Large enhancement of superconducting transition temperature of SrBi3 induced by Na substitution for Sr

The Matthias rule, which is an empirical correlation between the superconducting transition temperature (T-c) and the average number of valence electrons per atom (n) in alloys and intermetallic compounds, has been used in the past as a guiding principle to search for new superconductors with higher T-c. The intermetallic compound SrBi3 (AuCu3 structure) exhibits a T-c of 5.6 K. An ab-initio electronic band structure calculation for SrBi3 predicted that T-c increases on decreasing the Fermi energy, i.e., on decreasing n, because of a steep increase in the density of states. In this study, we demonstrated that high-pressure (similar to 3 GPa) and low-temperature (<350 degrees C) synthesis conditions enables the substitution of Na for about 40 at.% of Sr. With a consequent decrease in n, the T-c of (Sr,Na)Bi-3 increases to 9.0 K. A new high-T-c peak is observed in the oscillatory dependence of T-c on n in compounds with the AuCu3 structure. We have shown that the oscillatory dependence of T-c is in good agreement with the band structure calculation. Our experiments reaffirm the importance of controlling the number of electrons in intermetallic compounds.