Pressure dependence of the thermodynamic critical field $B_c$ in elemental aluminum was studied by means of the muon-spin rotation-relaxation technique. Pressure enhances the deviation of $B_c\left(T\right)$ from parabolic behavior, expected for a typical type-I superconductor, thus suggesting the weakening of the gap ratio $\langle \alpha \rangle =\langle \mathrm{\Delta }\rangle /k_B{T}_c$ ($\langle \mathrm{\Delta }\rangle$ is the average value of the superconducting energy gap, $T_c$ is the transition temperature, and $k_B$ is the Boltzmann constant). With the pressure increase from 0.0 to $\simeq 1.6$ GPa, $\langle \alpha \rangle$ decreases almost linearly from 1.73(1) to 1.67(1). Our results imply, therefore, that in elemental aluminum, the gap ratio $\langle \alpha \rangle$ is smaller than the weak-coupled BCS prediction ${\alpha }_{\mathrm{BCS}}\simeq 1.764$, and it is even further reduced under pressure.

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• Pressure effects
• Superconducting gap
• Superconducting phase transition
• Elemental superconductors
• Low-temperature superconductors
• Muon spin relaxation & rotation