THE COLLECTIVE NATURE OF A FLUX-CREEP IN HIGH-T-C SUPERCONDUCTOR YBA2CU3OX AS OBSERVED IN MICROWAVE-ABSORPTION EXPERIMENTS

Studies of the flux lattice dynamics via microwave absorption measurem ents with the help of an ESR spectrometer are presented. The experimen ts give evidence that for the temperatures below 35 K the absorbed pow er depends on the input microwave power in a non-linear way. For the c ase of absorption due to the motion of vortexes via thermally activate d flux creep this corresponds to the dependence of the activation barr ier on the current density. The measurements lead to the conclusion, t hat the activation barrier should grow with decreasing current. Fittin g of the expression for the dissipated power obtained in terms of the collective creep theory to the obtained experimental data enable us to determine the character of pinning and creep at small amplitudes of t he external exciting current. The obtained numerical values give evide nce that the flux lattice is frozen in a vortex-glass state and in the limit of zero current the resistance at finite temperatures vanishes.