横场中具有周期性各向异性的一维XY模型的量子相变

通过解析和数值计算的方法研究了横场中具有周期性各向异性的一维XY自旋模型的量子相变和量子纠缠.主要讨论了周期为二的情况,即各向异性参数交替地取比值为α的两个值.结果表明,与横场中均匀XY模型相比,α=-1所对应的模型在参数空间的相图存在着明显的不同.原来的Ising相变仍然存在,没有了沿x和y方向的各向异性铁磁(FM_x,FM_y)相,即各向异性相变消失,出现了一个新的相,并且该相内沿x和y方向的长程关联函数相等且大于零,我们称新相为各向同性铁磁(FM_(xx))相.这是由于系统新的对称性所导致的.解析结果还说明系统在FM_(xx)相中的单粒子能谱有两个零点,是一个无能隙的相.最后,利用冯·诺依曼熵数值地研究了系统在新相内各点的量子纠缠,发现该相内每一点的冯·诺依曼熵的标度行为与均匀XY模型在各向异性相变处的相似,即S_L～1/3㏒_2L+Const.     

面心立方(001)方向AB合金薄膜表面层的有序无序相变

采用平均场近似方法对两组元面心立方合金薄膜的有序无序相转变过程进行模拟计算，结果表明，合金薄膜的有序无序相变受薄膜层数奇偶性的影响.薄膜层数奇偶性不同，会导致薄膜具有不同的相结构和热力学性质.在弱表面偏析作用下，对于偶数层薄膜，由于薄膜边界对称性破缺，对应体组分x=0.5的化学势区间，偶数层薄膜有序无序相变过程中出现了中间温度相和浸润现象.而奇数层薄膜的有序无序相变类似体材料的相变.在强表面偏析作用下，由于受表面偏析作用和有限尺寸效应影响，对应体组分x=0.5的化学势区间，奇数层薄膜中出现AB(AB)关键词： 合金薄膜 有序无序相变 浸润现象 准热力学相变     

具有Dzyaloshinskii-Moriya相互作用的XY模型的量子相干性

研究了具有Dzyaloshinskii-Moriya(DM)相互作用的一维横场XY自旋链的量子相变和量子相干性.采用约旦-维格纳变换严格求解了哈密顿量,并描绘了体系的关联函数和相图,相图包含反铁磁相、顺磁相和螺旋相.利用相对熵和Jensen-Shannon熵讨论了XY模型的量子相干性.研究发现,相对熵与Jensen-Shannon熵所表现的行为都可以很好地表征该模型的量子相变.非螺旋相中量子相干性不依赖DM相互作用,而在螺旋相DM相互作用对量子相干性有显著影响.此外,指出了在带有DM相互作用的这一类反射对称破缺体系中关联函数计算的常见问题.     

磁性多层膜磁特性的表面效应

利用Monte-Carlo方法和转移矩阵法研究了具有不同表面交换耦合J_s和薄膜厚度 磁性多层 膜的表面和尺寸对磁相变的影响.模拟结果表明，系统的相变温度随薄膜层数的变化取决于J_s/J(J为体内交换耦合)，当J_s/J大于某一临界值时，由于表面磁 有序先于体内磁有序 ，系统的相变温度随薄膜层数的增多而降低，反之，表面磁无序可与体内磁有序共存，系统 的相变温度随薄膜层数的增多而升高；当J_s/J较小时，随J_s增大 ，系统的居里温度缓慢 升高，趋近于体内相变温度，而当J_s/J较大时，随J_s增大，系统的 居里温度 呈线性升高.模拟结果与用转移矩阵法推导出的结果相当符合，且很好地解释了实验事实. 关键词： 磁星多层膜 交换耦合 Monte-Carlo模拟 转移矩阵法     

面心立方(001)方向AB合金薄膜表面层的有序无序相变

采用平均场近似方法对两组元面心立方合金薄膜的有序无序相转变过程进行模拟计算,结果表明,合金薄膜的有序无序相变受薄膜层数奇偶性的影响.薄膜层数奇偶性不同,会导致薄膜具有不同的相结构和热力学性质.在弱表面偏析作用下,对于偶数层薄膜,由于薄膜边界对称性破缺,对应体组分x=0.5的化学势区间,偶数层薄膜有序无序相变过程中出现了中间温度相和浸润现象.而奇数层薄膜的有序无序相变类似体材料的相变.在强表面偏析作用下,由于受表面偏析作用和有限尺寸效应影响,对应体组分x=0.5的化学势区间,奇数层薄膜中出现AB(AB)nA相,它不存在严格热力学意义上的有序无序相变.     

参数修改对铁电薄膜相变性质的影响

在关联有效场理论的框架内, 利用微分算子技术, 详细地计算了基于横场伊辛模型描述的对称铁电薄膜系统的相变性质. 根据薄膜各层自旋平均值构成的一系列耦合方程, 推导出可以用来计算任意层的具有不同表面层的薄膜相图的解析通式方程, 讨论了参数修改对薄膜相互作用参数从FPD (铁电相占主导地位的相图)到PPD (顺电相占主导地位的相图)过渡值和参数空间中各相变区域的影响. 在与平均场近似进行比较的结果显示, 关联有效场理论所得到的铁电薄膜的铁电性在某种程度上比平均场近似下的结果减弱. 关键词： 铁电薄膜 横场伊辛模型 相图 居里温度     

应用保真度研究两腿XXZ梯子系统的量子相变

本文基于投影纠缠对态的自旋梯子系统的张量网络算法提供了一个有效的方法,来研究两腿自旋1/2的XXZ梯子量子系统的量子临界性,通过与单位格点基态保真度相结合,得到清晰的基态保真度三维图,来探测量子多体系统的量子相变,从而可以绘制出两腿自旋1/2XXZ梯子量子模型的基态相图,在相图中出现铁磁(FM)相、条纹铁磁(SF)相、Néel(N)相、条纹Néel(SN)相、Rung singlet(RS)相、Haldane(H)相、Rung triplet(RT)相、临界XY相(XY1相和XY2相)。在不知道系统是否存在局域序参量或非局域序参量的情况下,可以通过系统的基态波函数,在对称性破缺相中由约化密度矩阵得到局域序参量;或者在对称性相中,找出系统存在的非局域序参量,来完整地描述系统的量子相变和量子临界现象。     

微扰模型分析长周期光纤光栅薄膜传感器

提出了一种新的基于三层阶跃折射率光纤的微扰模型,分析长周期光纤光栅（Long Period Fiber Grating,LPFG）薄膜传感器,并从β2稳定性定理出发推导出适用于薄膜传感器的微扰公式.该模型不仅能够清晰反映薄膜参量与包层模传播常量变化量之间的关系,而且在计算量和计算难度远低于四层波导模型情况下获得与严格求解结果相当的计算准确度.考虑数值计算本身引入的计算误差,该模型能够满足定性和半定量理论分析需要.最后通过长周期光纤光栅液态水膜的挥发实验对该模型进行了初步验证.     

量子相变和量子临界现象

本文综述凝聚态物理学中的量子相变和量子临界现象,首先考察了相变中存在量子效应的可能性,通过横磁场Ising模型介绍了量子相变的基本特征;接下来对照热临界现象,引入了量子标度和量子重正化的基本概念和操作方式;然后利用量子临界现象的方案,分析了密度驱动、无序驱动和关联驱动的金属-绝缘体相变;继续利用量子临界性的概念探讨如重电子化合物、铜氧化物和巡游铁磁体这类复杂的相互作用多粒子系统;最后选择量子点、碳纳米管和单层石墨为例,介绍了量子临界性在低维和纳米系统研究中的作用.     

铁磁/非铁磁/铁磁层状薄膜的巨磁阻抗效应的计算

基于文献[17]和[18]提出的铁磁非铁磁铁磁层状薄膜的电磁模型,详细研究了层状薄膜的巨磁阻抗增强效应,以及磁性层和非磁性层厚度与层状薄膜的巨磁阻抗效应的关系.分析表明,铁磁层和非铁磁层薄膜的电阻率相差越大,越有利于获得显著的巨磁阻抗效应;对于总厚度要求一定的层状薄膜,铁磁层和非铁磁层薄膜存在一最佳厚度 关键词： 巨磁阻抗效应 磁性薄膜 趋肤效应     

Phase transitions and critical properties in the antiferromagnetic Ising model on a layered triangular lattice with allowance for intralayer next-nearest-neighbor interactions

The phase transitions (PTs) and critical properties of the antiferromagnetic Ising model on a layered (stacked) triangular lattice have been studied by the Monte Carlo method using a replica algorithm with allowance for the next-nearest-neighbor interactions. The character of PTs is analyzed using the histogram technique and the method of Binder cumulants. It is established that the transition from the disordered to paramagnetic phase in the adopted model is a second-order PT. Static critical exponents of the heat capacity (α), susceptibility (γ), order parameter (β), and correlation radius (ν) and the Fischer exponent η are calculated using the finite-size scaling theory. It is shown that (i) the antiferromagnetic Ising model on a layered triangular lattice belongs to the XY universality class of critical behavior and (ii) allowance for the intralayer interactions of next-nearest neighbors in the adopted model leads to a change in the universality class of critical behavior.     

Critical properties of XY model dimensional layered magnetic on two- films

Using Monte Carlo simulations, we have investigated the classical $XY$ model on triangular lattices of ultra-thin film structures with middle ferromagnetic layers sandwiched between two antiferromagnetic layers. The internal energy, the specific heat, the chirality and the chiral susceptibility are calculated in order to clarify phase transitions and critical phenomena. From the finite-size scaling analyses, the values of critical exponents are determined. In a range of interaction parameters, we find that the chirality steeply goes up as temperature increases in a temperature range; correspondingly the value of a critical exponent for this change is estimated.     

Critical properties of XY model on two-layer Villain－ferromagnetic lattice

We investigate phase transitions of the XY model on a two-layer square lattice which consists of a Villain plane (J) and a ferromagnetic plane (I), using Monte Carlo simulations and a histogram method. Depending on the values of interaction parameters (I,J), the system presents three phases: namely, a Kosterlitz－Thouless (KT) phase in which the two planes are critical for I predominant over J, a chiral phase in which the two planes have a chiral order for J predominant over I and a new phase in which only the Villain plane has a chiral order and the ferromagnetic plane is paramagnetic with a small value of chirality. We clarify the nature of phase transitions by using a finite size scaling method. We find three different kinds of transitions according to the values of (I,J): the KT transition, the Ising transition and an XY-Ising transition with ν=0.849(3). It turns out that the Ising or XY-Ising transition is associated with the disappearance of the chiral order in the Villain plane.     

Nematic and chiral order for planar spins on a triangular lattice

We propose a variant of the antiferromagnetic XY model which includes a biquadratic (J2) as well as the quadratic (J1) interaction on the triangular lattice. The phase diagram for large J2/J1 exhibits a phase with coexisting quasi-long-range nematic, and long-ranged vector spin chirality orders in the absence of magnetic order, which qualifies our model as the first instance of a classical spin model that exhibits a vector chiral spin liquid phase. The interplay of nematic and spin chirality orders is discussed. A variety of critical properties are derived by means of Monte Carlo simulation.     

Ferromagnetic phase transition in a Heisenberg fluid: Monte Carlo simulations and Fisher corrections to scaling

The magnetic phase transition in a Heisenberg fluid is studied by means of the finite size scaling technique. We find that even for larger systems, considered in an ensemble with fixed density, the critical exponents show deviations from the expected lattice values similar to those obtained previously. This puzzle is clarified by proving the importance of the leading correction to the scaling that appears due to Fisher renormalization with the critical exponent equal to the absolute value of the specific heat exponent alpha. The appearance of such new corretions to scaling is a general feature of systems with constraints.     

Superconductor-insulator transition of Josephson-junction arrays on a honeycomb lattice in a magnetic field

We study the superconductor to insulator transition at zero temperature in aJosephson-junction array model on a honeycomb lattice with f flux quantum perplaquette. The path integral representation of the model corresponds to a (2 + 1)-dimensional classical model, which isused to investigate the critical behavior by extensive Monte Carlo simulations on largesystem sizes. In contrast to the model on a square lattice, the transition is found to befirst order for f = 1 /3 and continuous for f = 1 / 2 but in a different universality class.The correlation-length critical exponent is estimated from finite-size scaling of vortexcorrelations. The estimated universal conductivity at the transition is approximately fourtimes its value for f =0. The results are compared with experimental observations on ultrathinsuperconducting films with a triangular lattice of nanoholes in a transverse magneticfield.     

Critical phenomena in ferromagnetic superlattices

Within the framework of the high-temperature series expansions technique, we examine the phase transition and the critical phenomena of a two-component superlattice with simple cubic structure, through three models: Ising, XY and Heisenberg. The reduced critical temperature of the system is studied as a function of the thickness of the constituents and the exchange interactions in each material, and within the interface. We show the existence of a critical thickness of the unit cell at which the reduced critical temperature of the binary superlattice remains insensitive to the exchange coupling within the interfaces. The values of the effective critical exponent γ _eff associated with the magnetic susceptibility agreed with the universal classes in the limit cases where the superlattice is still comparable to an infinite simple cubic lattice. We attribute the breakdown in the universality hypothesis to the crossover effects.     

Monte Carlo investigation of phase transitions in the frustrated Heisenberg model on a triangular lattice

The critical properties and phase transitions of the three-dimensional frustrated antiferromagnetic Heisenberg model on a triangular lattice have been investigated using the Monte Carlo method with a replica algorithm. The critical temperature has been determined and the character of the phase transitions has been analyzed using the method of fourth-order Binder cumulants. A second-order phase transition has been found in the three-dimensional frustrated Heisenberg model on a triangular lattice. The static magnetic and chiral critical exponents of the heat capacity α, the susceptibility γ and γ _k , the magnetization β and β _k , the correlation length ν and ν _k , as well as the Fisher exponents η and η _k , have been calculated in terms of the finite-size scaling theory. It has been demonstrated that the three-dimensional frustrated antiferromagnetic Heisenberg model on a triangular lattice forms a new universality class of the critical behavior.     

Ground state of a spin system with two- and four-spin exchange interactions on the triangular lattice

We study a spin system with both two- and four-spin exchange interactions on the triangular lattice as a possible model for the nuclear magnetism of solid ³He layers adsorbed on grafoil. The ground state is analyzed by the use of the mean-field approximation. It is shown that the four-sublattice state is favored by introduction of the fourspin exchange interaction. A possible phase transition at a finite temperature into a phase with the scalar chirality is predicted. Application of a magnetic field is shown to cause a variety of phase transitions.     

Phase transitions and critical properties in the antiferromagnetic Heisenberg model on a layered cubic lattice

Phase transitions and critical properties in the antiferromagnetic Heisenberg model on a layered cubic lattice with allowance for intralayer next nearest neighbor interactions have been studied using the replica Monte Carlo algorithm. The character of phase transitions has been analyzed using the histogram method and the Binder cumulant method. It has been found that a transition from the collinear to paramagnetic phase in the model under study occurs as a second order phase transition. The statistical critical exponents of the specific heat α, susceptibility γ, order parameter β, and correlation radius ν, as well as the Fisher index η, have been calculated using the finite-size scaling theory. It has been shown that the three-dimensional Heisenberg model on the layered cubic lattice with allowance for the next nearest neighbor interaction belongs to the same universality class of the critical behavior as the antiferromagnetic Heisenberg model on a layered triangular lattice.