各向异性三角反铁磁材料的量子磁性

在现代凝聚态物理领域中，围绕量子材料开展的研究逐渐成为领域内主流的 研究方向。量子材料中有许多系统存在竞争相互作用，但并非所有相互作用都会 导致同一种有序态的建立。足够强的竞争可以导致非常规的物理现象的出现，这 些现象包括但不限于非共线有序、丰富的相图，或者极低温下长程有序的缺失。常 见的几何阻挫的晶格类型有二维三角晶格、笼目晶格，三维钙钛矿晶格等。二维 三角晶格反铁磁体是最简单的几何阻挫系统之一，它凭借低维和阻挫的双重特性 常表现出非平庸的物理，因而受到广泛关注。在金属三角晶格体系中可能会出现 由磁场诱导的斯格明子，它们表现出自发的霍尔效应。在交换作用的影响下局域 磁矩会沿着导电电子移动的环路排列，在其波函数中诱导出贝里相。在绝缘体三 角晶格体系中，没有导电电子帮助建立长程有序，相互作用的尺度进一步减小，体 系量子涨落剧烈，可能有 1/3 磁化平台或其他量子态的出现。 本文以稀土基的金属体系 CePtAl₄Ge₂ 和过渡族钴基的绝缘体氧化物 Na₂SrCo(PO₄)₂ 为例，探究其晶体生长的改进方法和磁化率和比热等基本物性，着 重研究其极低温下的量子磁性行为。受晶体场劈裂和自旋轨道耦合的影响，两个 体系的基态为双重态，可以由有效自旋 1/2 的模型描述。我们在层状金属三角反铁 磁体 CePtAl₄Ge₂ 中观察到由磁场诱导的非公度相变，该相变可能与导电电子和几 何阻挫共同确定。在钴基畸变三角晶格反铁磁体 Na₂SrCo(PO₄)₂ 中我们观察到很 低的相变温度且伴随着体系很强的自旋涨落，而磁场破坏反铁磁相后诱导出了不 同于经典体系的无序态。CePtAl₄Ge₂ 和 Na₂SrCo(PO₄)₂ 分别代表着二维三角晶格 体系中的金属体系和绝缘体体系，且各自表现出奇特的物理现象，这两种物理现 象在目前的强关联体系中较为少见，具有一定的独特性，值得进一步的研究和理 论上的解释。

In the realm of strongly correlated electron systems, the exploration of quantum materials is paramount. These materials often feature competing interactions, where some interactions fail to align to promote a singular ordered state. Intense competition can lead to emergent phenomena, including non-collinear ordering, intricate phase diagrams, or a lack of long-range order at low temperatures. The two-dimensional triangular lattice systems, characterized by their inherent low dimensionality and frustration, often unveil novel physical phenomena, thereby attracting much attention. The discovery of Skyrmion crystals induced by a magnetic field might occur in the metallic triangular antiferromagnets. When conduction electrons traverse through a skyrmion spin texture, they manifest a spontaneous Hall effect. As a consequence of the exchange interaction, the underlying local moment texture aligns the spin of a conduction electron traversing a loop, inducing a Berry phase in its wavefunction. In insulating triangular lattice systems, absent the involvement of conducting electrons, the interaction scale further decreases, leading to pronounced quantum fluctuations and the potential emergence of quantum states such as 1/3 magnetization plateaus or other quantum phenomena. Under the influence of crystal field splitting and spin-orbit coupling, the ground states of both systems are described by doublets with effective spins of 1/2. In the layered metallic triangular antiferromagnet CePtAl₄Ge₂, we observe a fieldinduced inconmmensurate phase transition, which might be determined by the interplay of conducting electrons and geometric frustration. In the cobalt-based distorted triangular lattice antiferromagnet Na₂SrCo(PO₄)₂, we observe a very low transition temperature accompanied by strong spin fluctuations, and a disordered state distinct from classical systems is induced with the antiferromagnetic phase transition suppressed. These two peculiar physical phenomena mentioned in this study have unique characteristics, thereby warranting further research and theoretical explanation.

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