凝聚态物理中的高自旋衍生费米子系统及其物理性质研究

The research for the topological states of matter has been one of the central subjects in condense matter physics. This also owns great potentials in application to industry. In known topological states of matter, the spin-1/2 Dirac-type fermions play a key role. Theoretically, the emergent Dirac fermions have been exhaustively studied and well understood. The topological states of matter of higher spin emergent fermions and their physical properties are very different from those of Dirac fermions. The simplest ones are spin-1 and 3/2, which are also the most possible to be realized in materials. For the former, because there are already proposed candidates of 3D materials, we will focus on the studies of the physical properties, e.g., the anomalous magneto-electric transport, magneto-optics, quantum oscillation and quantum tunneling. The 2D materials are unknown and we will look for the material candidates and then study the quantum Hall effects. For the second type of fermions, we pay our attention to the relativistic spin 3/2 fermions, Rarita-Schwinger fermions. The realization of RS fermion in condensed matter system is very challenging and its physical properties are very interest, e.g., the superluminality in an external magnetic field, which is not allowed in a strictly relativistic system but in a condensed matter system since here the “speed of light” means the Fermi velocity. We will also study the topological, transport and correlated properties of these higher spin fermions.

拓扑物态研究已成为凝聚态物理的主流研究学科之一并具有极大的潜在应用前景。在已知拓扑物态中，自旋1/2衍生狄拉克型费米子起关键作用。在理论上，人们已经比较全面地理解了衍生狄拉克费米子的物理性质。自旋大于1/2衍生费米子相关的拓扑物态有与狄拉克费米子非常不同的物性。最可能在实际材料中实现的有两类：自旋1和3/2费米子。前者有比较明确的3维材料候选者，我们将着重于物性研究，例如，反常的磁电输运性质，磁光性质，量子振荡和量子隧穿性质。2维材料还没有明确的候选者，我们将着重于寻找材料和可能的量子霍尔效应研究。对后者，我们的研究重点是寻找能实现相对论性自旋3/2 Rarita-Schwinger费米子的材料。这类粒子的经典和量子物性非常特殊，例如，在外电磁场下超光速。在凝聚态系统中，“光速”是费米速度，“超光速”是允许的。我们也将研究这些高自旋费米子的拓扑性质、输运性质和关联性质。

凝聚态物理系统本身可以用非相对论多体相互作用理论描述。最近十几年来，多体系统中衍生出一系列“相对论性”准粒子。但是，它们没有衍生的洛伦兹对称性。因而，组成世界的基本粒子满足的统计-自旋约束对这些准粒子无效，可以有一些奇异的准粒子存在，例如，赝自旋为1的“相对论性”费米子，即类似光子的费米型准粒子。 没有物理自旋类光费米子气体是Weyl半金属的赝自旋为1的对应物，而Dirac半金属的对应物是有物理自旋自由度的类光费米子气体，称为Kane费米子气体。本项目中我们研究它们的反常磁输运性质、额外的量子振荡和磁光电导性质，发现可以很好解释其他理论研究不能解释的实验结果。为了研究这些衍生费米子系统中的量子混沌现象，我们首先研究Weyl半金属。这是一个没有研究就过的问题，我们的研究表明，在第一类和第二类Weyl半金属的界面上 ，可能存在霍金辐射，而且，在第二类Weyl半金属一侧，可以有Weyl费米子运动出现量子混沌现象。这是一个原创性的理论成果，当然，还有待实验验证。