Unconventional Pressure Dependence of the Superfluid Density in the Nodeless Topological Superconductor α − PdBi 2

We investigated the superconducting properties of the topological superconductor $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{PdBi}}_{2}$ at ambient and external pressures up to 1.77 GPa using muon spin rotation experiments. The ambient pressure measurements evince a fully gapped $s$-wave superconducting state in the bulk of the specimen. Alternating current magnetic susceptibility and muon spin rotation measurements manifest a continuous suppression of ${T}_{c}$ with increasing pressure. In parallel, we observed a significant decrease of superfluid density by $\ensuremath{\sim}20%$ upon application of external pressure. Remarkably, the superfluid density follows a linear relation with ${T}_{c}$, which was found before in some unconventional topological superconductors and hole-doped cuprates. This finding signals a possible crossover from Bose-Einstein to Bardeen-Cooper-Schrieffer like condensation in $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{PdBi}}_{2}$.