PLANE-CHAIN COUPLING IN YBA2CU3O7 - IMPURITY EFFECT ON THE CRITICAL-TEMPERATURE

We have studied the effect of impurities on the critical temperature f or 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 phono ns, while Coulomb repulsion induces in the chains an order parameter w ith opposite sign. Due to the anticrossing produced by hybridization b etween planes and chains, the order parameter changes sign on a single sheet of the Fermi surface, resulting in nodes in the gap. We find th at, in our model, the critical temperature is much less sensitive to i mpurities than in standard d-wave models. One reason is that impuritie s produce essentially plane-plane and chain-chain scattering, which do ss not affect the critical temperature. T-c is reduced by the scatteri ng between parts of the Fermi surface which have opposite signs for th e order parameter. just as in standard d-wave models. In our model thi s 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) (th at 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 d etail 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 whi ch can vary continuously from s-wavelike to d-wavelike depending on th e parameters. We expect a similar behavior and reduction to occur in t he general case.