Takafumi Aomine

Anisotropy and hysteresis of transport critical currents in high temperature Ln-Y-Ba-Cu-O superconductors

Abstract Following the measurements of anisotropy and hysteresis in transport critical currents with changing temperature and polarity of magnetic field, a.c. magnetic measurements by the Campbell method were performed for sintered Ln-Y-Ba-Cu-O superconductors. The information derived from them indicated that hysteresis does not occur in the intragrain current but in the intergrain one. The results are interpreted in terms of flux pinning, where the anisotropy originates from the texture structure and hysteresis is associated with the superconducting weak links of the sintered oxide pellets.

Transport critical currents and flux pinning mechanisms in single-crystalline thin films of YBa2Cu3O7−δ

Abstract In order to investigate the anisotropy of transport critical current densities J c and flux pinning properties, we have measured J c for YBa 2 Cu 3 O 7-δ single-crystalline thin films in magnetic fields H up to 12 T as a function of the angle between H and the crystal axes. The observed angular dependence of J c at 77 K and near 1.0 T could be explained by the model of intrinsic flux pinning proposed by Tachiki and Takahashi under the condition of transport current I ⊥ H . However, in a high field region near the upper critical field H c2 , the results disagreed with this model. We found that the reduced field H / H c2 dependence of the volume pinning force F p could be represented by F p ∝ ( H / H c2 ) 1 2 (1- H / H c2 ) 2 for all configurations for which we measured. The results indicate that both intrinsic and extrinsic pinning centers worked as a surface-type (two-dimensional) pinning. Furthermore, the angular dependence of H c2 was explained by the effective mass model.

Lorentz-force dependence of transport critical currents and anisotropic pinning properties in YBa2Cu3O7−δ single-crystalline thin films

Abstract The transport critical current density Jc of YBa2Cu3O7−δ single-crystalline thin films in the three-dimensional temperature region was measured in magnetic fields H up to 12 T applied at varied angles to the crystals axes. The observed angular dependence of Jc(α) and Jc(θ) could be described by the Lorentz-force-dependent critical state model by considering the anisotropy of the volume pinning force and the upper critical field. Here, α and θ are the angles between H and transport current I in the ab-plane and in the plane comprising H, I and the c-axis, respectively. Anisotropic pinning mechanisms were discussed based on the intrinsic pinning model proposing step-like flux lines. Jc(θ) had different values from Jc(φ) at θ=φ, where φ is an angle between H and the CuO2 layer under the conditions of I ⊥ H . We found that the difference between Jc(θ) and Jc(φ) comes from the different contribution of the Lorentz-force to the flux line segments parallel to the CuO2 layers.

Specific heat and entropy of dysprosium gallium garnet in magnetic fields

Abstract The specific heat of dysprosium gallium garnet (DyGaG) single crystal has been measured between 20 and 2 K in magnetic fields up to 5 T. From the specific heat and adiabatic demagnetization measurements, the temperature dependence of the entropy in various magnetic fields is determined. It is shown that DyGaG is a good material for magnetic refrigeration between 12 and 2 K.

Structural relaxation and crystallization of semiconducting vanadate glass accompanying a jump of the electrical conductivity

The 57Fe Mossbauer spectrum of semiconducting xK2O·(90–x)V2O5· 10Fe2O3 glasses (20⩽x⩽30) consists of a quadrupole doublet peak. The linewidth and the quadrupole splitting decreased gradually after heat treatment at the crystallization temperature, reflecting an increased uniformity of Fe–O bonds and a decreased distortion of FeO4 tetrahedra, respectively. FTIR spectra of heat-treated vanadate glasses revealed an evident peak separation between VO4 and FeO4 tetrahedra. The FeO4 units were not observed in non-treated glass because they occupy substitutional sites of VO4. An X-ray diffraction study of heat-treated glasses established the formation of a KV3O8 phase composed of VO5 pyramids. Additional diffraction peaks due to a K3V5O14 phase, composed of VO4 tetrahedra and VO5 pyramids, were observed in 30K2O · 60V2O5· 10Fe2O3 and 35K2O · 65V2O5 glasses after the heat treatment. Weak peaks due to K4V10O27 phase were also observed in 20K2O ·70V2O5· 10Fe2O3 glass. A Kissinger plot of the differential thermal analysis (DTA) data yielded activation energies of 2.0–2.9 (± 0.3) eV, indicating that cleavage of Fe–O bonds with bond energies of ca. 2.6 eV triggers the crystallization. Magnetic susceptibility measurements of 25K2O · 65V2O5· 10Fe2O3 glass revealed an antiferromagnetic behaviour with Curie–Weiss constants of –13, – 93 and –108 K after the heat treatment at 340 °C for 0, 2100 and 5000 min, respectively. A jump of the electrical conductivity from 6.3 × 10–8S cm–1 to 4.3 × 10–4 S cm–1 was observed when 25K2O · 65V2O5· 10Fe2O3 glass was heat treated at 380°C for 10 min, and this can be ascribed to an increased probability of electron hopping from V4+ to V5+ which was brought about as a result of a decreased distortion of VO4, FeO4 and VO5 units.

The Effects of Gaseous Helium and Nitrogen on the Thermopower Measurements: A Note of Concern for the Discrepancy of the Results Observed in High Temperature Superconductors

We investigated the effects of gaseous helium and nitrogen on the thermopower measurements. It was observed that for a constant power supply to the heater, the temperature gradient ΔT established across the sample, lead (Pb) in our case, depended upon the thermal conductivity of the gas with which the sample vessel was filled. This affected the estimation of the thermopower S since the measured ΔT was probably a pseudo value due to the heat losses that occurred in the presence of a heat conducting gas. A qualitative model was introduced to account for the physical processes taking place during the thermopower measurements for vessels filled with heat conducting gases. The effects of gas adsorption and desorption to and from the sample respectively are also discussed.

Hysteresis in magnetic field and field-direction dependence of critical current in Bi2Sr2CaCu2O8+y

Abstract We measured the dependence of the critical current density J c ( T,H ) on angles o and θ, the magnetic field H and the temperature T for Bi 2 Sr 2 CaCu 2 O 8+ y films with the preferred c -axis orientation prepared by a laser ablation technique, where o is an between H and the basal plane for the current density J ⊥ H , and θ an angle between H and J in the plane comprising the c -axis and J . Characteristic features were as follows. (1) J c ( T,H ) versus o and J c ( T,H ) versus θ had a characteristic hysteresis due to a characteristic pinning of the flux lines parallel to the c -axis. (2) When J c ( T,H ) versus o, J c ( T,H ) versus θ and J c ( T,H ) versus T were replotted with J c ( T,H ) versus the c -axis component of the magnetic fields, these dependences were almost identical. (3) This hysteresis was reduced if the magnetic field was applied by a special procedure.

Self-Heating Effects in Long Superconducting Thin Films over a Wide Temperature Range

Heating effects in current-voltage characteristics are investigated by theoretical and experimental methods. In order to apply the one-dimensional hotspot model over a wide temperature range, the theory of Skocpol et al. is modified. The modified theory agrees well with the experimental data in a wide temperature range.

Zero bias conductance peak enhancement in Bi2Sr2CaCu2O8+δ-SiO-Ag planar tunnel junctions

We report the experimental results of quasiparticle tunneling using Bi 2 Sr 2 CaCu 2 O 8+δ (BSCCO)-SiO-Ag planar junctions. For the {001}-oriented junctions, we have measured a well-known V-shaped gap structure expected for d -wave superconductors. For the {100}- and {110}-oriented junctions, a reproducible zero bias conductance peak (ZBCP) enhancement has been observed. The temperature dependence of the ZBCP is also presented. The ZBCP was observed below T c . Our results are consistent with those of recent studies on the Andreev reflection between a d -wave superconductor and a normal metal.

Nonlinear resistivity in the mixed state of superconducting aluminum films

Abstract In order to study the vortex-glass (VG) transition for conventional type-II superconductors, we have investigated the relation between the current density J and the resistivity ρ in the mixed state of aluminum films with different thickness and dirtiness. The aluminum films have the characteristics of lower Tc and longer coherence length in contrast with high-Tc superconductors. The J–ρ isotherms at various external magnetic fields have been analyzed by Fishers VG theory. The data for the dirty thick film and dirty thin film can be well collapsed onto a single function according to the theories for three dimensional (3D) and two dimensional (2D) systems, respectively, with use of vortex-glass transition temperature, and reasonable values of critical exponent determined from the data analysis. However, for the thick clean film, there are some discrepancies between the experiment and 3D scaling theory; very narrow region of liquid phase and a small value of dynamic critical exponent. The reason for the narrow critical region has been discussed from a different viewpoint.