We report measurements of the electrical resistivity and the specific heat at the structural transition and in the superconducting state at low temperatures for samples with composition ${\mathrm{La}}_{3\mathrm{-}\mathrm{x}}$${\mathrm{S}}_4$, ${\mathrm{La}}_{3\mathrm{-}\mathrm{x}}$${\mathrm{Se}}_4$, and ${\mathrm{La}}_3$${\mathrm{Te}}_4$. The results show that the structural transition and the anomalous superconducting properties of ${\mathrm{La}}_3$${\mathrm{S}}_4$ and ${\mathrm{La}}_3$${\mathrm{Se}}_4$ are due to a band-structure anomaly at the Fermi level.

• Received 13 November 1984

DOI:https://doi.org/10.1103/PhysRevB.32.2985