次近邻作用对随机量子Ising系统动力学性质的影响

利用递推关系方法在高温极限下研究了具有次近邻自旋耦合相互作用的一维随机量子Ising系统的动力学性质,求解了系统的自关联函数及谱密度.假设自旋耦合参量或横向磁场满足双高斯分布,研究发现当随机变量的标准偏差σJ(σB)较小时系统的动力学性质存在从集体模行为到中心峰值行为的交跨效应,当σJ(σB)较大时,交跨效应消失,系统只表现一种动力学行为.讨论了次近邻相互作用对系统动力学性质的影响,发现当KiJ2i(Ji和Ki分别为近邻和次近邻相互作用)时次近邻相互作用对系统动力学性质的影响不太明显,可以忽略;当Ki2Ji时,次近邻相互作用使得系统的中心峰值行为表现得更加明显,或使其集体模行为呈减弱的趋势.     

随机纵场对一维量子Ising模型动力学性质的影响

量子自旋系统的动力学性质是统计物理和凝聚态理论研究的热点问题.本文利用递推关系方法,通过计算系统的自旋关联函数及谱密度,研究了纵场对一维量子Ising模型动力学性质的影响.对于常数纵场的情况,发现当自旋耦合相互作用较弱时纵场能够引起不同动力学行为之间的交跨效应,且驱使系统出现了多种振动模式,但较强的自旋耦合相互作用会掩盖纵场的影响.对于随机纵场的情况,分别讨论了双模型随机纵场和高斯型随机纵场的影响,发现不同随机类型下的动力学结果有很大的差别,且高度依赖于随机分布中参数的选取,如双模分布的均值,高斯分布的均值和偏差等.尽管常数纵场和随机纵场下的动力学结果不同,但可以得到一个共同的结论:当纵场所占比重较大时,系统的中心峰值行为将得到保持.且此结论可以推广:系统哈密顿中非对易项的出现有利于中心峰值行为的保持.     

三模型随机场对一维量子Ising模型动力学性质的调控

量子自旋系统在外磁场下的动力学性质一直是凝聚态理论和统计物理研究的热点.本文利用递推关系方法,通过计算系统的自旋关联函数及其对应的谱密度,研究了三模型随机外场对一维量子Ising模型动力学性质的调控效应.在三模型随机横场下,利用r分支引入了非磁性杂质,研究表明:非磁性杂质使得系统的低频响应得到保持,中心峰值行为更加明显;非磁性杂质与横场之间的竞争能激发出新的频率响应,呈现多峰行为;但较多的非磁性杂质最终会限制系统对横场的响应.此外,研究还发现随机横场的三模分布参数满足qB_q=pB_p这一条件,是使中心峰值行为得到保持的有利条件.在三模型随机纵场下, r分支仅仅起到调节纵场强度的作用,且r分支所占比重的增大不利于低频响应,与三模型随机横场下r分支的调控作用是相反的.     

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.     

非线性相互作用的自旋-轨道耦合玻色-爱因斯坦凝聚体的淬火动力学

研究了在二维自旋-轨道耦合的相互作用超冷玻色气体中存在一维光晶格时,超流条纹相到超固相的非平衡动力学.通过研究这一动力学过程中的缺陷(位相空间中的涡旋)及波函数的变化行为,利用涡旋数及波函数的交叠等描述方法,确定了考虑光晶格深度随时间线性变化的量子淬火动力学过程的转变时间.发现在转变时间之前,体系对于淬火过程没有响应.当演化时间超过转变时间后,系统开始迅速响应,涡旋数及体系的波函数开始迅速变化.当演化时间足够长时,系统将达到稳态.另外还发现,在上述动力学过程中,由于体系中自旋-轨道耦合的存在,系统在空间中的密度分布与自旋在空间中的结构始终相伴生,即具有拓扑结构的磁斯格明子(反斯格明子)的中心位置始终与体系密度分布的极小值位置相对应.     

磁性硅烯超晶格中电场调制的谷极化和自旋极化

研究了基于硅烯的静电势超晶格、铁磁超晶格、反铁磁超晶格中谷极化、自旋极化以及赝自旋极化的输运性质,分析了铁磁交换场、反铁磁交换场以及化学势对输运性质的影响,讨论了电场对谷极化、自旋极化以及赝自旋极化的调控作用.结果表明:当3种超晶格的晶格数达到10以上时,在硅烯超晶格中很容易实现100%的谷极化、自旋极化和赝自旋极化,而且通过调节超晶格上的外加电场可以使极化方向发生翻转,从而在硅烯超晶格中实现外电场对谷自由度、自旋自由度以及赝自旋自由度的操控.     

随机外磁场作用下Ising自旋体系的随机共振

通过采用数字仿真手段，研究了经平均场处理的Ising自旋体系在弱确定性周期外场和随机外场（噪声场）混合驱动下的动态响应行为.着重考察了不同强度混合驱动外场作用下，Is ing自旋体系的非平衡动态转变所表现出区别于单纯确定性周期场作用下的新特征——随机 共振.选定体系非平衡动态转变的动态序参量Q为表征参量，系统模拟计算了混合驱动外场在多种参数组合下体系的动态响应特征，高场低温下的非连续动态转变和低场高温下的连续动态转变.模拟计算表明在适当混合驱动外场的作用下，Ising自旋体系具有随机共振现象，并诱发形成非 关键词： Ising自旋体系 随机共振 动态相变 对称性     

带电粒子系统在双外场中的高温弱简并效应

用Thomas-Fermi半经典统计理论和方法,研究处于匀强电场加谐振场约束的带电粒子系统的热力学性质.首先导出外场约束下带电粒子系统的量子状态数密度,进而给出高温弱简并情况下带电粒子系统的化学势、内能和热容量的解析式,并分析电场和谐振场对系统化学势、内能和热容量的具体影响. 研究结果表明,带电粒子系统在双外场约束下具有高温弱简并效应. 关键词： 带电粒子 匀强电场 谐振场 高温弱简并效应     

钻石分形晶格上Ising模型的配分函数与关联函数

用参数变换方法,研究了钻石分形晶格上Ising自旋模型.精确求解了零场和非零场中系统的配分函数、自由能和关联函数,尤其推得了关联函数方程及其渐近行为的解析形式.结果表明，其关联行为与平移对称晶格上的Ising自旋系统类似. 关键词： Ising模型 外磁场 钻石分形晶格 关联函数     

偶极-偶极相互作用下双势阱中旋量玻色-爱因斯坦凝聚磁化率的非线性动力学性质

利用平均场理论和单空间模近似，研究了偶极-偶极相互作用下双势阱中总自旋F=1的旋量玻色-爱因斯坦凝聚磁化率的非线性动力学性质.在给定的初态条件下，研究结果表明：当λ_A+2λ_d=0时，凝聚体只表现为磁化振荡行为；当λ_A+2λ_d≠0时，凝聚体既存在磁化振荡行为，又存在磁自陷俘现象. 关键词： 玻色-爱因斯坦凝聚 自旋 磁化率     

Dynamics of one-dimensional random quantum XY system with Dzyaloshinskii–Moriya interaction

In this paper, the effects of random variables on the dynamics of the s=1/2 XY model with the Dzyaloshinskii-Moriya interaction are studied. By means of the recurrence relation method in the high-temperature limit, we calculate the spin autocorrelation functions as well as the corresponding spectral densities for the cases that the exchange couplings between spins or external magnetic fields satisfy the double-Gaussian distribution. It is found that when the standard deviation of random exchange coupling δ_j (or the standard deviation of random external field δ_B) is small, the dynamics of the system undergoes a crossover from a collective-mode behavior to a central-peak one. However, when δ_J (or δ_B) is large, the crossover vanishes, and the system shows a central-peak behavior or the most disordered one. We also analyze the cases in which the exchange couplings or the external fields satisfy the bimodal and the Gaussian distributions. Our results show that for all the cases considered, the dynamics of the above system is similar to that of the one-dimensional random XY model.     

Temperature dependence of static and dynamic properties of an anisotropic ferrimagnet

The temperature dependences of the magnetizations of sublattices and the spectra of elementary excitations of an anisotropic ferrimagnet have been investigated. It has been shown that, in the anisotropic ferrimagnet, even a weak single-ion anisotropy induces the quantum spin reduction effect and, as a consequence, changes the behavior of the magnetizations of the sublattices. This gives rise to a new branch of magnetic excitations with the purely longitudinal dynamics of the average value of the spin due to the quantum spin reduction effect.     

Thermodynamic Properties of Random Transverse Field Mixed Spin System in the Presence of Single-Ion Anisotropy

We study the thermodynamic properties of random transverse field mixed spin system in the presence of single-ion anisotropy on a square lattice. By making use of the effective field theory and a cutting approximation, the detailed phase diagrams are described and some interesting results are found under trimodal random transverse field distribution. A small single-ion anisotropy can magnify magnetic ordering region at low temperatures and existence of a large transverse field can assist the occurrence of reentrant phenomena. With increasing disorder, second-order phase transitions are shown to change into first-order phase transitions. The trajectory of the tricritical point in the phase space as a function of disorder is presented. These indicate a strong correlation with the corresponding to trimodal transverse field distribution.     

Specific features of spin ordering in an Ising antiferromagnet with single-ion easy-plane anisotropy

A thermodynamic analysis of spin ordering in an Ising antiferromagnet with single-ion easy-plane anisotropy is carried out. The solutions of the equations of state are analyzed, and the phase diagram is constructed. It is shown that an increase in the single-ion anisotropy constant as a result of increasing competition between single-ion anisotropy and exchange interaction brings about a crossover from the continuous phase transition to the abrupt phase transition. This is initiated by a change in the ion state populations due to single-ion anisotropy and by the freezing of ionic states with a nonzero spin projection. An increase in the single-ion anisotropy also leads to a crossover of the phase transformation between the paramagnetic and antiferromagnetic states from the order-disorder to order-order type.     

Intrinsic spin fluctuations reveal the dynamical response function of holes coupled to nuclear spin baths in (In,Ga)As quantum dots

The problem of how single central spins interact with a nuclear spin bath is essential for understanding decoherence and relaxation in many quantum systems, yet is highly nontrivial owing to the many-body couplings involved. Different models yield widely varying time scales and dynamical responses (exponential, power-law, gaussian, etc.). Here we detect the small random fluctuations of central spins in thermal equilibrium [holes in singly charged (In,Ga)As quantum dots] to reveal the time scales and functional form of bath-induced spin relaxation. This spin noise indicates long (400 ns) spin correlation times at a zero magnetic field that increase to ～5 μs as dominant hole-nuclear relaxation channels are suppressed with small (100 G) applied fields. Concomitantly, the noise line shape evolves from Lorentzian to power law, indicating a crossover from exponential to slow [～1/log(t)] dynamics.     

Random field effects on the anisotropic quantum Heisenberg model with Gaussian random magnetic field distribution

The effect of a zero-centered Gaussian random magnetic field distribution on the phase transition properties of the anisotropic quantum Heisenberg model has been investigated on a honeycomb lattice within the framework of effective field theory (EFT) for a two-spin cluster (which is abbreviated as EFT-2). Particular attention has been devoted to investigation of the effect of the anisotropy in the exchange interaction on a system with Gaussian random magnetic field distribution. The variation of the critical temperature with the randomness parameter (i.e., the width of the distribution) has been obtained for several anisotropy parameters. Critical Gaussian distribution width values, which make the critical temperature zero, have been obtained. Moreover, it has been concluded that all critical temperatures are of second order, and that reentrant behavior does not exist in the phase diagrams.     

Correlated projection superoperators in relaxation and transport investigations

The time-convolutionless (TCL) projection operator technique allows a systematic analysis of the dynamical properties of open quantum systems. Unfortunately, using the standard projector to investigate the decay to equilibrium fails to describe the dynamics of a small system coupled to a finite environment correctly. However, a recently introduced correlated projection operator leads to the appropriate dynamical behavior and shows a reasonable good agreement with the exact Schr?dinger dynamics. Furthermore, a similar projector can be used to describe the transport through a quantum system. Using the technique in Fourier space allows for a detailed analysis of transport in the context of its length scale dependence as well. Finally, the method is also applicable to realistic systems, e.g., spin models.     

Quantum analogue of the spin-flop transition for a spin pair

Quantum (step) magnetization curves have been analyzed for a spin pair with antiferromagnetic interaction in the presence of a magnetic field that is parallel to the easy magnetization axis. Both semiclassical and numerical analyses have been performed for a wide range of the anisotropy parameter and spins up to S ? 100. In the dependence of the anisotropy character (single-ion or exchange), a spin jump larger than unity can appear in the magnetization curve or jumps can be concentrated in a narrow range of the field. In addition, regions of the problem parameters have been revealed where behavior is semiclassical for low spins on the order of S = 5 and where behavior is substantially quantum even for S → ∞.     

Entanglement and quantum phase transition in a mixed-spin Heisenberg chain with single-ion anisotropy

We study the ground-state and thermal entanglement in the mixed-spin (S,s)=(1,1/2) Heisenberg chain with single-ion anisotropy D using exact diagonalization of small clusters. In this system, a quantum phase transition is revealed to occur at the value D=0, which is the bifurcation point for the global ground state; that is, when the single-ion anisotropy energy is positive, the ground state is unique, whereas when it is negative, the ground state becomes doubly degenerate and the system has the ferrimagnetic long-range order. Using the negativity as a measure of entanglement, we find that a pronounced dip in this quantity, taking place just at the bifurcation point, serves to signal the quantum phase transition. Moreover, we show that the single-ion anisotropy helps to improve the characteristic temperatures above which the quantum behavior disappears.     

Quantum phase transition in the two-dimensional XY model with single-ion anisotropy

In this paper we study the quantum phase transition and low temperature behavior in a square lattice quantum two-dimensional XY model with single-ion anisotropy and spin S=1. Starting with the Villain representation, a Landau-Ginzburg expression is written. The large D phase is studied using the bond operator formalism.