High-temperature Superconductor YBa2Cu3O7-x Research Articles

Flux Pinning Enhancements of YBa2Cu3O7-x with Nanosize Magnetic Additions

Different methods of flux pinning are being tested world-wide to enhance critical currents (Ics) of high temperature superconductor YBa2Cu3O7-x (YBCO) coated conductors exposed to high magnetic fields. Magnetic materials are interesting to consider as flux pinning additions because of their potential for very strong pinning strength. To our knowledge, there have been limited demonstrations of magnetic pinning additions to YBCO conductors. This paper describes the study of different M magnetic phase additions to YBCO including M = BaFe12O19, La0.67Ca0.33MnO3, and other oxide phases. Nanosize additions were incorporated by depositing multilayer (M/YBCO)N films to minimize degradation of Tc, and testing volume % additions of M phase from 0.5 % to 5%. Results indicate that Tc onsets are depressed with magnetic additions, however in some cases interestingly without degrading the transition width. With optimization of magnetic additions a 50% increase of critical current density has been obtained, for low magnetic fields of < 10,000 Oe at 65K to 77K. Microstructural and superconducting properties are summarized, including SEM analysis.

In operando evidence of deoxygenation in ionic liquid gating of YBa2Cu3O7-X

Field-effect experiments on cuprates using ionic liquids have enabled the exploration of their rich phase diagrams [Leng X, et al. (2011) Phys Rev Lett 107(2):027001]. Conventional understanding of the electrostatic doping is in terms of modifications of the charge density to screen the electric field generated at the double layer. However, it has been recently reported that the suppression of the metal to insulator transition induced in VO2 by ionic liquid gating is due to oxygen vacancy formation rather than to electrostatic doping [Jeong J, et al. (2013) Science 339(6126):1402-1405]. These results underscore the debate on the true nature, electrostatic vs. electrochemical, of the doping of cuprates with ionic liquids. Here, we address the doping mechanism of the high-temperature superconductor YBa2Cu3O7-X (YBCO) by simultaneous ionic liquid gating and X-ray absorption experiments. Pronounced spectral changes are observed at the Cu K-edge concomitant with the superconductor-to-insulator transition, evidencing modification of the Cu coordination resulting from the deoxygenation of the CuO chains, as confirmed by first-principles density functional theory (DFT) simulations. Beyond providing evidence of the importance of chemical doping in electric double-layer (EDL) gating experiments with superconducting cuprates, our work shows that interfacing correlated oxides with ionic liquids enables a delicate control of oxygen content, paving the way to novel electrochemical concepts in future oxide electronics.

Thermal-physical and thermoelectric properties of solids near the polymorphic and superconducting transitions

The relationships are presented for the effective thermal conductivity and thermoelectric power coefficients near the superconducting and polymorphic phase transitions. The relationships are verified using the experimental data obtained for porous media saturated with a fluid. The experimental data obtained for the high-temperature superconductor YBa2Cu3Ox (x = 6.8, 6.9) and n-InAs are analyzed.

Contactless Estimation of Critical Current Density and Its Temperature Dependence Using Magnetic Measurements

Contactless Estimation of Critical Current Density and Its Temperature Dependence Using Magnetic Measurements A. Youssef, L. Banicova, Z. Svindrych and Z. Janůb,∗ Charles University in Prague, Faculty of Mathematics and Physics, 121 16 Prague 2, Czech Republic Institute of Physics of the AS CR, v.v.i., Na Slovance 2, 182 21 Prague, Czech Republic We have measured magnetic response of second-generation high temperature superconductor YBa2Cu3Ox wire and Nb thin film in perpendicular ac field as a function of temperature. We compare experimental complex ac susceptibility to the calculated susceptibility based on the model of Bean’s critical-state response in two-dimensional disk in perpendicular field. The harmonic analysis is needed for comparison between the model and the experimental data. We present a method of linking model and experimental susceptibility. Good agreement of experimental susceptibility with model susceptibility of 2D disk allows contactless estimation of critical depinning current density and its temperature dependence.

Study of the long-range proximity effect in LSMO/YBCO bilayers

Superconductor-ferromagnet thin film heterostructures in lateral geometry were prepared to study the long-range proximity effect. The high temperature superconductor YBa2Cu3Ox (YBCO) as superconductive electrode and La0.67Sr0.33MnO3 (LSMO) as ferromagnetic separation were used. The length of the ferromagnetic layer between two YBCO microstrip electrodes was about 3 μm. The temperature dependence of the resistance of these structures was measured. The results obtained showed that the separation of the superconducting YBCO microstrips is too large for observation of a long-range proximity effect; further optimization of the structures is, therefore, needed.

Analysis of magnetic response of critical state in second-generation high temperature superconductor YBa2Cu3Ox wire

We have measured magnetic response of second-generation high temperature superconductor YBa2Cu3Ox wire in perpendicular ac field as a function of temperature. We compare complex ac susceptibility to the calculated susceptibility based on the model of Bean’s critical-state response in two-dimensional (2D) disk in perpendicular field and infinitely long cylinder in parallel field. We present a method of linking model and experimental susceptibility. Good agreement of experimental susceptibility with model susceptibility of 2D disk allows noncontact estimation of critical depinning current density and its temperature dependence.

The Origin of Electrical Activity at Grain Boundaries in Perovskites and Related Materials

In the last few years, the combination of atomic-resolution Z-contrast microscopy, electron energy loss spectroscopy and first-principles theory has proved to be a powerful means for structure property correlations in complex materials1. Here we demonstrate the effectiveness of this combined approach by demonstrating the origins of electrical activity at grain boundaries in the prototypical perovskite SrTiO3 and the high-temperature superconductor YBa2Cu3O7-x, materials that are closely related in structure. We show, both experimentally and theoretically, that grain boundaries in SrTiO3 are intrinsically non-stoichiometric. Electron energy-loss spectroscopy (EELS) provides direct evidence of non-stoichiometry, in agreement with total- energy calculations that predict non-stoichiometric grain boundaries to be energetically favorable. The predicted structures are consistent with atomic-resolution Z-contrast micrographs. These results provide a consistent explanation of the grain boundary charge that was previously inferred from electrical measurements, and provides a microscopic explanation of the resulting “double-Schottky barriers”. We also present experimental evidence for non-stoichiometry at grain boundaries in the high-temperature superconductor YBa2Cu3O7-x, where the same phenomenon explains the observed exponential reduction of critical currents with grain boundary misorientation.

Current transport along the [001] axis of YBCO in low-temperature superconductor-normal metal-high-temperature superconductor heterostructures

The electrophysical properties of heterojunctions several microns in size, obtained by successive deposition of the metal-oxide high-temperature superconductor YBa2Cu3Ox, a normal metal Au, and the low-temperature superconductor Nb, were studied experimentally. Current flows in the [001] direction of the epitaxial YBa2Cu3Ox film. It is shown, by comparing the experimental data with existing theoretical calculations, that for the experimentally realizable transmittances (D=10−5–10−6) of the YBa2Cu3Ox—normal metal boundary the critical current of the entire heterostructure is low (of the order of the fluctuation current) because of a sharp change in the amplitude of the potential of the superconducting carriers at this boundary. The current-voltage characteristics of the heterostructure studied correspond to tunnel junctions consisting of a superconductor with \(d_{x^2 - y^2 } \) type symmetry of the superconducting wave function and a normal metal.

Radio-frequency properties of inhomogeneous superconductors in low magnetic fields (SQUIDs)

It is experimentally demonstrated how closed looped objects of inhomogeneous superconductors can exhibit quantum interference device (SQUID) behaviour. It appears that the physical origin is the ability of exceedingly small changes in magnetic field to induce macroscopic redistributions of magnetic flux. The samples studied range from ceramic tubes of the high-temperature superconductor YBa2Cu3O7-x to thin-film tubes of ordinary metallic superconductors with artificially introduced defects. In contrast to the ordinary SQUID, the observed period with flux in the response does not usually correspond to a single flux quantum in the geometrically calculated applied flux. This is associated with incomplete penetration of the flux lines through the tube walls.

超電導磁気軸受の試作と評価

High-temperature superconductor YBa2Cu3O7-x with strong pinning force has allowed the realization of stable magnetic suspension with no control. Superconducting magnetic bearing (SMB) using YBCO and permanent magnet, and SMB operation system were assembled. TWO SMB cooled down in a field cooling state by liquid nitrogen were proved to support a rotor weighing 2.5 kg up to 42 000 rpm. A radial run out of the rotor was 8 μm or less. The value can be considered for practical applications. The rotating performance at high speed has been disclosed by an active control assist in radial direction during accelerations.

Possible flux pinning centers in YBa2Cu3O7-x

The critical current density Jc of a superconductor is one of its most important properties for the purposes of applications. Various investigators have made numerous attempts to increase the value of Jc in the high temperature superconductor YBa2Cu3O7-x (Y123) through the introduction of defects into the crystal structure. References 1, 2, and 3 provide a few examples of some of the more recent efforts. In general, defects of an appropriate size should act as flux pinning centers in the material.We have attempted to increase Jc in Y123 samples both through bulk processing techniques and through irradiation by fast neutrons. In an attempt to correlate the microstructures of the materials with the superconducting properties we have studied the materials using TEM. Here, we report on results from two samples. The first is an Y123 sample prepared by the decomposition of YBa2Cu4O8 (Y124). The decomposition process results in Y123 material with a value of Jc which is enhanced by approximately a factor of 10 at low magnetic field at 77 K as compared with Y123 prepared by standard processes. The second sample is one which was irradiated with fast neutrons (&gt; 0.1 MeV) to a fluence of 6.0 x 1017 cm-2. This sample also shows an increase in Jc.

Microwave absorption by vortex cores in the high-temperature superconductor YBa2Cu3O7-x.

Direct absorption measurements of microwave energy having energy less than the superconducting gap in the granular composites of the 90-K superconductor ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$ as a function of dc magnetic-field strength between 0.1 and 1.0 T reveal a nonlinearly increasing absorption of microwave energy with increasing field strength. The absorption can be detected in regions of the T-H phase diagram in which thermally activated flux flow does not occur. The temperature and field dependences of the absorption are different from those for low-magnetic-field absorption (H0.06 T) and are shown to be consistent with the behavior expected by the absorption of microwave energy by normal carriers at the cores of vortices in type-II superconducting materials.

Crucible corrosion in crystal growth of YBa2Cu3O7-x

Crucible corrosion is one of the major obstacles to obtaining large crystals of the high-temperature superconductor YBa2Cu3O7-x. The crucible-melt interfaces of three types of ceramics (alumina, zirconia and tin oxide) have been studied in order to understand the corrosion mechanism. This may help to reduce the crucible problem in crystal growth of YBCO and to indicate alternative crucible materials of improved corrosion resistance.

Electron spin resonance and infrared studies of Cu2+sire symmetry in the high-Tcsuperconductor YBa2Cu3O7-x

The author reports detailed electron spin resonance (ESR) studies (300-77 K) of the high-temperature superconductor YBa2Cu3O7-x for samples (type A) have a broad transition (Tc0-Tconset=13.5 K) as well as for samples (type B) with a sharp transition (Tc0-Tconset=3 K). The room-temperature axial symmetry around the Cu2+ ion in both cases is maintained down to Tc0+or-2 K (Tc0 equals 86.5 K for type A samples and 96 K for type B samples) below which the symmetry changes gradually to orthorhombic at 77 K. In type B samples the orthorhombicity in the g tensor is larger than in type A samples. When the sample undergoes as many as five repeated cooling and heating cycles (within a week for case II), the superconducting transition becomes broader with Tconset=96 K and Tc0 is correlated with the superconducting properties. The infrared spectrum of case II is significantly different from that of case I. The EST linewidth in both cases (types A and B) is almost temperature independent; this is a rather surprising result for the ESR of Cu complexes. The nearly linear variation in ESR signal intensity IS with temperature in type A samples shows the possible existence of Curie susceptibility whereas in type B samples the weak dependence of IS on T may indicate possible Pauli paramagnetism.

Growth of Large Single Crystals of Oxide Superconductor YBa&lt;sub&gt;2&lt;/sub&gt;Cu&lt;sub&gt;3&lt;/sub&gt;O&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;

Single crystals of high-temperature superconductor YBa2Cu3Ox have successfully been prepared using a CuO or a BaO-CuO flux method. Several shapes of crystals are obtained in a single batch. Two types of crystals having the same YBa2Cu3Ox composition are foud: one is thin plates which are semiconductive but not superconductine above 4.2K, and another shows a very sharp superconducting transition above 90K in the as-grown state. The former is supposed to form under a solution growth condition, whereas the latter by the use of a peritectic reaction between Y2BaCuO5 and surrounding liquid phase. As the former is extremely thin whereas the latter is thick, a large volume of block crystals can possibly be obtained in the latter reaction. For the purpose of obtaining crystals of higher quality, an experiment was held at various growth-temperatures and compositions of flux. As a result, growth temperature should be set at a little higher than the peritectic temperature. If the max. temperature is too high, the separation of Y2BaCuO5 from the co-existing liquid phase occurs because of the coagulation of the liquid at high temperatures, and this separation prevents the progress of the peritectic reaction. An application of CuO-BaO flux in the place of pure CuO effectively prevents the crystals from taking in flux. Due to the low melting point, CuO-BaO flux can easily be discharged from the crystals and the strong impetus to advance the peritectic reaction.

Growth of YBa2Cu3Ox single crystals

We report a set of conditions for crystal growth of the high-temperature superconductor YBa2Cu3Ox. The as-grown single crystals have critical temperatures up to 85 K. Preliminary studies have shown that the transition temperatures can be increased by thermal annealing in oxygen, as in ceramic samples. The crystals are in suitable dimensions for definitive magnetic, optical, and transport measurements.

Ion Channeling Studies of Crystallinity, Amorphization, and Thermal Annealing in Single-Crystal YBa2Cu3Ox.

ABSTRACTWe have performed Rutherford backscattering spectroscopy (RBS) and axial channeling on single crystals of the high-temperature superconductor YBa2Cu3Ox. The results demonstrate good crystal quality and cation stoichiometry in the as-grown samples. We observe some surface disorder, and estimate from the surface-peak intensities that there is roughly one formula unit of misregistered atoms per unit cell, or one molecular monolayer of surface disorder.We have studied the surface disorder induced in these crystals by various surface processing treatments, and have tried to achieve solid-phase epitaxial regrowth of disordered surface layers. Preliminary results have been obtained on the annealing of 100 nm amorphous layers formed on the surface of single crystals by 30 keV oxygen ion implantation. The Ba in the amorphous layer was found to segregate strongly to the surface for annealing temperatures as low as 500°C, although the crystalline phase does not decompose even at much higher temperatures.