各向异性晶格调控对BiS2基超导体物质特性的影响研究

Since the discovery of BiS2 based superconductors in 2012, they, who share similarities in many aspects with Cu- and Fe-based superconductors, have drawn wide attention in the research area of superconductivity. Well known similarities include, for example, (a) that their structures consist of stacked blocking layers and superconducting layers, (b) that their critical superconducting temperature depends substantially on doping and on variation of lattice parameters, and (c) that their Fermi surfaces are close to a topological change at optimal doping, all of which being reminiscent of the properties of Cu- and Fe-based superconductors. These similarities, which come from different superconducting systems, provide much room in comparison study on their superconducting and other properties, and the outcome of such study is believed to be crucial for understanding the mechanisms of high temperature superconductivity. In this work, a relatively new lattice tuning method is going to be applied, i.e. piezo-electric based anisotropic strain tuning method, to disturb the lattice parameters and Fermi surface topology of La(O,F)BiS2, and to study the response of the critical temperature, the temperature dependence of resistivity and upper critical magnetic field as well as other properties of the system. The purpose of this study is to investigate the relationship between the microscopic structure, the bonding angle and distance, the characteristics of the Fermi surface and the physical properties of the system. The outcome of this study is going to be helpful in unrevealing the superconducting mechanism of BiS2 based superconductors, and is also going to be meaningful in a wider range to understand the influence to a superconductor of Lifshitz transition and Fermi surface nesting, and to understand the interplay between structural, magnetic, and superconducting or other instabilities. In the long term this work will contribute to clarifying the mechanisms of high temperature superconductivity.

由于在多个方面具有与铜基和铁基超导体类似的特性，近年来发现的BiS2基超导体引起了广泛关注。这些跨越材料体系的相似点，如结构上由超导层和隔离层交替组成、临界温度对掺杂浓度和晶格参数敏感、费米面靠近拓扑相变等，给高温超导领域带来了很大的对比研究空间。本项目将利用一种比较新的晶格调控技术，即基于压电陶瓷组件的各向异性形变产生技术，在不改变化学组成的情况下从不同方向扰动La(O,F)BiS2超导体的晶格参数和费米面形状，并观察材料电阻率、临界温度等物理特性的变化，来研究材料的微观结构、原子间成键距离和角度、费米面特征等与材料物理性质的关系。本项目的研究将不仅对理解BiS2基超导体的超导机制提供重要支撑，还将在更广泛的范围内，对理解费米面嵌套和费米面拓扑相变对材料特性的影响、低温下结构、磁和超导等各种不稳定性的合作或竞争关系等提供重要帮助，远期将为深入理解高温超导机制贡献力量。

项目针对近年来发现的BiS2基超导体开展了其在无应变调控和有应变调控情况下的超导特性研究。研究的主要背景为BiS2基超导体系与铜基和铁基超导体等著名高温超导体系在微观结构特性上具有相似性（如结构上具有层状特性，由超导层和隔离层交替组成），同时在特性上具有临界温度Tc对掺杂浓度和晶格参数敏感、费米面靠近拓扑相变等相似之处——这给高温超导领域带来了很大的对比研究空间。本项目利用一种各向异性晶格调控技术（即基于压电陶瓷组件的各向异性形变产生技术），在不改变化学组成的情况从特定方向扰动La(O,F)BiS2超导体的晶格参数，并观察材料临界温度Tc和临界磁场Bc2等物理特性的变化，来研究材料的微观结构、原子间成键距离和角度、费米面特征等与材料物理性质的关系。研究中本项目团队对处于最优掺杂附近的LaO0.5F0.5BSi2超导体施加了沿晶体面内主轴方向的力，并使其产生了显著的轴向应变，同时通过磁导率的变化观察和测量了样品在不同磁场和应变条件下的临界超导温度的变化。项目获得了这一材料的超导温度-磁场相图及其随应变的变化，发现其临界温度与临界磁场的关系曲线具有正的曲率，并发现其在应变状态下的Bc2与Tc平方的比值不是常数，预示着该超导体费米面处存在范霍夫奇点。项目的研究成果不仅对理解BiS2基超导体的超导机制有重要价值，同时对低温下结构、磁和超导等各种不稳定性的相互关系等具有启发意义，为深入理解高温超导机制提供了重要素材。