SURFACE BARRIERS AND BULK PINNING IN YBA2CU4O8

Strong surface barrier effects have been observed for the motion of vortices in single crystal YBa2Cu4O8. They are most easily visible between 25 and 50 K where a typical hysteresis loop for fields less than 1 tesla shows a classic Bean-Livingston shape with magnetization close to zero for the leg of the hysteresis loop where vortices are exiting the sample. At higher fields, bulk pinning grows dramatically with a magnetic hysteresis loop that rises and falls with a classic fishtail shape. For these YBa2Cu4O8 single crystals, surface barriers dominate at low field with little bulk pinning. At 35 K, bulk pinning turns on at about 1 T.