ACTIVATION-ENERGIES FOR ULTRAHIGH PURITY YBCO SINGLE-CRYSTALS

Strong flux pinning is essential in high-T-c superconductors for high current applications. A number of methods are in use to increase the c ritical current of MTG-grown ReBCO material where Re stands for rare e arth or Y. Further improvement requires additional research into the n ature of flux pinning in high-T-c superconductors. We present effectiv e activation energies derived from thermopower measurements on high-pu rity YBCO crystals grown in yttria crucibles. These measurements show that there is a larger activation energy for quasiparticles travelling parallel to the c-axis compared with those travelling through the a-b -plane. In addition, there is a maximum in the effective activation en ergy for both orientations near the oxygen content 7 - delta = 6.93, w hich coincides with the optimal doping level. For underdoped and overd oped crystals the effective activation energy is reduced.