Comparative studies on unconventional superconductivity in Cr$_3$Ru compounds with bcc and A15 structures

Chromium (Cr) is a transition metal element with 3$d$ orbital electrons. In most compounds containing Cr, due to the correlation effect, twofold features of localization and itinerancy are expected. The localization gives rise to a magnetic moment, while the latter exhibits as the effective coherent weight for conductivity. Here we report the physical properties of Cr$_3$Ru compounds with bcc or A15 phases by using multiple experimental tools. The resistivity measurements show sharp superconducting transitions at $T_{\rm c}$ = 2.77 K and $T_{\rm c}$ = 3.37 K for the bcc and A15 structures. A high residual resistivity exists for both samples with the mean-free-path about 2 nm. Magnetization measurements also show rather narrow transitions, with a clear hump structure at high temperatures around 150 K, which may be ascribed to the remaining antiferromagnetic spin fluctuations. The specific heat coefficient reveals a clear jump at $T_{\rm c}$. We find that s-wave gaps can be adopted to fit the low temperature specific heat data yielding ratios of about $2\Delta/k_{\scriptscriptstyle B}T_{\rm c}$ $\approx$ 3.6, indicating a moderate pairing strength. Interestingly, the Wilson ratios are 3.81 and 3.62 for the bcc and A15 phases, suggesting a moderate correlation effect of conducting electron in the normal state. Besides, for samples with A15 structure, another specific heat anomaly occurs at about 0.85 K and is sensitive to magnetic fields. By applying high pressures, both system exhibit an enhancement of $T_{\rm c}$ with a rate of about 0.019 K/GPa and 0.013 K/GPa for the bcc and A15 phases. We also conduct tunneling spectrum measurements and found that the coherence peaks are strongly smeared out. The strong suppression to the coherence peaks may be ascribed to the strong scattering. Our combinatory results point to an unconventional superconductivity in these Cr based compounds.