RPA Analysis of a Two-orbital Model for the BiS2-based Superconductors

TL;DR

This study applies RPA to a two-orbital model for BiS2 superconductors, revealing competing pairing states influenced by spin fluctuations and Fermi surface nesting, with implications for experimental observations.

Contribution

It demonstrates the near degeneracy of A1g and B2g superconducting states in a BiS2 model using RPA, highlighting the role of spin fluctuations and Fermi surface nesting.

Findings

Competing A1g and B2g superconducting states with similar pairing strengths.

Fermi surface segments near wavevector(,) influence pairing interactions.

Spin fluctuations promote local pair scattering in k-space.

Abstract

The random-phase approximation (RPA) is here applied to a two-orbital model for the BiS2-based superconductors that was recently proposed by Usui et al., arXiv:1207.3888. Varying the density of doped electrons per Bi site, n, in the range 0.46 < n < 1.0, the spin fluctuations promote competing A1g and B2g superconducting states with similar pairing strengths, in analogy with the A1g-B1g near degeneracy found also within RPA in models for pnictides. At these band fillings, two hole-pockets centered at (0,0) and (\pi,\pi) display nearly parallel Fermi Surface segments close to wavevector(\pi/2,\pi/2), whose distance increases with n. After introducing electronic interactions treated in the RPA, the inter-pocket nesting of these segments leads to pair scattering with a rather "local" character in k-space. The similarity between the A1g and B2g channels observed here should manifest in experiments on BiS2-based superconductors if the pairing is caused by spin fluctuations.