Comparison of physical properties in BaAlSi and CaAlSi

CaAlSi, with the AlB2-type structure, is a superconductor with relatively high Tc of 7.7 K and the related compounds, SrAlSi and BaAlSi have the same crystal structure. Despite the fact that the density of states in SrAlSi and BaAlSi is higher than that of CaAlSi, Tc of SrAlSi is 4.2 K and BaAlSi does not show superconductivity down to 2 K. We synthesize polycrystalline and single crystalline BaAlSi and compare their physical properties with those in CaAlSi. The transport and the specific heat measurements show the higher density of states in BaAlSi, consistent with the band structure calculations. We interpret that the absence of superconductivity in BaAlSi is due to the much weaker electron-phonon coupling constant compared with that in CaAlSi, because of the absence of the soft-phonon mode. In the course of the study, we find a superconductor with a composition close to Ba2Al4Si3.