Abstract
The compound Sr0.5Ce0.5FBiS2 belongs to the intensively studied family of layered BiS2 superconductors. It attracts special attention because superconductivity at Tsc = 2.8 K was found to coexist with local-moment ferromagnetic order with a Curie temperature TC = 7.5 K. Recently it was reported that upon replacing S by Se TC drops and ferromagnetism becomes of an itinerant nature. At the same time Tsc 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−xSex 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 TC = 3.4 K and TC = 3.3 K, respectively. For x = 1.0 (Tsc = 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.
Highlights
The interplay between superconductivity and magnetism has been a central issue in superconductivity research for several decades
We report muon spin relaxation and rotation experiments on Sr0.5Ce0.5FBiS2−xSex conducted to investigate the coexistence of superconductivity and ferromagnetism on the microscopic scale. μSR is the technique par excellence to probe small magnetic moments, as well as to determine the superconducting and magnetic volume fractions in a crystal[22]
These results sharply contrast with μSR spectra measured for the superconducting itinerant ferromagnet UCoGe with Tsc = 0.5 K and TC = 3.0 K24
Summary
The interplay between superconductivity and magnetism has been a central issue in superconductivity research for several decades now. The general idea is that the ferromagnetic exchange field impedes the formation of spin-singlet Cooper pairs that is prescribed by the microscopy theory of Bardeen, Cooper and Schrieffer (BCS)[6] Notwithstanding this restriction, the search for ferromagnetic superconductors continued unremittingly. In a small group of uranium-based correlated metals formed by UGe2 (under pressure15), URhGe16 and UCoGe17, ferromagnetism and superconductivity do coexist on the microscopic scale and are carried by the same 5f electrons This is corroborated by the itinerant nature of the ferromagnetic state. They argue magnetic order and superconductivity are carried by the same type of electrons for x ≥ 0.5 This and the small size of the ordered moment lead them to draw a close parallel between Sr0.5Ce0.5FBiS2−xSex and UCoGe as regards the coexistence of superconductivity and itinerant ferromagnetism.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
