Superconducting Properties of the Non-Centrosymmetric Superconductor Re6Hf

The synthesis and detailed characterization of the noncentrosymmetric superconductor Re$_{6}$Hf using powder x-ray diffraction (XRD), magnetization, transport and thermodynamic measurements are reported. XRD confirms the crystal structure of Re$_{6}$Hf to be cubic noncentrosymmetric $\alpha$-\textit{Mn} with a lattice parameter a = 9.6850(3)\text{\AA}. Resistivity, DC and AC magnetization confirms the bulk superconducting transition T$_{c}^{onset} \sim $ ~ 5.96 K. The lower critical field H$_{C1}(0)\approx$ 5.6 mT and upper critical H$_{C2}(0)$ was found to be around 12.2 T which is greater than the Pauli limiting field, indicating unconventional behavior in Re$_{6}$Hf and leads to the possibility of triplet pairing. A sharp discontinuity in the specific heat data was observed around T$_{c}$ indicating bulk superconductivity, and the normalized specific heat jump is $\Delta C_{el}$/$\gamma_{n}$T$_{c}$ = 1.53 which is in close agreement with BCS value. Superconducting electronic specific heat data is fitted reasonably well for an exponential relation with Sommerfeld coefficient ($\gamma$) showing a linear relation with magnetic field suggesting that in Re$_{6}$Hf dominant pairing channel is s-wave with isotropic superconducting gap parameter without nodes.