Tomoyuki Naito

Vortex phase diagram as a function of oxygen deficiency in untwinned YBa{sub 2}Cu{sub 3}O{sub y}

Magnetization and resistivity measurements were performed in un-twinned YBa2Cu3Oy(YBCO) single crystals with different oxygen content. We find that the vortex phase diagram is a strong function of the oxygen deficiency; (1) For fully oxidized YBCO (y ≃ 7, Tc ≃ 87.5 K), the first-order vortex lattice melting line Tm(H) is observed up to 30 T. (2) For slightly overdoped YBCO (Tc≃ 92 K), Tm(H) and the vortex glass transition line Tg(H) terminate at the multicritical point and the field-driven disordering transition line separates the vortex solid phase into the Bragg glass and the vortex glass phases. (3) For slightly underdoped YBCO (Tc≃ 92.3 K), Tg(H) terminates well below the critical point of Tm(H), indicating the existence of the vortex slush regime by the introduced oxygen deficiency.

Anomalous magnetization due to the vortex lattice melting transition in YBa{sub 2}Cu{sub 3}O{sub y}

Magnetization measurements in YBa2Cu3Oy single crystals are performed as a function of the temperature T and the magnetic field H. It is found that an anomalous magnetization step which provides the thermodynamic evidence of the first-order vortex lattice melting transition and the resistive kink with the hysteresis are observed in the same phase boundary in H-T plane. For samples with a small value of the irreversibility field Hirr(T) as compared with the vortex lattice melting line Hm(T), the vortex lattice melting is observed in the reversible magnetization region and the entropy change per vortex per CuO2 layer at the transition is estimated to be Δ s≃0.5–1.0 kB. For samples with a slightly larger value of Hirr(T) comparable to Hm(T), on the other hand, the jump height and the value of Δs is enhanced. Thus, the value of Δs is affected significantly by the relation between Hirr(T) which varies from sample to sample and Hm(T) which is an universal line.

Thermal transport in 90 K- and 60 K-phase YBa2Cu3O7-δ High-Tc oxides

The thermal diffusivity α and conductivity x of YBa 2 Cu 3 O 7-δ sintered crystals with oxygen deficiencies δ 0.10 (90 K-phase) and δ 0.35 (60 K-phase) have been quasi-simultaneously measured between 15 and 170 K. The phonon and electron components of α and x are separated and analyzed from viewpoints of both the d- and s-wave superconducting energy gap models. It is found that the enhancement of x below T c , which is commonly observed for 90 K- and 60 K-phase YBa 2 Cu 3 O 7-δ , is consistent only with the phonon origin model under d-wave energy gap anisotropy with the maximum gap 2Δ max /k B T c 5. The phonon scattering mechanisms are synthetically analyzed for both 90 K- and 60 K-phase YBa 2 Cu 3 O 7-δ and the oxygen vacancy is found to be one of the dominant phonon scattering centers in this system.

Magnetic phase diagram of untwinned YBa2Cu3Oy single crystals annealed in high pressure oxygen atmosphere

Abstract Magnetization and resistivity for untwinned YBa 2 Cu 3 O y single crystals with controlled oxygen contents were measured at high fields ( H‖c ) up to 27 T. With increasing the oxygen content above the optimum condition, the superconducting transition temperature T c changed from ≃93 K to ≃88 K and the hysteresis width of the magnetization M ( H ) curve decreased. The resistivity measurement for the heavily overdoped sample showed a discontinuous jump which results from the first-order vortex lattice melting transition (VMLT) even at 26 T. The result indicates that the vortex pinning is strongly reduced for heavily overdoped sample and the oxygen vacancy has great influence on its phase diagram in untwinned YBa 2 Cu 3 O y .

Phase transition in vortex-state of untwinned YBa2Cu3O7−δ single crystals at high magnetic fields up to 260 kOe

Abstract In order to study the thermodynamic phase transition in the vortex-state of YBa 2 Cu 3 O 7−δ , we measured the temperature dependence of the electrical resistivity both of untwinned and twinned singel crystals in high magnetic fields H parallel to the c -axis. For untwinned crystals, the resistive jump which results from the first-order vortex lattice melting transition completely disappears and the resistivity shows gradual temperature dependence above certain field H ∗ . The first-order and the second-order phase transition line is determined up to 260 kOe and the vortex-state is discussed for untwinned and twinned crystals.

Superconducting properties of untwinned YBa2Cu3Oy single crystals annealed in high-pressure oxygen

Abstract The effect of the oxygen content on the superconducting properties have been studied in untwinned YBa2Cu3Oy single crystals. The oxygen content was controlled by annealing at several temperatures in 8 bar oxygen. The heavily overdoped sample (Tc∼88 K) annealed at Tan=350°C shows the first-order vortex lattice melting transition up to 26xa0T. With decreasing oxygen content, the critical point of the melting transition line decreases to 4xa0T for the slightly underdoped sample (Tan=600°C, Tc∼92.5 K). The magnetization hysteresis curves also show a remarkable change as a function of the oxygen content.

Effects of the weak disorder on the vortex phase diagram and the second peak in untwinned YBa2Cu3Oy single crystals

Abstract Magnetization and resistivity measurements have been performed in untwinned YBa 2 Cu 3 O y single crystals with slightly different oxygen contents near the optimal condition. For slightly overdoped samples, the steep increase of the magnetization at H *, which is connected with the critical point, is consistent with the field-driven disordering transition. For the slightly underdoped sample, on the other hand, the vortex phase diagram shows no remarkable feature near the critical point due to the enhanced disorder by the oxygen deficiency.

Vortex-lattice melting transition in La1.85Sr0.15CuO4 single crystal

We have measured the magnetization of La1.85Sr0.15CuO4 single crystal as a function of the temperatureT and the magnetic fieldH. Above the irreversibility fieldHirr, the magnetization shows small anomaly at the magnetic fieldHm. This anomaly is attributed to the first-order vortex-lattice melting transition. The temperature dependence ofHm is described asHmα (1−T/Tc)n withn=1.8, which agree with the vortex-lattice melting theory. The Lindemann criterioncL is estimated to be 0.12 which is as large as the reported values for YBa2Cu3O7−δ or Bi2Sr2CaCu2O8+δ system.

Superconducting and magnetic properties of (La,Ca)MnO3/YBa2Cu3O7-δ multilayer films

Abstract YBCO/LCMO multilayer thin films are prepared by the magnetron sputtering method and the electrical resistivity is measured in magnetic fields. Both CMR effect at high temperatures and the superconducting transition at low temperatures are observed. The angular dependence of the resistivity is scaled by the perpendicular field component, because vortices behave as the two dimensional pancakes in the multilayer films.

Reentrant peak effect and irreversibility line in untwinned single crystals of YBa2Cu3Oy

Abstract Magnetization for untwinned single crystals of YBa 2 Cu 3 O y is measured and the anomalous reentrant pinning behaviour is observed. The reentrant magnetization and the irreversibility line are caused by the increase of the pinning force due to the change from the vortex lattice phase to the vortex glass phase. The behaviour is also related to the thermal depinning effect which reduces the pinning potential of the quenched disorder.