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.     

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.     

Nature of contact resistance in high-T c YBa2Cu3O7−δ Superconductors

The contacts between various metals and the high-T_c superconductor YBa₂Cu₃O_7– are characterized by contact resistance,R _c, andI–V measurements from 300 K to 90 K. The contacts with bulk superconductor were made by vacuum deposition. Four metals, Au, Ag, Al, and Bi were investigated. The current transport across the contact is by carrier tunneling. All contacts were ohmic as theirI–V characteristics were symmetrical with respect to current direction.R _c values range between 10^–2 to 10¹ cm² and increase linearly as the temperature is lowered. The contact resistance originates from two distinct physical processes. One is the modification of the carrier concentration at the interface by the contact metal. The second is the nature of carrier injection at the free surface of the superconductor. TheR _c values depend on the contact metal-oxygen interaction parameter signifying the need for oxygen passivation for obtaining low contact resistances.     

Femtosecond relaxation dynamics of electronic processes and electron-phonon coupling in YBa2Cu3O7-δ and PrBa2Cu3O7 epitaxial films

The room temperature femtosecond relaxation dynamics of reflection spectra of YBa₂Cu₃O_7- with different oxygen contents and PrBa₂Cu₃O₇ epitaxial films have been studied. The relaxation times of R(t) in YBa₂Cu₃O_7- increase from less than 100 fs to more than 1 ps with increasing and with substitution of Pr for Y. The electron and lattice temperature profiles at these quasi-equilibrium state are calculated from the nonequilibrium energy-balance model. The process of electronphonon energy transfer was found to be dominated in our experiments. These results reveal that in 123 phase superconductors decrease of carrier concentration slows down the carrier-phonon coupling and the type of the electron-phonon interaction would be localized. The dependence of electron-phonon coupling to oxygen defects in YBa₂Cu₃O_7- was found to be exp(–).     

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.     

Fluctuation-enhanced conductivity and magnetoconductivity of high-quality YBa2Cu3O7−δ crystals

We present measurements of the in-plane resistivity _ab of YBa₂Cu₃O_7– single crystals withT _c 92 K and _ab (100 K)50 cm. The temperature dependence of the fluctuation conductivity and of the magnetoconductivity aboveT _c is analyzed in terms of direct and indirect fluctuation contributions for layered superconductors. The combination of fluctuation conductivity and magnetoconductivity allows to determine both coherence lengths _ab (0) and _c (0) as well as the phase-relaxation time of the pairs in an unequivocal manner. Evidence for clean limit type-II superconductivity in our crystals is given by large values of the mean free pathl _{ab ab} (0).Dedicated to Prof. Dr. F. Hund on the occasion of his 95th birthday     

Thermal and magnetic properties of some YBa2Cu3O7−δ samples

Thermal and magnetic measurements have been performed on several YBa₂Cu₃O_7−δ compounds, some ones showing a large content of high T_c (93 K) superconducting phase. A jump in the specific heat ΔCp, is well evidenced at the transition allowing a determination of the ratio ΔCp/T_c ≅ 23 ± 5. mJ/ (mole Cu)K². In addition, an estimation of the γ value (≅ 11 mJ/(mole Cu). K²) has been drawn from the determination of the electronic entropy at T_c. The samples have been characterized by susceptibility, magnetization and resistivity experiments. The critical field slopes at T_c were found to be dHc₁/dT ≅ 17 Oe/K and dHc₂/dT ≅ 20 kOe/K. The results are discussed in the framework of the Ginzburg-Landau theory.     

An investigation of the PrBa2Cu3O7-δ system

Members of the PrBa₂Cu₃O_7– system possessing the orthorhombic structure over a wide range of oxygen stoichiometry (=–0.5 to +0.5) have been prepared and characterized. Similar compositions with a tetragonal structure have also been prepared. None of the oxides is superconducting, independent of the structure or stoichiometry. Praseodymium seems to be present to a small extent in the 4+ state in oxygen-excess (negative ) samples. Orthorhombic PrBa₂Cu₃O_7– samples show the presence of twins suggesting that twins arise from orthorhombicity and have no relation to the superconductivity. A neutron diffraction study of a near-stoichiometric sample has shown a disordered orthorhombic structure with 72% occupancy of the 01 (chain) sites and with no interchange between Pr and Ba sites.Contribution No. 503 from the Solid State and Structural Chemistry Unit     

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.     

Magnetoresistance hysteresis of granular YBa2Cu3O7−δ high-temperature superconductor in weak magnetic fields

The variation of the transverse magnetoresistance of YBa₂Cu₃O_～6.95 high-temperature superconducting ceramic with external magnetic field intensity H _ext first increasing from zero to ～500 Oe (Δρ⁺ / ρ_{273 K}) and then decreasing from about 500 Oe to zero (Δρ^? / ρ_{273 K}) is studied for transport current densities varying from j/j _c ≈ 0.01 to ≈0.99 (where j _c is the critical current density in the absence of the magnetic field) at 77.3 K. It is found that the field dependence of the magnetoresistance of YBa₂Cu₃O_～6.95 high-temperature superconductor is characterized by pronounced hysteresis, the difference Δρ⁺ / ρ_{273 K} ? Δρ^? / ρ_{273 K} increasing with j/j _c. As j/j _c grows, the effective critical fields of Josephson weak links, H _c2J , and the lower critical fields of superconducting grains, H _c1A , decline. When field H _ext rises, the critical fields are lower than when H _ext diminishes: H _c2J ⁺ < H _c2J ^? and H _c1A ⁺ < H _c1A ^? .     

Oxygen dynamics in epitaxial YBa2Cu3O7-δ thin films

We have investigated epitaxial YBCO films by Perturbed γγ-Angular Correlation Spectroscopy (PAC) using ¹¹¹In probe atoms. The probes substitute on the yttrium lattice site and hyperfine parameters have been determined unambiguously. Excellent agreement is achieved with full potential electric field gradient (efg) calculations. Good reproducibility has been achieved by our preparation method allowing us to study the influence of oxygen motion on the structural orthorhombic to tetragonal phase transition. This revised version was published online in August 2006 with corrections to the Cover Date.     

The Hubbard model and pressure effects of YBa2Cu3O7−δ superconductors

We apply a mean field approach to the extended Hubbard model on a square lattice to the YBa₂Cu₃O_7−δ family of superconductors under pressure. The parameters of the tight-binding band are taken from experiments, and the coupling strength U and V are estimated by the zero pressure phase diagram (T_c×n_h). This scheme yields the non-trivial dependence of the superconductor critical temperature T_c as a function of the hole concentration n_h in the CuO₂ plane. With the assumption that the pressure P modifies the potential V and the on-plane hole content n_h, we can distinguish the charge transfer and the intrinsic contribution to T_c(P). We show that the changes on T_c(P) for the YBa₂Cu₃O₇ optimally doped compound at low pressures are almost entirely due to the intrinsic term.     

Superlattice structures in solid solution of high-Tc YBa2Cu3O7−δ and (Pb,Cu)Sr2(Ca,Y)Cu2O7−δ superconductors

A complete solid solution range exists between the systems YBa₂Cu₃O_7−δ and (Pb,Cu)Sr₂(Ca,Y)Cu₂O_7−δ has been found with general stoichiometry (Pb_0.75xCu_1−0.75x)(Sr_2xBa_2−2x)(Ca_0.5xY_1−0.5x)Cu₂O_7−δ. Energy dispersive spectrometry and X-ray diffraction identified that a true solid solution exists. Superlattice structures observed by electron diffraction across the solid solution range have a modulation range have a modulation periods along a* which can be varied by altering both the compositional parameter x and the overall oxygen content. The existence of these superlattices infers that the solid solution is non-random and therefore thermodynamically non-ideal. The superconducting transition temperatures, T_c, across the solid solution range are also strongly dependent on the composition, x, but no direct relationship with the modulation period has been established. From these studies it may be concluded that the solid solution between known superconductors is possible, although involving some partial ordering of the lattice, but ordering of cations in the rock-salt to charge reservoir layer is not a significant factor in determining the superconducting properities, which depend more closely on the overall composition and hence on the ability of the charge reservoir layer to transfer charge to the superconducting layers.     

Far IR transmission spectra of an YBa2Cu3O7−δ thin film

Abstract

The FIR transmission of an YBa₂Cu₃O_7-δ film 1000 Å thick deposited on an MgO plate has been studied from 20 cm^?1 to 4000 cm^?1 at T = 300 K, and at 120 K, 80 K and 7 K. i) The spectra for the normal state are well fitted if a mid-IR oscillator of high strength and high damping is added to the simplest Drude model. ii) The spectra for the superconductive state do not show significant variations of transmission vs. temperature for ω > 120 cm^?1, which should be in agreement with a weak BCS coupling 2Δ = 3.5 kT_c . iii) The FIR transmission at 7 K for ω = 20 cm^?1 is not zero (around 1%) and seems to confirm that the low-temperature perovskite is made of two phases: a superconducting, and a normal one, the proportion of the first one increasing when the film temperature is decreased.     

YBa2Cu3O7-δ和PrBa2Cu3O7-δ1/f噪声尺寸效应的研究

我们分别测量了YBa2Cu3O7-δ（YBCO）和PrBa2Cu3O7-δ（PRBCO）微桥样品在正常态沿ab面方向的1/f噪声，发现这两种材料分别存在着数量级为10^1μm和10^2μm的特征尺度，在此尺寸以下，噪声水平急剧下降，即噪声的尺寸效应，我认为这一结果反映出样品是由尺寸为10^0μm的独立小区组成，小尺寸样品噪声表现是这些小区涨落叠加的结果。     

Raman study of the 90° grain boundaries in YBa2Cu3O7−δ thin films

YBa₂Cu₃O_7−δ (YBCO) films containing two different types of 90° grain boundaries were fabricated on the same substrates. Raman spectra from several parts of the basal-plane-faced tilt (TL) grain boundaries were measured and compared with those from the twist (TW) grain boundaries. The Raman results show that I(500)/I(340), the relative intensity of the A_1g mode near 500 cm⁻¹ with respect to that of the B_1g mode near 340 cm⁻¹, is suppressed in the TL boundaries, relative to the TW boundaries. The results can be interpreted to mean that the stress is stronger in the TL boundaries than in the TW boundaries. This may offer an alternative explanation for the weak-link behavior of the step-edge Josephson junctions.