Berlin 2005 – wissenschaftliches Programm
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TT: Tiefe Temperaturen
TT 18: Superconductivity - Properties, Electronic Structure, Order Parameter II
TT 18.7: Vortrag
Montag, 7. März 2005, 11:45–12:00, TU H2053
Are intercalated metallochloronitrides electron-phonon mediated superconductors? — •Rolf Heid and Klaus-Peter Bohnen — Institut für Festkörperphysik, Forschungszentrum Karlsruhe
The layered metallochloronitrides XNCl, X=Zr,Hf, have surprised with rather high superconducting transition temperatures of up to 25.5 K after intercalation with Li or Na [1], raising the question to what extent the conventional electron-phonon coupling mechanism is at work. Here we present results of an ab initio investigation of the lattice dynamics and electron-phonon coupling of undoped and Li-intercalated ZrNCl applying a density-functional perturbation approach, which allows a calculation of the full momentum dependency of these quantities. The theoretical phonon spectra are found to be in very good agreement with those obtained by neutron scattering experiments [2]. On intercalation, small Fermi surface pockets develop around the K points. This topology gives rise to strongly momentum dependent electron-phonon coupling, which is carried predominantly by two in-plane vibrations of Zr and N. We find that the integrated coupling constant is ≈ 0.5, a value significantly larger than estimated before [3], but still rather small in view of the high Tc values observed. Implications for the superconducting mechanism will be discussed.
[1] S. Yamanaka et al., Nature 392, 580 (1998)
[2] P.Adelmann et al., J. Low Temp. Phys. 117, 449 (1999)
[3] R. Weht et al., Europhys. Lett. 48, 320 (1999)