Activation energies for ultrahigh purity YBCO single crystals

Strong flux pinning is essential in high-Tc superconductors for high current applications. A number of methods are in use to increase the critical current of MTG-grown ReBCO material where Re stands for rare earth or Y. Further improvement requires additional research into the nature of flux pinning in high-Tc superconductors. We present effective activation energies derived from thermopower measurements on high-purity 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 energy for both orientations near the oxygen content 7-δ=6.93, which coincides with the optimal doping level. For underdoped and overdoped crystals the effective activation energy is reduced. (author)