Probing the superconducting gap symmetry of $\alpha$-PdBi${}_2$: A penetration depth study

We report measurements of the in-plane London penetration depth $\lambda$ in single crystals of the $\alpha$-PdBi${}_2$ superconductor — the $\alpha$-phase counterpart of the putative topological superconductor $\beta$-PdBi${}_2$, down to 0.35~K using a high-resolution tunnel-diode-based technique. Both $\lambda$ and superfluid density ${\rho }_s$ exhibit an exponential behavior for $T\le$ 0.35$T_c$, with $\mathrm{\Delta }(0)\mathrm{/}{k}_B{T}_c\sim$2.0, $\mathrm{\Delta }C\mathrm{/}\gamma T_c$$\sim$2.0 and $\lambda (0)$$\sim$140~nm, showing that $\alpha$-PdBi${}_2$ is a moderately-coupling, fully-gapped superconductor. The values of $\mathrm{\Delta }(0)$ and $\mathrm{\Delta }C\mathrm{/}\gamma T_c$ are consistent with each other via strong-coupling corrections.