Theoretical Study Of The Physical Properties In BiS₂ Superconductors

Since the discovery of superconductivity in Bi4O4S3,the BiS2-based supercon-ductors have attracted a lot of research interests. Similar to cuprates and iron-based superconductors, the BiS2-based superconductors are also layered in structure. In the past two years, a lot of new BiS2 superconductors are discovered and some anoma-lous properties of BiS2 superconductors are revealed by experiments. While ongoing efforts are aimed at new superconductors with the BiS2-layers, the underlying pairing mechanism also becomes an important issue, and whether BiS2 superconductors are conventional superconductors is still under debate. Recent angle-resolved photoemis-sion reveals that the doping level in some BiS2-based superconductors is much lower than the nominal one, which was widely used in previous theoretical study. So here we determine the pairing function at such a realistic low doping level by functional renor-malization group (FRG), and study the resulting physical properties by an effective mean field theory.We find that the pairing is a mixture of singlets and triplets due to spin-orbital coupling (SOC), and the triplet components dominate. The gap function transforms as d*/χ2-y2-wave under simultaneous rotation of spin and lattice, but is nodeless on the Fermi pockets which avoid the nodal lines. By the FRG-based mean field theory, we examine the behaviors of the superconducting phase. The specific heat, superfluid density and Knight shift are indistinguishable to that in a fully gapped conventional s-wave superconductor, in agreement with available experiments. However, the absence of the Hebel-Slithcer peak in spin-lattice relaxation rate (in the dirty limit), the impurity induced in-gap states and the gapless edge states (due to the weak topological nature of the pairing) are defining properties of the nodeless d*/χ2-y2-wave pairing to be measured in future experiments.