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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