Unconventional superconducting gap via spin fluctuations in iron-vacancy-ordered AyFe2-xSe2

Based on an effective 12-orbital tight-binding model, we examine the superconducting states induced by the antiferromagnetic fluctuations for iron-vacancy-ordered A(y)Fe(2-x)Se(2). It is shown that, due to the broken reflection symmetry induced by the iron vacancies, new superconducting states with C-4h symmetry emerge. In particular, we show that in the C-4h symmetry, symmetric axes of the pairing momenta do not need to coincide with axes of the unit cell. As a result, in addition to the magnitude of the pairing gap, the relative orientation of the pairing wave function to the lattice forms another degree of freedom for characterizing the superconducting gap and can further help in gaining the condensation energy. Nonetheless, similarly to other iron-based superconductors, the singlet ground state is still dominated by the s wave or d wave, which are nearly degenerate with anisotropic gaps. Furthermore, s-wave and d-wave superconducting states are separated by a quantum critical point controlled by the Hund's rule coupling J(H).