Nobumitsu Usui

Bio: Nobumitsu Usui is an academic researcher from Nihon University. The author has contributed to research in topics: Superconductivity & Type-II superconductor. The author has an hindex of 7, co-authored 8 publications receiving 160 citations.

Magnetic Behavior and Effect of Transport Current on It in Superconducting Nb-Zr Wire

Abstract: The magnetic behaviors of superconducting Nb-Zr (25%) wire have been studied at 4.2°K. The magnetization curve exhibits an irreversible behavior. In transverse fields the magnetization is reduced due to the transport current, but in longitudinal fields the influence of the transport current is not recognized. The results are explained by a phenomenological model for inhomogeneous type II superconductors. The model, based on Ginzburg-Landau-Abrikosov-Gor'kov (GLAG) theory, was proposed by Silcox and Rollins and extended by the authors to consider a pinning effect of imperfections on fluxoids. The calculated magnetization curve is in good agreement with the experimental values. The influence of the transport current is qualitatively explained by introducing the Lorentz force on fluxoids due to current into the theory. The dependence of critical current density of a short wire sample on an applied field is also calculated and compared with the experimental values.

Effect of External Current on the Magnetization of Non-Ideal Type II Superconductors

Abstract: The influence of the external current on the magnetization of non-ideal type II superconductors is explained. It is shown how in the mixed state the external current brings about a particular state of the critical distribution of flux threads allowing a finite total transport current. In the case where the external current is perpendicular to the external magnetic field, the reduction of the magnetization due to the external current and its dependence on the history of the application of current and field are explained quantitatively in Nb-Zr(25%) alloy. The transverve paramagnetic effect observed by LeBlanc is also discussed.

Magnetization, Flux Flow Resistance and Hall Effect in Superconducting Vanadium

Abstract: The magnetization, the flux flow resistivity and the Hall voltage have been measured for three vanadium specimens with different purities. The parameters pertinent to the magnetic properties of type II superconductors near the upper critical field H c 2 –the Ginzburg-Landau-Maki parameters κ 1 and κ 2 , and the Helfand-Werthamer parameters h * and (d h /d t )–were obtained from the data. It was found that the values of κ 1 , κ 2 and h * at 0°K increased with increasing purity and the magnitude of the increase of them was larger than the value expected from the microscopic theory. This discrepancy between experiment and theory seems to correspond to the effect of the non-spherical Fermi surface suggested by Hohenberg and Werthamer. Resistivity measurements showed that with increasing purity, the behavior of the slope of the flow resistivity near H c 2 changed from that of the dirty material predicted by Caroli-Maki to that of intermediate material. This result is different from the Kim's empirical relation...

Effects of Paramagnetic Impurities on Superconducting Properties in the La 3− x Gd x Al System

Abstract: Effects of the magnetic impurity on superconducting properties were investigated on the La 3- x Gd x Al system. The superconducting transition temperature T c , the upper critical field H c2 and the magnetic susceptibility in the normal state were measured for various concentrations of gadolinium. A remarkable departure of the T c versus impurity concentration curve from the Abrikosov-Gor'kov curve was found in the region of high impurity concentrations (>0.4 at.% GD). Magnetic susceptibility versus temperature curves of compounds with Gd concentrations large enough to suppress the superconductivity showed no peak above 4.2°K. The H c2 versus temperature curve of the compound with 0.3 at.% Gd crossed the curve of La 3 Al. The upper critical field at 0°K, H c2 (0), had a maximum at a certain Gd concentration near La 3 Al. This fact is qualitatively explained by the theory of Fulde-Maki.

Flux vortices and transport currents in type II superconductors

Abstract: This article is concerned with the mechanisms by which type II superconductors can carry currents. The equilibrium properties of the vortex lattice are described and the generalized driving force in gradients of temperature and field is derived using irreversible thermodynamics. This leads to expressions for thermal cross effects which can include pinning forces. The field distributions which occur in a range of situations are derived and a number of useful solutions of the critical state given. In particular, the distribution in a longitudinal field is obtained, and the conditions under which force-free configurations can break down by the cutting of vortices discussed. The effects of lattice rigidity on the summation of pinning forces is considered and it is shown that a summation based on statistical arguments uses the same approximations and leads to the same results as a dissipation argument. Theoretical expressions are derived for the vortex pinning interaction to a number of different metallurgical...

Temperature dependence of the dielectric function of germanium

Abstract: Ellipsometric measurements of the dielectric constant of undoped Ge were performed between 1.25 and 5.6 eV in the temperature range of 100 to 850 K. The dependence of the ${E}_{1}$, ${E}_{1}+{\ensuremath{\Delta}}_{1}$, ${E}_{0}^{\ensuremath{'}}$, and ${E}_{2}$ critical energies on temperature was obtained. It can be represented either with Varshni's empirical formula or with an expression proportional to the Bose-Einstein statistical factor of an average phonon. Broadening parameters, amplitudes, and phase angles for the corresponding critical points were also obtained. A decrease of the excitonic interaction with increasing temperature was found. The results are discussed in the light of recent calculations of the effect of temperature on the band structure of Ge containing Debye-Waller and self-energy contributions.

Theory of Flux Motion in Non-Ideal Type-II Superconductors

Abstract: A phenomenological expression of the pinning force density is proposed, and the equation of force balance including this expression is shown to describe well the motion of magnetic fluxoids in non-ideal type-II super-conductors in cooperation with the equation describing the continuity of magnetic flux flow. The parameters related to an individual pinning force and pin distribution are included in the present expression which contains those from the models of Bean-London, Silcox-Rollins and Yasukōchi as its special cases. The calculated magnetizations agree well with observed data by adjusting the parameters. The hysteresis losses in the course of magnetization are calculated and the field dependence of the a.c. losses are explained. The formulas of the a.c. dynamic losses are also shown. The field dependence of the critical current in the resistive state and the relaxation time of flux penetration are calculated and compared with observed data.

Pinning of flux vortices in type-ii superconductors.

Abstract: The superconducting properties of Pb-Bi alloys are reported. Measurements were made on the epsilon phase (a reversible material with K ∼ 12) and the eutectic, which is this epsilon phase with pure Bi precipitates. It is shown that the irreversible properties of the eutectic are consistent with the pinning of vortices in a reversible matrix of epsilon phase. The critical state is shown to be the same in measurements of magnetization, transverse critical currents and zero field critical currents, and it is extended to cover a composite ferromagnetic material. Experimentally it is found that the variation of critical current with flux density, microstructure and temperature is given by the expression J c = 33S v M(rev.)/B1/2 amps/sq.cm. (S v is the phase boundary area per unit volume and M (rev.) the magnetization of the matrix.) There is strong evidence that the irreversibility is due to pinning at the phase boundaries of precipitates and it is found that the surface currents observed in the epsilo...