Superconducting and normal-state properties of the noncentrosymmetric superconductor Re6Zr

We systematically investigate the normal and superconducting properties of non-centrosymmetric Re${}_6$Zr using magnetization, heat capacity, and electrical resistivity measurements. Resistivity measurements indicate Re${}_6$Zr has poor metallic behavior and is dominated by disorder. Re${}_6$Zr undergoes a superconducting transition at $T_{$\mathrm{c}$} = \left(6.75\pm0.05\right)$ K. Magnetization measurements give a lower critical field, ${\mu }_0{H}_{\mathrm{c}1}=(10.3\pm 0.1)$ mT. The Werthamer-Helfand-Hohenberg model is used to approximate the upper critical field ${\mu }_0{H}_{\mathrm{c}2}=(11.2\pm 0.2)$ T which is close to the Pauli limiting field of 12.35 T and which could indicate singlet-triplet mixing. However, low-temperature specific-heat data suggest that Re${}_6$Zr is an isotropic, fully gapped s-wave superconductor with enhanced electron-phonon coupling. Unusual flux pinning resulting in a peak effect is observed in the magnetization data, indicating an unconventional vortex state.