$B_{1g}$-like pairing states in two-leg ladder iron superconductors

TL;DR

This paper investigates the pairing symmetry in two-leg ladder iron superconductors, revealing a $B_{1g}$-like pairing state with sign-changing order parameters between orbitals, distinct from the typical $s_{ ext{±}}$ state, and identifies a first-order transition between phases.

Contribution

It introduces a detailed mean-field analysis of multi-orbital ladder models, identifying a novel $B_{1g}$ pairing state and the transition from planar to ladder geometries.

Findings

Superconducting order parameters change sign between $d_{xz}$ and $d_{yz}$ orbitals.

The leading pairing symmetry is $B_{1g}$, different from $A_{1g}$ $s_{ ext{±}}$ state.

A first-order transition occurs when interpolating from 2D planes to ladders.

Abstract

Motivated by the recent report of superconductivity in Fe-based ladder materials, we study the pairing state of a multi-orbital t-J model defined on two-leg ladders using the standard mean-field theory. We find that the superconducting order parameters change sign between the $d_{xz}$ and $d_{yz}$ orbitals in most of the phase diagram. By analogy with the two-dimensional Fe planes, we conclude that the leading pairing channel of this state belongs to the $B_{1g}$ symmetry class, which is distinct from the common $s_{\pm}$ gap with the $A_{1g}$ symmetry. By smoothly interpolating from planes into ladders, we show that a first-order transition occurs between these two competing phases when the dimension of the system is reduced.