Unconventional superconducting properties of noncentrosymmetric Re5.5Ta

Rhenium based noncentrosymmetric superconductors crystallizing in alpha-Mn structure have become potential candidates to exhibit an unconventional superconducting ground state. Here we report a detailed investigation on the superconducting and normal state properties of Re5.5Ta, that also has the alpha-Mn structure. Magnetization, specific heat, and transport measurements confirm the bulk superconducting transition T-C at 8.0 K. Upper critical field value [H-C2(0)] calculated from magnetization, specific heat, and AC transport measurements exceed the Pauli paramagnetic limit (14.7 T), indicating that the superconducting properties of Re5.5Ta are probably unconventional in nature. However, low-temperature specific heat and transverse-field muon spin rotation measurements suggest a surprising nodeless isotropic superconducting gap, although with strong electron-phonon coupling.