Strong bilayer coupling induced by the symmetry breaking in the monoclinic phase of BiS$_2$-based superconductors

TL;DR

This study reveals that symmetry breaking in the monoclinic phase of BiS$_2$-based superconductors significantly enhances interlayer coupling, leading to notable band splitting and Fermi surface anisotropy, as shown by first-principles calculations.

Contribution

It demonstrates the strong bilayer coupling induced by symmetry breaking in the monoclinic phase of BiS$_2$ superconductors through detailed band structure analysis.

Findings

Large Bi $6p_{x,y}$ band splitting at key extbf{k}-points in monoclinic structure

Enhanced interlayer coupling due to symmetry breaking

Fermi surface becomes anisotropic in the monoclinic phase

Abstract

We perform first-principles band structure calculations for the tetragonal and monoclinic structures of LaO$_{0.5}$F$_{0.5}$BiS$_2$. We find that the Bi $6p_{x,y}$ bands on two BiS$_2$ layers exhibit a sizable splitting at the X = ($\pi$, 0, 0) and several other \textbf{k}-points for the monoclinic structure. We show that this feature originates from the inter-BiS$_2$ layer coupling strongly enhanced by the symmetry breaking of the crystal structure. The Fermi surface also shows a large splitting and becomes anisotropic with respect to the $k_x$ and $k_y$ directions in the monoclinic structure, whereas it remains almost flat with respect to the $k_z$ direction.