Fermi-surface topology and pairing symmetry in BiS2-based layered superconductors

• We theoretically investigate superconducting properties of LaO1−xFxBiS2LaO1−xFxBiS2.
• The superconducting transition temperature TcTc becomes maximum at x=0.6.
• For x<0.45x<0.45, we obtain the d-wave gap of which line nodes avoid the Fermi surface.
• For x>0.45x>0.45, the extended s-wave gap is formed on the large Fermi surface.
• The Fermi-surface topology plays a key role for the gap symmetry of BiS2 systems.

In order to clarify superconducting properties of BiS2-based layered superconductors LaO1−xFxBiS2LaO1−xFxBiS2, we evaluate the superconducting transition temperature TcTc and analyze the gap function in a weak-coupling limit on the basis of the two-dimensional two-orbital Hubbard model. It is found that TcTc becomes maximum at x  =0.6 in the present calculations. For x<0.45x<0.45, we obtain the d  -wave gap of which line nodes do not cross the pocket-like Fermi-surface curves with the centers at X and Y points. On the other hand, for x>0.45x>0.45, the extended s-wave gap is found for a couple of large Fermi-surface curves with the centers at M and X points. The variation of the Fermi-surface topology plays a key role for the gap symmetry in BiS2-based superconductors.