Effective carrier type and field dependence of the reduced-T-c superconducting state in SrFe2-xNixAs2

Measurements of the Hall effect, thermoelectric power, magnetic susceptibility, and upper and lower critical fields were performed on single crystals of SrFe2-xNixAs2, an FeAs-based superconducting system that exhibits a reduced superconducting transition temperature T-c in comparison to most other iron-pnictide superconductors. Studies of the Hall and thermoelectric responses indicate that Ni substitution in this system results in a dominant electronlike response, consistent with electron doping in other similar systems but with a weaker change in the Hall coefficient and a more gradual change in the thermoelectric response with Ni concentration. For optimally doped samples with full superconducting volume fraction, the lower and upper critical fields were determined to be H-c1(1.8 K)=0.08 T and H-c2(0)=25 T, respectively, with lower-T-c samples showing reduced values and indications of inhomogeneous superconductivity. Comparable to other higher-T-c FeAs-based materials, the temperature dependence of the upper critical field, delta H-c2/delta T, is linear over a wide temperature range, and the large values of H-c2(0) greatly exceed conventional estimates of paramagnetic and orbital limits.