Two superconducting gaps with quasiparticle-lifetime broadening in single-crystal V₃Si revealed by field and temperature dependence of NMR spin-lattice relaxation rate 1/T₁ of ⁵¹V

The A15 superconductor V₃Si – though the subject of nuclear magnetic resonance (NMR) studies for decades – continues to yield insights into the superconducting transition. The measurement of spin-lattice longitudinal relaxation rate, 1/T1, is a probe into the energy density of states near the Fermi energy, allowing us to model the superconducting gap. We have found that 1/T1 could be measured more reliably: (1) with a high-purity, single crystal, which in turn (2) allows for an orientation with field along the [111] direction, thus (3) producing a spectrum with one, central transition that follows a single-exponential relaxation curve. Our model for temperature dependence down to less than 100 mK, under fields up to 15 T, clearly indicates two gaps and a “rounding” of the internal gap edge, attributed to the broadening of quasiparticle lifetimes, following a model originally by Dynes et al in 1978. This behavior suggests an additional mechanism through which vortex penetration accelerates the closing of the gap as critical temperature or field is approached. Gap width and quasiparticle broadening were also found to have contrasting field dependence for the two gaps, in contrast to results from other recent measurements.