Superconducting energy gap from break-junction tunneling spectroscopy in the ternary silicide CaAlSi

Tunneling spectroscopy using a break-junction technique has been conducted to determine the superconducting gap structure of the ternary silicide-layered superconductor CaAlSi with and without superstructure. We find that the tunneling conductance in CaAlSi shows a single gap feature, which can be described by the BCS density of states. The gap size $\ensuremath{\Delta}$ obtained from the tunneling conductance in CaAlSi without superstructure is estimated to be approximately $1.0\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, corresponding to a weak-coupling gap ratio. On the other hand, for a specimen with superstructure, the largest gap, $\ensuremath{\Delta}=1.5\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, significantly deviates from the BCS value, indicating strong-coupling superconductivity. These results demonstrate systematic variations in the pairing interaction with superstructure formations, which are consistent with heat capacity measurements.