We present a combined density-functional-perturbation-theory and inelastic neutron scattering study of the lattice dynamical properties of YNi2B2C. In general, very good agreement was found between theory and experiment for both phonon energies and l
ine widths. Our analysis reveals that the strong coupling of certain low energy modes is linked to the presence of large displacements of the light atoms, i.e. B and C, which is unusual in view of the rather low phonon energies. Specific modes exhibiting a strong coupling to the electronic quasiparticles were investigated as a function of temperature. Their energies and line widths showed marked changes on cooling from room temperature to just above the superconducting transition at Tc = 15.2 K. Calculations simulating the effects of temperature allow to model the observed temperature dependence qualitatively.
We report an inelastic neutron scattering investigation of phonons with energies up to 159 meV in the conventional superconductor YNi$_2$B$_2$C. Using the SWEEP mode, a newly developed time-of-flight technique involving the continuous rotation of a s
ingle crystal specimen, allowed us to measure a four dimensional volume in (Q,E) space and, thus, determine the dispersion surface and linewidths of the $A_{1g}$ (~ 102 meV) and $A_u$ (~ 159 meV) type phonon modes for the whole Brillouin zone. Despite of having linewidths of $Gamma = 10 meV$, $A_{1g}$ modes do not strongly contribute to the total electron-phonon coupling constant $lambda$. However, experimental linewidths show a remarkable agreement with ab-initio calculations over the complete phonon energy range demonstrating the accuracy of such calculations in a rare comparison to a comprehensive experimental data set.
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 $2Delta$ 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.
We report high-resolution inelastic x-ray measurements of the soft phonon mode in the charge-density-wave compound TiSe$_2$. We observe a complete softening of a transverse optic phonon at the L point, i.e. q = (0.5, 0, 0.5), at T ~ T_{CDW}. Renormal
ized phonon energies are observed over a large wavevector range $(0.3, 0, 0.5) le mathbf{q} le (0.5, 0, 0.5)$. Detailed ab-initio calculations for the electronic and lattice dynamical properties of TiSe2 are in quantitative agreement with experimental frequencies for the phonon branch involving the soft mode. The observed broad range of renormalized phonon frequencies is directly related to a broad peak in the electronic susceptibility stabilizing the charge-density-wave ordered state. Our analysis demonstrates that a conventional electron-phonon coupling mechanism can explain a structural instability and the charge-density-wave order in TiSe_2 although other mechanisms might further boost the transition temperature.
We show that the superconducting energy gap $Delta$ can be directly observed in phonon spectra, as predicted by recent theories. In addition, since each phonon probes the gap on only a small part of the Fermi surface, the gap anisotropy can be studie
d in detail. Our neutron scattering investigation of the anisotropic conventional superconductor YNi$_2$B$_2$C demonstrates this new application of phonon spectroscopy.
