Compound YBa2Cu3O7 Research Articles

Search for Novel Phases in Y-Ba-Cu-O Family

In order to search for possible residual minor phases in the Y-Ba-Cu-O family, powdered mixtures of Y2O3 + BaCO3 + CuO and, independently, superconducting compound YBa2Cu3O7−x have been treated in evacuated cells and elevated temperatures. YBa2Cu3O7−x was reduced to YBa2Cu3O5 by use of the special home-designed Taconis–Knudsen vacuum device. Subsequent doping by oxygen converts produced insulator YBa2Cu3O5 to semiconductor or metal YBa2Cu3O5+x (0 < x < 0.3). In addition to YBa2Cu3O5, 0.05 volume percent of the minor delafossite phase Y2Cu2O4 was spotted in the powder mixture 1/2 Y2O3 + 2BaCO3 + 6Cu2O, heated up to 818 °C in an inert gas atmosphere. An attempt to prepare the insulating bulk delafossite samples was successful, and subsequent doping by oxygen produced novel metallic phases.

Strain effect on the high T c superconductor YBa2Cu3O7: an ab initio study comparing bulk and monolayer models

In this work, the effect of strain on the vibrational and electronic properties of the YBa2Cu3O7 compound was studied through ab initio calculations. For this, two structural models were used: a bulk model and a surface model (a monolayer with CuO2 and BaO as the terminating layers). The phonon spectra was calculated for both structures under different levels of c axis strain. The most appreciable change occurs in the vibrational properties, and in the surface case. From the simulation of the Raman spectra, we were able to quantify the Raman shift ratio as a function of the applied strain, and analyzed its behavior in terms of the overlap population of the different bonds and the reduced mass of selected phonons. The effect of the level of deformation on the band structure and the electronic density of states is small for both structures, although more noticeable in the case of the surface model. In both cases, tendencies are observed when the fine features of the band structure are analyzed by means of the tight binding model. Due to the lower symmetry, the surface model also shows modifications of the bands related to the CuO2 planes.

Влияние облучения протонами и ионами высоких энергий на структуру и свойства композитных ВТСП-2 лент

The paper considers the effect of radiation defects caused by irradiation with protons (2.5 MeV), heavy ions 132Xe27+ (167, 80, 40 MeV), 86Kr17+(107 MeV), 40Ar8+(48 MeV), on the critical parameters of HTSC-2 tapes based on compounds YBa2Cu3O7 – x and GdBa2Cu3O7 – x. The results of calculations based on the model of the thermal peak of the ion track sizes are presented. The projective ranges of ions and protons in these samples are calculated. The radiation resistance of the studied samples to ion and proton radiation of the indicated energies is determined. The performed studies made it possible to detect, at low fluences of irradiation with heavy ions, an increase in the critical current (Ic), an improvement in the adhesion between the superconducting layer and the substrate, and a decrease in internal stresses in the HTSC layer. At higher values of fluences, the critical current and critical temperature decrease. It is important that the decrease in Ic begins at lower fluences than Tc.

Doping of Sm3Ba5Cu8O19 Compound with Praseodymium

An investigation has been carried out on the effects of praseodymium doping in Sm3−xPrxBa5Cu8O19 for different values of x. Results indicate that the transition temperature reduces almost linearly with x. Based on the Abrikosov–Gor’kov pair-breaking theory, it has been concluded that the exchange interaction constant is large (Jex = 110 meV) in the Sm3Ba5Cu8O19 compound. In addition, it seems that the hybridization of Pr4f states with O2p–Cu3d states has an important role in the depression of transition temperature of Sm3−xPrxBa5Cu8O19 for the Pr content < 10%. However, for the higher values of Pr, the two observed plateaus at the transition temperatures of ~ 60 K and ~ 17 K, respectively, resemble the structural phase transitions due to oxygen reduction in the YBa2Cu3O7 compound and the antiferromagnetic phase formation in the PrBa2Cu3O7 compound. It seems that depending on the Pr content, the pair-breaking, structural effects and hole localization are responsible for the reduction in the transition temperature.

Investigation of the effect of Al plasma treatment on the superconducting properties of YBa2Cu3O7−δ

The effect of the partial substitution of Cu for Al on the structural, compositional, and magnetic properties of polycrystalline compounds YBa2Cu3O7−δ was studied. All prepared samples turned out to be single-phase with a small fraction of Ba secondary phases. Substitution of more than 2% aluminum causes an increase in spurious phases. DC susceptibility measurements show that the superconducting transition temperature T C decreases. Hysteresis loops show that magnetic irreversibility decreases with increasing Al content. The critical current density J C obtained using the Bean formula does not follow the same change. Al compensates for a decrease in J C and an increase in its content near the solubility limit gives a higher J C than in an undissolved sample.

Ядерное квадрупольное взаимодействие и эффективные заряды атомов в кристаллах YBa2Cu3O7_x и La2_xSrxCuO4

Проведено сопоставление параметров эмиссионных мессбауэровских спектров на изотопах 57Co(57mFe), 67Cu(67Zn), 67Ga(67Zn), 155Eu(155Gd) и параметров ядерного магнитного резонанса на изотопах 17O, 137Ba с расчетными параметрами решеточного тензора градиента электрического поля для всех узлов кристаллических решеток высокотемпературных сверхпроводников на основе металлооксидов меди YBa2Cu3O7 и La2-xSrxCuO4. Согласие экспериментальных и расчетных параметров тензора градиента электрического поля может быть получено только для моделей, когда дырка, возникающая за счет нестехиометричности соединения YBa2Cu3O7 или аливалентного замещения La3+ на Sr2+ в соединениях La2-xSrxCuO4, находится преимущественно в подрешетке цепочечного (в YBa2Cu3O7) или плоскостного (в La2-xSrxCuO4) кислорода. Подтверждение предложенной модели пространственного распределения электронных дефектов в решетках La2-xSrxCuO4 (x = 0.1 - 1.0) было получено путем сравнения расчетных зависимостей отношения главных компонент тензоров градиента электрического поля в узлах лантана и меди с отношением постоянных квадрупольного взаимодействия в этих узлах для зондов 57mFe3+, 67Zn2+ и 155Ga3+. Наблюдаемые зависимости могут быть количественно объяснены, если дырка локализуется преимущественно в позициях атомов планарного кислорода. С использованием этих моделей определены коэффициенты Штернхеймера для зондов 67Zn2+, 137Ba2+ и 17O2-.

The Effect of γ Irradiation on Phase Transitions in the Polyimide–YBa2Cu3O6 + x System

The effect of γ irradiation on optical properties of films of the polyimide–YBa2Cu3O6.7 system with different concentrations of the filler in the form of the finely crystalline compound YBa2Cu3O6.7 (0.05 wt %, 0.10 wt %, and 0.50 wt %) has been investigated. The properties were studied on an IKS-29 infrared spectrophotometer in a spectral range of 400–4200 cm–1 at room temperature. The samples were irradiated in air under room conditions on an RKhM-γ-20 by doses of 150, 250, and 600 kGy. The power of the exposure dose of the 60Co source was 0.16 rad/s. It has been established that irradiation with a dose of up to 600 kGy of the abovementioned system slightly affects the structure of the polyimide matrix but decreases the number of free radical groups in the film structure with the formation of hydrogen bonds. In the range of 400–4200 cm–1 at D = 150 kGy, the most intense changes in spectra of polymeric composite samples have been revealed. In the range of 2850–3350 cm–1, the wide absorption spectrum band is preserved, which is caused by the presence of Cu, Y, and Ba crystallohydrates. The band is related to deformation vibrations of the Cu–O bond in YBa2- Cu3O6.7 (YBCO) chains due to properties of barium carbonate, formation of oxygen vacancies in the filler, and vibrations of the Cu(I)–O(I) bonds near one or several O(4) vacancies. Irradiation by doses of 250 and 600 kGy for different concentrations of the filler leads to a decrease in the signal intensity in the range of 1800–3600 cm–1 by 20–35% on average. Peaks related to interband transitions (transitions of the Cu(I)–Cu(IY)–Cu(II) system) and bending vibrations O(I)–Cu(I) are not changed by strains of O(IY)–Cu(I) bonds. This testifies to the radiation-stimulated diffusion of oxygen atoms to O(IY) sites determining the orthorhombic phase of YBa2Cu3O6.7 from the occupied positions in the Cu–O sublattice.

Oxygen-induced internal pressure in superconductors

For the first time, analytical methods are used to define and investigate the properties of oxygen-created internal (chemical) pressure Pi in HTS YBa2Cu3O7−y and Tl2Ba2CuO6+x samples. We predict values for the critical temperature Tc, values of dTc/dPi for the main crystallographic axes, and also the possibility of synthesizing the compounds YBa2Cu3O7−y and Tl2Ba2CuO6+x in which Tc is equal to 166 and 92.3 K, respectively.

Superconductivity. Broken translational and rotational symmetry via charge stripe order in underdoped YBa₂Cu₃O(6+y).

After the discovery of stripelike order in lanthanum-based copper oxide superconductors, charge-ordering instabilities were observed in all cuprate families. However, it has proven difficult to distinguish between unidirectional (stripes) and bidirectional (checkerboard) charge order in yttrium- and bismuth-based materials. We used resonant x-ray scattering to measure the two-dimensional structure factor in the superconductor YBa2Cu3O(6+y) in reciprocal space. Our data reveal the presence of charge stripe order (i.e., locally unidirectional density waves), which may represent the true microscopic nature of charge modulation in cuprates. At the same time, we find that the well-established competition between charge order and superconductivity is stronger for charge correlations across the stripes than along them, which provides additional evidence for the intrinsic unidirectional nature of the charge order.

Band Structure and Quantum Oscillations in YBa2Cu3O7: A Local Spin Density Approximation with the On-Site Coulomb Interaction Study

First-principles calculations have been performed on the Bravais lattice, the density of states (DOS), the band structure (BS) and the Fermi Surface (FS) topology for high-Tc superconductor YBa2Cu3O7. We used the method of the Full-Potential non-orthogonal Local-Orbital Minimum-Basis Band-structure Scheme (FPLO) [in the local spin density approximation with the on-site Coulomb interaction (LSDA+U) and the coherent potential approximation (CPA)] coupled with the XFSF program. Our FPLO Fermi surface consists of four large quasi-2D cylinders centered on the Brillouin zone corners with mostly CuO2 plane character and two quasi-1D sheets with mostly Cu–O apex (or apical O) chain character. The main feature of this Fermi surface is that each CuO chain-sheet shows up in its center a small tubular pocket with a funnel-like shape (Fermi pockets) with mainly Ba–O apex character which could give rise to the quantum oscillations observed by Doiron-Leyraud et al. (Nature 447: 565, 2007). Our FPLO band structure (BS) shows that three flat bands cross the Fermi level (FL) which suggests that the two Fermi pockets could be produced by the conjunction near the Fermi level of two flat bands arising from the apex oxygen (BaO layer). This is consistent with the density of states (DOS) where the Fermi level is dominated by the two O apex (BaO) peaks (∼90 %). Moreover, the strong narrow peak exhibited in the DOS just below the Fermi level (FL) corresponds to the van Hove singularity (VHS) and originates from the hybridization of Cu 3d and apex O2p (BaO) orbitals. In other hand, the calculated value of the bare band-structure Sommerfeld constant γ0band=10.60 mJ/(mol K2) corresponds to a very high Fermi level density of states N(EF)=4.72 states/(eV×cell) and the electron–phonon coupling constant λ is found to be about 1.64 which implies that YBa2Cu3O7 compound is a strong-coupling superconductor and the cyclotron effective mass m∗ is enhanced in comparison with the band mass mb [m∗=(1+λ)mb]. Also, the discrepancy between our FPLO calculated Debye temperature (274.15 K) and that obtained experimentally (437 K) may originate from a phonon anisotropy likely generated by the antiferromagnetic spin fluctuations. Our results therefore provide evidence of changes in the topology of the Fermi surface materialized by the Ba–O apex small pockets formed in the center of the Cu–O apex Fermi surface sheets.

Mechanism of thermal decomposition of the nonstoichiometric compound YBa2Cu3O6.8

Thermal decomposition of the nonstoichiometric high-temperature superconductor YBa2Cu3O6.8 at a temperature of 200°C in air has been investigated using the full-profile analysis of X-ray diffraction lines. Two mechanisms of decomposition are revealed. The first mechanism, i.e., separation into two phases with a different oxygen content, occurs continuously. The second mechanism, i.e., disordering of the heavy atoms Y, Ba, Ba, Y along the crystallographic axis c, begins to occur after a 20- to 35-h annealing and progresses with a further annealing.

Fine structure and the mechanism of the low-temperature decomposition of the nonstoichiometric compounds YBa2Cu3O6.8 and YBa2Cu3O6.8 doped with Ce

The resistance of the nonstoichiometric compounds YBa2Cu3O6.8 (single crystals, ceramics) and YBa2Cu3O6.8 (Ce) (single crystal) to low-temperature decomposition (at 200°C) in air and an argon atmosphere is studied by X-ray diffraction and electron microscopy. At the first stage, the compounds are found to undergo oxygen separation into two phases, namely, oxygen-rich and oxygen-lean phases as compared to the initial state, that have different lattice parameters. After 20-to 35-h annealing, the disordering of the heavy Y, Ba, Ba, Y atoms along the c axis in ceramic pellets begins mainly because of the action of elastic stresses. This process occurs via the formation of the packets of numerous stacking faults on (001) planes. The disordering of the ceramic matrix ends in the formation of a CsCl-type cubic phase upon 100-h annealing, which is accompanied by a significant decrease in the diamagnetic response and an increase in Tc from 75 K in the initial state to 90 K. The retained superconductivity and the increase in Tc are caused by the presence of a large number of oxygenrich ortho-phase particles in a nonsuperconducting matrix; these particles are correlated with each other and form a multicoupled system of superconducting filaments.

Small phonon contribution to the photoemission kink in the copper oxide superconductors

Despite over two decades of intense research efforts, the origin of high-temperature superconductivity in the copper oxides remains elusive. Angle-resolved photoemission spectroscopy experiments have revealed a kink in the dispersion relations (energy versus wavevector) of electronic states in the copper oxides at binding energies of 50-80 meV, raising the hope that this anomaly could be a key to understanding high-temperature superconductivity. The kink is often interpreted in terms of interactions between the electrons and a bosonic field. Although there is no consensus on the nature of the bosons (or even whether a boson model is appropriate), both phonons and spin fluctuations have been proposed as possible candidates. Here we report first-principles calculations of the role of phonons and the electron-phonon interaction in the photoemission spectra of La(2 - x)Sr(x)CuO4. Our calculations within the standard formalism demonstrate that the phonon-induced renormalization of the electron energies and the Fermi velocity is almost one order of magnitude smaller than the effect observed in photoemission experiments. Therefore, our result rules out electron-phonon interaction in bulk La(2 - x)Sr(x)CuO4 as the exclusive origin of the measured kink. Our conclusions are consistent with those reached independently in a recent study of the related compound YBa2Cu3O7.

Effect of bulk Ca-substitution in YBaCuO thin films for microwave applications

Calcium diffusion into defects in the high-temperature superconductor (HTS) compound YBa2Cu3O7−x (YBCO) has been previously investigated [D. Seron, D.E. Oates, G. Hammerl, J. Mannhart, P.J. Hirst, R.G. Humphreys, A.C. Anderson, M.A. Hein, J. Derov, Phys. Rev. B 72 (2005) 104511] as a possible way to improve the dc, ac and rf transport properties and to clarify nucleation, growth, and doping of YBCO. In this paper, the microwave properties of homogeneously deposited Ca-substituted YBCO, Y1−yCayBa2Cu3O7−x (YCBCO) films are presented and compared with the previously published Ca-free YBCO films with a Y0.7Ca0.3Ba2Cu3O7−x capping layer. By using different substrates and growth process detailed in the paper, particular defects are created in the YBCO or YCBCO films. In every situation, Ca-substitution is probed and its effect is compared with very high quality epitaxial YBCO films close to the intrinsic limit of microwave nonlinearity for YBCO. Information on the percolation, doping effect, and the role of the defects obtained from the dc properties in the normal state are used to help interpret the nonlinear microwave behavior.

Analysis of self-organization processes among the hole states in cuprates and the features of their manifestation in YBa2Cu3O6+δ

A string model proposed previously, based on the concept of quasi-one-dimensional correlations in the overlapping 2p shells of the oxygen sublattice, is used to analyze the influence of self-organization processes in the hole community on the evolution of the electronic properties of cuprates as the doping level is increased. The proposed scenario implies, in particular, that the formation of bosonic stripes within bunches of fermion-like hole excitations in the cuprates should be reflected in their electron spectra, first in the form of bulges and then in the form of coherent peaks adjacent to humps of incoherent origin on the low-energy side. In light of the approaches developed, the central thrust is to ascertain the specifics of the ordering of hole states in the compounds YBa2Cu3O6+δ, where, owing to alignment of the basal oxygen into chains, more favorable conditions can be created for the formation of a frustrated network of superconducting nanochannels consisting of second-rank bosonic stripes, which can maintain stability at temperatures up to 1200K at δ≈0.22. It is shown that the theoretical results, calculated without recourse to adjustable parameters, are in good agreement with the results of measurements for the main characteristics of the electron spectra of the cuprates.

Phenomenon of structural recovery in nonstoichiometric compounds YBa2Cu3O7−δ and EuBa2Cu3O6.8

Experimental results are presented on the phenomenon of recovery (characteristic of aging metallic alloys) in single crystals of nonstoichiometric oxides YBa2Cu3O∼6.6 and EuBa2Cu3O∼6.8. After single low-temperature and high-temperature annealings (200 and 300°C), all signs of spinodal decomposition in these single crystals were found; after double heat treatment (200°C + 300°C), the restoration of structure inherent in the initial state (antiphase domains of twin orientation) occurred.

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

Unconventional consideration of the symmetry properties—New techniques and computer codes: Application for high‐temperature superconductors

AbstractThe conventional technique for solving the equations of quantum chemistry is extended to the structures possessing certain symmetries. This proposal allows the release of unoccupied electronic states located lower than the ground‐state Fermi level of a specific system. Such states can be treated as “spectral holes.” Application of this technique, in particular, when calculating the electronic structure of the high‐temperature superconductor (HTSC) compound YBa2Cu3O7δ (0 ≤ δ ≤ 1) results in the following. For all versions of the examined charge distributions over a crystal lattice, spectral holes of high spatial localization are found. The “spatial spectral holes” are located mainly at the py‐orbitals of the apex oxygens. These orbitals overlap and form linear chains that are parallel to the known Cu(1)O chains, disappearing when δ is close to 1. One can suppose that the linear chains of the overlapping hole states form a “superconducting channel.” Some other parameters closely related to the critical characteristics of HTSC materials are also calculated. The calculations show that the superconducting channel is broken when the oxygen chain atoms O(1) are removed (δ &gt; 0). © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002

Routes to obtain high performance, high Tc superconducting bulk materials

This paper reviews our investigations for understanding the fundamental relations between physical properties, crystal chemistry, phases relation and processings on the compounds YBa2Cu3O7−x and Bi(Pb)2Sr2Ca2Cu3O10+x. These two systems have in common the complexity of their crystal structure and multi-components chemistry which require challenging material processing for applications. In the YBaCuO system, horizontal bridgman and melting zone processes have demonstrated the possibility of obtaining high performances in this system, with different shaping. A high reproducibility with low material losses have been achieved with a top seeding melt texturing process. For BiSSCO based compounds, sol–gel and polymer matrix methods have been studied and developed with the achievement of promising critical current densities at 77 K.

Dispersion characteristics of magnetostatic waves propagating at a ferrite/perovskite interface

The propagation of magnetostatic waves at the interface between an yttrium-iron-garnet (YIG) film and a layer of perovskite compound LaAlO3, SrTiO3, or YBa2Cu3O7−x (YBCO) was experimentally studied. The microwave dispersion characteristics were measured at room temperature and at 77 K. An effective method for controlling the propagation speed and the phaseshift of magnetostatic waves in the YIG/YBCO structures is suggested. Specifically, the phaseshift at 3 GHz is changed by more than 1.5π by reducing the critical current density from 1 to 0.7× 106 A/cm2.