Fusao Ichikawa

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.

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.

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.

Temperature dependence of activation energy due to intrinsic pinning for flux lines in (Nd1−xCex)2CuO4−δ films

Abstract Using electron superconductor (Nd 1− x Ce x ) 2 CuO 4−δ films prepared by a laser ablation technique, we measured the temperature dependence of the resistivity in magnetic fields and the magnetic field-direction dependence of the critical current. The temperature dependence of the resistivity in magnetic fields was analyzed using an activation type of formula. The temperature dependence of the activation energy for some oxide superconductors for H ∥ the basal plane was calculated using the intrinsic pinning model proposed by Tachiki and Takahashi. The temperature dependence changed systematically depending on the ratio of the coherence length along the c -axis to the inter-distance between CuO conductive planes. The magnetic field-direction dependence of the critical current was also explained using the model. From these analyses, the origin of the temperature dependence of the activation energy is discussed.

Magnetoresistance in a Nd2−xCexCuO4 thin film below Tc

Abstract The electrical resistivity ϱ of a superconducting Nd2−xCexCuO4 (x ∼ 0.15) thin film was measured in magnetic fields B parallel to the c-axis below the critical temperature Tc. At low temperatures, ϱ(B) passes through a characteristic value after the nonzero ϱ appears with increasing B. With further increasing B, ϱ(B) has a maximum at a certain value and decreases above it. These features are discussed on the basis of a field tuned superconductor-insulator transition and weak localization.

Analysis of the superconducting-fluctuation conductance due to the Maki-Thompson contribution in aluminum films

Abstract The excess conductance σ′ and the magnetoconductance Δσ in a field H due to superconducting fluctuations have been measured at temperatures near Tc for Al films in the wide range of the sheet resistance R □ , 5.8 Ω/□ ⪅- R □ ⪅- 160 Ω/□ . The contributions due to the Maki-Thompson (MT) correction σ′MT and ΔσMT are well described by the recent Reizer theory, which was obtained from reconsidering the theory of Keller and Korenman. As predicted by the recent theory, the dirtiness dependence of σ′MT and ΔσMT of weak-coupling superconducting films is explained by only R□ without any adjustable parameter. The T dependence of σ′ and the T and H dependence of Δσ for all films are satisfactorily explained by theories including the Aslamazov-Larkin contributions, using R□ as the fitting parameter within a factor of 0.7–1.3 of experimentally determined values.

Phase-locking of a triangular loop with three Josephson junctions having zero capacitance

A voltage‐locking of a triangular loop containing three Josephson junctions with zero capacitance has been studied by calculating phase‐space trajectories. Attractors show that there are both phase‐locking and quasiperiodicity in the region of the uneven changes. It is found from the figure of applied current versus winding number that phase‐locking occurs in some intervals of an applied current.

Dynamics of a triangular loop containing three Josephson junctions with zero capacitance

Dynamical properties of a triangular loop, each side of which consists of one Josephson junction with zero capacitance and inductance, are considered in this paper. For two kinds of applied currents, phase‐time and current‐voltage relations are calculated by integrating a set of derived nonlinear differential equations numerically. Time dependence of a fluxoid number in the loop is given by phase‐time relations, and it is found that the calculated current‐voltage relations exhibit uneven changes slightly above the threshold current for the voltage shifting from two to three junctions. The analytical result for the threshold currents under some conditions is in good agreement with the numerical one.

Quantum corrections to the magnetoconductance in superconducting aluminum films above the upper critical magnetic field

Abstract Measurements of magnetoconductance (MC) have been made on aluminu films up to large magnetic fields. It has been found that (1) near the transition temperature the MC reaches the minimum at a temperature-independent field H m0 ; (2) for films in a wide range of disorder the values of H m0 are about twice that of the mean-field upper critical field H c2 (0); (3) above H m0 , the MC shows ln H dependence. The magnitudes of the coefficient of ln H have been quantitatively explained by only the sum of contributions from the weak localization and electron-electron interaction (orbital effect). This result suggests that the superconducting fluctuation is limited to the magnetic field region below H ≅ H m0 in the phase diagram in the H - T plane. However, the present theory for MC due to the fluctuation cannot explain such a characteristic field H m0 .

Hysteresis of temperature dependence and scaling law in time dependence of sound velocity in sintered Bi2Sr2CaCu2O8+δ

Abstract The temperature T dependence of the mechanical resonance frequency f r of longitudinal sound waves in sintered bulk samples of Bi 2 Sr 2 CaCu 2 O 8+δ has been measured from room temperature down to liquid-nitrogen temperature. The f r increases steeply around 200 K with decreasing T and on the other hand it decreases steeply around 230 K with increasing T . This hysteresis of f r has a width depending on the change rate of T . Keeping T at a point T f between 230 K and 200 K, the time dependence of f r has been measured. The f r increases toward a saturated value f r max on the hysteresis loop determined by T f . When the time t is scaled by a characteristic time t 1 2 when f r reaches the middle point between the initial and the saturated ones, the plots of f r / f r max versus t/t 1 2 at each T f are almost reduced to one universal curve independent of T f . These results suggest that the sample experiences a first-order phase transformation around those temperatures.