Unconventional superconducting gap via spin fluctuations in iron-vacancy ordered A$_{y}$Fe$_{2-x}$Se$_{2}$

TL;DR

This paper investigates unconventional superconducting states in iron-vacancy-ordered A$_{y}$Fe$_{2-x}$Se$_{2}$, revealing new symmetry-induced pairing states influenced by spin fluctuations and vacancy-induced symmetry breaking.

Contribution

It introduces a 12-orbital model showing how iron vacancies induce new superconducting states with $C_{4h}$ symmetry and explores their properties and phase transitions.

Findings

Emergence of $C_{4h}$ symmetric superconducting states due to vacancy-induced symmetry breaking.

Degenerate s-wave and d-wave pairing states separated by a quantum critical point.

Relative orientation of pairing wave function adds a new degree of freedom for superconducting gap characterization.

Abstract

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, similar to other iron-based superconductors, the singlet ground state is still dominated by 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 $J_{H}$.