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 YNi${}_{2}$B${}_{2}$C. In a previous study [Phys. Rev. Lett. 101, 237002 (2008)] it was shown that certain phonons exhibit a clear signature of the superconducting gap $2\ensuremath{\Delta}$ 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 ${B}_{c2}$.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 $B=3\phantom{\rule{0.28em}{0ex}}\mathrm{T}$, where the intervortex distance is less than $300\phantom{\rule{0.16em}{0ex}}$\AA{}, the phonon line shape still shows evidence for a variation of the gap.