Spin-orbit coupling, minimal model and potential Cooper-pairing from repulsion in BiS$_2$-superconductors

TL;DR

This paper develops a minimal model for BiS$_2$ superconductors incorporating spin-orbit coupling, analyzing potential pairing mechanisms from repulsive interactions and predicting gap symmetries and Fermi surface features.

Contribution

It introduces a realistic low-energy model including spin-orbit coupling and explores unconventional pairing instabilities in BiS$_2$ superconductors.

Findings

Dominant $d_{x^2-y^2}$-wave and $s_{ ext{±}}$-wave instabilities at different doping levels

Superconducting gap anisotropy consistent with ARPES data

Fermi surface topology at high doping resembles iron-based pnictides

Abstract

We develop the realistic minimal electronic model for recently discovered BiS$_2$ superconductors including the spin-orbit coupling based on a first-principles band structure calculations. Due to strong spin-orbit coupling, characteristic for the Bi-based systems, the tight-binding low-energy model necessarily includes $p_x$, $p_y$, and $p_z$ orbitals. We analyze a potential Cooper-pairing instability from purely repulsive interaction for the moderate electronic correlations using the so-called leading angular harmonics approximation (LAHA). For small and intermediate doping concentrations we find the dominant instabilities to be $d_{x^2-y^2}$-wave, and $s_{\pm}$-wave symmetries, respectively. At the same time, in the absence of the sizable spin fluctuations the intra and interband Coulomb repulsion are of the same strength, which yields the strongly anisotropic behaviour of the superconducting gaps on the Fermi surface in agreement with recent ARPES findings. In addition, we find that the Fermi surface topology for BiS$_2$ layered systems at large electron doping can resembles the doped iron-based pnictide superconductors with electron and hole Fermi surfaces with sufficient nesting between them. This could provide further boost to increase $T_c$ in these systems.