PINNING FORCES AND LOWER CRITICAL FIELDS IN YBA2CU3OY CRYSTALS - TEMPERATURE-DEPENDENCE AND ANISOTROPY

Measurements of the field-dependent radiofrequency penetration depth (H,T) are used to delineate the lower-critical-field Hc1-T phase boundary, and to study flux dynamics in YBa2Cu3Oy crystals. For both Hc and HSc Hc1 obeys a BCS temperature dependence with a temperature-independent anisotropy of 3.4±0.3. In the mixed state, the data obey 2(H)=[0/0±(T)]B(H), and yield both the functional dependence B(H) and the pinning force constant ±(T). The latter is anisotropic, obeys an approximately (1-t2)2 temperature dependence, and vanishes slightly below the bulk transition temperature. © 1990 The American Physical Society.