Anisotropy of lower critical field and critical currents in YBa 2Cu 3O 6+x single crystal

Abstract

The anisotropy of the first critical field H c1 is found to increase from H c1 //c(0)/H c1 ⊥c(0)=4 to to x=0.64 . For x=0.93, H c1 //c(0)=(950 ±200)Oe, H c1 ⊥c(0)=(230±20)Oe. For x=0.64, H c1 //c(0)=(270±30)Oe, H c1(0)=(27±2)Oe. An anomalous linear T-dependence was found for H //c at low temperatures. Critical currents j c were also determined showing a j c(T)=j oexp(−T/T o) dependence and an important anisotropy (x=0.93, j o1=10 6 A/cm 2 (in a-b planes), j o2=1.4 10 5 A/cm 2 (along c axis); for x=0.64, j o1=3.6 10 5 A/cm 2, j o2=2.2 10 4 A/cm 2). The anisotropy ratio j o1/j o2 is enhanced from 7 (x=0.93) to 16 (x=0.64).