Superconductivity and the effects of pressure and structure in single-crystalline SrNi2 P2

Heat capacity, magnetic susceptibility, NMR, and resistivity of SrNi2 P2 single crystals are presented, illustrating a purely structural transition at 325 K with no magnetism. Bulk superconductivity is found at 1.4 K. The magnitude of the transition temperature Tc, fits to the heat-capacity data, the small upper critical field Hc2 =390 Oe, and Ginzburg-Landau parameter κ=2.1 suggest a conventional fully gapped superconductor. With applied pressure a second structural phase transition occurs which results in an 8% reduction in the c/a ratio of lattice parameters. We find that superconductivity persists into this high-pressure phase, although the transition temperature is monotonically suppressed with increasing pressure. Comparison of these Ni-P data as well as layered Fe-As and Ni-As superconductor indicates that reduced dimensionality can be a mechanism for increasing the transition temperature.