强关联材料霍尔热导率实验测量综述

Experimental Progress in Thermal Hall Conductivity Research on Strongly Correlated Electronic Systems

在材料中输入热流并在垂直于热流的方向上施加磁场时,热载流子将可能被磁场偏转,获得横向速度,从而导致材料在横向出现一个温度梯度。这种效应被称为热霍尔效应(THE)。与电霍尔效应类似,热霍尔效应被预言将在一些拥有非平庸贝利曲率的材料中出现,因此它可以揭示材料的拓扑性质。然而,热霍尔效应并不像电霍尔一样,只局限于载流子带电的体系;相反,任何种类的准粒子都可以导热。因此,热霍尔效应也可以用来探索强关联电子体系材料(尤其是绝缘体)的奇异性质。因此,热霍尔效应更具有普适性,并日益成为探测电中性激发,如声子和磁振子的强有力手段。不仅如此,有如手性声子这样超越一般非平庸贝利曲率图像的因素仍可导致热霍尔效应;探查其中的热霍尔效应将为理解材料中复杂的微观机理指明方向。但是,热信号比电信号要微弱得多。尤其是测量热霍尔效应,往往要在较大背景噪音中提取微弱的有效信号,这使霍尔热导的测量极具挑战性。但是得益于科研工作者大量的努力,该领域在近几年发展迅速,得到了许多十分有趣的结果。在本文中,我们将简要总结现有的一些令人兴奋的在霍尔热导率测量方面的成果,指出尚未解决的问题,并提出未来可能的方向。

Thermal Hall effect(THE)is to describe the phenomenon where heat carriers are deflected by an external magnetic field applied perpendicular to the heat flow,and thus the carriers gain transverse velocity,leading to a finite temperature gradient on the two sides orthogonal to the heat flow and field.THE is predicted to occur in systems with nontrivial Berry curvatures and thus can reveal topological properties,similar to the electrical Hall effect.However,THE is not limited to charge excitations as in the electrical Hall effect,but rather,to all kinds of excitations that are able to conduct heat,making it possible to explore the exotic properties in strongly correlated electronic systems,which are typically insulators.Therefore,THE is more universal than the electrical form and has become a powerful probe in detecting charge-neutral excitations,such as phonons and magnons.Moreover,there are some sources such as chiral phonons,which are beyond a simple nontrivial-Berry-curvature scenario,that can also give rise to THE;examining THE wherein will shed light on the complex microscopic mechanism hidden in materials.Despite these,heat signals are much weaker than electrical ones.Especially for measurements of the thermal Hall conductivity,it is often needed to collect weak signals on top of a large background.This makes measuring the THE challenging—but thanks to the sustained efforts of the community,this field is developing rapidly in recent years,with many interesting results on the measurements of the thermal Hall conductivity.In this review article,we try to summarize some of these exciting accomplishments,point out remaining outstanding issues,and suggest possible future directions.