自旋团簇中的交换作用和量子效应强磁场研究

Spin clusters have received much attention because of quantum criticality and Higgs excitations arising from crossover between zero- and three-dimensionalities. To understand the magnetism of spin clusters, it is crucial to obtain precise Hamiltonian parameters--- exchange interactions. In this project, by combining high magnetic-field ESR and magnetization measurements, we plan to study the exchange interactions and quantum effects of spin clusters by high-field-induced “magnetic saturation”. First, series of Cu, Ni and Mn clusters (trimer, tetramer etc.) with S=1/2-5/2 will be synthesized and their crystal structure and basic magnetism will be characterized. Then, high magnetic field will be applied to suppress the spin gap and make the systems achieve “magnetic saturation”. With the ESR modes in the saturated phase, exchange interactions will be extracted through classical harmonic spin-wave theory. Finally, based on the obtained exchange interactions we will study the magnetization processes, try to find field-induced quantum phase transitions including fractional magnetized plateaus, possible Bose-Einstein condensates and new matter states, and finally clarify the relationship between quantum phase transitions and exchange interactions. Special attention will be paid to the magnetism on the “ordered side” near the quantum critical point.

自旋团簇因“零维-三维交叉”而引起的量子临界性、Higgs激发备受关注。理解自旋团簇的磁性，准确的哈密顿量参数---磁交换作用至关重要。本项目拟组合强磁场ESR和磁化测量技术，尝试通过强磁场诱导“磁饱和”的方法，研究自旋团簇的交换作用和量子效应。首先，合成系列S=1/2～5/2的Cu、Ni和Mn等自旋团簇（三聚体、四聚体等）样品，并表征其晶体结构和基本磁性；然后，利用强磁场抑制自旋隙，使系统达到“磁饱和”，并利用“磁饱和”相的ESR激发模式，通过经典的谐波自旋波理论，获得交换作用参数；最后，基于获得的交换作用参数，进一步研究磁化过程，探索强磁场诱导的量子相变包括分数磁化平台、可能的玻色-爱因斯坦凝聚现象，以及新的物质形态，阐明量子相变与交换作用之间的关联。特别关注量子临界点附近“磁有序端”的磁性。

自旋团簇因“零维-三维交叉”而引起的量子临界性、Higgs激发备受关注。理解自旋团簇的磁性，精确的交换作用参数至关重要。依照项目计划书，本项目首先合成了系列S=1/2~1的Cu、Co、Ni等自旋团簇化合物和“团簇化”的自旋链，并表征了其晶体结构和磁性。然后，利用强磁场研究了这些材料中磁场诱导的相变，观察到了量子磁化平台、异常磁激发和可能的超固态等量子效应。通过建立哈密顿量模型，利用精确对角化、量子蒙特卡洛模拟阐述了磁化平台的起源，获得了交换作用参数。发现的一般规律是：（1）由于存在团簇间交换作用，自旋团簇一般发生长程磁有序，而磁化平台的出现说明了经典磁性与量子磁性的共存，耦合团簇的基态一般位于经典的“磁有序端”；（2）对于磁无序的自旋隙团簇，足够强的磁场可抑制自旋隙，使体系达到“磁饱和”；（3）自旋团簇中的量子相变、量子临界性取决于团簇间与团簇内交换作用的比值。本项目的研究有助于理解自旋团簇量子临界点附近的磁性。