ANISOTROPY OF THE UPPER CRITICAL-FIELD IN UNTWINNED SINGLE-CRYSTAL YBA2CU3O7-DELTA (DELTA-CONGRUENT-TO 0.1 AND 0.4)

We report contact-free measurements of the temperature dependence of the upper critical field (most probably the onset field of flux line motion), Hc2(T), of an untwinned single-crystal YBa2Cu3O7-δ for δ {all equal to} 0.1 and δ {all equal to} 0.4. The sample was oriented in situ in the field along the three principal crystal axes and the measurements were done in magnetic fields up to 22T. For YBa2Cu3O6.9 the highest critical field is found along the b-axis and the anisotropy in the basal plane amounts to 20% at 85K, where dHc2/dT at 22T is between -3.8 and -3.1 T/K. Along the c-axis dHc2/dT was found to be -1.1 T/K at 22T. The 60 K superconductor YBa2Cu3O6.6 shows pronounced 2D behavior and is characterized by the largest critical field along the a-axis where dHc2/dT is -20 T/K at 22T. Along the c-axis dHc2/dT is reduced to -0.9 T/K. © 1990.