Charge and quadrupole fluctuations and gap anisotropy in BiS2-based superconductors

Recent angle-resolved spectroscopy in BiS2-based superconductors has indicated that the superconducting gap amplitude possesses remarkable anisotropy and/or a sign change on a small Fermi pocket around the X point. It implies a possibility of an unconventional pairing state. Here we study the gap anisotropy in superconductivity mediated by inherent charge and quadrupole fluctuations in an extended Hubbard model, which includes intersite interaction between Bi and S atoms. The first-principles downfolded band structure is composed of Bi 6p(x)/p(y) and S 3p(x)/p(y) orbitals on a BiS2 single layer. Evaluating the linearized gap equation, we find that the ferroic charge and quadrupole fluctuation driven by the intersite interaction leads to a fully gapped dx(2)-y(2)-wave pairing state, in which the gap amplitude has sizable anisotropy on the Fermi surface.