Anisotropic superconductivity in layered silicides

CaAlSi is a layered silicide with the AlB2-structure and shows superconductivity below 7.7 K. The angular dependence of the upper critical field, Hc2(θ), in CaAlSi shows a cusp-like anomaly, suggesting the presence of decoupled two-dimensional superconducting layers. Concomitant with this feature, a strong lattice modulation along the c-axis exists in this material. On the other hand, Hc2(θ) in CaGaSi is well fitted by the anisotropic Ginzburg-Landau (GL) model, and the superstructure along the c-axis is absent. In order to get some insight into the origin of the anomalous Hc2(θ) in CaAlSi, we have performed magneto-optical observations of the vortex penetration for fields applied parallel and perpendicular to the c-axis. Vortices are found to penetrate the crystal inhomogeneously when the field is applied perpendicular to the c-axis, indicating the presence of macroscopic inhomogeneities in CaAlSi. We have also grown single crystals of CaAl1−xGaxSi and studied the evolution of anomalous Hc2(θ) and the lattice modulation along the c-axis. With increasing Ga content in CaAl1−xGaxSi, the superstructure along the c-axis survives up to x=0.2, although the anomaly in Hc2(θ) is strongly suppressed. It may suggest irrelevance of the two features, although it seems to be closely correlated in CaAlSi and CaGaSi. Possible scenarios for the appearance of the anomalous Hc2(θ) are proposed.