Magnetism and Superconductivity in the Rare Earth Nickel Borocarbides | ||||
| Egyptian Journal of Solids | ||||
| Article 1, Volume 25, Issue 2, 2002, Page 153-170 PDF (458.89 K) | ||||
| DOI: 10.21608/ejs.2002.150472 | ||||
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| Abstract | ||||
| The single layer borocarbide and triple layer boronitride superconductors have in common a dual band characteristic at the Fermi level which leads to a complex Fermi surface consisting of several sheets. The theoretical description of the thermodynamic properties and of the upper critical field showing a pronounced upward curvature close to TC requires a strong-coupling "Eliashberg" theory which accounts for both the anisotropy of the Fermi surface and the anisotropy of the electron phonon interaction. Using such a theory for conventional electron-phonon mediated polycrystalline superconductors with an s-wave order parameter {schachinger} we obtained good agreement with the experimental data with a comparable anisotropy parameter for the Fermi velocity <bk 2> ≅ 0.25 in both systems, the borocarbides and the boronitride, but with a significantly larger anisotropy of electron-phonon interaction <ak 2> ≅ 0.08 for La3Ni2B2N3-δ compared to <ak 2> ≅ 0.03 in the single layer borocarbides. The larger <ak 2> explains the weak-coupling signatures of the boronitride, though its coupling strength is of similar magnitude as that of the borocarbides, namely, moderately strong. | ||||
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