B₁g-like pairing states in two-leg ladder iron superconductors

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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 dxz and dyz 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 B₁g symmetry class, which is distinct from the common s± gap with the A₁g 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.