BiS $_2$基超导体中的电荷/四极涨落和带隙各向异性
Charge/quadrupole fluctuations and gap anisotropy in BiS$_2$-based superconductors
摘要：
最近的角分辨光谱研究表明，在$_2$基超导体中，超导能隙振幅具有显著的各向异性和/或在$X$点附近的小费米口袋上的符号变化。这意味着一种非常规配对状态的可能性。在扩展的哈伯德模型中，考虑Bi和S原子之间的相互作用，研究了由电荷/四极涨落引起的超导能隙各向异性.第一性原理下折能带结构由BiS $_2$单层上的Bi $6p_x/p_y$和S $3p_x/p_y$轨道组成。通过对线性化的能隙方程的计算，我们发现铁电电荷/四极子涨落由位间相互作用驱动，导致了完全能隙的$d_{x^2-y^2}$ -波配对态，在这种状态下，能隙振幅在费米面上具有相当大的各向异性.
英文摘要：
Recent angle-resolved spectroscopy in BiS$_2$-based superconductors has indicated that the superconducting gap amplitude possesses remarkable anisotropy and/or a sign change on a small Fermi pocket around $X$ point. It implies a possibility of an unconventional pairing state. Here we study the gap anisotropy in superconductivity mediated by inherent charge/quadrupole fluctuations in an extended Hubbard model, which includes inter-site interaction between Bi and S atoms. The first-principles downfolded band structure is composed of Bi $6p_x/p_y$ and S $3p_x/p_y$ orbitals on a BiS$_2$ single layer. Evaluating the linearized gap equation, we find that the ferroic charge/quadrupole fluctuation driven by the inter-site interaction leads to a fully-gapped $d_{x^2-y^2}$-wave pairing state, in which the gap amplitude has sizable anisotropy on the Fermi surface.
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