T. Fujiyoshi

Flux pinning properties of ErBa2Cu3Oy thin films with BaZrO3 nanorods

ErBa2Cu3Oy (ErBCO) thin films with BaZrO3 (BZO) nanorods were prepared by a PLD method for an enhancement of the critical current density Jc. The values of Jc for the ErBCO thin film containing 1.5 wt% BZO (Er15) in magnetic fields are higher than those for the ErBCO thin film containing 0.5 wt% BZO (Er05). The peaks of Jc have been observed in the angular dependence of Jc in both the films when the magnetic field is applied parallel to the c-axis. It has been found that the peak is attributed to the flux pinning by BZO nanorods oriented parallel to the c-axis. The vortex glass–liquid transition temperature Tg and the pinning parameter m were derived by fitting observed electric transport properties to the theoretical expression based on the percolation transition model. The value of Tg of Er15 is higher than that of Er05. This result indicates that the vortex glass phase extends to a higher temperature region on increasing the fraction of BZO. The peak of m has been found in the magnetic field dependence. This fact is probably due to matching the density of BZO nanorods with that of fluxoids, which was confirmed by TEM observations.

Critical current density of MgB2 thin film with pinning centres introduced by deposition in oxygen atmosphere

Critical current densities Jc of MgB2 thin films have been measured; these were prepared by an electron beam evaporation method without any post-annealing. In order to improve the characteristics of Jc in high magnetic fields, a MgB2 thin film was deposited in an O2 atmosphere. The values of Jc of the O2-doped film have exceeded those of the non-doped film in magnetic fields applied perpendicular and also parallel to the c-axis. The peak of Jc caused by the c-axis correlated pinning centres has been observed in the angular dependence of Jc. The upper critical field Bc2 and the vortex glass–liquid transition field Bg have been estimated from temperature dependences of resistivity curves in both directions of the magnetic fields perpendicular and parallel to the c-axis. It is found that Bc2 and Bg are increased by the introduction of O2 in both the directions of the applied field. This fact indicates that effective pinning centres have been introduced by the deposition of the MgB2 thin film in an O2 atmosphere.

Transport characteristics near the glass–liquid transition temperature before and after heavy-ion irradiation in a YBa2Cu3O7−δ thin film

Abstract We investigated the influence of flux pinning on the behavior of flux lines in a c-axis oriented YBa2Cu3O7−δ thin film, by analyzing the critical scaling of current–voltage characteristics at various temperatures under the magnetic field parallel to the c-axis before and after irradiation. In the irradiation process, splayed columnar defects were produced in the film by tilting 200 MeV Au-ion beams off the c-axis by ±15°. The enhancement of the glass–liquid transition temperature and the increment of the dynamic critical exponent z were observed after the irradiation. The depinning model taking into account a distribution of pinning strength can reasonably describe the origin of the changes of both the glass–liquid transition temperature and z owing to the introduction of defects. This fact indicates that the flux line dynamics is governed by the pinning interaction rather than the intrinsic property of the flux line system.

A theory of thermal-fluctuation-induced depinning line in weakly pinning high-Tc superconductors

Abstract The expression for the thermal-fluctuation-induced depinning line, T = TP (Be), is derived theoretically for weakly pinning high-Tc superconductors. In the derivation, it is shown that the thermal displacement of pinned fluxoids, uth reduces the critical current density as J c = J c0 ( − 〈u 2 th 〉/d 2 p ) 3 2 , where J c0 is the critical current density in the absence of u th and d p is the effective half width of the summed-up pinning potential. Then T p (itB e) is determined as the temperature at which Jc becomes zero due to the increase of 〈u2th〉 with the increase of T in the external flux density of Be. The relation of Tp(Be) with the irreversibility line of Ti(Be) and the flux melting line of Tm(Be) is also discussed.

High critical current density under magnetic fields in as-grown MgB2 thin films deposited by molecular-beam epitaxy

As-grown MgB2 thin films were prepared by a MBE method under the conditions of low temperature, low deposition rate and high vacuum for applications in electric devices. The MgB2 thin films deposited on MgO and Ti buffered ZnO substrates have considerably higher Jc under magnetic fields among MgB2 thin films reported before. The value of Jc for the MgB2 thin film deposited on Ti buffered ZnO has been 5.8 × 105 A cm−2 at 10 K, 5 T in the magnetic field applied parallel to the c axis. In the angular dependence of Jc, the peak of Jc attributable to c-axis-correlated pinning centres has been observed when the magnetic field was applied parallel to the c axis.

On the magnetic field dependence of critical current density in single crystals of high-Tc superconductors

Abstract The critical current density, Jc, is discussed theoretically to explain the exponential dependence of Jc on the flux density, B, given typically by Jc∝exp( −B B 0 (T) ), which has been widely observed for single crystals of high-Tc superconductors in an external magnetic field parallel to the crystal c-axis. It is pointed out that Jc shows an exponential-type dependence, Jc∝exp( −ƒf(B, T) ), when the tilt modulus, C44, of fluxoids shows a remarkable softening for higher values of the wave number. It is also shown that the present expression of Jc is reduced to the observed B dependence of ƒ(B, T)⋍ B B 0 (T) by taking account of th e effects of the layered crystal structure on C44 and the correlation length, l44.

A theory of the thermal depinning transition in type-2 superconductors

Abstract It is pointed out theoretically that the electric field, E , vs. current density, J , characteristic of the vortex glass state is different from that predicted by M.P.A. Fisher; that is, the electric resistivity, ρ = E / J has a finite value even at J →0, while the Fisher theory predicts ρ →0 with J →0. It is also pointed out that the vortex glass–liquid transition is merely a kind of the bifurcation transition in the mixed state of type-2 superconductors containing pinning centers, which may be called the thermal depinning transition between the pinning state and the depinning state of fluxoids resulting from the thermal agitation on fluxoids. Furthermore, it is shown theoretically that only the flux flow resistivity obeys the scaling law near the thermal depinning transition temperature, T dp , while the flux creep resistivity approaches a finite value as J is decreased to zero, even in the pinning state below T dp . When ρ is measured down to a very small level of E , therefore, the noticeable deviation from the scaling law of ρ against J that was predicted by Fisher is expected to appear due to the above-mentioned behavior of the flux creep resistivity. The above theoretical conclusion that is contrary to the theoretical prediction by Fisher, however, seems to be supported by the recently observed data over a very wide range of the electric field, E provided by Kodama et al., because the present theoretical expression for the E vs. J , characteristics agrees quantitatively with these observed data.

Sweep rate dependence of magnetic hysteresis loops and peak effect in high- superconductors

The origin of the peak effect in QMG-processed YBCO was investigated using the sweep rate dependence of magnetic hysteresis loops. A numerical analysis of magnetic hysteresis loops demonstrated that electric field E versus current density J characteristics can be accurately derived from the sweep rate dependence using the Mawatari method rather than the conventional Bean one. The pinning potential energy can be obtained from the slope of the log E versus log J plot accurately. By analysing the experimental data with the peak effect using the Mawatari method, it was found that the pinning potential energy and the critical current density have the peak values at different magnetic fields. This anomalous phenomenon incidental to the peak effect can be explained by the Klein model which is based on the concept of the field-induced flux pinning.

Anisotropic thermal diffusivity in crystalline YBCO superconductors

Abstract The thermal diffusivity of crystalline YBCO for the vertical and parallel directions to c-axis has been measured to clarify the electron-phonon interaction below T c , by using the transient method. From the diffusivity vs temperature, the scaled BCS gap and electron-phonon coupling strength are anisotropic.

Influence of the columnar defect densities on the current–voltage characteristics in high-Tc superconductors

Abstract We investigated the influence of columnar defect densities on the critical exponents estimated from the critical scaling analysis of the current–voltage characteristics in high- T c superconductors. It was found that the dynamic critical exponent z increased with increasing columnar defect densities, systematically. The peak of z was observed near the magnetic field B / B Φ =1/3. These results are explained by the depinning model considering the distribution of the pinning strength rather than the vortex-glass model.