Plane-chain coupling in YBa2Cu3O7: Impurity effect on the critical temperature

We have studied the effect of impurities on the critical temperature for a model of Y-Ba-Cu-O involving pairing both in the CuO2 planes and in the CuO chains. In this model pairing in the planes is due to phonons, while Coulomb repulsion induces in the chains an order parameter with opposite sign. Due to the anticrossing produced by hybridization between planes and chains, the order parameter changes sign on a single sheet of the Fermi surface, resulting in nodes in the gap. We find that, in our model, the critical temperature is much less sensitive to impurities than in standard d-wave models. One reason is that impurities produce essentially plane-plane and chain-chain scattering, which doss not affect the critical temperature. T-c is reduced by the scattering between parts of the Fermi surface which have opposite signs for the order parameter. just as in standard d-wave models. In our model this is due to plane-chain scattering. We have found that this scattering, whatever its origin, will be smaller by a factor of order t/E(F) (that is, hybridization coupling over the Fermi energy) compared to plane-plane and chain-chain scattering. Accordingly the sensitivity of T-c to impurities in our model is reduced by a similar factor. compared to the d-wave situation. In the specific case which we have studied in detail and which reduces to the two-band model, we have found a further reduction of the sensitivity of T-c to impurities with a behavior which can vary continuously from s-wavelike to d-wavelike depending on the parameters. We expect a similar behavior and reduction to occur in the general case.