Three-dimensional superconductivity in nominal composition Fe1.03Se with Tc zero up to 10.9 K induced by internal strain

Iron selenide with the nominal composition Fe1.03Se was synthesized by a self-flux solution method which shows a zero resistance temperature up to 10.9 K and a Tconset (90% ρ_n, ρ_n: normal state resistivity) up to 13.3 K. The residual resistivity ratio RRR=R(300 K)/R0=12. Scanning electron microscopy-EDX study shows that the composition of as grown sample is near stoichiometric FeSe. The decrease in superconducting transition temperature by heat treatment indicates that internal crystallographic strain which plays the same effect as external pressure is the origin of its high T_c. The fluctuation conductivity was studied which could be well described by three-dimensional (3D) Aslamazov-Larkin power law. The estimated value of coherence length ξ_c=9.2 Å is larger than the distance between conducting layers (∼6.0 Å), indicating the 3D nature of superconductivity in this compound.