自旋S=3/2和S=2 Blume-Capel模型相变的Monte Carlo研究

利用计算机MonteCarlo模拟,研究自旋S=3/2和S=2Blume-Capel模型的相图。与最近刚发展起来的适用于研究S>1的关联有效场相比较,得出如下结论:对于S=3/2和S=2,关联有效场过高地估计了它们的相变温度对于S=3/2的相图,除了有二级相变线外,还发现了一级相变线对于S=2,关联有效场同样过高地估计了三临界点的值。 关键词：     

自旋S=1淬灭键稀释蜂窝格子伊辛模型

本文提出一种新的缀饰方法,将交换作用参数处在子空间exp(K)cosh(J)=1的自旋S=1淬灭键稀释伊辛模型映象到混合自旋淬灭座稀释缀饰格子系统。对此混合自旋缀饰系统,应用退火模型近似求得自旋S=1淬灭键稀释蜂窝格子伊辛系统的临界温度、磁化强度与键浓度之间的解析关系。 关键词：     

具有单轴各向异性场的无规横向伊辛模型(S=1)的临界行为

应用我们提出的对近似和离散路径积分技术相结合的方法,研究了具有单轴各向异性场的无规横场伊辛模型(自旋s=1)的临界行为,并且讨论了单轴各向异性场和横场无规分布对系统相图结构的影响。 关键词：     

半无限无规场混合伊辛自旋系统的临界行为

应用平均场重整化群方法，讨论表面无规场（三峰分布）存在时S=1和σ=1/2混合自旋伊辛系统的表面临界行为，得到了系统的表面相图． 关键词：     

无规场非晶化表面的临界行为

应用平均场重整化群方法,讨论表面无规场和非晶化存在对S=1和σ=1/2混合自旋伊辛系统表面磁性临界行为的影响,得到在各种情况下系统的表面相图.计算结果表明,非晶化在一定范围内与无规场竞争仍可出现三临界点和重入现象. 关键词：     

面心立方格子上自旋S=1的Ising反铁磁体的基态研究

本文发展了文献[1]中提出的利用费密变换求Ising模型基态的方法,根据由费密变换后,激发态能量大于零求得相边界,提出了直接比较各真空态能量来求解各基态的相边界。采用此方法,研究了具有近邻对相互作用的最一般的自旋S=1的Ising反铁磁体模型,严格地求得它在面心立方格子上的基态能量和零温相图。同时,还利用多相点方程,将参数k,l平面分成60个小区域,它们分别对应于各个拓扑不等价的零温相图。 关键词：     

自旋S=1座稀释蜂窝格子的伊辛模型

本文对于自旋S=1的淬灭座稀释蜂窝格子的伊辛模型,在约束条件exp(K)cosh(J)=1下,求得了系统的临界温度和磁化强度与自旋浓度之间的关系。 关键词：     

随机横场与晶场作用混合自旋系统的热力学性质

利用有效场理论和切断近似，在伊辛模型的框架内考虑随机横场与晶场作用的混合自旋1/2 和自旋1系统的热力学性质。重点研究了晶场、横场和随机浓度对混合自旋系统相变的影响 ，研究发现在随机横场条件下，较小的晶场存在并不能改变临界横场阈值；取较大横场值时 在某些随机浓度的范围内出现重入相变现象，而取较小横场值时则没有出现重入相变现象。 给出了有关相图并进行了讨论。 关键词： 混合自旋系统 伊辛模型 热力学性质 随机横场     

混合自旋(S=1,1/2)模型具有再次近邻反铁磁相互作用晶格基态

本文发展文献[1]的方法到两种不同自旋的原子(S_a=1,S_b=1/2)构成的晶格中,计算了简单立方晶格在具有再次近邻反铁磁相互作用下在外磁场中的基态自旋结构、能量和相界。从文中给出的相图可知:这种晶格有两种反铁磁自旋结构,有四种亚铁磁自旋结构。 关键词：     

Ising自旋（S=1／2）反铁磁超晶格低温热力学

利用费米子变换方法,将 Ising 自旋(S=1/2)反铁磁超晶格体系变换为粒子数不守恒的费米子体系.再利用量子统计微扰方法,在二级微扰近似下对周期为L=L_a+L_b(L_a=L_b)的 Ising 自旋(S=1/2)反铁磁超晶格进行了计算,求得了这种超晶格体系的自由能等一些热力学量和温度(T)、周期 L 的关系式.     

Ising自旋S=1,具有次近邻相互作用的面心立方格子的反铁磁体在外场下的基态能量

本文用文献[4]中提出的方法,变S=1的Ising体系成一个粒子数不守恒的费密体系,严格地求得了具有最近邻及次近邻相互作用的反铁磁Ising晶格在任意外场下的基态能量,得到了零温的相图。 关键词：     

THE ΔS=l WEAK RADIATIVE DECAYS OF HYPERONS

We calculate the rates and asymmetry parameters of radiative hyperon decays based on the hybrid structure of hyperons.We exploit an effective Hamiltonian derived from a penguin-Like Feynman diagram. We also consider a model of a static mean gluon field. A plausible recoil effect is taken into account in the final results. The resultant asymmetry parameters are reasonably consistent with data.     

S为1/2体系粉末或冷冻玻璃态电子自旋共振谱的模拟

本文较详细地介绍了模拟s为1/2体系粉末或冷冻玻璃态电子自旋共振谱的理论基础。给出了具体计算公式,利用这些公式完全可以在微机上进行计算机模拟电子自旋共振谱。     

THE PHASE STRUCTURE OF 4-d U(1) HIGGS MODEL WITH RADIAL EXCITATION

On 4⁴ regular lattice, the phase structure of U(1) Higgs model-a U(1) lattice gauge theory coupled to a scalar (Higgs) field-has been studied by using modified Metropolis algorithm without freezing the radial excitation of the Higgs field carrying fundamental charge. The result shows that Higgs and confined regions are completelg separated by a first-order phase transition line and the phase diagram of our model.consists of three distinct phases. This feature greatly differs from that of the model with radially frozen Higgs field discussed by D.J.E.Callaway and L.J. Carson.^[1]     

Magnetic phase diagram of the dimerized spin S = 1/2 ladder

The ground-state magnetic phase diagram of a spin S=1/2 two-leg ladder with alternating rung exchange J_⊥(n)=J_⊥[1 + (-1)ⁿ δ] is studied using the analytical and numerical approaches. In the limit where the rung exchange is dominant, we have mapped the model onto the effective quantum sine-Gordon model with topological term and identified two quantum phase transitions at magnetization equal to the half of saturation value from a gapped to the gapless regime. These quantum transitions belong to the universality class of the commensurate-incommensurate phase transition. We have also shown that the magnetization curve of the system exhibits a plateau at magnetization equal to the half of the saturation value. We also present a detailed numerical analysis of the low energy excitation spectrum and the ground state magnetic phase diagram of the ladder with rung-exchange alternation using Lanczos method of numerical diagonalizations for ladders with number of sites up to N = 28. We have calculated numerically the magnetic field dependence of the low-energy excitation spectrum, magnetization and the on-rung spin-spin correlation function. We have also calculated the width of the magnetization plateau and show that it scales as δ^ν, where critical exponent varies from ν = 0.87±0.01 in the case of a ladder with isotropic antiferromagnetic legs to ν = 1.82±0.01 in the case of ladder with ferromagnetic legs. Obtained numerical results are in an complete agreement with estimations made within the continuum-limit approach.     

Quadrupolar phases of the s=1 bilinear-biquadratic Heisenberg model on the triangular lattice

Using mean-field theory, exact diagonalizations, and SU(3) flavor theory, we have precisely mapped out the phase diagram of the S = 1 bilinear-biquadratic Heisenberg model on the triangular lattice in a magnetic field, with emphasis on the quadrupolar phases and their excitations. In particular, we show that ferroquadrupolar order can coexist with short-range helical magnetic order, and that the antiferroquadrupolar phase is characterized by a remarkable 2/3 magnetization plateau, in which one site per triangle retains quadrupolar order while the other two are polarized along the field. Implications for actual S=1 magnets are discussed.     

Bose-Einstein condensation of S = 1 nickel spin degrees of freedom in NiCl2-4SC(NH2)2

It has recently been suggested that the organic compound NiCl2-4SC(NH2)2 (DTN) undergoes field-induced Bose-Einstein condensation (BEC) of the Ni spin degrees of freedom. The Ni S = 1 spins exhibit three-dimensional XY antiferromagnetism above a critical field H(c1) approximately 2 T. The spin fluid can be described as a gas of hard-core bosons where the field-induced antiferromagnetic transition corresponds to Bose-Einstein condensation. We have determined the spin Hamiltonian of DTN using inelastic neutron diffraction measurements, and we have studied the high-field phase diagram by means of specific heat and magnetocaloric effect measurements. Our results show that the field-temperature phase boundary approaches a power-law H - H(c1) proportional variant T(alpha)(c) near the quantum critical point, with an exponent that is consistent with the 3D BEC universal value of alpha = 1.5.     

缺陷对滞后标度性的影响

对二维Ising铁磁系统中的非磁杂质，利用Monte Carlo方法，获得不同杂质浓度、不同磁场变化速率下的磁滞(?)线，从而得到滞后(?)线面积与磁场变化速率的关系．结果表明，在温度和缺陷浓度组成的相图上，存在一条相变线．低温和低浓度时，(?)线面积与磁场速率的标度指数约为1/3，只微弱地依赖于温度及杂质浓度；而对高温和高浓度的杂质，指数强烈地依赖于温度和浓度 关键词：