SURFACE FLUX-PINNING IN YBA2CU3O7-DELTA

We report on the temperature and field dependence of the ac susceptibi lity and magnetic hysteresis of a number of YBa2CU3O7-delta Single cry stals with very low twin boundary densities and with defects added in a controlled manner to the crystal surfaces. It is shown that surface damage is by far the most important source of flux pinning, and that s ubstantial shifts in the irreversibility line can be controlled by the number and type of defects added to the surface.