Magnetic fluctuations in FeSe(1-delta) and Cu-doped FeSe(1-delta): (77)Se NMR experiments

Abstract

The roles of Se deficiency and Fe atoms, for the superconductivity of FeSe, have been, respectively, investigated in FeSe(1-delta) and (Fe(1-x)Cu(x))Se(1-delta), by our NMR experiments. The data, for nuclear spin-lattice relaxation rate (1/T(1)), show that the spin fluctuations are weakened at a larger delta, and are correlated with the superconductivity in FeSe. The superconducting volume fraction, estimated by our ac susceptibility experiments, is found to vary inversely with delta. Our findings suggest that the Se-deficient FeSe has an inhomogeneous phase, where the superconductivity is associated with the regions having few or no Se vacancies. As for (Fe(1-x)Cu(x))Se(1-delta), T(c) is rapidly suppressed by Cu doping and vanishes around x=0.03. The (77)Se and the (63)Cu NMR linewidths suggest that a local moment is induced at the Fe sites and not at the Cu sites. However, 1/T(1) shows no obvious change with the Cu doping. We suspect that other effects, such as disorder or change in the density of states, have more influence on T(c) suppression since a metal-insulator transition, induced by Cu substitution, occurs in the resistivity measurements.