  Superconductivity in the C32 compounds CaAlSi and SrAlSi is investigated by
heat capacity (Cp) measurements as well as high-pressure ac-susceptibility
experiments. The heat capacity in the superconducting state is well explained
by the BCS theory in the weak-coupling limit. The jump of Cp at the
superconducting transition (compared to the normal state electronic heat
capacity) is 1.62 and 1.21 for CaAlSi and SrAlSi, respectively, and agrees well
with the BCS value of 1.43. The magnitudes of the superconducting gaps are
estimated as 2.7 meV (CaAlSi) and 1.4 meV (SrAlSi). The pressure effect on the
superconducting transition temperature, dlnTc/dp, is positive in CaAlSi (+0.026
GPa-1) but negative in SrAlSi (-0.024 GPa-1). The opposite sign of the pressure
coefficients of Tc indicates distinct differences in the electronic structure
and the density of states at the Fermi energy of the two compounds in agreement
with recent band structure calculations.
