SWC Seminar - Nanoscale Phase Separation in an Iron Based Superconductor | Shull Wollan Center
The AxFe2-ySe₂ (A = K, Rb, Cs) iron selenide superconductors have been intensely studied since their discovery in part due to the interplay between magnetism and superconductivity, and in part due to the possible role of Fe-vacancy order in the superconducting mechanism, and whether or not phase separation occurs between superconducting (SC) and non-superconducting (NSC) regions. In the high temperature phase, the vacancies are randomly distributed. Upon cooling below the structural transition, superlattice structures appear due to Fe vacancy and alkali metal order. Several scenarios have been proposed regarding the nature of the crystal structure below the transition, includig one that describes a lattice that is phase separated into a minority I4/mmm phase which is compressed in-plane and extended out-of-plane in comparison to the high temperature centrosymmetric phase and has no Fe vacancies, and an I4/m phase with the Fe vacancies ordered in different superlattice patterns. Moreover, magnetic ordering is characteristic of the vacancy-ordered phase. The antiferromagnetic state, commonly reported in the literature, is robust unlike what has been observed in all other Fe-based superconductors, and its coexistence with the superconducting state has raised concerns about the validity of the s+/- coupling mechanism coupled with the absence of hole pockets at the Fermi surface and the lack of nesting in this system. The I4/mmm phase with no Fe vacancy has largely been attributed to be the host of superconductivity in part because of the absence of magnetism and vacancies at least at the Fe site. It is understood at present that by post-annealing and quenching, superconductivity can be enhanced in this system even though the actual mechanism remains unknown. Magnetic refinement from neutron powder diffraction measurements revealed that magnetic order does not exclude the presence of a SC phase. To investigate this issue further, high energy X-ray scattering measurements were performed on two kinds of KxFe2-ySe2single crystals, one annealed and SC, and the other as-grown and NSC. In combination with Monte Carlo simulation, it is shown that superconductivity in quenched crystals is most likely present in regions between the I4/m domain boundaries, bordering the I4/mmm domains with no Fe vacancies. Thus superconductivity in this system appears at the crossover of the vacancy order-disorder transition. Quenching increases the boundary walls around the I4/m domains, leading to an increase of the percolation paths and an enhancement of the SC volume fraction.