Quantum Monte-Carlo and Series-Expansion Analysis of Spin-Models for Molecular Magnets

本课题主要研究分子磁体的热力学、随时间变化及零温度性质。首先，我们将分析未失效的单分子磁体（SMM）的磁化率和磁化强度，这些磁体太大，无法用目前的精确对角化（ED）方法处理。这将使用随机序列展开（SSE）来完成，这是一种新颖的量子蒙特卡罗（QMC）模拟方法。为此目的，我们打算改进和采用SSE。特别是，我们将SSE应用于SMM [Mn9(2POAP-2H)2]6+。其次，我们将研究二聚体耦合、局部各向异性和磁场在二聚体阶梯中的作用。草酸盐体系Na2Me2(C2O4)3(H2O)2，其中Me = Mn, Fe, Co, Ni，自旋S =(5,4,3,2)/2。本研究将以SSE为基础，辅以分析债券算子理论（BOT）。最后，利用基于算子重整化群（ORG）方法的零温度下的高阶级数展开，研究了弱耦合四核分子磁体的团簇间交换和自旋受挫。与实验组合作研究了四面体间交换在eg自旋动力学中的作用。[L2Ni4(N3)(O2CAda)4]（CIO4）将被考虑。

This project is devoted to thermodynamic, time-dependent, and zero temperature properties of molecular magnets. First, we will analyze the susceptibility and magnetization, of unfrustrated single-molecule magnets (SMM)s which are too large to be handled with present days exact diagonalization (ED) methods. This will be done using the stochastic series expansion (SSE), which is a novel Quantum Monte-Carlo (QMC) simulation method. We intend to improve and adopt the SSE for this purpose. In particular we will apply the SSE to the SMM [Mn9(2POAP-2H)2]6+. Second, we will study the role of dimer-coupling, local anisotropies, and magnetic fields in dimer-ladders as eg. the oxalate systems Na2Me2(C2O4)3(H2O)2, with Me = Mn, Fe, Co, Ni and spin S = (5, 4, 3, 2)/2. This study will be based on the SSE, complemented by an analytic bond-operator theory (BOT). Finally, using a high order series expansion at zero temperature, which is based on an operator renormalization group (ORG) method we will study inter-cluster exchange and spin-frustration in weakly coupled tetra-nuclear molecular magnets. In collaboration with experimental groups the role of inter-tetrahedral exchange on the spin-dynamics of eg. [L2Ni4(N3)(O2CAda)4](CIO4) will be considered.