非中心对称超导序参量研究

Order parameters of non-centrosymmetric superconductors

非中心对称超导体是近年发现的一类新型超导材料.在这类材料中,非中心对称的晶体势场产生一个有效的反对称自旋-轨道耦合(ASOC)并导致自旋简并的能级发生分裂,从而在超导配对态中允许自旋单态和自旋三重态混合.这一性质有别于先前研究的大部分超导体,需要从概念上突破BCS理论框架.此外,理论研究还表明非中心对称超导可能还是一类潜在的拓扑超导材料.这些独特的物理性质已激发了广泛的研究兴趣,并且越来越受到关注.超导序参量的对称性是认识和理解超导形成机理的一个重要物理量.本文将介绍基于隧道二极管的伦敦穿透深度测量技术,并简要综述非中心对称超导的研究现状以及穿透深度测量在非中心对称超导序参量研究中的应用.通过对比研究具有不同反对称自旋-轨道耦合强度的非中心对称超导材料,我们发现其混合超导配对态与反对称自旋-轨道耦合强度缺乏简单的对应关系,但与能带劈裂(EASOC)相对于超导转变温度(Tc)的比值(Er=EASOC/Tc)紧密相关.

The non-centrosymmetric（NCS） superconductors（SCs）, a class of novel superconducting materials, have recently attracted considerable interests. As a result of antisymmetric spin-orbital coupling（ASOC） arising from the absence of inversion symmetry, the superconducting pairing state of these compounds allows the admixture of spin-singlet and spin-triplet components. This is in contrast to other previously studied superconductors, which usually possess an inversion symmetry in their crystal structure, and therefore their pairing state is of either spin-singlet/even parity or the spin-triplet/odd parity due to the restrictions of the Pauli principles and parity conservation.Determination of the gap structure is crucial for unveiling the pairing state of NCS SCs. In this article, we first describe a method of measuring the precise temperature dependence of the changes in the London penetration depth using the tunnel-diode-oscillator（TDO）, which provides an important evidence for the superconducting gap structures.Then the pairing states of NCS SCs are briefly reviewed, putting the emphasis on a few compounds with different ASOC strengths. It is proposed that the ASOC may tune the ratio of the spin-triplet to the spin-singlet component and,therefore, the spin-triplet state may become dominant while the ASOC effect is sufficiently strong in NCS SCs. However,our investigations demonstrate that the actual case is more complicated and there is no simple correspondence between the ASOC size and the pairing states. Instead, it is found that the band splitting due to the ASOC effect divided by the superconducting transition temperature Tc may better characterize of the superconducting pairing states in NCS SCs.