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

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 (π,π) display nearly parallel Fermi Surface segments close to wavevector(π/2,π/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.

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