Field-Induced Quasiparticle Excitation in Ca(Al${}_{0.5}$Si${}_{0.5}$)${}_2$: Evidence for unconventional Superconductivity

The temperature ($T$) and magnetic field ($H$) dependence of the magnetic penetration depth, $\lambda (T,H)$, in Ca(Al${}_{0.5}$Si${}_{0.5}$)${}_2$ exhibits significant deviation from that expected for conventional BCS superconductors. In particular, it is inferred from a field dependence of $\lambda (H)$ ($\propto H$) at 2.0 K that the quasiparticle excitation is strongly enhanced by the Doppler shift. This suggests that the superconducting order parameter in Ca(Al${}_{0.5}$Si${}_{0.5}$)${}_2$ is characterized by a small energy scale ${\mathrm{\Delta }}_S\mathrm{/}{k}_B\le 2$ K originating either from anisotropy or multi-gap structure.