High-temperature Superconductor YBa2Cu3O7 Research Articles

Deposition and characterization of thin HTS and magnetic perovskite films

The in-situ growth and physical properties were investigated of thin films and bilayers of high-temperature superconductors (HTS) YBa2Cu3O7 (YBCO) and ferromagnetic (FM) manganites La0.7Sr0.3MnO3 (LSMO) and La0.7Ca0.3MnO3 (LCMO) with thicknesses of several tens of nm obtained by magnetron sputtering. Similar twin structures were observed by an optical microscope in areas on the LaAlO3 substrate surface void of films, in areas containing YBCO, in LSMO films, and in YBCO/LCMO bilayers, although the main spatial period of the twin structure seemed to be slightly different in the areas containing the LSMO film. The resistance (and its temperature dependence) of the LCMO films strongly depends on the annealing conditions. The resistance of the LSMO and LCMO films grown on Al2O3 substrates decreased as the temperature (T) was increased in the lower and higher temperature ranges, and increased as T was increased at medium temperatures.

Highly efficient solid state catalysis by reconstructed (001) Ceria surface

Substrate engineering is a key factor in the synthesis of new complex materials. The substrate surface has to be conditioned in order to minimize the energy threshold for the formation of the desired phase or to enhance the catalytic activity of the substrate. The mechanism of the substrate activity, especially of technologically relevant oxide surfaces, is poorly understood. Here we design and synthesize several distinct and stable CeO2 (001) surface reconstructions which are used to grow epitaxial films of the high-temperature superconductor YBa2Cu3O7. The film grown on the substrate having the longest, fourfold period, reconstruction exhibits a twofold increase in performance over surfaces with shorter period reconstructions. This is explained by the crossover between the nucleation site dimensions and the period of the surface reconstruction. This result opens a new avenue for catalysis mediated solid state synthesis.

Lattice dynamical signature of charge density wave formation in underdopedYBa2Cu3O6+x

We report a detailed Raman scattering study of the lattice dynamics in detwinned single crystals of the underdoped high temperature superconductor YBa2Cu3O6+x (x=0.75, 0.6, 0.55 and 0.45). Whereas at room temperature the phonon spectra of these compounds are similar to that of optimally doped YBa2Cu3O6.99, additional Raman-active modes appear upon cooling below ~170-200 K in underdoped crystals. The temperature dependence of these new features indicates that they are associated with the incommensurate charge density wave state recently discovered using synchrotron x-ray scattering techniques on the same single crystals. Raman scattering has thus the potential to explore the evolution of this state under extreme conditions.

Gold nanocrystals in high-temperature superconducting films: creation of pinning patterns of choice

One of the major drawbacks for applications of high-temperature superconducting films is that magnetic flux is not completely expelled but penetrates the film in the form of flux vortices. Any motion of these vortices is accompanied by magnetic noise and prevents larger supercurrents. Thus, an effective pinning of flux vortices is a prerequisite for applications based on thin films of high-temperature superconductors such as coated conductors or magnetic sensor devices. However, particular superconducting structures such as tunnel barriers or flux guides suffer from increased pinning forces. We report that adding thin gold layers to the production process of epitaxial films of the high-temperature superconductor YBa2Cu3O7−δ allows the fabrication of superconducting films with spatially varying flux pinning properties. This paves the way for an easy realization of custom tailored current carrying capabilities in arbitrary patterns. Critical current densities of already strong pinning films can be locally enhanced up to 150% to prepare a material of choice at a position of choice for the realization of high-quality electronic devices with improved performance.

The critical current density of grain boundary channels in polycrystalline HTS and LTS superconductors in magnetic fields

We provide evidence that a single mechanism—flux flow along channels—can explain the functional form of the critical current density (Jc) in the low-temperature superconductor Nb3Sn and in the high-temperature superconductors (HTS) YBa2Cu3O7−δ (YBCO) and (Bi,Pb)2Sr2Can−1CunOx (BiSCCO) in low and high magnetic fields. In this paper, we show that standard flux pinning theories, used for the past four decades to describe Jc in low-temperature superconductors (LTS), cannot explain the strain dependence of Jc in YBCO because Jc is a function of strain but the average superconducting properties are not. We conclude that in the polycrystalline samples presented here, the channels are grain boundaries that are narrow and metallic in Nb3Sn and YBCO but wide and semiconducting in BiSCCO. In Nb3Sn, strain alters Jc by changing the superconducting properties of the grains, whereas in YBCO, strain alters Jc by changing the properties of the grain boundaries.

Microwave spectroscopy of vortex dynamics in ortho-II YBa2Cu3O6.52

We present measurements of the vortex-state surface impedance, Z_s = R_s + i X_s, of a high quality, ortho-II-ordered single crystal of the cuprate high temperature superconductor YBa2Cu3O6.52 (T_c = 59K). Measurements have been made at four microwave frequencies (\omega/2\pi = 2.64, 4.51, 9.12 and 13.97 GHz), for magnetic fields ranging from 0 to 7 T. From these data we obtain the field, frequency and temperature dependence of the vortex viscosity, pinning constant, depinning frequency and flux-flow resistivity. The vortex viscosity, \eta(\omega,T), has a surprisingly strong frequency dependence and bears a striking resemblance to the zero-field quasiparticle conductivity, \sigma_qp(\omega,T), suggesting that the dominant dissipative mechanism for the flux lines is induced electric fields coupling to bulk, long-lived d-wave quasiparticles outside the vortex cores. This is in sharp contrast to the conventional Bardeen-Stephen picture, in which dissipation takes place inside quasi-normal vortex cores. The strong frequency dependence of the vortex viscosity in the microwave range requires us to treat it as a complex response function, with an imaginary part that is predicted to contribute to the apparent pinning force on the vortices. Measurements of the frequency dependence of the pinning force confirm that this term is present, and in a form consistent with the requirements of causality. At low temperatures the flux-flow resistivity, \rho_ff \propto 1/\eta, has the form \rho_ff(T) = \rho_0 + \rho_1 \ln(1/T), reminiscent of the DC resistivity of cuprates in the pseudogap regime.

Electron mass anisotropy of BaZrO3 doped YBCO thin films in pulsed magnetic fields up to 30 T

The high anisotropy of the critical current density in the high-temperature superconductor YBa2Cu3O6+x can be compensated by changing the pinning landscape, for example, through doping with BaZrO3. We measure the change due to BaZrO3 doping in the effective electron mass anisotropy, γ, by resistive measurements of the upper critical field in pulsed high magnetic fields. It is found that the angular dependence of the upper critical field follows the Blatter scaling up to 30 T but the irreversibility field does not. We also report a significant reduction in the effective electron mass anisotropy from 6.0 to 3.4 by BaZrO3 doping.

Coexistence of Superconductivity and Ferromagnetic Phases in YBa2Cu3O7−δ Nanoparticles

High-temperature superconductor YBa2Cu3O7−δ (YBCO) nanopowders were synthesized by the citrate-gel route, which is a modification of the sol-gel method. The fine powders were calcinated at 860 and 900 °C. They were of small size, in the range of 30–35 nm. X-ray diffraction (XRD) patterns verified production of the orthorhombic superconducting phase in all samples. Measuring the magnetic properties of these nanoparticles at room temperature, via a vibrating sample magnetometer (VSM), indicated ferromagnetism behavior in the YBa2Cu3O7−δ nanoparticles. As the size of the nanoparticles decreased, the magnetic saturation of all samples increased. The development of the ferromagnetism effect was attributed to the presence of surface oxygen vacancies that lead to electron redistribution on the different ions at the surface. Thus, in an innovative work, the produced samples were annealed at 700 °C for 5 h under 0.8–0.9 bar of air atmosphere. The results showed that a small increase in the nanoparticle size provided a dramatic increase of magnetic saturation in all samples. Thus, we can say that the annealing process at vacuum improves the ferromagnetic properties of YBCO nanoparticles.

Synthesis, characterization and thermostability of barium β-diketonate with tetraethylenepentamine ligand

The metal-organic chemical vapor deposition (MOCVD) technique is a promising process for high-temperature superconductor YBa2Cu3O7−δ(YBCO) preparation. In this technique, it is a challenge to obtain barium precursors with high volatility. In addition, the purity, evaporation characteristics and thermostability of adopted precursors in the whole process would decide the quality and reproducible results of YBCO film. In the present report, the barium precursor containing 2,2,6,6-tetramethylheptane-3,5-dionate and tetraethylenepentamine ligands (Ba(TMHD)-tetraen) was synthesized and identified by FTIR, 1H NMR, 13C NMR and ESI-MS spectroscopy. Subsequently the thermal property and the kinetics of decomposition were systematically investigated by combining non-isothermal thermogravimetric analysis methods (TGA), Ozawa, Kissinger and Friedman methods. On the basis of the apparent activation energy of the evaporation process, the thermostability and evaporation characteristics of the precursors were discussed. All results show that Ba(TMHD)-tetraen has higher volatility than Ba(TMHD)2, but it is unstable and highly sensitive to the change of temperature during the whole evaporation process. Therefore, it is very important to choose suitable volatilization technology and conditions for avoiding Ba(TMHD)-tetraen breakdown (or thermal aging) during the MOCVD process.

Ineluctable complexity

The discovery of charge-density-wave order in the high-temperature superconductor YBa2Cu3O6+y places charge order centre stage with superconductivity, suggesting that they are intertwined rather than competing.

Thermoelectric properties of YBa2Cu3O7−δ–La2/3Ca1/3MnO3 superlattices

We report measurements of the thermoelectric power and electrical resistivity of superlattices composed of the high-temperature superconductor YBa2Cu3O7−δ (YBCO) and the metallic ferromagnet La2/3Ca1/3MnO3 (LCMO) with individual layer thicknesses between 5 and 50 nm. Whereas YBCO and LCMO reference films prepared under the same conditions exhibit negative Seebeck coefficients, in excellent agreement with data on bulk compounds of identical composition, those of all superlattices are positive, regardless of the individual layer thickness. Having ruled out lattice strain and incomplete oxygenation, we attribute the observed sign reversal of the Seebeck coefficient to a long-range electronic reconstruction nucleated at the YBCO-LCMO interfaces.

Fermi surface reconstruction from bilayer charge ordering in the underdoped high temperature superconductor YBa2Cu3O6+x

We show that a Fermi surface in underdoped YBa2Cu3O6+x yielding the distribution of quantum oscillation frequencies observed over a broad range of magnetic field can be reconciled with the wavevectors of charge modulations found in nuclear magnetic resonance and x-ray scattering experiments within a model of biaxial charge ordering occurring in a bilayer CuO2 planar system. Bilayer coupling introduces the possibility of different period modulations and quantum oscillation frequencies corresponding to each of the bonding and antibonding bands, which can be reconciled with recent experimental observations.

Synthesis, characterization, and thermostability of bis(2,2,6,6-tetramethyl-3, 5-heptanedionato)copper(II)

The metal–organic chemical vapor deposition (MOCVD) technique is a promising process for high-temperature superconductor YBa2Cu3O7−δ (YBCO) preparation. In this technique, it is a challenge to obtain barium precursors with high volatility. In addition, the purity, evaporation characteristics, and thermostability of adopted precursors in whole process will decide the quality and reproducible results of YBCO film. In the present report, bis(2,2,6,6-tetramethyl-3,5-heptanedionato)barium(II) (Ba(TMHD)2) was synthesized, and its structure was identified by FTIR, 1H NMR, 13C NMR, and ESI–MS spectroscopy. Subsequently, the thermal properties and the kinetics of decomposition were systematically investigated by nonisothermal thermogravimetric analysis methods. Based on the average apparent activation energy evaluated by the Ozawa, Kissinger, and Friedman methods, the volatilization process was discussed, and all results show that Ba(TMHD)2 is unstable and highly sensitive to the change of temperature during the whole evaporation process. Therefore, it is very important to choose suitable volatilization technology and conditions for avoiding Ba(TMHD)2 breakdown (or thermal aging) during MOCVD process. Subsequently, the possible conversion function is estimated through the Coats–Redfern method to characterize the evaporation patterns and follows a phase boundary reaction mechanism by the contracting surface equation with average activation energy of 118.7 kJ·mol−1. During the metal–organic chemical vapor deposition (MOCVD) process for YBa2Cu3O7−δ preparation, the evaporation characteristics and thermostability of Ba(TMHD)2 will decide the quality and reproducible results of YBCO film. Based on the average apparent activation energy evaluated by the Ozawa, Kissinger, and Friedman methods, the stability of Ba(TMHD)2 can be evaluated by the activation energy changes in evaporation process. All results show that Ba(TMHD)2 is unstable and highly sensitive to the change of temperature during the whole evaporation process.

Long-range transfer of electron–phonon coupling in oxide superlattices

The electron-phonon interaction is of central importance for the electrical and thermal properties of solids, and its influence on superconductivity, colossal magnetoresistance and other many-body phenomena in correlated-electron materials is the subject of intense research at present. However, the non-local nature of the interactions between valence electrons and lattice ions, often compounded by a plethora of vibrational modes, presents formidable challenges for attempts to experimentally control and theoretically describe the physical properties of complex materials. Here we report a Raman scattering study of the lattice dynamics in superlattices of the high-temperature superconductor YBa(2)Cu(3)O(7) (YBCO) and the colossal-magnetoresistance compound La(2/3)Ca(1/3)MnO(3) that suggests a new approach to this problem. We find that a rotational mode of the MnO(6) octahedra in La(2/3)Ca(1/3)MnO(3) experiences pronounced superconductivity-induced line-shape anomalies, which scale linearly with the thickness of the YBCO layers over a remarkably long range of several tens of nanometres. The transfer of the electron-phonon coupling between superlattice layers can be understood as a consequence of long-range Coulomb forces in conjunction with an orbital reconstruction at the interface. The superlattice geometry thus provides new opportunities for controlled modification of the electron-phonon interaction in complex materials.

Transport Properties of YBa2Cu3O7 Josephson Junctions at THz Frequencies

Abstract For application in a THz scanning microscope we employ Josephson junctions from the high-temperature superconductor YBa2Cu3O7 on a so called Josephson cantilever to measure the local microwave power density in the half space above a device under test. The Josephson junctions are prepared by pulsed laser deposition and argon ion etching on symmetric LaAlO3 30 ∘ -bicrystal substrates. To achieve high sensitivity at THz frequencies, a high I c R n product is required. Therefore, the Josephson junctions are operated on a cryo-cooler at low temperature of about 36 K under vacuum conditions. The high frequency characterization is accomplished in the near field of a Gaussian far infrared gas laser beam optically pumped by a CO2 laser. We use strong emission lines of formic acid at 762 GHz and 1.2 THz for the investigations and report on the electrical transport properties determined from the current-voltage characteristics of the Josephson junctions under influence of external magnetic field and far-infrared radiation.

Chemical potential oscillations from nodal Fermi surface pocket in the underdoped high-temperature superconductor YBa2Cu3O6+x

The electronic structure of the normal state of the underdoped cuprates has thus far remained mysterious, with neither the momentum space location nor the charge carrier type of constituent small Fermi surface pockets being resolved. Whereas quantum oscillations have been interpreted in terms of a nodal-antinodal Fermi surface including electrons at the antinodes, photoemission indicates a solely nodal density-of-states at the Fermi level. Here we examine both these possibilities using extended quantum oscillation measurements. Second harmonic quantum oscillations in underdoped YBa₂Cu₃O(₆+x) are shown to arise chiefly from oscillations in the chemical potential. We show from the relationship between the phase and amplitude of the second harmonic with that of the fundamental quantum oscillations that there exists a single carrier Fermi surface pocket, likely located at the nodal region of the Brillouin zone, with the observed multiple frequencies arising from warping, bilayer splitting and magnetic breakdown.

HTS Josephson Junction Cantilever With Integrated Near Field THz Antenna

Josephson junctions from the high-temperature superconductor YBa2Cu3O7 are routinely used on a cantilever to detect microwave emission up to 762 GHz from room temperature microwave devices with spatial resolution. The Josephson junctions on the cantilever are operated in a temperature range between 40 K and 80 K cooled by a cryocooler. With this measurement setup and a 3-axis table we are able to scan the 3D half-space in the near and far field over the microwave devices. To measure the near field of a microwave source, the antenna coupled to the Josephson junction has to be smaller than the wavelength to achieve high spatial resolution but large enough to be able to detect sufficient power. Also the characteristic frequency, antenna impedance, bandwidth up to 1 THz and connection filters have to be optimized for a LaAlO3-bicrystal substrate. We report on the first measurement at 762 GHz with a new design of the Josephson cantilever.

Strong pinning in thick YBa2Cu3O7 layers mediated by catalysis of a new long-period metastable cuprate phase

Catalysts are widely utilized to promote reactions in liquid and gaseous phases but are rarely encountered in solid state synthesis. Here we use catalytically active (001) ceria buffers to modify the structure of the epitaxial high temperature superconductor YBa2Cu3O7. The modification is achieved by catalytically-assisted synthesis of a previously unknown metastable phase. The new phase, a long-period (3.5 nm) perovskite, intercalates into the YBa2Cu3O7 matrix without negatively affecting the critical temperature of the film. Analysis of electron microscopy and synchrotron x-ray diffraction data allow identification of the phase as a long-period YBa2Cu3O7 derivative formed through short-range cation displacement. The 0.8 μm thick films exhibit strong enhancement of the critical current density, reaching a maximum of 4.2 MA/cm2 at 77 K. The result emphasizes the critical role of catalysis for synthesis of novel complex materials.

Multiple quantum oscillation frequencies in YBa2Cu3O6+δ and bilayer splitting

Experiments have revealed multiple quantum oscillation frequencies in underdoped high-temperature superconductor YBa2Cu3O6+δ, corresponding to approximately 10% doping, which contains CuO bilayers in the unit cell. These unit cells are further coupled along the c-axis by a tunneling matrix element. A model of the energy dispersion that has its roots in the previously determined electronic structure, combined with twofold commensurate density waves, reveals multiple electron and hole pockets. To the extent that quasiparticles of the reconstructed Fermi surface have finite residues, however small, the formation of Landau levels is the cause of these oscillations, and the bilayer splitting and warping of the electronic dispersion along the direction perpendicular to the CuO-planes are firm consequences. The goal here is to explore this possibility from various directions and provide a better understanding of the rapidly developing experimental situation involving multiple frequencies. An important conclusion is that bilayer splitting is considerably renormalized from the value obtained from band structure calculations. It would be extremely interesting to perform these experiments for higher values of doping. We roughly expect the splitting of the frequencies to increase with doping, but the full picture may be more complex because the density wave order parameter is also expected to decrease with doping, vanishing around the middle of the superconducting dome.

On the incorporation of beryllium into the biotemplated synthesis ofYBa2Cu3O7−δ

This work represents the first SQUID (superconducting quantuminterference device) magnetometry study on the effect of incorporatingberyllium into the synthesis of the high temperature superconductorYBa2Cu3O7−δ (Y123). Through a biopolymer-mediated synthetic approach, the homogeneous nature ofthe precursor sol and the preferred nucleation and growth of the Y123 phase allow for thegreatest possible yield of the superconducting phase. In contrast to a previous report,our finding, through crystallographic studies of the as-synthesized material, isthat in spite of a beryllium excess being provided in the precursor, berylliumdoes not substitute into the Y123 crystal lattice, instead forming an associatephase which comes to dominate at high doping levels, to the detriment of theY123 phase. In addition, we are able unambiguously to refute the previouslyreported increase in superconducting transition temperature at low beryllium dopinglevels.