Browsing by Author "Heid, R."

We show that the superconducting energy gap 2 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 studied in detail. Our neutron scattering investigation of the anisotropic conventional superconductor YNi(2)B(2)C demonstrates this new application of phonon spectroscopy.

We report an inelastic neutron scattering investigation of phonons with energies up to 159 meV in the conventional superconductor YNi2B2C. Using the sweep mode, a newly developed time-of-flight technique involving the continuous rotation of a single 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) (approximate to 102 meV) and A(u) (approximate to 159 meV) type phonon modes over 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.

Phouons in a metal interact with conduction electrons which give rise to a finite linewidth. In the normal state, this leads to a Lorentzian shape of the phonon line. Density functional theory is able to predict the phonon linewidths as a function of wave vector for each branch of the phonon dispersion. An experimental verification of such predictions is feasible only for compounds with very strong electron-phonon coupling. YN2B2C was chosen as a test example because it is a conventional superconductor with a fairly high T-c (15.2 K). Inelastic neutron scattering experiments did largely confirm the theoretical predictions. Moreover, they revealed a strong temperature dependence of the linewidths of some phonons with particularly strong electron phonon coupling which can as yet only qualitatively be accounted for by theory. For such phonons, marked changes of the phonon frequencies and linewidths were observed from room temperature down to 15 K. Further changes were observed on entering into the superconducting state. These changes can, however, not be described simply by a change of the phonon linewidth.

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 line 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 T-c = 15.2 K. Calculations simulating the effects of temperature allow us to model the observed temperature dependence qualitatively.