Surface Flux Pinning in YBCO Crystals.

Various etching and abrasion techniques have been used to alter the surface of high quality YBCO crystals. These untwinned or slightly twinned crystals exhibit almost no flux pinning above 70 K in the pristine condition. It is found that in sufficiently clean crystals, surface defects become the dominanat source of pinning at high temperatures, and control the position of the irreversibility line and properties of the magnetization loop.