EFFECT OF OXYGEN DEFICIENCY AND DISORDER ON MICROWAVE LOSSES OF EPITAXIALLY GROWN YBA2CU3O7-DELTA FILMS

We have investigated the influence of oxygen content and disorder on t he surface impedance of epitaxial YBa2Cu3O7-delta (YBCO) films at 87 G Hz. For this purpose, five films prepared by DC-sputtering on MgO subs trates were annealed for 1 h at 500-degrees-C in different oxygen pres sures between 80 and 1000 mbar. The degree of oxygen disorder was vari ed by a change of the cooling procedure or a subsequent annealing at 2 50-degrees-C. The absolute values as well as the slope of the resistiv ity in the normal conducting state and the penetration depth in the su perconducting state are continuously increasing with delta. The transi tion temperatures change only slowly and a maximum in T(c) vs. delta o ccurs. As a main result, we observe a strong influence of delta on the surface resistance below 0.8T(c) which first increases at the maximum of T(c) and then decreases for higher oxygen deficiencies. Oxygen dis order leads to a degradation of all measured quantities, but R(s) was found to be most affected. These observations can be explained in the context of a recent model by Kresin and Wolf which proposes two conduc tive subsystems and, as a result, a two-gap structure of YBCO. Charge transfer between the Cu-O planes and chains leads to an induced superc onducting state in the chains. Oxygen content and order affect the val ue of the smaller energy gap of the chains, which in turn has a large impact on R(s). However, T(c) changes only slowly, reflecting its main correlation to the large energy gap given by the planes.