Micromechanical model for self-organized secondary phase oxide nanorod arrays in epitaxial YBa2Cu3O7−δ films

A micromechanical model based on the theory of elasticity has been developed to study the configuration of self-assembled secondary phase oxide nanostructures in high-temperature superconducting YBa₂Cu₃O_7?δ films. With the calculated equilibrium strain and elastic energy of the impurity doped film, a phase diagram of lattice mismatches vs. elastic constants of the dopant was obtained that predicts the energetically preferred orientation of secondary phase nanorods. The calculation of the nanorod orientation and the film lattice deformation has yielded excellent agreement with experimental measurements.     

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.     

Thermal stability of YBa2Cu3O7−δ films deposited on Ni-W textured substrates by laser ablation

The degradation processes of YBa₂Cu₃O_6.77(3) films 600 nm thick deposited on substrates from a textured (100) foil of the Ni-W alloy with the YSZ (Y₂O₃ + ZrO₂) (100 nm) and CeO₂ (50 nm) buffer layers at 200°C in air have been investigated. It has been shown that the degradation proceeds in the same manner as in the bulk ceramic materials with the same oxygen content, namely, initially the single-phase material with the orthorhombic structure laminates into the fractions depleted (the main fraction) and enriched with oxygen. After holding for more than 20 h, the oxygen-depleted fraction is first subjected to the structural phase transformation with the transition of the orthorhombic structure into the cubic one, which is associated with the redistribution of barium and yttrium atoms over their sites in the presence of water vapors.     

Features of the twin structure of YBa2Cu3O7−x epitaxial films

X-ray diffraction is used to investigate YBa₂Cu₃O_7?x (YBCO) films on NdGaO₃(110) and a ${{(100)CeO_2 } \mathord{\left/ {\vphantom {{(100)CeO_2 } {(1\bar 102)}}} \right. \kern-0em} {(1\bar 102)}}Al_2 O_3 $ heterostructure. Symmetric, asymmetric, and axial geometries for θ and θ/2θ scans are used to obtain diffraction spectra from different crystallographic planes. The orientational and quantitative twinning characteristics of the films are determined. While the crystallographic parameters of these two types of film (the films are c-axis oriented with c=11.67 Å) are similar, there are differences in the twin structure. In particular, the features of the NdGaO₃ structure lead to the appearance of an angle differing from 90° (90.20°) between the possible (110) and θ/2θ twin planes in a YBCO film and a different number of twin components in each system of twins. It is concluded from an analysis of the broadening of reflections, which are sensitive to twinning, that there is not twinning in a 60% film of YBCO on Al₂O₃ with a CeO₂ buffer layer.     

Study of interaction between ZrO2 (YSZ) substrates and YBa2Cu3O7−δ superconducting films

The interactions between YBa₂Cu₃O_7– superconducting films and ZrO₂ substrates with different crystal orientations were investigated. Our results show that the crystal orientation of the substrate influences the orientations of the YBa₂Cu₃O_7– crystals, whose c-axis have preferential orientations parallel to the surface of 100 ZrO₂ substrates. The copper segregation in Y-Ba-Cu-O/ZrO₂ films results from substrate reactions. Y, Ba, and Cu atoms diffusing into substrates were detected by AES and RBS techniques. A thin silver film evaporated on ZrO₂ substrate as a buffer layer effectively slows down or eliminates the substrate interactions. The strong preferential c-axis orientation perpendicular to the substrate surface has been observed in the YBa₂Cu₃O_7– /Ag/ZrO₂ samples.This work has been supported by Science Foundation of Academia Sinica     

Structural phase transition in YBa2Cu3O7−δ: the role of dimensionality for high temperature superconductivity

We have performed detailed high temperature X-ray diffraction to study the nature of the structural phase of YBa₂Cu₃O_7−δ. The results indicate the existence of a reversible orthorhombic to tetragonal phase transition at a temperature close to 750°C. If the high temperature tetragonal phase is quenched-in at low temperatures the critical superconducting temperature is considerably reduced from 92.5 K. This suggests that the one dimensional CuO chains present in the orthorhombic structure are necessary for high temperature superconductivity.     

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.     

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)     

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)     

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(–).     

The effect of low-temperature electron irradiation on the transition temperature of YBa2Cu3O7−δ superconductors

Measurements of the complex susceptibility =–i of electron-irradiated YBa₂Cu₃O_7– show a strong influence of the electron irradiation dose, ·t on the transition temperatureT _c . For irradiation doses of ·t=2.2·10¹⁹ e^–/cm² we find a damage rate of T _c /(·t)=–1.6·10^–19 K/(e^–/cm²). It is assumed that the decrease ofT _c is mainly a bulk effect due to the production of atomic defects like vacancies and interstitials in the Cu–O–Cu chains and in the basal planes of the unit cells.     

Permittivity of BaTiO3 epitaxial films grown on the YBa2Cu3O7−δ(001) surface

The epitaxial heterostructures YBa₂Cu₃O_7?δ and YBa₂Cu₃O_7?δ/(5 nm)SrTiO₃/BaTiO₃/(5 nm)SrTiO₃/YBa₂Cu₃O_7?δ are grown by the laser evaporation method on an LaAlO₃(100) substrate. The permittivity of a BaTiO₃ layer is approximately doubled (T=300 K) when a SrTiO₃ thin layer is inserted between a ferroelectric layer and superconducting cuprate electrodes. A maximum in the temperature dependence of the permittivity for a barium titanate layer in the YBa₂Cu₃O_7?δ/(5 nm)SrTiO₃/BaTiO₃/(5 nm)SrTiO₃/YBa₂Cu₃O_7?δ heterostructure is shifted by 70–80 K toward the low-temperature range with respect to its location in the corresponding dependence for the BaTiO₃ bulk single crystal. The bias voltage dependence of the permittivity for the BaTiO₃ grown layers exhibits a clearly pronounced hysteresis (T=300 K). The superconducting transition temperature for the lower YBa₂Cu₃O_7?δ electrode in a superconductor/ferroelectric/superconductor heterostructure considerably depends on the rate of its cooling after the completion of the formation process.     

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.     

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.     

Epitaxial growth of solution deposited YBa2Cu3O7-δ films

The solution deposition of YBa2Cu3O7- (Y123) high temperature superconducting thin films was studied. The films were prepared from a polymer-containing precursor onto SrTiO3 (001) and LaAlO3 (001) substrates and mineralized at high temperatures. The process depended on details of the film preparation in a delicate fashion, resulting either in superconducting or non-superconducting thin films. To elucidate this difference in conductivity, scanning electron microscopy and several X-ray diffraction techniques were used to characterize the resulting Y123 layers. Both the morphology of the film and the percentage of non-superconducting minority components are likely to be the origin of the difference in the conductance behavior at low temperatures.     

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.     

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.     

Specific heat anomaly of single phase YBa2Cu3O7-δ at superconducting transition temperature

The specific heat of single phase YBa₂Cu₃O_7-δ has been measured using non-adiabatic method between 4.2K and 120K. There is a specific heat anomaly Δc at 90K (about 3.2% of total specific heat) approximately, due to superconducting transition. From the measured value of ΔC and transition temperature T_c, the electronic density of state at Fermi level N(E_F) and Sommerfeld parameter γ calculated are 2.55±0.30states/eV.Cu-atom and 2.77±0.30 mJ/mole.K², respectively. The experimental result of N(E_F) is consistent with that of the band calculation by Mattheiss. The Debye temperature above T_c in this material deduced from Debye function is about 340K. Below 20K, the relation C=γ'T+βT³ is satisfied. But the value of γ' is smaller. That means, most of the electrons have formed superconducting Cooper pairs which give no contribution to specific heat below 20K.     

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.     

Spin crossover in the magnetic phase transition in YBa2(Cu1−x Fex)3O7±δ superconductors

All iron ions in the Cu1 and Cu2 local lattice sites of the YBa₂(Cu_0.9 ⁵⁷Fe_0.1)₃O_7.01 superconductor with T _c=31 K experienced magnetic ordering below T _m=22 K. Therefore, at T < T _m, magnetic ordering coexisted with superconductivity. According to the Mössbauer spectroscopy data, iron ions in Cu2 (Fe2) sites were in the low-spin state at T < T _m(S= 3/2 or 1/2), whereas an equal number of iron ions in Cu1 (Fe1) sites were in the high-spin Fe³⁺ state (S=5/2). The magnetic transition near T _m changed iron ion spin states-low-spin ions turned into high-spin ions, and vice versa. This preserved the spin balance between iron ions in the Cu1 and Cu2 layers. Control measurements on other samples of the YBa₂(Cu_{1? x} Fe_x)₃O_7±δ series substantiated these conclusions.