Possible pairing symmetries of the ordered honeycomb network superconductor BaPtSb

The series of superconductors with ordered honeycomb network structures has been discovered, and we focus in this paper on the recently discovered one BaPtSb with inversion-symmetry breaking (Tc=1.6K, the point group; D3h) [1]. The first principle calculation reveals that the anti-symmetric spin-orbit coupling works significantly and causes band splitting [2]. Although it is still preliminary, an observation of spontaneous field below Tc by the muon spin relaxation experiment has been reported. The result seems to be a sign of broken time-reversal symmetry breaking in the superconducting phase [3].

In this paper, we classify the pairing symmetry based on the group theoretical analysis and examine the pairing instability using the extended Hubbard type attractive interaction. Among the unconventional states with time-reversal symmetry breaking, we find that the preferable state with gap structure compatible with Fermi surfaces is a spin-triplet chiral state with Sz=0. This pairing symmetry supports the chiral surface state at the sample boundary.