Effect of oxygen deficiency and disorder on microwave losses of epitaxially grown YBa2Cu3O7−δ films

We have investigated the influence of oxygen content and disorder on the surface impedance of epitaxial YBa2Cu3O7-delta (YBCO) films at 87 GHz. For this purpose, five films prepared by DC-sputtering on MgO substrates were annealed for 1 h at 500-degrees-C in different oxygen pressures between 80 and 1000 mbar. The degree of oxygen disorder was varied by a change of the cooling procedure or a subsequent annealing at 250-degrees-C. The absolute values as well as the slope of the resistivity in the normal conducting state and the penetration depth in the superconducting state are continuously increasing with delta. The transition temperatures change only slowly and a maximum in T(c) vs. delta occurs. 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 disorder 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 conductive subsystems and, as a result, a two-gap structure of YBCO. Charge transfer between the Cu-O planes and chains leads to an induced superconducting state in the chains. Oxygen content and order affect the value 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.