High-magnetic-field study of high- Tc cuprates

Abstract

We present our recent studies of high- T c superconductors in very high pulsed magnetic fields exceeding 100 T (megagauss fields). In YBa 2Cu 3O 7− δ , the super to normal transition was observed in transport measurements for B∥ c under short pulse fields up to 120 T. A plot of the upper critical field H c2 as a function of temperature demonstrates that the curve obeys the conventional relation well established in other type-II superconductors in the dirty limit. From the resistivity versus temperature curve at high magnetic fields above the super to normal transition, it was found that the hole system shows a metallic temperature dependence for B∥ c. In Nd 2− x Ce x CuO 4 as electron-doped cuprate, a large magneto-resistance was observed above the transition field. It was also found that the temperature dependence of the resistivity above the critical field is insulator-like showing a prominent logarithmic up-turn at low temperatures. A similar logarithmic up-turn was observed in several other cuprates such as La 2− x Sr x CuO 4 (LSCO), La 2− x Ce x CuO, and Pr 2− x Ce x CuO 4. The experimental results for thin-film samples of LSCO with different strains suggest that the Kondo effect plays an important role in these materials. A new phase diagram of LSCO is proposed on the basis of a viewpoint of the interplay between superconductivity and the Kondo effect.