Phonons 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.
KeywordsElectron-phonon coupling density functional theory inelastic neutron scattering
PACS Nos63.20.dd 63.20.dk 63.20.kd
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- The DFT calculations were carried out within the framework of the mixed basis pseudopotential method using the local density approximation. A density functional perturbation approach was used for calculating properties of the lattice dynamics and electron-phonon coupling. Details can be found in R Heid and K-P Bohnen, Phys. Rev. B60, R3709 (1999)ADSGoogle Scholar
- F Weber, A Kreyssig, L Pintschovius, R Heid, W Reichardt, D Reznik, O Stockert and K Hradil, to be publishedGoogle Scholar
- F Weber, W Reichardt, L Pintschovius, A Kreyssig, R Heid, D Reznik, O Stockert and K Hradil, in preparationGoogle Scholar
- For the Gaussian smearing used in our calculations, a smearing by 1 meV corresponds approximately to a temperature of 5.5 KGoogle Scholar
- The calculations required about three months of computer time for each value of the smearing parameter, using a very fast work station. Reducing the smearing parameter to 10 meV would require usage of a much denser mesh in q-space and hence an order of magnitude longer computer timeGoogle Scholar
- W Reichardt, R Heid and K-P Bohnen, J. Superconductivity 18, 759 (2005)Google Scholar
- We note that many calculations published in the literature were done with δE = 200 meV, i.e. the largest value used in our studyGoogle Scholar
- A better approximation was recently proposed by M Calandra and F Mauri, Phys. Rev. B71, 064501 (2005), which needs, however, a very large computational effortGoogle Scholar