Macroscopic phase separation of superconductivity and ferromagnetism in Sr 0.5 Ce 0.5 FBiS 2?x Se x revealed by μ SR

摘要

The compound Sr0.5Ce0.5FBiS2 belongs to the intensively studied family of layered BiS2 superconductors. It attracts special attention because superconductivity at T sc =?2.8 K was found to coexist with local-moment ferromagnetic order with a Curie temperature T C =?7.5 K. Recently it was reported that upon replacing S by Se T C drops and ferromagnetism becomes of an itinerant nature. At the same time T sc increases and it was argued superconductivity coexists with itinerant ferromagnetism. Here we report a muon spin rotation and relaxation study (μSR) conducted to investigate the coexistence of superconductivity and ferromagnetic order in Sr0.5Ce0.5FBiS2?x Se x with x?=?0.5 and 1.0. By inspecting the muon asymmetry function we find that both phases do not coexist on the microscopic scale, but occupy different sample volumes. For x?=?0.5 and x?=?1.0 we find a ferromagnetic volume fraction of ~8 % and ~30 % at T?=?0.25 K, well below T C =?3.4 K and T C =?3.3 K, respectively. For x?=?1.0 (T sc =?2.9 K) the superconducting phase occupies most (~64 %) of the remaining sample volume, as shown by transverse field experiments that probe the Gaussian damping due to the vortex lattice. We conclude ferromagnetism and superconductivity are macroscopically phase separated.