ANISOTROPIC MICROWAVE LOSSES OF PARTIALLY UNTWINNED YBA2CU3O7-X SINGLE-CRYSTALS

We have built up a highly sensitive system to measure the temperature- dependent anisotropy of the microwave losses of HTSC crystals. By rota ting a crystal in the microwave field of a 21 GHz cylindrical Nb reson ator the relative amount of surface currents into the different crysta l directions can be varied. The lowest surface resistance of 150 mu Om ega at 50K and resistivity of 39 mu Omega cm at 100K have been obtaine d for a 40 mu m thin twinned YBa2Cu3O7-x (YBCO) crystal. Preliminary m easurements on two partially untwinned YBCO crystals have confirmed th e much reduced conductivity in c-direction and the improved one in cha in (b)-direction, both in the normal state. In contrast, enlarged loss es have been obtained below T-c in chain direction, suggesting a weake ned superconductivity in the chains of the YBCO crystals.