Teruo Matsushita

Flux creep and irreversibility line in high‐temperature oxide superconductors

The irreversibility line in high‐temperature oxide superconductors is theoretically investigated from a viewpoint of dependence on the flux‐pinning strength and a general relation between the effective pinning potential and the critical current density is derived. It is shown that the irreversibility magnetic field at 77 K in strongly pinned oxide superconductors is sufficiently high for application.

Estimate of Attainable Critical Current Density in Superconducting YBa2Cu3O7-δ

Attainable critical current density in oxide superconductor YBa2Cu3O7-δ is estimated by an ordinary analyzing method in flux pinning phenomena with superconducting parameters obtained from a magnetization measurement. The pinning mechanisms considered here are an anisotropy in the upper critical field and an electron scattering effect for grain boundaries and twinning planes. While the elementary pinning force of these planar defects is small due to a small thermodynamic critical field at 77 K, a critical current density comparable to that of Nb-Ti at 4.2 K is expected to be attained by increasing the pin concentration.

A theory of thermally activated flux creep in nonideal type II superconductors

Abstract The relaxation of the magnetization due to thermally activated flux creep in a bulk sample of a nonideal type II superconductor is discussed theoretically. Firstly, an expression for the average volume of a flux bundle characterizing its coherent thermal hopping motion is derived by starting with an equation for the local force balance on pinned fluxoids. Secondly, a thermal hopping rate of flux bundles is derived from the Fokker-Planck equation of pinned flux bundles. These results are used to derive expressions for both the time dependence of the magnetization and the induced electric field due to flux creep. It is shown that the present theory can quantitatively explain the existing observed data of the flux-creep rate, not only in conventional superconductors such as Nb-Ta and Pb-Tl but also in strongly-pinning bulk samples of the high- T c oxide superconductors such as Y-Ba-Cu-O.

Anomalous Magnetic Behavior due to Reversible Fluxoid Motion in Superconducting Multifilamentary Wires with Very Fine Filaments

It was found experimentally that the hysteresis loss in a superconducting multifilamentary wire was drastically decreased with the decrease of the filament diameter in spite of inappreciable change in the critical current density. This tendency is incompatible with the conventional theoretical prediction from the critical state model. In the present letter it is pointed out that this anomaly can be explained by taking account of a reversible motion of fluxoids in very fine filaments.

Relationship among flux depinning, irreversibility and phase transition in a disordered HTS material

The influence of flux pinning on the vortex glass-liquid (GL) phase transition and irreversibility in HTSs has been examined on the basis of systematic measurements of electric field (E) versus current density (J) curves in a YBCO thin film. It has been shown that the GL transition is identical to the thermal depinning determined by the minimum value of the macroscopic pinning strength. The isothermal scaling of the E-J characteristics is directly related to the nature of the pinning. The irreversibility line can be described as an iso-J line governed by the statistical pin distribution. If we determine the criteria for the irreversibility line, we can describe the irreversibility line based on the depinning line. The essential physical quantity, therefore, can be attributed to the flux depinning.

Electromagnetic phenomena and hysteresis losses in superconductors

Abstract Hysteresis losses in superconductors are caused by irreversible motion of fluxoids. This motion is, in most cases, described by the critical state model. In this article, various electromagnetic phenomena due to flux pinning effects are reviewed and explanations of these phenomena are given using the critical state model. The phenomena which cannot be well described by the present model, such as reversible fluxoid motion and the longitudinal field effect, are also introduced.

Highly sensitive open-flat coil magnetometer for the λ(H,T) measurements in plate-like high-Tc cuprates

The “LC resonator” method was greatly improved for high resolution measurements of magnetic-field penetration depth, λ, of radio frequency into thin films or plate-like high-Tc superconductors by replacing a solenoid testing coil by an open-flat coil (OFC) driven by a tunnel diode oscillator of a low power and a highly stable frequency. The assembled OFC magnetometer showed new technical advantages such as simplicity, convenience, and high resolution of Δλ∼1–3u200aA (Δλ/λ∼10−6), ability of reliable operation in wide ranges of temperature (4.2–300 K) and magnetic field (up to 12 T), etc. This enables us to use it for studying peculiarities of the vortex dynamics with weak signal in small sample volume high-Tc cuprates. As a first test of the assembled 23 MHz measuring setup, the temperature dependence of the upper critical field, HC2(T), is determined from the magnetic transition curves at different temperatures for Y1Ba2Cu3O7−δ thin-film ring. The HC2(T) data obey the formula: [1−(T/Tc)2]β with β=1.22±0.03 at...

Critical current densities in superconducting Y-Ba-Cu-O prepared by the quench-and-melt growth technique

The critical current characteristic of a Y-Ba-Cu-O specimen prepared by the quench-and-melt growth (QMG) technique was investigated by the ac-inductive method. It was found that there were two kinds of currents. The bulk critical current density is 1.0×108 A/m2 at 77 K and 0.12 T. This value is comparable to a previous estimate from dc-magnetization measurements. The local current density amounts to 3.2×1010 A/m2 under the same conditions. Appearance of two kinds of currents seems to be caused by nonsuperconducting second-phase layers formed along the direction of grain growth, which disturb uniform current.

Critical current density of superconducting NbTa tapes in a longitudinal magnetic field

Abstract It is empirically known that the superconducting critical current density is much larger in a longitudinal field geometry than in a transverse one. Flux cutting and flux pinning have been considered as two possible reasons for the origin of large longitudinal critical current density. Critical current density in superconducting Nbue5f8Ta tapes with normal precipitates was measured in this field geometry in a wide range of pinning strength in order to investigate this problem. Critical current density depended strongly on the pinning strength and increased with increasing pinning strength. This result insists that flux pinning dominates in the determination of critical current density. If the cutting threshold exists, it is estimated to be quite small from extrapolation of the present results to the limit of weak pinning. This value is compared with the theoretical estimate of the cutting threshold.

New paramagnetic peculiarity of the superconductive transition detected by a highly sensitive OFC magnetometer

The improved `LC-resonator technique (the open-flat coil magnetometer), based on a low-power stable-frequency tunnel diode oscillator with a single-layer open-faced coil, is very sensitive to changes of position. About 1 A absolute and better than 10-6 relative resolution can be reached in distance change measurements. It is an excellent MHz-range scientific instrument with which one may study weakly expressed fine peculiarities of the superconductive state in small-volume thin plate-like high-Tc superconductors (HTS). It enables changes to be detected of about 1 pH of the HTS films magnetic inductance (changes of ~1-3 A of the magnetic penetration depth, λ) with very high (~10-6) relative resolution. Owing to the pick-up coils flat design, relatively low operation frequency and high relative resolution, the present method has advantages over others. It is essential for non-destructive studies of thin flat HTS materials (with a small signal) especially near Tc (at the beginning of the formation of `Cooper pairs). Mainly as the result of these advantages a new `paramagnetic peculiarity of the superconductive transition has been detected in Y-Ba-Cu-O film, which precedes the well known `diamagnetic ejection. A possible explanation of the observed effect is given, and the influence of some external physical parameters on the effect is revealed and discussed.