Fe Vacancy Order and Domain Distribution in A x Fe2−y Se2

Abstract

The Fe vacancy and its ordering pattern is a unique property of the A x Fe2−y Se2 (A = K, Rb, Cs) superconductors. Below the structural transition temperature T S ∼ 550 K, the system phase separates into the I4/mmm phase with no Fe vacancy and the I4/m phase with the $\sqrt 5\times \sqrt 5\times 1$ vacancy order. In this work, an Ising-like Hamiltonian defined on the Fe-sublattice is constructed and employed in a Metropolis Monte Carlo simulation to study the Fe vacancy order and the domain distribution under different thermal treatments. A third phase with randomly distributed Fe vacancies and space group I4/mmm is observed in the simulation. The response of the volume fraction of this simulated phase under thermal treatment shows a direct connection to the change of the superconducting shielding fraction under annealing and quenching processes, indicating a close relation between the vacancy disordered I4/mmm phase and the superconducting phase.