Vanadium-based superconductivity in the breathing kagome compound Ta2V3.1Si0.9

Superconductivity in V-based kagome metals has recently raised great interest as they exhibit the competing ground states associated with the flat bands and topological electronic structures. Here we report the discovery of superconductivity in ${\mathrm{Ta}}₂{\mathrm{V}}_3.1{\mathrm{Si}}_0.9$ with a superconducting transition temperature ${T}_c$ of 7.5 K, a record high for kagome metals at ambient pressure. While the V ions form a two-dimensional breathing kagome structure, the length difference between two different V-V bonds is just $0.05\phantom{\rule{0.16em}{0ex}}\AA{}$, making it very close to the perfect kagome structure. Our results show that ${\mathrm{Ta}}₂{\mathrm{V}}_3.1{\mathrm{Si}}_0.9$ is a moderate-coupled type-II superconductor with a large upper critical field that is close to the Pauli limit. DFT calculations give a Van Hove singularity band located at Fermi energy, which may explain the relatively high ${T}_c$ observed in this material. Furthermore, electron-phonon coupling calculations indicate that the in-plane vibrations of the V atoms play a significant role in driving the superconductivity.