Kuniyuki Miyahara

Fluxoid Pinning in Nonideal Type-II Superconductors. I. Force between a Fluxoid and a Lattice Defect

A general expression of the interaction energy between fluxoids and lattice defects is obtained by perturbational calculations of the generalized Ginzburg-Landau equations, which are derived by minimizing the free energy of the mixed state including elastic terms. The present expression is the same as Labuschs obtained only for a special type of lattice defects with a pure shear, but is shown to be applicable to any type of defects. The interaction force between a fluxoid and a lattice defect is derived for some typical types of defects by the use of the present expression of interaction energy, and the results of previous authors are shown to be unsatisfactory, even qualitatively. The present results give the first step for the derivation of a general expression of the so-called “pinning force”, which is the final purpose of this series.

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.

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.

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.

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.

In situ measurements of transport characteristics in heavy-ion irradiated YBa2Cu3Oy under magnetic field

Abstract We produced splayed columnar defects in a c-axis oriented YBa2Cu3Oy thin film by irradiating 200 MeV Au ions at low temperature (∼80 K ) from two different directions; ±15° with respect to the c-axis. Then the transport critical current density Jc as a function of magnetic field direction θ with respect to c-axis was measured in situ. A symmetric peak for Jc was observed in θ-dependence of Jc around θ=0°.

Amorphous gallium phosphide (a-GaP) pn-diode by ion beam assisted deposition

Abstract Amorphous gallium phosphide (a-Ga x P) has been formed as pn-diode by using electron beam deposition under hydrogen and/or nitrogen ion beam assistance. The rectification performance limited due to localized states, is improved to an optical device application grade through a higher current source of hydrogen and/or nitrogen ions. It is suggested, that a selective usage of the two ions can optimize the carrier type and junction states. In the case of an optimized ion-assisted amorphous film on crystal GaP, blue light luminescence has been observed.

Fluxoid Pinning in Nonideal Type II Superconductors. II. Time-Dependent Ginzburg-Landau Equations

A set of usual Ginzburg-Landau Equations is extended to discuss a slowly time-dependent phenomenon such as a flux flow in nonideal type II superconductors. A Hamiltonian is composed of interaction terms between electrons and elastic strain fields, together with a usual Hamiltonian including the BCS potential for superconductors. A derivation of the Ginzburg-Landau equations started from the above Hamiltonian is carried out by the scheme of the GLAG theory, except that the strain field is dealt with semi-classically. The extensions of the Ginzburg-Landau equations to the case of nonideal samples so far have been carried out in phenomenological ways, besides the existing ones are only applicable to static phenomena and cannot be used to discuss dynamic phenomena such as a flux flow.

Critical current density of YBa2Cu3Oy containing inclined columnar defects

Abstract Effects of columnar defects on the critical current density have been investigated by the in situ measurement of current–voltage characteristics for a c -axis oriented YBa 2 Cu 3 O y (YBCO) thin film after the irradiation with 200 MeV Au ions from the direction θ irr =45° off the c -axis. The dependences of the critical current density, J c , on the magnetic field, the magnetic field angle, and the density of columnar defects are discussed. With increasing the density of the columnar defects, B φ , the contribution of the columnar defects to the pinning force per unit volume, ΔF p , increases monotonically. The peak field for ΔF p as a function of the magnetic field seems to be almost independent on B φ .