Theoretical models for high-temperature superconductivity

Summary A semi-phenomenological analysis is given of the effects of certain band structure features on the gap ratios 2Δ/k _B T _c for high-T _c superconductors, including multigap systems. In addition to phonons other intermediate bosons (IB) mediating the superconducting interaction are considered. Interesting results emerge when the IB energy exceeds the widths of possible narrow peaks in the density of states associated with subbands presumably belonging to substructures such as stacked Cu−O planes. Comparison with experiment is made. In particular, data obtained by Warrenet al. via nuclear-spin relaxation on Ba₂YCu₃O_7−δ can be interpreted within the present framework in terms of a model having an IB of energy ≳1 eV, which exceeds the predicted width (≲0.3 eV) of a peak in the density of states containing the normal-state Fermi level. This suggests that the IB is not a phonon. To speed up publication, the author of this paper has agreed to not receive the proofs for correction.