{-\frac{5}{4}}$} ( k ?? 2 ) ? 5 4.