IMPURITY EFFECT ON T-C IN THE COUPLED PLANE-CHAIN MODEL OF YBA2CU3O7

We have studied the effect of impurities on T-c for a model of YBCO in volving pairing both in the CuO2 planes and in the CuO chains. In this model pairing in the planes is due to phonons, while Coulomb repulsio n 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 surfac e resulting in nodes in the gap. We find that T-c is much less sensiti ve to impurities than in standard d-wave models because impurities pro duce essentially plane-plane and chain-chain scattering, which does no t affect T-c. The reduction of T-c is due to plane-chain scattering wh ich is smaller by a factor hybridization over Fermi energy compared to plane-plane and chain-chain scattering. In addition, as in a two band model, we have a further reduction of the sensitivity of T-c to impur ities with a behaviour varying continuously from s-wave like to d-wave like depending on the parameters.