Strain-sensitive superconductivity in the kagome metals KV 3 Sb 5 and Cs V 3 Sb 5 probed by point-contact spectroscopy

The kagome lattice is host to flat bands, topological electronic structures, Van Hove singularities, and diverse electronic instabilities, providing an ideal platform for realizing highly tunable electronic states. Here, we report soft and mechanical point-contact spectroscopy (SPCS and MPCS) studies of the kagome superconductors ${\mathrm{KV}}_{3}{\mathrm{Sb}}_{5}$ and $\mathrm{Cs}{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$. Compared to the superconducting transition temperature ${T}_{\mathrm{c}}$ from specific heat and electrical resistance measurements, significantly enhanced values of ${T}_{\mathrm{c}}$ are observed via the zero-bias conductance of SPCS, which become further enhanced in MPCS measurements. While the differential conductance curves from SPCS can be described by a two-gap $s$-wave model, a single $s$-wave gap reasonably captures the MPCS data, likely due to a diminishing spectral weight of the other gap. The enhanced superconductivity probably arises from local strain caused by the point contact, which also leads to two-gap or single-gap behaviors observed in different point contacts. Our results demonstrate highly strain-sensitive superconductivity in kagome metals $\mathrm{Cs}{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$ and ${\mathrm{KV}}_{3}{\mathrm{Sb}}_{5}$, which may be harnessed in the manipulation of possible Majorana zero modes.