Influence of iron vacancies on the transport properties of (Fe-Ni)Te_0.65Se_0.35 crystals

The role of iron vacancies in the improvement of superconductivity is studied in Fe_1−yNi_yTe_0.65Se_0.35 single crystals, with y in the range from 0 to 0.08, grown by Bridgman’s method using different crystallization rates. As shown previously [1], slow crystallization leads to crystals with single tetragonal phase, but with superconducting properties inferior to crystals obtained by fast crystallization, which contain nanometer size iron deficient regions. Using matrix formalism for multicarrier systems we extract carrier concentrations and their mobilities from the low-temperature Hall effect data for crystals with y>0.03, in which superconductivity is suppressed. The evolution of the majority carriers with increasing y is similar for all crystals, from holes for y<0.06 to electrons for y>0.06, indicating electron doping by Ni substitution. However, at low T the T-dependence of the majority and minority carrier concentrations and their mobilities differ significantly in crystals with and without iron vacancies. These differences lead to the conclusion that inhomogeneity induced by iron vacancies enhances electron doping, what most likely contributes to the enhancement of superconducting fluctuations. Ref.: [1] D. J. Gawryluk, et al., Supercond. Sci. Technol. 24, 065011 (2011).