Phonon line shapes in the vortex state of the phonon-mediated superconductor YNi₂B₂C

Abstract

We present an inelastic neutron-scattering study of phonon line shapes in the vortex state of the type II superconductor YNi2B2C. In a previous study [ Phys. Rev. Lett. 101 237002 (2008)] it was shown that certain phonons exhibit a clear signature of the superconducting gap 2Δ on entering the superconducting state. Our interest was to find out whether or not the line shape of such phonons reflects the inhomogeneous nature of the vortex state induced by a magnetic field smaller than the upper critical field Bc2.We found that this is indeed the case because the observed phonon line shapes can be well described by a model considering the phonon as a local probe of the spatial variation of the superconducting gap. We found that even at B=3 T, where the intervortex distance is less than 300 Å, the phonon line shape still shows evidence for a variation of the gap.