Unusual superconductivity in the noncentrosymmetric compound Re$_{6}$Zr

We report the electronic structure, synthesis, and measurements of the magnetic, transport, and thermal properties of polycrystalline Re$_{6}$Zr. A bulk superconducting transition temperature was observed at $T_{c}$ $sim$ 6.7 K. The density of states at the Fermi surface was found to be dominated by Re emph{d}-bands. X-ray diffraction confirmed the noncentrosymmetric $alpha$-Mn cubic structure of the compound. Using the measured values for the lower critical field $H_{c1}$(0), upper critical field $H_{c2}$(0), and the specific heat emph{C}, the superconducting parameters, such as the coherence length $xi$(0), penetration depth $lambda$(0), and the Ginzburg-Landau parameter $kappa$(0) were estimated. The specific heat jump at emph{$T_{c}$} suggests a strong electron-phonon coupling, while the electronic specific heat below $T_{c}$ suggests an isotropic gap structure. An increase in the total thermal conductivity $kappa$(T) below $T_{c}$ was attributed to an enhancement of the phonon contribution, due to a decrease in electron-phonon scattering. In applied magnetic fields, phonon-vortex scattering in the mixed state suppressed the peak in the thermal conductivity. The low temperature magnetic penetration depth was measured and found to follow isotropic gap behavior. The effect of chemical doping and physical pressure on $T_{c}$ were also investigated. Collectively, the measurements suggest the contribution from triplet paring to the superconducting order parameter is rather weak.