Identification of Various Coexisting Phases in Superconducting and Non-superconducting Samples of RbxFe2−ySe2

Abstract

87Rb NMR, muon spin relaxation (μSR), and neutron diffraction studies have been carried out to investigate how the superconducting (SC) fraction is determined in RbxFe2−ySe2 samples. We observe several phases with different compositions, crystal structures, and magnetic properties of each sample. Samples containing a phase with superconductivity (SC phase) are separated into two phases, an antiferromagnetic (AFM) phase with the block-type antiferromagnetism (AFM1 phase; space group I4/m, Neel temperature TN1 ∼ 550 K) and the SC phase (I4/mmm, SC transition temperature Tc ∼ 30 K). This SC phase has a very small amount of Fe vacancies, if any. In samples having no SC phase, besides the AFM1 phase, there are two other phases, an AFM phase (AFM2 phase; Pmna, TN2 ∼ 150 K) and a phase with an appreciable amount of Fe vacancy (I4/mmm; non-SC phase). Thus, the presence of the coexisting phases is an origin of the complicated physical behavior dependent on the \((x,y)\) values observed in the RbxFe2−ySe2 samples.