Anomalous gap ratio in anisotropic superconductors: aluminum under pressure

Pressure dependence of the thermodynamic critical field $B_{\mathrm{c}}$ in elemental aluminum was studied by means of the muon-spin rotation/relaxation technique. Pressure enhances the deviation of $B_{\mathrm{c}}(T)$ from the parabolic behavior, expected for a typical type-I superconductor, thus suggesting the weakening of the gap ratio $\langle\alpha\rangle=\langle \Delta\rangle/k_{\mathrm{B} }T_{\mathrm{c}}$ ($\langle\Delta\rangle$ is the average value of the superconducting energy gap, $T_{\mathrm{c}}$ is the transition temperature and $k_{\mathrm{B}}$ is the Boltzmann constant). With the pressure increase from 0.0 to $\simeq1.6$ GPa, $\langle\alpha\rangle$ decreases almost linearly from 1.73 to 1.67. Our results imply, therefore, that in elemental aluminum the gap ratio $\langle\alpha\rangle$ is smaller than the weak-coupled BCS prediction $\alpha_{\mathrm{BCS}}\simeq1.764$ and it is even further reduced under pressure.