Superconductor YBa2Cu3O7 Research Articles

Superconductivity in the Y-Ba-Ca-Cu-O system

The search for the mechanisms of high-temperature superconductivity and novel superconducting materials are the most challenging tasks of condensed-matter physicists as well as material scientists [1–3]. The yttrium barium copper oxide (YBCO) YBa2Cu3O7 or CuBa2YCu2O7, often called “Y-123” or “Cu-1212” superconducting compound, is the first member of the homologous series of compounds of general formula Y2Ba4Cu6+nO14+n (n = 0, 1, 2). Cu-1212 phase exhibits a highest superconducting transition temperature (TC) inside YBCO family, at around 90–92 K, and till now remains the most extensively studied high-TC superconductor [4–8]. By the analogy to the Hg-based cuprates [9], we have tried to introduce additional CaO layers to the structure of CuBa2YCu2O7−δ (Cu-1212 or Cu-12102) superconductor. The purpose of the present work was to prepare new superconducting phases using the sol–gel synthesis method: (a) having a single layer of CaO between two CuO2 sheets with the nominal composition of CuBa2YCaCu3O9−δ (Cu–12113), and (b) having two CaO layers between triple CuO layers with the composition of CuBa2YCa2Cu4O11−δ (Cu–12124). The results of this study are presented herein. CuBa2YCaCu3O9−δ and CuBa2YCa2Cu4O11−δ samples were prepared by an acetate-tartrate sol–gel method developed previously for the synthesis of pure YBa2Cu4O8 superconductor [10]. As starting compounds stoichiometric amounts of analytical grade Y2O3, Cu(CH3COO)2·H2O, Ba(CH3COO)2 and Ca(CH3COO)2·H2O were used. In the sol–gel process, Y2O3 was first dissolved in 0.2 M acetic acid solution by stirring at 55–60 ◦C. After stirring the mixture for 10 hr in a beaker covered with a watch-glass a clear solution was obtained. In the next step, mixtures of the appropriate amounts of Ba(CH3COO)2, Ca(CH3COO)2·H2O, and Cu(CH3COO)2·H2O, all of them dissolved in small amounts of distilled water, were added with continuous stirring during several hours at the same temperature. An

State of the art simulations in electronic structure and total energy for the high temperature superconductor YBa 2Cu 3O 7

We present an ab initio total-energy density functional theory investigations for the stoichiometric high temperature cuprate YBa 2Cu 3O 7 using a FP-LAPW code. Two different schemes, the local density approximation (LDA) and a generalized gradient approximation (GGA) are employed for the treatment of electronic exchange and correlation effects. From theoretical point of view, total-energy calculations and the full-self-consistent energy bands along some high-symmetry points of the Brillouin zone have been carried out. A consistent picture of total and site-projected densities of states, the partial charges and their symmetry-decompositions is as well proposed via calculations. We compare our results to experimental and/or ab initio results, and find that in the case of the LDA, the resulting response show a considerable deviation from experimental values. Invoking GGA, the resulting values are significantly improved in good agreement with experimental findings. On the whole, a consequence of the DFT/LDA–GGA conjecture would be that first-principles calculations based DFT are an appropriate tool in studying such complicated materials as high-temperatures cuprates where the treatment of exchange and correlation effects in an important issue for this task.

Magnetic Properties of YBCO Superconducting Bulk

Magnetic properties of a field cooled YBaCuO superconductor beneath the toroidal permanent magnet was examined by means of improved magneto-balancing method at 77 K. Magnetic flux measurements of a toroidal magnet revealed a concave shaped field distribution with a single minimum and a null field along the axis of the torus at the point where the field reversed. The observed values of both the suspension position and the force exerted upon the superconductor specimen were in good agreement with those calculated from the magnetization curve of the specimen and the intensity of the magnetic field of the used permanent magnet.

OIM and X‐ray texture analysis of melt‐textured YBCO superconductors

The results of two independent texture measurements on melt-textured Y-Ba-Cu-O (YBCO) high-Tc superconductors are compared with each other. The texture data are acquired by means of an X-ray based pole figure texture analysis and of a local texture analysis provided by an automated electron-backscatter diffraction (EBSD) or often called orientation imaging microscopy (OIM) analysis. As samples, we employ two different melt-textured YBCO samples; one fully processed (orthorhombic) and one without oxygen treatment (tetragonal). Pole figures in [103] direction are used to enable a direct comparison. We find a clear coincidence between the results obtained by the two measurement techniques on the melt-textured YBCO samples, however, the EBSD results are much more detailed, yielding the local grain orientation distribution and quantitative results of the grain or subgrain misorientation angles. Therefore, the EBSD measurements give informations which are not accessible by the X-ray texture analysis. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The enhancement of vortex pinning in ferromagnet/superconductor bilayers

The magnetic-domain induced vortex pinning is studied in the ferromagnet/superconductor bilayers (FSB's), in which the F layers are Co/Pt multilayers with perpendicular magnetic anisotropy, and the S layers are either niobium or high temperature superconductor YBa2Cu3O7 (YBCO). The magnetization measurements reveal the enhancement of the flux pinning in both types of FSB's during the reversal of the magnetization of the F layer, but the details of the behavior depend on the type of the S layer. In the case of niobium FSB the maximum of pinning appears when the F layer is in the final stage of the magnetic reversal process, while the FSB with YBCOshows the maximumwhen the F layer is saturated. The possible origins of these differences are discussed. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The relationship between levitation force and stiffness in symmetrical and unsymmetricalapplied fields

The two key parameters for levitation performance over a permanent magnet (PM)guideway are the levitation force and the stiffness of the high temperature superconductor(HTS) YBaCuO bulk. Their distributions along a lateral displacement (LD) were measuredin zero-field-cooling (ZFC) and in field-cooling (FC) conditions. There are two cases ofapplied magnetic field for a single HTS bulk: symmetrical or unsymmetrical about the bulkaxis. Although the levitation force has a linear relationship with the applied field in thesymmetrical case, such a linear relationship disappears once the applied field becomesunsymmetrical. Furthermore, the levitation stiffness is measured in FC, and they stillchange more drastically than the levitation force in ZFC. In particular, the levitationstiffness has a linear relationship with the associated levitation force, whether the appliedfield is symmetrical or unsymmetrical. Its lateral distribution at a gap of 20 mmin FC agrees with the levitation force distribution at a gap of 10 mm in ZFC.

Spin Dynamics in High- T c Superconductor YBa 2 Cu 3 O 6.9

Dynamical structures in the vicinity of Q = ( \pi , \hspace{0.167em} \pi ) of optimally-doped YBa2Cu3O6.9 were measured by means of an inelastic neutron scattering technique utilizing pulsed neutron source. Obtained incommensurability δ of energy transfer at E = 36 \hspace{0.167em} meV was found to be ∼0.1 [rlu] which is not the value to be expected from the formation of spin-charge stripes. In higher energy above resonance energy (42 meV), a ring structure centered at (π, π) was observed up to 54 meV. Combined with the low energy part of the dynamical structure, the obtained results provide a total picture of the dynamical aspects in the high-T c superconductor YBa2Cu3O6.9.

Measurement of mass attenuation coefficients for YBaCuO superconductor at differentenergies

The mass attenuation coefficients forY2O3,BaCO3, CuO compounds, andsolid-state forms of YBa2Cu3O7 superconductor were determined at energies of 57.5, 59.5, 65.2, 74.8, 77.1,87.3, 94.6, 98.4, 122, and 136 keV. The samples were irradiated using a241Am point source emitting 59.5 keV photon energies and a57Co point source emitting 122 and 136 keV photon energies. Theother energies were obtained using secondary targets such as Ta,Bi2O3,and (CH3COO)2UO22H2O. The gamma- and x-rays were counted by a Si(Li) detector with a resolution of 160 eV at5.9 keV. Samples were selected on the basis of known composition and mass densities weremeasured using a densitometer. The experimental results obtained in this study arecompared with theoretical values.

Electric field gradients in cuprates: Does LDA+U give the correct charge distribution?

AbstractWe present calculations for the electric field gradients (EFG) on the Cu sites in La2CuO4, YBa2Cu3O6, and YBa2Cu3O7 using standard LDA and GGA exchange‐correlation functionals, but also by using the LDA+U method for the correlated Cu‐d electrons. LDA or GGA calculations yield proper EFGs in agreement with experiment for all sites except the planar Cu(2) site, where the theoretical EFGs are too small by a factor of 2 to 3. This happens for both the metallic (superconducting) YBa2Cu3O7 and the antiferromagnetic insulators. For the latter, these calculations also lead to a nonmagnetic and metallic solution. Application of LDA+U with various double‐counting corrections and varying values for the Hubbard parameter U gives the correct magnetic and insulating ground state and the corresponding EFGs are substantially enhanced. Best agreement with experiment is found using the LDA+U(FLL) method with U around 6 eV, because this method leaves the Cu(1) EFG unchanged, even when U is applied to its d‐states. For the metallic compound YBa2Cu3O7, the EFG results favor the presence of local magnetic moments; however, in bandstructure calculations the necessary long‐range order leads to some artifacts and should be replaced by random spin fluctuations. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Effect of anisotropic temperature factors on convergent-beam electron diffraction intensities and measurement of the local change in the oxygen concentration of YBa2Cu3O y

The local oxygen concentration of the high-T c superconductor YBa2Cu3O y was measured from large-angle convergent-beam electron diffraction patterns using a transmission electron microscope equipped with a field emission gun with an in-column-type energy filter. The anisotropic effect of thermal vibrations of atoms was considered in the analysis of the oxygen concentration, based on the many-beam dynamic theory. The observed fluctuation in the oxygen concentration can explain the peak effect in the J c–H curve, the critical current density J c plotted as a function of applied magnetic field H.

Energy balance of pulsed laser ablation: thermal model revised

We present the results of modeling of nanosecond laser ablation of graphite, niobium, YBa2Cu3O7 superconductor, and silicon performed on the basis of a modified thermal model which includes a thorough calculation of the solid–melt interface. The model is based on experimental data on mass removal and takes into account plasma absorption. The numerical results of the laser energy balance are presented over a wide range of laser fluences. The probable critical temperatures for the materials studied have been estimated on the basis of the modeling.

Survival of the d-wave superconducting state near the edge of antiferromagnetism in the cuprate phase diagram.

In the cuprate superconductor YBa2Cu3O6+x, hole doping in the CuO2 layers is controlled by both oxygen content and the degree of oxygen ordering. At the composition YBa2Cu3O6.35, the ordering can occur at room temperature, thereby tuning the hole doping so that the superconducting critical temperature gradually rises from 0 to 20 K. Here we exploit this to study the c-axis penetration depth as a function of temperature and doping. The temperature dependence shows the d-wave superconductor surviving to very low doping, with no sign of another ordered phase interfering with the nodal quasiparticles. The only apparent doping dependence is a smooth decline of superfluid density as T(c) decreases.

Two-dimensional geometry of spin excitations in the high-transition-temperature superconductor YBa2Cu3O6+x.

The fundamental building block of the copper oxide superconductors is a Cu4O4 square plaquette. The plaquettes in most of these materials are slightly distorted to form a rectangular lattice, for which an influential theory predicts that high-transition-temperature (high-T(c)) superconductivity is nucleated in 'stripes' aligned along one of the axes. This theory received strong support from experiments that indicated a one-dimensional character for the magnetic excitations in the high-T(c) material YBa2Cu3O6.6 (ref. 4). Here we report neutron scattering data on 'untwinned' YBa2Cu3O6+x crystals, in which the orientation of the rectangular lattice is maintained throughout the entire volume. Contrary to the earlier claim, we demonstrate that the geometry of the magnetic fluctuations is two-dimensional. Rigid stripe arrays therefore appear to be ruled out over a wide range of doping levels in YBa2Cu3O6+x, but the data may be consistent with liquid-crystalline stripe order. The debate about stripes has therefore been reopened.

Doping dependence of superconducting gap in YBa 2Cu 3O y from universal heat transport

Thermal transport in the T→0 limit was measured as a function of doping in high-quality single crystals of the cuprate superconductor YBa 2Cu 3O y . The residual linear term κ 0/ T is found to decrease as one moves from the overdoped regime towards the Mott insulator region of the phase diagram. The doping dependence of the low-energy quasiparticle gap extracted from κ 0/ T is seen to scale closely with that of the pseudogap, arguing against a non-superconducting origin for the pseudogap. The presence of a linear term for all dopings is evidence against the existence of a quantum phase transition to an order parameter with a complex (ix) component.

Influence of the Ta doping on the peritectic transformation of YBaCuO superconductor

In this work we present a systematic study of the influence of Ta doping on the YBa 2Cu 3O 7− δ (Y123) high T c superconductor. The results reveal that Ta ion substitutes the yttrium site in the lattice and the stoichiometry can be represented by Y 1− x Ta x Ba 2Cu 3O y in the interval 0⩽ x⩽0.08. The differential thermal analysis curves show displacement of the peritectic temperature from 1020 °C in x=0 to 950 °C in x=0.08. These significant displacements in the peritectic line suggest that the material can be textured at lower temperature compared with pure Y123. The results show that a second phase of the YTa 1.6Ba 3.5Cu 0.7O y composition segregates within of the grain. This fact suggests this phase can act as pinning center. All the results indicate that Ta content does not change the onset and offset of the superconducting transition temperature. The X-ray diffraction data show that textured material presents only peaks related to the reflections (0 0 l) and the excellent preferential orientation is confirmed by rocking curve which presented FWHM ∼0.2°.

Theoretical Investigation of the EPR g-factors for Yb3+ in YBa2Cu3O7-δ

The EPR g factors g‖, g⊥ for Yb3+ in YBa2Cu3O7−δ are studied with perturbation formulas based on the cluster approach of the spin-Hamiltonian parameters for a 4f13 ion in tetragonal symmetry. In these formulas, the contributions to the EPR parameters of the covalency effects, the admixture between the J = 7/2 and J = 5/2 states and the second-order perturbation terms are all included. The used crystal-field parameters are calculated with the superposition model and the local structural data of Yb3+ in YBa2Cu3O7−δ . The resulting EPR g factors for Yb3+ ions in the superconductor YBa2Cu3O7−δ agree reasonably with the experimental values. The results are discussed.

Effect of neutron irradiation on pinning in SmBaCuO and YBaCuO superconductors

In the present work, we have investigated the effect of neutron irradiation on the pinning and transport properties of melt-textured SmBaCuO samples. It was observed that the critical current density is strongly enhanced at various temperatures and magnetic fields up to 5 T. The enhancement is larger at low temperatures compared to high temperatures. This difference is discussed in terms of the defect structure. Our investigations on YBCO superconductors also show a similar behavior. But the critical current density enhancement with neutron irradiation is smaller in this system compared to SmBaCuO system. Possible reasons for this difference are also discussed.

Mechanism of the magnetic effect occurrence of YBaCuO superconductor

We report fabrication of a superconducting magnetometer of Wheatstone bridge type, consisting of a superconductor in liquid N 2 bath and two variable resistors, a null detector. Superconducting sensor was field cooled in the magnetic field of a 0.15 T permanent magnet by placing it near the N or S pole respectively, and it was assumed that both superconducting sensors had the same magnitude of trapped magnetic field with opposite polarity. The two variable resistors were used to balance the bridge circuit. Passing a current along the superconductor and applying a transverse magnetic field to it, a voltage is developed between the ends of the superconductors and varies as the current varies. The magnetic field dependence of the induced output voltage of the HTS magnetometer has been tested in the 10 −5 to 10 −4 T range by changing the distance between the pole of the permanent magnet to the S pole of the superconducting sensor, in this case the positive voltage increased, whereas the S pole of the magnet approached the N pole of the sensor.

Evidence for c-axiscorrelated vortex pinning in Y Ba2Cu3O7−δ films on sapphire buffered with an atomically flatCeO2 layer having a high density of nanodots

Experimental evidence of c-axis correlated vortex pinning in superconductingY Ba2Cu3O7−δ (YBCO) thin filmsdeposited on annealed CeO2-buffered sapphire was obtained. YBCO films were grown by pulsed laser deposition onCeO2-buffered () sapphire substrates. Prior to the superconducting film deposition,a self-assembly process was initiated where high-temperature(1025 °C)O2 annealing inducedsurface reconstruction of CeO2 on sapphire substrates. The result was an atomically flatCeO2 layer, self-assembled formation of nanodots on top of theCeO2 wetting layer, and high critical current density of the YBCO films depositedon such buffered sapphire substrates. The magnetic field dependence ofJc at various temperatures and the dependence ofJc on the orientation of the magnetic field were investigated. YBCO films with an annealedCeO2 buffer layershowed a high Jc peak when the applied field was directed along thec-axisof the YBCO. The surface morphology and microstructure of YBCO films on sapphire with as-grown andannealed CeO2 buffer layers were investigated using transmission electron microscopy. The results suggest that theJc peaks in YBCO withan annealed CeO2 bufferlayer are caused by c-axis correlated pinning sites, such as threading dislocations possibly induced byCeO2 nanodots and a high density of egg-shaped nanometre-sized precipitates that are elongated along thec-axis of YBCO.

Conductivity of the interface between La 0.7Ca(Sr) 0.3MnO 3 and underdoped YBa 2Cu 3O 7− δ

We report resistance/temperature, conductance/voltage and voltage/magnetic field measurements in a range of junctions between collosal magnetoresistive materials and the high temperature superconductor YBa 2Cu 3O 7− δ . We identify two forms of behaviour. In one form the resistance of the junction falls monotonically with decreasing temperature and is associated with zero bias conductance peaks. In the other form the resistance rises below some characteristic temperature T ∗ . In this latter case there is no zero bias conductance peak, but there are peaks in the voltage/magnetic field behaviour below T ∗ . We tentatively associate T ∗ with the pseudogap temperature.