Phonon lineshapes in the vortex state of the phonon-mediated superconductor YNi$_2$B$_2$C

TL;DR

This study uses inelastic neutron scattering to investigate how phonon lineshapes in YNi$_2$B$_2$C reflect the inhomogeneous vortex state in a superconductor under magnetic fields below the upper critical field.

Contribution

It demonstrates that phonon lineshapes can serve as local probes of the spatial variation of the superconducting gap in the vortex state.

Findings

Phonon lineshapes reflect the inhomogeneous vortex state.

Even at 3 T, gap variation is detectable via phonon lineshapes.

Phonon lineshapes can be modeled considering local superconducting gap variations.

Abstract

We present an inelastic neutron scattering study of phonon lineshapes in the vortex state of the type-II superconductor YNi$_2$B$_2$C. In a previous study [Phys. Rev. Lett. \textbf{101}, 237002 (2008)] it was shown that certain phonons exhibit a clear signature of the superconducting gap $2\Delta$ on entering the superconducting state. Our interest was to find out whether or not the lineshape 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 lineshapes 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 $B=3\,\rm{T}$, where the inter-vortex distance is less than $300\,$\AA, the phonon lineshape still shows evidence for a variation of the gap.