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

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 2&Dgr; 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 Â, the phonon line shape still shows evidence for a variation of the gap. [ABSTRACT FROM AUTHOR]