Quantum and Randomness Effects on Cooperative Phenomena in Spin systems

[1]利用连续-虚时间环算法的量子蒙特卡罗（MC）方法研究了海森堡反铁磁（HAF）模型，并阐明了模型所表现出的以下新型量子磁现象的机理。1)在二维（2D） HAF模型中引入位点或键稀释时出现的非磁杂质诱导的Neel长程序；2)键交替n-leg梯形HAF模型中的量子相变；3)准一维（准2D） HAF模型中Neel温度对链间（面间）相互作用的依赖及其自一致RPA理论的解释。4)通过分析“扭序参数”得到S=1随机键交替HAF模型的基态相图（目前已提交论文3,4）。[2]利用经典自旋系统的普通MC方法，研究了受挫性和随机性并存的自旋玻璃（SG’s）的老化现象，得到以下新结果：1)发现四维Ising EA (IEA) SG模型的自旋弛豫表现出临界区与热活化过程的交叉。2)发现3DIEA-SG模型中温度(T)移时效过程中的自旋弛豫仅在T移幅度较小时表现出累积记忆效应，而当T移幅度较大时，自旋弛豫表现出SG相的温度混沌特性。3)通过对三维ea -SG模型场移老化过程的分析，我们发现S相在静磁场作用下存在不稳定性，这是SG研究早期一直存在争议的基本问题之一。

1]By means of the quantum Monte Calro (MC) method by continuous-imaginary-time loop-algorithm we studied Heisenberg antiferromagnetic (HAF) models and clarified the mechanism of the following novel quantum magnetic phenomena exhibited by the models. 1)The nonmagnetic-impurity-induced Neel long-range order which occurs when site or bond dilution is introduced in the 2-dimensional (2D) HAF model, 2)quantum phase transitions in the bond-alternating n-leg ladder HAF models, 3)the dependence of the Neel temperatures on the interchain (interplane) interactions in the quasi-1D(quasi-2D) HAF models and its interpretation in terms of the self-consistent RPA theory, and 4)the ground-state phase diagram of the S=1 random bond-alternating HAF model by analyzing the "twist order parameter" (the papers for 3,4 are now submitted).2]By means of the ordinary MC method on classical spin systems, we investigated aging phenomena in spin glasses (SG's) in which frustration and randomness are coexisting, and obtained the following new results. 1)the spin relaxation in the 4D Ising EA (IEA) SG model was found to exhibit the crossover from the critical region to that thermal-activation processes. 2)The spin relaxation in the temperature (T)-shift aging processes in the 3DIEA-SG model was found to exhibit only the cumulative memory effect when a magnitude of the T-shift is small, while it exhibits a peculiar aspect attributable to the temperature-chaos nature of the SG phase when a T-shift is relatively large. 3)Frm analyses of the field-shift aging processes in 3D IEA-SG model, we found the result which strongly suggests the instability of the S phase under a static magnetic field, one of the fundamental issues which has been disputed since the early stage of the SG study.