Regensburg 2004 – wissenschaftliches Programm
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TT: Tiefe Temperaturen
TT 9: Supraleitung: Eigenschaften, elektronische Struktur, Ordnungsparameter
TT 9.8: Vortrag
Dienstag, 9. März 2004, 11:45–12:00, H20
Phonons and electron-phonon coupling in nickel borocarbides — •R. Heid, W. Reichardt, and K.-P. Bohnen — Institut für Festkörperphysik, Forschungszentrum Karlsruhe, D-76021 Karlsruhe
Superconductivity in nickel borocarbides exhibits various unusual properties [1], which raises the question if electron-phonon coupling is the main source of pairing in this class of materials. This subject can be elucidated by modern first principles methods based on density-functional theory which provide a detailed microscopic picture of the electron-phonon interaction. We present calculations of the bandstructure, the phonon dispersion, and the electron-phonon coupling for superconducting YNi2B2C (Tc=15.5 K) and compare them with results for the non-superconducting reference compounds LaNi2B2C and Y(Ni.75Co.25)2B2C. The calculations reproduce a pronounced phonon anomaly in the vicinity of the wave vector (0.6,0,0) in a low frequency branch of Δ4 symmetry that has been observed experimentally both in YNi2B2C and LuNi2B2C [2]. We find that the magnitude of the anomaly and of the electron-phonon coupling depend very sensitively on the chosen structural parameters (volume, c/a ratio). Another pronounced anomaly is predicted in an optic branch of the same symmetry with predominant B and C elongations. Although the low frequency phonon spectrum is strongly dominated by the vibrations of the heavy metal ions we find that B and C contribute more than 50% to the electron-phonon coupling constant λ.
[1] K. Izawa et al., Phys. Rev. Lett. 89, 137006 (2002)
[2] C. Stassis et al., Physica C 317-318, 127 (1999)