TEMPERATURE-DEPENDENCE OF THE PINNING POTENTIAL IN YBA2CU3O7-X SUPERCONDUCTORS

Voltage-current curves obtained from transport measurements on c-axis- oriented thin films of YBa2Cu3O7-x allow us to determine the temperatu re, field, and field-orientation dependence of the pinning potential o n the basis of the simple Kim-Anderson model for the thermal activatio n of flux lines. A strong increase of the pinning potential is found a t low temperatures, followed by a field-dependent maximum between 60 a nd 70 K and a decrease towards the transition temperature T-c. The res ults are compared to literature data of the pinning potential U-o(T) o btained from relaxation measurements and discussed in terms of thermal activation theory and related models for the current dependence of U- o. The analysis of the E-J curves, results on the angular dependence o f J(c)(ab), calculations of the pinning potential based on the core co ntribution, and due consideration of the different nature of the flux lines at low and high temperatures lead to a consistent picture, in wh ich all results are explained in a qualitative way, without the need o f invoking recent models on complex pinning interactions and on new ph ases of the flux-line lattice. It is shown that the pinning potential at low temperatures is of the order of the core contribution of a smal l number of pancake vortices and therefore intrinsically limited to sm all values, while at higher temperatures the correlation length along the field direction increases, which naturally leads to higher potenti als.