FeSe基浅带超导体的红外光谱研究

FeSe-based shallow-band superconductors exhibit many fascinating properties, resulting from the large ratio of the superconducting gap to Fermi energy. For example, as the ratio of the superconducting gap to Fermi energy approaches unity, the system falls into the BCS-BEC crossover regime, where electronic correlation and superconducting fluctuations are significantly enhanced; the change in the kinetic energy of carriers upon superconducting condensate becomes experimentally detectable; a Lifshitz transition may be induced more easily, leading to new superconducting phases. A detailed investigation into these properties can promisingly reveal the pivotal ingredients that promote high-Tc superconductivity, shedding new light on the pairing mechanism of iron-based superconductors. In this project, we will study the optical properties of the FeSe-based shallow-band superconductors using Fourier transform infrared spectroscopy, focusing on the following points: (1) We will track the optical spectral weight upon superconducting condensate to obtain the change in the kinetic energy of carriers, providing important information about the driving force of superconducting condensate and pairing interaction. (2) Through a detailed analysis of the optical conductivity measured on a series of samples with different doping levels, we aim to qualitatively describe the effects of shallow bands on electronic correlation and quasiparticle scattering process. (3) By performing optical spectral weight analyses in the normal state, we will tackle the question of whether strong superconducting fluctuations emerge in the BCS-BEC crossover regime as expected by theories, and also extract the corresponding temperature scale. (4) We will examine the impact of Lifshitz transition on the optical conductivity spectra, aiming to reveal the critical factors for the high-Tc superconducting phase induced by the Lifshitz transition.

FeSe基浅带超导体，由于其超导能隙和费米能接近，展现出许多奇特的性质。例如，系统处于BCS-BEC过渡区，电子关联强度、超导涨落大大增强；超导凝聚发生时费米面附近电子动能的变化处于实验可观测量级；容易诱导出Lifshitz相变，产生新超导相。实验观测和研究这些性质对理解铁基超导机理有重大意义。本项目利用傅立叶变换红外光谱对FeSe基浅带超导体进行研究，着重解决以下科学问题：(1)通过观测超导凝聚时费米面附近电子动能的变化，获得超导驱动机制，配对相互作用等信息；(2)分析系列掺杂FeSe基超导体的光电导谱，定性描述浅带效应对电子关联强度和准粒子散射过程的影响；(3)通过正常态光电导谱重分析，确定浅带超导体中是否存在BCS-BEC过渡区理论预期的强超导涨落，并确定其温度范围；(4)研究Lifshitz相变对光电导的影响，找到Lifshitz相变诱导高Tc超导相的关键因素。

浅带超导体中能带的带顶（或带底）到费米能级的距离很小，以至于接近超导能隙。这类超导体展现出许多奇特的性质。例如：系统处于BCS-BEC渡越区域，电子关联、超导配对强度、超导涨落都大大增强；可能出现s+is或s+id形式的超导能隙函数；容易诱导出Lifshitz转变，从而产生新的超导相或其它新颖量子态。本项目围绕浅带超导体以及具有浅带特征的其它相关材料，利用红外光谱手段研究浅带对超导电子配对势、能隙对称性、电子关联强度、准粒子散射过程的影响，并探索浅带诱导的BCS-BEC渡越区域的强耦合配对以及赝能隙等。在本项目支持下，获得了大量关键数据并取得了一系列重要结果，共发表SCI论文10篇。主要结果包括：(i)研究了具有浅带和初始带的铁基超导体KCa2Fe4As4F2的光电导谱，发现KCa2Fe4As4F2在超导态打开无节点强耦合超导能隙，且正常态存在赝能隙。而Ni掺杂消除浅带和初始带之后，超导能隙表现出BCS的弱耦合性质，赝能隙消失。这些结果表明强耦合配对和赝能隙是浅带和初始带诱导的，为铁基超导体配对机制的研究提供了重要信息；(ii)对拓扑绝缘体Sn-Bi1.1Sb0.9Te2S的红外光谱研究揭示了由墨西哥帽形反转能带形成的新型范霍夫奇点。这种费米能级附近的墨西哥帽形能带（浅平带）可能是拓扑绝缘体中容易诱导出超导和其它新颖量子态的原因，为寻找拓扑超导提供了新思路；(iii)Kagome材料CsV3Sb5在布里渊区的M点费米能级附近存在鞍点，这一特征和浅带具有相同的效应。系统测量了该材料不同温度的红外光谱，分析发现该材料发生电荷密度波转变后，鞍点处打开能隙，而其它能带上无能隙打开，这些实验结果表明费米面附近的鞍点之间的散射可能是驱动电荷密度波的主要因素，为该体系中电荷密度波驱动机制的研究提供了关键信息；(iv)系统研究了浅带铁基超导体FeSe的红外光谱，发现伴随电子向列相发生的dxz/dyz轨道劈裂导致光电导谱重从低频向高频转移，同时红外活性的Eu声子线型、线宽、共振频率和强度都发生异常，表明FeSe中的Eu声子和dxz/dyz轨道之间存在强耦合，为该材料中电子向列相的驱动机制研究提供了重要信息。