COMPLEX MICROWAVE CONDUCTIVITY OF YBA2CU3O7 IN MAGNETIC-FIELDS UP TO 500T

We have measured the complex conductivity of thin films of YBa2Cu3O7 ( YBCO) superconductor down to temperatures of 4K and magnetic fields up to 500T. The highly oriented films were probed by 94 GHz radiation, w ith the external magnetic field applied perpendicular to the c-axis. A s discussed below, these measurements allowed us to assign a value of 340+/-40 T for the upper critical field at T=0. The measurements were recently carried out at the pulsed field facility of the National High Magnetic Field Laboratory at Los Alamos using both Russian and Americ an magnetic flux compression generators.