Electric field effect on superconducting YBa2Cu3O7−δ films

Experimental evidence for a significant electric field effect in thin superconducting films of YBa₂Cu₃O_7– is presented. MISFET-type structures have been developed which allow the application of electric fields larger than 4×10⁶ V/cm across insulating SrTiO₃ barriers into thin cpitaxial YBa₂Cu₃O_7– channel layers. The results demonstrate that with these structures the electrical resistivity aboveT _c (R=0) and the density of free carriers in the YBa₂Cu₃O_7– films can be modified by 1–2% with gate voltages smaller than 50 V.     

Growth of YBa2Cu3O7−x thin films on sapphire with a cerium-oxide sublayer

The possibility is demonstrated of preparing high-quality films of the high-temperature superconductor YBa₂Cu₃C_7-δ with thicknesses up to 2.6 μm by dc magnetron sputtering. It is found that inclusions consisting of CuO and YBa₂Cu₃O₈ coexist with the growing film and are “sinks” for defects, nonstoichiometric atoms, and mechanical stresses. Using x-ray diffraction and Rutherford backscattering, we find that the structural perfection of the films is improved by increasing the thickness when using the proposed fabrication technique. Zh. Tekh. Fiz. 69, 94–98 (January 1999)     

Field-suppressed interfacial spin scattering in YBa2Cu3O7−δ/La0.67Sr0.33MnO3 heterostructures

The superconductivity and magnetoresistance (MR) of epitaxial bilayers consisting of YBa₂Cu₃O_7?δ (YBCO) and La_0.67Sr_0.33MnO₃ (LSMO) are investigated with respect to the proximity and spin polarization effects. In the region of the mixed vortex state near the onset temperature of the superconducting transition, the diffusion of the spin-polarized quasiparticles from the LSMO to the YBCO is evidenced by their distinct magnetotransport behaviors, which are characterized by two superconducting transitions. It is revealed that the second transition is strongly dependent on the applied magnetic field, demonstrating an abnormal pair-breaking effect. This effect, together with the reduced MR, is believed to arise from an increase in the spin injection efficiency induced by the suppressed interfacial spin scattering, disclosing a new perspective for understanding the magnetotransport phenomena in proximity-coupled superconductor/ferromagnet (S/F) hybrids.     

Enhanced grain growth in YBa2Cu3O7−δ doped with Ag,Cu, and CuO

Powders of YBa₂Cu₃O_7– were mixed with Ag, Cu, and CuO powders, pressed and sintered in air and oxygen. Grain-growth enhancement has been observed in the doped samples on sintering in oxygen at 980°C and its attributable to a partial melting and decomposition followed by crystallization during cooling. While the mechanisms of the partial melt may differ with dopant, grain growth enhancement appears to be a common feature of partial melt followed by slow cooling. The possibility of using suitable dopants in aiding an aligned grain structure is pointed out.     

Study of YBa2Cu3O7−δ ceramics/Al interface

Properties of oxide layers at the interface between Al metal and YBa₂Cu₃O_7−δ ceramics were investigated using X-ray diffraction (XRD), X-ray photoelectron spectra (XPS) and impedance measurements. There have been found the oriented grains having their c-plane parallel to the surface of YBa₂Cu₃O_7−δ. When Al metal was evaporated onto the ceramics sample, the aluminum oxide layer was produced at the interface between Al and YBa₂Cu₃O_7−δ because Al metal oxidizes more easily. The oxygen-deficiency was observed at the ceramics side of the interface as examined by X-ray photoelectron spectra. This oxygen-deficiency can be partly replenished by post annealing whereas it can be fully replenished for the sample to which the mechanical polishing is applied beforehand in order to remove the oriented grains. The thickness of aluminum oxide layer was evaluated by means of the impedance measurements using the alternating current three-terminal method.     

Superconducting cluster in YBa2Cu3O7−x ceramic

The method of point laser heating of a sample is used to perform experimental investigations of the electrical conductivity of YBa₂Cu₃O_7−x ceramic in the vicinity of its critical state. It is found that, in the vicinity of transition to the superconducting state, the electrical conductivity of the ceramic exhibits a clusterlike behavior substantially nonuniform over the sample cross section. The topology of a superconducting cluster is investigated, as well as its spatial localization in the sample. A model of the formation of a superconducting cluster in a ceramic superconductor is suggested.     

Misfit stresses in YBa2Cu3O7−x films

Misfit and thermoelastic stresses in HTSC layers 1-2-3 grown on various substrates are, analyzed with the use of ultrasonic measurements of Young's moduli inYBa ₂ Cu ₃ O _7−x . It is shown that the misfit stress gives the main contribution to the formation of the HTSC layer strain. Military Academy of Strategic Rocket Forces. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 87–90, April, 2000.     

Normal state resistivity of YBa2Cu3O7−x

A systematic study has been carried out on the normal state resistivity of YBa₂Cu₃O_7–x . Samples were cut from the same well-annealed material and were quenched from different temperatures (T _Q ). The resistivity is metallic whenT _Q 500 °C and becomes semiconductor-like whenT _Q 600 °C. The data on the latter was interpreted in terms of the three-dimensional localization model proposed by Mott.The work at USCD was supported by the California MICRO program     

Analysis of anisotropic electrical resistivity and fluctuation conductivity of YBa2Cu3O7−δ single crystals

For a large number of YBa₂Cu₃O_7– single crystals which were grown by a flux method in different crucibles the anisotropic resistivity has been measured. An analysis of the metallic in-plane resistivity _ab gives evidence for different carrier concentrations in the individual samples caused by different amounts of oxygen disorder effects in theab-planes. The temperature dependence of the out-of-plane resistivity c which is also metallic for the best samples is determined by the applied crucible material. From the luctuation conductivity in theab-plane aboveT _c the coherence length _c (0)2.5 Å can be determined in good agreement with values obtained fromH _c2 measurements.Dedicated to Prof. Dr. G. von Minnigerode on the occasion of his 60th birthday     

Transient photoimpedance response of YBa2Cu3O7−0δ epitaxial films in the mixed state

This paper reports on an investigation of transient photoimpedance response to radiation, in other words, real-time variations of the impedance induced by femtosecond optical pulses in superconducting films transferred to the mixed state by an external magnetic field applied parallel to the c-axis. When the films were in a state characterized by the absence of dc resistivity, the response amplitude increased with the magnetic field faster than expected owing to the contribution of magnetic vortices to the impedance of a superconductor with s-wave pairing of electrons. It turned out that the effect is due to a growth in the effective density of quasiparticle states in the mixed state of a d-wave superconductor. In the absence of magnetic field, however, the response amplitude was higher at lower temperatures, which contradicts the predictions of both models. Possible reasons for this feature in the response as a function of temperature are discussed. Zh. éksp. Teor. Fiz. 116, 1035–1047 (September 1999)     

Pinning potential in thick PrBa2Cu3Ox/YBa2Cu3O7−δ quasi-multilayers

Using Pulsed Laser Deposition we have fabricated thick quasi-multilayers composed of incomplete layers of PrBa₂Cu₃O_x (PrBCO) nano-dots and layers of YBa₂Cu₃O_7?δ (YBCO). The number of such sequences was between 2 and 6, with the thickness of individual YBCO layers between 565 and 885 nm, and total thickness between 1.13 and 5.31 μm. For the thinner quasi-multilayer, DC magnetization studies showed an increase in the critical current density J_c at all fields in comparison with a pure YBCO reference sample, while the thicker samples showed an increased J_c only in high fields. We have also investigated the frequency dependence of J_c from AC susceptibility studies and found that the pinning potential is well described by a logarithmic dependence on current density. Pinning potentials in PrBCO/YBCO quasi-multilayers also proved to be higher than in the reference sample at high fields. From angle-dependent transport measurements we have found indications of strong pinning centres induced by the (PrBCO) nano-dots, both isotropic and c-axis correlated.     

Growth of ultrathin YBa2Cu3O7−x films on the SrTiO3 substrate

The growth of ultrathin (1.7–7.3 nm) YBa₂Cu₃O_7?x films on the SrTiO₃(100) substrate is investigated by the mean-energy ion scattering technique. It is found that the growth of islands proceeds according to the two-dimensional and three-dimensional models and depends on the growth conditions.     

Effects of heating on the Raman spectra of YBa2Cu3O7−δ

We report effects of heating to the intensities of Raman modes in the spectra of single-crystals embedded in ceramic YBa₂Cu₃O_7– pellets, and compare the results with those from YBa₂Cu₃O_7– thin-films. Heating was done by either increasing the laser-beam power or resistively heating in a heating cell. From the measurements on a single-crystal in z(xx) polarization, we find that the relative intensity of the B_1g mode of the plane oxygens to that of the A_1g mode of apical oxygens decreases strongly as the samples are heated. For a crystal in near x(zz) polarization, the relative intensity of the two modes increases up to certain value. In thin-films, however, no such change was observed even when heated to higher temperatures than the crystals were. Therefore, we interpret that the change in the relative intensity of the two modes of single-crystals is not due to electronic or structural changes nor thermal decomposition in the heated samples. We propose that a single-crystal, when heated, is fractured into many micro-crystals and these micro-crystals thermally fluctuate in orientations. For thin-films, such fluctuation in orientations might be hindered because there are only a few layers of micro-crystals on top of the substrate. Scanning electron micrographs (SEM) of the heated part of the samples seem to provide the evidence of the micro-crystals.     

Transverse resistance of YBa2Cu3O7−δ single crystals

Measured is the transverse electrical resistance of YBa₂Cu₃O_7−δ single crystals with different oxygen deficiency values (δ) in the temperature range T_c − 300 K. The experimental data are approximated by an empiric expression accounting for the fluctuation conductivity near T_c and the semiconductor-like resistance regime. Our analysis of the concentration dependences of the fitting parameters, in particular, reveals that the resistance temperature dependence is largely affected by the sample's non-homogeneity. The latter, in turn, causes a T_c anisotropy and variable-range hopping conductivity between different phases. The deduced maximal values of the basal-plane coherence length, ξ_xy(0), are comparable with those for low-temperature superconductors.     

Local electronic structure of Cu2O,CuO and YBa2Cu3O7−δ

Cu 3d and O 2p electronic states of Cu₂O, CuO, and the highT _c compound YBa₂Cu₃O_7– have been probed by means of high resolution x-ray emission spectroscopy (XES). The CuL and OK XES bands are compared in detail with recently reported x-ray photoelectron and ultraviolet photoelectron spectroscopy (XPS and UPS) measurements and densities of states obtained by local density functional (LDF) theory. The XES data show that the hybridization between Cu 3d and O 2p states is completely modified in CuO and YBa₂Cu₃O_7–, whered-d correlation energy is large, as compared to LDF predictions. Such is not the case for Cu₂O where agreement between theory and experiment is good.The Cu 3d states are found to be highly localized in YBa₂Cu₃O_7– (though less so than in CuO). The O 2p states lie at lower binding energies than in the simpler oxides and are mainly situated above the Cu 3d states. The respective positions of the centre of gravity of the OK emission bands on an x-ray energy scale indicate that the oxygen sites are less well screened by the O 2p states in the highT _c compound. This provides indirect evidence for the presence ofd-like states at the oxygen sites.     

Low temperature acoustic properties in YBa2Cu3O7−δ

Temperature dependencies of acoustic lossesQ ^–1 and of relative sound velocity change v/v in YBa₂Cu₃O_7– up to 60 K are calculated by the tunneling model theory. The tunneling systems are related to the off-centered positions of the apical oxygen atoms O(A) and are described through the pseudo-Jahn-Teller effect. Tunneling systems' parameters are distributed in narrow range of values and are in correspondence with the experimentally observed infrared phonon spectra and thermal ellipsoids of O(A). Respective relaxation times are calculated by the adapted reaction rate method. The calculatedQ ^–1(T) and v(T)/v dependencies are in good agreement with the experimental data, which is an additional support to the conclusion about the existence of tunneling systems in YBa₂Cu₃O_7– due to the pseudo Jahn-Teller effect.     

Production of thick YBa2Cu3O7-δ films on sapphire with a cerium oxide sublayer

The feasibility of producing high-quality films of the high-temperature superconducting material YBa₂Cu₃O_7-δ with thicknesses up to 2.6 μm by dc magnetron sputtering is demonstrated. It is found that inclusions consisting of CuO and YBa₂Cu₃O₈ coexist with the growing film and are “sinks” for defects, nonstoichiometric atoms, and mechanical stresses. X-ray diffraction and Rutherford backscattering studies show that the structural perfection of the films increases as the thickness is increased in the proposed production regime. Zh. Tekh. Fiz. 69, 132–136 (September 1999)     

YBa2Cu3O7纳米桥rfSQUID

我们采用电子束曝光和氩离子刻蚀技术制备了ＹＢＣ纳米桥，在此基础上得到高温ｒｆＳＱＵＩＤ，器件的磁通噪声为４×１０＾－４Φ／√Ｈｚ，磁场分辨率为２．０ｐＴ／√Ｈｚ，对没有三角波的器件，经过ＥＤＴＡ后腐蚀技术对纳米进行弱化处理，部分器件出现了三角波。     

Effect of deoxygenation on the weak-link behavior of YBa2Cu3O7−δ superconductors

A systematic study of the weak-link behavior for YBa₂Cu₃O_7−δ polycrystalline samples has been done using the electrical resistivity and AC susceptibility techniques. The experiments were performed with two samples of similar grain, a sample of well-coupled grains, and a deoxygenated sample in such a way that the oxygen mostly comes from the intergrain region. Analysis of the temperature dependence of the AC susceptibility near the transition temperature (T_c) has been done employing Bean's critical state model. The observed variation of intergranular critical current densities (J_c) with temperature indicates that the weak links are changed from superconductor normal–metal superconductor (SNS) for well-coupled samples to superconductor insulator normal–metal–superconductor (SINS) type of junctions for the deoxygenated sample. These results are interpreted in terms of oxygen depletion from grain boundaries, which in turn decreases the intergranular Josephson coupling energy with a concomitant decrease of pinning of the intergranular vortices.     

Specific heat studies of high-Tc superconductor YBa2Cu3O7−x

Specific heat studies of the high-T_c superconducting compound YBa₂Cu₃O_7−x with bulk transition temperature at 92K are reported. A distinct anomaly of electronic origin in the specific heat is observed with granular-like behavior corresponding to a Sommerfeld constant γ = 7±2mJ(moleCuK²)^±1 Debye temperature (φo ≈ 400K) is obtained by fitting the experimental data with the theoretical Debye specific heat.