考虑次近邻相互作用的正方形点阵完全阻挫XY模型的Ising型相变

利用有限尺寸标度分析方法,研究了考虑次近邻相互作用的正方点阵完全阻挫XY模型的Ising型相变现象.结果表明对于次近邻耦合系数x=0.5的情况,系统存在Ising型相变,临界温度Tc=0.4043±0.0005,临界指数ν=0.80±0.05.     

Influence of the Boundary Condition on the Short—Time Dynamic Behaviour of the Ising—Like Phase Transition in Square—LOattice Fully Frustrated XY Models

We investigate the influence of the boundary condition on the short-time dynamic behaviour of the Ising-like phase transition in square-lattice fully frustrated (FF) XY models with periodic and fluctuating twist boundary conditions.The transition temperature Tc and the dynamic and static critical exponents z,2β/υ and υ are estimated for both cases using short-time dynamic scaling analysis.The results show that both models have the same critical exponents,indicating that the boundary condition has nearly no effect on the short-time dynamic behaviour of the FFXY model.     

AN EFFECTIVE APPROACH FOR TWO-DIMENSIONAL POLARONS IN AN ASYMMETRIC QUANTUM DOT

Within the framework of Feynman-Haken path integral theory, we calculate the ground-state energy of two-dimensional polarons in asymmetric quantum dots for arbitrary electron-phonon coupling constants. From a general three-dimensional Hamiltonian, some interesting problems, such as polarons in quasi-one-dimensional quantum wires and quasi-zero-dimensional asymmetric or symmetric quantum dots can be easily discussed only by taking different limit in the whole coupling regime. After the numerical calculation, we find that the relative polaronic correction increases monotonically with the decrease of effective dot size, and it becomes more pronounced with increasing dimension and asymmetry. Moreover, despite the insensitivity of relative polaronic enhancement to the variation of coupling constant at weak coupling, the correction is related to the coupling constant as the latter becomes larger.     

Phase Diagram of XY Model with Nematic Coupling on the Simple Cubic Lattice

Using cluster Monte Carlo method,we numerically investigate the criticality in the XY model with nematic coupling on the simple cubic lattice.We determine critical lines belong to the three-dimensional XY universality class in variable of θ(2θ) between the XY-ferromagnetic(nematic) and disordered states.Furthermore,the phase transition between the XY-ferromagnetic and the nematic states is found to be in the three-dimensional Ising universality class.The critical points are determined from the intersections of Binder ratios for various system sizes.With two sets of critical points obtained,we finally construct the phase diagram on the-J plane.     

VARIATIONAL CALCULATION ON GROUND-STATE ENERGY OF BOUND POLARONS IN PARABOLIC QUANTUM WIRES

Within the framework of Feynman path-integral variational theory, we calculate the ground-state energy of a polaron in parabolic quantum wires in the presence of a Coulomb potential. It is shown that the polaronic correction to the ground-state energy is more sensitive to the electron－phonon coupling constant than the Coulomb binding parameter, and it increases monotonically with decreasing effective wire radius. Moreover, compared to the results obtained by Feynman Haken variational path-integral theory, we obtain better results within the Feynman path-integral variational approach (FV approach). Applying our calculation to several polar semiconductor quantum wires, we find that the polaronic correction can be considerably large.     

Variational Study on a Dissipative Two-Level System

A new variational approach is proposed to study the ground-state of a two-level system coupled to a dispersionless pbonon bath. By the extended coherent state, where the more phonon correlations are easily incorporated, we can obtain very accurate ground state energy and the tunnelling reduction factor in all regime of tunnelling matrix element δ0 and coupling parameter s. The relative difference between the present ones and those by exact numerical diagonalization is less then 0.001%. In addition, some simple analytical results are given in the limits of δ0/s → 0 and ∞.     

稳定性参数选优设计的新探讨——可控因子分类,综合调节变量法

本文将试验的可控因子分类,用两指标综合分析试验结果,优先控制误差波动(由于噪声因子干扰所引起的),然后调节系统偏倚,从而达到稳定性参数选优设计的目的。本文用两个例子作模拟试验,取得了理想的试验效果。     

第2类超导体中无序钉扎环境下的尖峰效应

用分子动力学方法模拟了第2类超导体的尖峰效应,结果表明尖峰效应的位置正好在单磁通钉扎转变到多磁通钉扎的温度区域.在温度接近Tc时,涡旋运动速度出现大幅度下降,暗示系统的临界电流大幅度增加.此外,借助Wang-Landau算法,对涡旋在Tc附近的行为进行了分析,结果表明：温度接近于临界温度时,磁通的尺寸大幅度增加,钉扎作用范围变大,钉扎能够容纳涡旋的个数也逐渐增加.     

INFLUENCE OF PINNING CENTRE DISTRIBUTION ON THE DYNAMICS OF TWO-DIMENSIONAL VORTEX SYSTEM

Numerical study of the influence of the distribution of pinning centres on the dynamics of a two-dimensional vortex system is performed. The superconductor sample has a periodic structure with a pinned region of length L_p and an unpinned region of length L_x-L_p along the direction of driving force (Lorentz force). Results show that, at zero temperature, the critical force F_c increases with the increase of L_p, indicating that the homogeneity of pinning centres helps to enhance the critical electric current of the superconductor. At large driving forces, vortex static channels form in the pinned region even for L_px.