RUI: Quantum Micromagnetism

9978882Chudnovsky这是一个支持量子和无序系统微磁学理论研究的瑞奖。特别是，将开展两个项目：(1)固体中磁化强度的宏观量子隧穿。这项研究是对PI提出的亚稳态磁态从量子温度到热衰变温度的两种交叉的进一步研究。这项工作引发了最近在计算不同对称性的自旋哈密顿量的交叉图方面的重大活动。在MN-12中发现了一级交叉的实验证据。通过将自旋问题映射到粒子问题，并通过Caldeira-Leggett方法考虑了与环境的相互作用，考虑了隧道预因子和耗散，得到了从隧道到热激活的交叉区域。PI还将尝试开发一种等价于Caldeira-Leggett方法的方法来求解Landau-Lifshitz方程的磁矩。对量子和热行为的两种交叉的研究将扩展到磁区壁的量子去钉扎和磁区的量子成核问题。(2)还将继续研究高自旋分子团簇Mn-12和Fe-8中的自旋隧穿，以及单磁区磁粒子中共振自旋隧穿的可能性及其实验检测。对于分子团簇，重点将放在热辅助共振和非共振隧穿理论、量子雪崩、量子相干和Berry相振荡的可能性上。Wernsdorfer和Sessoli最近在Fe-8中报道了这种振荡。我们将研究耗散对这些振荡的影响。我们将发展连续和离散能级模型来定量解释Mn-12和Fe-8的量子经典交叉图。PI还将研究由外部磁场触发的激光对自旋能级的影响的可能性。对于单域粒子，问题是能否在具有一定尺寸分布的多粒子系统中检测到自旋能级的量子化。在考虑能级量子化的情况下，我们将计算铁磁性和反铁磁性粒子的驰豫和阻塞温度的场依赖关系。这是一个支持量子和无序系统微磁性理论研究的RUI奖。特别是，将开展两个项目：(1)研究固体中磁化强度的宏观量子隧穿；(2)研究高自旋分子团簇中的自旋隧穿和单畴磁性粒子中共振自旋隧穿的可能性。一般说来，这项工作研究的是在宏观长度尺度上表现出量子效应的系统的物理学。

9978882ChudnovskyThis is an RUI award to support theoretical research on micromagnetism in quantum and disordered systems. In particular, two projects will be undertaken:(1) Macroscopic quantum tunneling of magnetization in solids. This research will follow up on the PI's suggestion that there are two kinds of crossover on temperature from quantum to thermal decay of a metastable magnetic state. This work has induced significant recent activity on computing crossover diagrams for spin Hamiltonians of different symmetry. Experimental evidence of the first-order crossover has been found in Mn-12. The vicinity of the crossover from tunneling to thermal activation will be worked out, with account of the tunneling prefactor and dissipation, by mapping the spin problem onto a particle problem and including the interaction with the environment via the Caldeira-Leggett method. The PI will also attempt to develop an equivalent of the Caldeira-Leggett approach for the Landau-Lifshitz equation for the magnetic moment. The study of two kinds of crossover between quantum and thermal behavior will be extended to problems of quantum depinning of domain walls and quantum nucleation of magnetic domains.(2) Research will also continue on spin tunneling in high-spin molecular clusters, Mn-12 and Fe-8, and on the possibility of resonant spin tunneling and its experimental detection in single-domain magnetic particles. For the molecular clusters, the emphasis will be on the theory of thermally assisted resonant and non-resonant tunneling, quantum avalanches, the possibility of quantum coherence and Berry-phase oscillations. Such oscillations have been recently reported by Wernsdorfer and Sessoli in Fe-8. The effect of dissipation on these oscillations will be studied. The continuous and discrete-level models will be developed to explain quantitatively the quantum-classical crossover diagrams in Mn-12 and Fe-8. The PI will also study the possibility of the laser effect on spin levels, triggered by the external magnetic field. For single domain particles the question is whether one can detect quantization of spin levels in a many-particle system with a certain size distribution. The field dependence of the relaxation and of the blocking temperature will be computed, with account of level quantization, for ferro-and antiferromagnetic particles.%%% This is an RUI award to support theoretical research on micromagnetism in quantum and disordered systems. In particular, two projects will be undertaken: (1) research will be done on macroscopic quantum tunneling of magnetization in solids, and (2) research will be done on spin tunneling in high-spin molecular clusters and on the possibility of resonant spin tunneling in single-domain magnetic particles. Generally, this work investigates the physics of systems in which quantum effects manifest themselves at the macroscopic length scales.***