Seiya Haraguchi

Important Role of Josephson-Vortex Pinning in the Mixed State Properties in Oxygen-Content Controlled YBCO Single Crystals

In-plane anisotropy of the resistivity and phase transition of the Josephson-vortex system have been studied in YBCO crystals with different oxygen-contents. Angular dependences of the resistivity are represented by superposition of four-fold anisotropy with a sharps dip for H ∥ a - and b -axes and two-fold one in under-doped and over-doped crystals. Only two-fold in-plane anisotropy appears in optimally-doped ones. The anisotropic behaviors are described by the superconducting fluctuation and the Josephson-vortex pinning effect. The resistive transition for H ∥ a - and b -axes shows sharp drop to zero resistivity accompanied with the first-order phase transition in the optimally-doped crystals. It is characterized by the second-order vortex-glass transition in over-doped and under-doped ones. It is indicated that the Josephson-vortex pinning is important in the determination of the in-plane anisotropy of the resistivity and the phase transition.

Four-fold symmetry of 90 K YBCO single crystals in magnetic fields

We have measured the angular dependence of the resistivity ρ(θ) in 90 K-YBCO single crystals, where θ is the angle between the direction of a magnetic field H and the direction of [0 1 0] which is perpendicular to the current I in the single crystal. The obtained ρ(θ) shows the four-fold symmetry. On the other hand, Kwok et al. had observed only the two-fold symmetry in 90 K YBCO single crystals. The origin of the difference between the symmetry of each case is speculated that it comes from the difference of anisotropy that depends on the concentration of oxygen in the CuO-chains.

Vortex glass transition of the Josephson vortex system in LSCO crystals

Abstract Electrical resistivity has been measured under magnetic fields parallel to the CuO2 planes in LSCO single crystals, where Josephson vortices form in the region between the CuO2 planes. In the crystals over the wide Sr content from the under-doped to the over-doped the vortex glass melting transition is observed in the field range of 1–17.5 T. The glass transition curves strongly depend on the anisotropy of the system, that is, it is suppressed to the low field in the under-doped crystal with large anisotropic parameter. The Josephson vortex glass transition is discussed in terms of the anisotropic parameter.

In-plane anisotropy of the resistivity in 60 K YBCO single crystal under magnetic field

Abstract The in-plane anisotropy of the resistivity ρ ab ( θ ) has been measured on a twinned 60 K YBa 2 Cu 3 O 7− δ (YBCO) single crystal under magnetic fields H precisely parallel to the CuO 2 planes. ρ ab ( θ ) curves are described by the superposition of the twofold and fourfold symmetry, and the dip structure appears at θ =0° and 90°. The fourfold anisotropy is first observed in YBCO system in the resistivity measurements. We show that the obtained results are explained by the superconducting fluctuation, the symmetry of the superconducting energy gap of the d-wave pairing and the effect of the vortex pinning.

Anomalous in-plane anisotropy of the resistivity on single crystalline 60-K YBa2Cu3O7−δ in high magnetic fields

Abstract We have measured the in-plane anisotropy of the in-plane resistivity on 60-K YBa 2 Cu 3 O 7− δ single crystals in magnetic fields parallel with the ab -plane. In high magnetic fields, the temperature dependence of the resistivity ρ ( T ) for H ⊥ I shows the jump near the zero-resistivity transition, while the ρ ( T ) for H ∥ I the gradual behavior. Furthermore, the transition temperature of the latter is lower than that of the former, which is inconsistent with the vortex dynamics induced by the Lorentz force. We discuss the observed anomalous behavior in terms of the phase transition of the Josephson vortex system.