PLANE-CHAIN COUPLING IN YBA2CU3O7 - TEMPERATURE-DEPENDENCE OF THE PENETRATION DEPTH

We have studied the penetration depth for a model of YBa2Cu3O7, involv ing pairing, both in the CuO2, planes and in the CuO chains. In this m odel pairing in the planes is due to an attractive interaction, while Coulomb repulsion induces in the chains an order parameter with opposi te sign. Due to the anti-crossing produced by hybridization between pl anes and chains, one obtains a d-wave like order parameter which chang es sign on a single sheet of the Fermi surface and has nodes in the ga p. We find that our model accounts quite well for the anisotropy of th e penetration depth and for the absolute values. We reproduce fairly w ell the whole temperature dependence for both the a-and the b-directio ns, including the linear dependence at low temperature. We use a set o f parameters which are all quite reasonable physically. Our results fo r the c-direction are also satisfactory, although the situation is les s clear both experimentally and theoretically. (C) 1997 Elsevier Scien ce B.V.