Thermodynamic functions within the van Hove BCS model: symmetry mixing effects

All the new layer perovskite superconductors seem to show a phenomenon of s ymmetry mixing with respect to the order parameters. An analysis of the dif ferent alternatives of mixing and how far they could be presented is carrie d out. For the particular case of s + id symmetry of the gap, the temperatu re dependence of the specific heat (C-es) and the thermodynamic critical ma gnetic field (H-c) are calculated, A double peak transition is observed on C-es(T) in the mixed regime while the single peak behavior is recovered for a purely symmetric state (s or d). C-es presents a quadratic law at low te mperatures for a d-wave gap and the typical exponential attenuation for the s-wave. The temperature dependence of H-c shows a clear phase transition o f second order at temperatures where the d-wave component becomes negligibl e. A comparison with other results and experiments is done. (C) 1999 Publis hed by Elsevier Science B.V. All rights reserved.