重费米子半导体的探索和奇异输运行为研究

The phenomena of heavy fermion forms one of the main research fields that are intimately related to superconductivity, magnetism and other many-body physics. Except for the commonly known heavy-fermion metals and superconductors, there exists a class of materials showing semiconducting behavior, known as heavy-fermion semiconductor or Kondo insulator. Interesting physics wherein concerns the opening of the energy gap,the existence of a spin gap beside the charge gap, the relevance to magnetic semiconductor, anomalous magnetic and transport properties, etc., in addition to the potential for application as functional materials. Heavy-fermion semiconductors (or Kondo insulators) are on one hand different to Mott insulator, in the sense that its gap originates from hybridization between flat f/d level with conduction band. On the other hand, they differ even more to conventional band semiconductors, with largely renormalized bands and effective mass at the gap edges, and a spin gap besides the charge gap. This project first focuses on exploring new heavy-fermion semicondutors and some known systems that are suspected to be heavy-fermion semicondutors or semimetals, with a completely or partially opened gap, for example, the recently discovered CeFe2Al10 and CeRu2Al10 which shows heavy-fermion behavior with however a metal-semiconductor transition at around 30 K. Furthermore, by employing our home-made setup, we plan to perform detailed measurements of xx and xy components of various transport tensors, i.e., the electrical conductivity tensor, thermal conductivity tensor, and thermoelectric tensor. Such a multi-functional measurement systems operating down to very low temperatures and high magnetic fields requires specific experiences and outstanding techniques, therefore is rather rare in the world. Some fundamental aspects are expected to be clarified through this project, for instance, how can a combination of electron correlation and a small energy gap lead to an enormous thermopower, how does the high-T incoherent transport develops into the low-T coherent transport? In addition, very recently, theoretical and experimental studies confirmed the first topological surface state in a heavy-fermion semicondutor, SmB6, defined as topological Kondo insulator or topological heavy-fermion semiconductor. Similar transport features, like a saturate resistivity at low temperatures, have been observed in many other Kondo insulating systems, for example, CeNiSn. Therefore, we also plan to do a comparative investigation on different such systems that might be relevant to a topological Kondo insulator.

重费米子现象主要发生在含有稀土类元素的化合物中，是推动多体物理发展的重要研究方向。除了重费米子金属和超导外，部分重费米子体表现为半导体。这类体系既不同于Mott绝缘体，也不同于传统能带半导体。能隙的产生机理，非相干输运，和磁性半导体的关系等是其涉及的关键基础问题，并具有巨大的应用前景。该项目首先探索新型的含Ce,Yb等的重费米子半导体，深入研究一些最近发现的重费米子半导体（例如，CeFe2Al10等）。用自行开发的低温磁场下热和电输运张量测量技术研究这类半导体异常输运的微观机理，具体包括，重费米子和能隙的结合如何引起巨大热电势，电荷输运如何从高温非相干过程演变为低温相干态等。近期，近藤半导体SmB6的低温电阻饱和现象被认为是表面拓扑态所引起。低温下类似电阻饱和现象在很多重费米子半导体里都有，例如CeNiSn，但是通常被认为是低温相干态。我们计划对不同近藤体系该现象的起源进行比较研究。

重费米子现象主要发生在含有稀土元素的化合物中，是重要的多体物理研究领域。传统上，该领域的主要研究对象是具有重费米子行为的金属或超导体。在重费米子体系中，也有个别材料会在费米能级附近打开一个窄能隙，形成重费米子半导体或者重费米子半金属。和传统窄能隙半导体不同，重费米子半导体的能隙打开过程是典型的多体关联的结果，且随着温度，压力，磁场等外参量发生敏感变化。另外，也和莫特绝缘体也不同，当考虑了电子关联效应后，重费米子半导体的能隙形成过程一定程度上可以基于能带理论进行计算。由于结合了窄能隙和电子关联等特点，重费米子半导体近年来在基础物理和应用物理方面都表现出了重要的研究价值。尤其是，最近理论和实验相继发现重费米子半导体也可以表现出拓扑非平庸的电子结构。典型例子是SmB6。该材料作为第一个近藤拓扑半导体而备受关注，进一步引发了人们对重费米子半导体的关注。在本项目中，针对重费米子半导体，我们重点开展了以下几个方面的工作。（一）着眼于已知的重费米子半导体材料，详细研究了低温和磁场下的热电输运。发现了重费米子体系中的准粒子弛豫过程是导致巨大热电效应的主要原因。（二）重费米子半导体CeRu2Al10在27K发生了奇特的有序结构。通过热输运测量，我们发现了其热导率和著名的隐藏序材料URu2Si2非常类似，为进一步研究二者的隐藏序提供了新的信息。（三）CeRu4Sn6被理论预测为第一个重费米子外尔半金属。我们首次成功制备了该材料的单晶，并研究了低温下的输运特性，发现了该材料低温下稳定的电阻平台。