Contribution to the theory of high-temperature superconductivity in YBa2Cu3O7

Applying the wave functions and exciton energies determined land described in two previous articles) for YBa2Cu3O7, we calculated the effective potent ial V-eff assuming as a first approximation of the "two-bands" model an exc itonic mechanism for superconductivity. With the help of the obtained V-eff , We solved the gap equation for the cases Delta(k1)(T-c) = 0 and Delta(k1) (T = 0). We obtained a critical temperature T-c value of 87 K and Delta(max ) = 19 meV. These results which give for the ratio 2 Delta(max)/k(B)T(c) = 5.03 agree very well with the corresponding experimental values (T-c = 92 K , Delta(max) approximate to 20 meV, 2 Delta(max)/k(B)T(c) approximate to 5. 0). In investigating the symmetry of the gap as a function of k(1) [Delta(k (1))], we found a dominantly d(x)2(-y)2 Wave with a small s wave admixture in good agreement with experiment. It should be mentioned that in this case starting from first-principle band structures (no adjustable parameters; t he density functional methods still cannot provide exciton energies), the m ost important characteristics of the normal and superconducting state of a cuprate high T-c superconductor were quantitatively calculated in very good agreement with experiment. (C) 1999 John Wiley & Sons, Inc.