Effective activation energy U( T, H) in Na-doped MTG-YBCO crystals

Abstract

We have measured the resistivity of Na-doped melt textured growth method MTG-YBCO ( YBa 2−x Na x Cu 3 O y+40 mol% Y211 , x=0.0, 0.1, 0.2) crystals as a function of temperature T and magnetic field H up to 7.5 T. The results show that for the pure MTG-YBCO ( x=0.0) crystal the effective activation energy U follows U∝(1−T/T c ) ln H for H‖ ab and U∝(1− T/ T c) H −0.82 for H‖ c. With certain Na addition, U follows U∝(1− T/ T c) m H − α and there is a crossover from the exponent α≈0.5 at lower field to α≈1 at high field and a value of m≈2 for H‖ ab. While for the configuration H‖ c, we get large values of m≈2.5 and α≈1.75. Our results suggest that the flux pinning behaviors become more complicated with Na addition. The possible reasons responsible for the effect of Na addition on the flux pinning behaviors are discussed.