Takanobu Kiss

A NEW INTERPRETATION OF THE GLASS-LIQUID TRANSITION OF PINNED FLUXOIDS IN HIGH-TC SUPERCONDUCTORS

Abstract The present paper proposes a possibility that the transitions between the vortex glass state and the vortex liquid state, which have been observed by the resistive measurements in high- T c cuprate superconductors, can be interpreted as the thermal-fluctuation-induced percolative transitions of pinned fluxoids. Since weaker flux-pinning centers become successively ineffective due to thermal agitation as the temperature increases, this kind of percolative transition occurs when a narrow channel of unpinned fluxoids penetrates throughout the whole specimen, and hence the flux flow begins to occur through the channel of unpinned fluxoids, where the long range order among fluxoids recovers. The results of computer simulation on the motion of pinned fluxoids support strongly the present mechanism. The present theoretical expression for the scaled master curves of the induced electric field versus the transport current density shows good agreement with existing observed data. The relations of the transition temperature and the scaling exponents to the flux-pinning characteristics are also discussed.

Current-voltage characteristics near the glass-liquid transition in high-Tc superconductors

Abstract Theoretical expressions for the induced DC electric field, E , are derived as a function of the applied current density, J , near the glass-liquid transition temperature of T = T GL as E(J) = E (J)[1 + ( δE(J) E (J) )] , where E (J) is the scalable part and δE(j) E (j) is the unscalable part given by a power series of | T GL − T | nv ( z + 2 − D ) wi Then the condition of δE E ⪡1 gives a measure of the width of the critical regime, in which E = E ( J ) is well scalable. The general scaling characteristics of E= E (J) are the same as those predicted by Fisher and coworkers based on the conventional theories of phase transitions with the scaling hypothesis, while the present theory provides a concrete expression for E (J) . Furthermore, the expressions for the scaled master curves of the AC impedance are derived. The reason, why the observed E vs. J characteristics are scalable over wider ranges of the temperature and flux density than the critical regime, is also discussed.

Conceptual Design of HTS Coil for SMES Using YBCO Coated Conductor

High Tc superconducting (HTS) toroidal coil using YBCO coated conductor is designed for 70 MJ class superconducting magnetic energy storage (SMES) for power system control. Its configuration and shape are optimized by means of genetic algorithm (GA) to minimize the required length of the conductor. The optimization is performed for two kinds of constrains, i.e., maximum electric field or flux flow loss of the coil, which are calculated by means of finite element method (FEM). The FEM analysis considers quantitative current density (J)-electric field (E) expressions based on percolation transition model. It is shown that the great transport performance against magnetic field of YBCO coated conductor can realize a very compact SMES coil compared with an existing Nb-Tis one.

Critical current properties in HTS tapes

Abstract The author’s recent studies on the critical current properties in Bi-2223 multifilamentary tapes as well as YBCO coated IBAD tapes have been reviewed. Extended electric field vs. current density ( E – J ) characteristics were studied over wide range of temperature, T , magnetic field, B , and angle, θ . Based on the analysis of statistic J c distribution and scaling, we derived an analytical expression for J as a function of E , T , B , and θ . This method allows us to describe and extrapolate the nonlinear E – J characteristics even in extremely low E region and/or high B region based on rather simple measurements.

Current Transport Properties of 200 A-200 m-Class IBAD YBCO Coated Conductor Over Wide Range of Magnetic Field and Temperature

Current transport properties in 200 A-200 m-class IBAD-YBCO coated conductor (CC) have been investigated over a wide range of temperature and magnetic flux density up to 26 T. YBCO CC fabricated by reel-to-reel pulsed laser deposition equipment with a multi-plume and multi-turn deposition system showed high critical current of 245 A in length of 212.6 m. Critical current density, Jc, in 5 T parallel to c-axis at 65 K and 4.2 K were 0.44 MA/cm2 and 5.5 MA/cm2, respectively. Moreover, even in very high magnetic flux density, the superior Jc property remains at lower temperature, e.g. Jc at 4.2 K in 26 T was 2 MA/cm2. We also showed analytical expressions of electric field vs. current density (E-J) characteristics based on a framework of percolation model and scaling law of the flux pinning properties. Using the expressions, E-J characteristics and Jc value at any conditions of temperature and magnetic flux density can be predicted quantitatively.

Estimation of E–J characteristics in a YBCO coated conductor at low temperature and very high magnetic field

Abstract E – J characteristics in a YBCO coated conductor at low temperature and very high magnetic field up to 27 T have been estimated using experimental E – J characteristics below 12 T along with a scaling analysis based on the percolation model. 1 mm thick YBCO films was deposited on a biaxially aligned Y 2 O 3 /YSZ/Hastelloy substrate fabricated by the IBAD technique. The critical current I c for the 1 cm wide tape was 85 A at 77 K in self-field. We measured E – J characteristics in perpendicular field up to 12 T using 100 μm wide and 1 mm long bridge patterned in the tape. Statistic J c distribution and pinning properties were analyzed from the E – J characteristics. Using the pinning parameters determined by the scaling analysis, we can calculate the critical current properties as a function of temperature and magnetic field. E – J characteristics at low temperature and very high magnetic field up to 27 T were estimated analytically and compared with the experimental data. Estimated value of J c and n -value, and their error analysis were also discussed.

Percolative transition and scaling of transport E-J characteristics in YBCO coated IBAD tape

We have investigated extended electric field-vs.-current density E-J characteristics in YBCO coated IBAD tapes. The results of a Monte-Carlo study on the depinning in a random pin medium were compared with measurements. Using a low temperature scanning laser microscope, we examined the percolative behavior of the local resistive transition in the YBCO tape. It was also shown that the depinning probability, which is proportional to the dynamic resistance of the E-J curves, is scaled as a function of reduced current density with the aid of a power index. Consequently, the E-J characteristics in the tapes can be described by the combination of the two kinds of scaling laws: one is the scaling law of the depinning probability predicted in a random network and the other is the scaling law for the flux pinning force. These properties agree well with the percolation model of depinned clusters.

Stability and quench development study in small HTSC magnet

Stability and quench development in a HTSC magnet have been experimentally studied with the transport current in the magnet being below or above the “thermal quench current” level. The magnet was tested at both cryocooler cooling and liquid nitrogen cooling, with and without background magnetic field (up to 4 T). The temperature and electrical voltages in different sections of the magnet were measured using 20 thermocouples and 24 potential taps embedded in the winding. In this paper, the experimental procedure and the results are described. The results are compared with those obtained earlier in the experiments with the smaller HTSC specimens and are analysed by using the scaling theory of the thermal quench.

Highly effective and isotropic pinning in epitaxial Fe(Se,Te) thin films grown on CaF2 substrates

We report on the isotropic pinning obtained in epitaxial Fe(Se,Te) thin films grown on CaF2(001) substrate. High critical current density values – larger than 1 MA/cm2 in self field and liquid helium – are reached together with a very weak dependence on the magnetic field and a complete isotropy. Analysis through transmission electron microscopy evidences the presence of defects looking like lattice disorder at a very small scale, between 5 and 20 nm, which are thought to be responsible for such isotropic behavior in contrast to what was observed on SrTiO3, where defects parallel to the c-axis enhance pinning in that direction.

Angular dependence of critical current properties in YBCO coated tape under high magnetic field up to 18 T

Abstract Critical current properties in a YBCO coated tape have been studied as a function of temperature, magnetic field and field angle under high magnetic field up to 18 T. 1 μm thick YBCO film was deposited on a bi-axially aligned Y 2 O 3 /YSZ/Hastelloy substrate fabricated by the IBAD technique. The value of critical current, I c , of the 1 cm wide tape reached as high as 85 A at 77 K in self-field condition. After etching the tape into 100 μm wide and 1 mm long bridge, we measured extended E – J characteristics. The value of critical current density, J c , is maintained higher than 10 5 A/cm 2 at 70 K in 5 T perpendicular to the tape surface. Moreover, the power index n and the so called glass–liquid transition field, B GL , of the tape in perpendicular field are superior to those of epitaxial film deposited on a SrTiO 3 single crystalline substrate. J c vs. angle relationship shows broad peak around 90° where field is perpendicular to the tape surface, in addition to a sharp peak at 0°. These results suggest that the tape is highly uniform and correlated defects along c -axis direction contribute to flux pinning.