Phase separation in YBa2Cu3O7−δ single crystals near δ = 0

X-ray diffraction, resistivity, AC and DC magnetization data on high-quality single crystals of YBa₂Cu₃O_7−δ are presented. We demonstrate that for δ<0.08, the (0 0 l) diffraction lines are split into two, indicating that at these high oxygen concentrations the crystals are no longer single phase but actually consist of two (or more) different phases with slightly different c-axis parameters. In the two-phase region, the electrical resistivity and the AC and DC magnetic susceptibilities show a broadening of the superconducting transition. This broadening is thought to be due to the proximity effect or strains in two finely dispersed phases with slightly different transition temperatures.     

Dynamic softening of vortex lines in YBa2Cu3O7−δ single crystals

Transport measurements in the mixed state of oxygen-deficient YBa₂Cu₃O_7−δ single crystals using the flux transformer configuration show that the flux liquid changes with increasing anisotropy from strongly correlated to uncorrelated in the field direction. For intermediate coupling, the current inducing loss of vortex correlation has a maximum near the irreversibility temperature. Thus, an effective softening of vortex lines with decreasing temperature is detected. We propose a simple model that accounts for this behavior by including the effects of the pinning potential on the dynamics of vortices.     

Tunneling studies on NdBa2Cu3O7−y

Tunneling measurements were performed at 4.2 K on well-characterized NdBa₂Cu₃O_7−y samples using a sandwich configuration with an artificially grown barrier layer (sputtered indium oxide) and Pb_0.5In_0.5 counter electrode. The conductance spectra exhibited well-defined structures characteristic of gap opening. Fitting the data to a life-time broadened BCS density of states function yielded the following values: 20 meV for the energy gap Δ, 1.5 meV for the line width Γ and 5.3 for 2Δ/k_BT_c.     

Peak effect and oxygen ordering in YBa2Cu3O7−δ single crystals

We have investigated the second magnetization peak in pure YBa₂Cu₃O_7−δ single crystals with various oxygen contents (6.91<7−δ<6.97) and degrees of oxygen vacancy ordering, as achieved by low (1 bar) and high (100 bar) oxygen pressure annealing. Although the position of the peak changes drastically with oxygen stoichiometry, no dependence on the distribution of oxygen vacancies has been found for temperatures below 70 K. For T>70 K, however, ordering effects become important as demonstrated by the disappearance of the peak for the high pressure annealed samples. These results suggest that while at low temperatures, pinning of the vortex system by clusters or a more homogeneous distribution of oxygen vacancies is similar, at elevated T, the former are much stronger pinning sites leading to larger hysteresis and the presence of the peak.     

Low temperature magnetocaloric effects in Dy1Ba2Cu3O7−δ and Y1Ba2Cu3O7−δ high-Tc single crystals

The heat release has been measured in high temperature superconductors during and after a change of the external magnetic field μ₀H_a ≤ 0.5 T applied parallel to the c-axis at 0.1 K ≤ T ≤ 0.45 K. Two Y123 and one Dy123 twinned single crystals were investigated. An analysis of the heat release based on the Bean model and on a model for thin superconductors in perpendicular geometry recently proposed by Brandt et al. has been carried out. During field sweep we measured ∝ H_a²dH_a/dt. This dependence can be understood within the Bean model for longitudinal geometry. The critical current densities obtained with this method are in good agreement with those from magnetization measurements. We also present measurements, and a first quantitative analysis, of dissipation due to the time relaxation of the flux line lattice after a field sweep. For the heat release a t⁻¹ dependence has been found which corresponds to a logarithmic time dependence of the magnetization. The normalized relaxation rate we obtained is in agreement with literature values.     

Ramans study of Y1−xPrxBa2Cu3O7−δ and YB2Cu3−xZnxO7−δ single crystals

Single crystals with known T_c values of Y_1−xPr_xBa₂Cu₃O_7−δ (Y---Pr1:2:3) and YBa₂Cu_3−xZn_3−xZn_xO_7−δ (Y---Zn1:2:3) systems are studied by Raman measurements. The Raman spectra for (Y---Pr1:2:3) single crystals show that the frequencies of Ba and O_z modes increase as the Pr content increases. The results are consistent with the hole-localization scheme proposed for the suppression of superconductivity in the polycrystalline Y---Pr1:2:3 systems. On the other hand, in the Y---Zn1:2:3 system, all the Raman modes do not change in frequencies. However, the FWHM of the Cu(2) mode increases with the decrease of T_c, indicating strong scattering of charge carriers by the substituted Zn ions in the CuO₂ planes. The induced disorder in the CuO₂ planes may be related with suppression of T_c in the Y---Zn1:2:3 system. Thus, the suppression mechanism in the Y---Zn1:2:3 systems seems to be different from that in the Y---Pr1:2:3 systems.     

Fabrication and growth rate estimation of PrBa2Cu3O7−δ single crystal by the modified top seeded crystal pulling method

Recently, we succeeded in fabricating single crystals of PrBa₂Cu₃O_7−δ by a modified top seeded crystal pulling method called the SRL-CP (Solute Rich Liquid-Crystal Pulling) method. Y₂O₃ and MgO polycrystalline crucibles and a MgO single crystal crucible were used to grow the single crystals. The crystal growth temperature was set in the range of 968°C to 972°C. The grown crystals were identified as PrBa₂Cu₃O_7−δ by X-ray diffraction. In the case of using Y₂O₃ crucibles the composition of the grown crystals was Y_xPr_1−xBa₂Cu₃O_7−δ (0.48 < x < 0.57) and in the case of using MgO crucibles a relatively small amount of Mg contamination to the grown crystals occurred at a typical concentration of approximately 1 at.% of the sum of cations. According to the crystal growth model of the SRL-CP method [1–5], a maximum growth rate of 1.7 × 10⁻⁵ cm/s was calculated with the aid of the phase diagram studies we reported earlier [6]. This value is reasonably in agreement with the experimental results.     

Unusual resistivity broadening in the mixed state of NdBa2Cu3O7−δ crystals grown by a crystal-pulling technique

In order to clarify the origin of a strong peak effect in the magnetization curve of NdBa₂Cu₃O_7−δ crystals, we investigated the superconductivity transition behavior of the in-plane resistivity in static magnetic fields up to 8 T. Comparing the results for the samples exhibiting and not exhibiting the peak effect, we found that the former (peak effect sample) shows lower resistivity above the vortex melting temperature. This implies that the pinning force is effective in the vortex liquid state. We also found that the normal state resistivity behaviors of some samples were unusual, indicating inhomogeneous current flow. These results suggest an existence of sheet-like pinning centers perpendicular to the conduction planes.     

Phonon Raman scattering in Nd1+xBa2−xCu3O7−δ and Pr1+xBa2−xCu3O7−δ single crystals

The polarized Raman spectra of Nd_1+xBa_2−xCu₃O_7−δ (−0.023≤x≤0.107) and Pr_1+xBa_2−xCu₃O_7−δ (0.01≤x≤0.15) single crystals have been investigated. It was found that the Cu(2) A_g mode softens by 6 cm⁻¹ in Nd 1:2:3 and 4 cm⁻¹ in Pr 1:2:3 as x increases. These frequency shifts cannot be explained by the change in the relevant bond lengths due to Nd(Pr)-substitution for Ba. The variations with x of the two low frequency modes may be affected by change of their hybridization and/or change of their force constants. The linewidths of Ba mode in Pr 1:2:3 are broader than those in Y 1:2:3. This result suggests that the Pr substitution on Ba sites occurred even in a very small value of x. In x(yy) geometry the relative intensity of the Ba and O(4) modes in Nd 1:2:3 is greater than those in Pr 1:2:3. The difference between Nd 1:2:3 and Pr 1:2:3 in the relative intensity of the Ba and O(4) modes may be produced by the chains.     

Positron annihilation in the hydrogenated granular superconductor of YBa2Cu3O7−δ

In this paper, we have used the sputtering neutral-particles mass spectroscopy (SNMS) and positron-annihilation technique to investigate the effect of hydrogenation on the physical properties of different oxygenated YBa₂Cu₃O_7−δ superconductors. Under the same of hydrogenation treatment, the hydrogenation effects on the superconductors are compared to the non-superconductors. It was shown that the hydrogen concentration in the superconductors is about eight times of the non-superconductor's. It was proven that the long lifetime of positrons in the annihilation process is determined by the variation of the concentrations of monovacancies and microvoids, which takes place in both of intragrain and intergrain samples. The hydrogenation effect can be classified into four stages. At the first stage, the hydrogen atoms fill both monovacancy and microvoid. At the end of the first stage, the long lifetime τ₂ reaches the maximum value which is determined by the lifetime of the positron in the monovacancy-free and nearly microvoid-exhausted YBCO sample. In the second stage, the hydrogen charging will lead to creation of new monovacancies; this will make the long lifetime τ₂ drop monotonically to its minimum value. In the third stage, further hydrogen charging promotes the formation of microvoids, and leads to an increasing τ₂ up to a saturation value, which indicates the equilibrium concentrations of monovacancy and microvoid at that temperature.     

Flux jump avalanches in torque studies of single crystal YBa2Cu3O7−δ

A series of avalanche-like jumps are observed in the mixed state of single crystal YBa₂Cu₃O_7−δ (YBCO) superconductors. Emerging as a saw-tooth pattern in torque vs. sample orientation in magnetic field, these jumps are discontinuous on our most resolute angular scale. While reminiscent of the classical flux jump instability, the present jumps are instead proposed to be associated with the layered nature of the material and twin boundary (TB) pinning, the combination of which promotes a crossover from a tilted to a kinked vortex structure.     

Etch pits and tunnel barrier in chemically etched YBa2Cu3O7−δ crystals by scanning tunneling microscopy

By imaging in air the (001) surfaces of YBa₂Cu₃O_7−δ single crystals etched in dilute CH₃OH : Br₂ solution with scanning tunneling microscopy we have observed etch pits and followed their modification with time. The current-voltage characteristics of the junction are measured and described in terms of electron tunneling through a trapezoidal potential barrier of average thickness (≈ 25 Å). Such a barrier results from the passivated layer formed on the superconductor surface exposed to room air after the etching. The superconductor surface is metallic. The normalized conductance G(V) measured at room temperature is that of a metal-insulator-metal junction except at small bias. Sharp peaks assigned to local density-of-states features within 1 eV from the Fermi level are observed.     

The weak center nature and their pinning anisotropy of radiation damage defects in Y1Ba2Cu3O7−δ single crystals

A field induced weak pinning center, in MeV proton irradiated Y₁Ba₂Cu₃O_7−δ (YBCO) single crystal and in ⁶Li-doped sintered YBCO exposed to thermal neutron irradiation, has been identified. The pinning contribution of those radiation caused weak centers (RWCs) showed strong field dependence, which resembles that of the common weak centers of oxygen deficient regions (OWCs) in most of the bulk YBCO materials. The radiation dose dependence of the pinning enhancement, in proton irradiated single crystal, showed strong angular dependence, when the field orientation relative to the crystal was changed. This is believed to be due to the different flux line size (orξ) in different directions, and the so caused volume or line pinning effects. No angular dependence has been observed in neutron irradiated YBCO, which is polycrystalline in nature.     

Enhanced diamagnetism in YBa2Cu3O7−δ by hydorgen-plasma treatment

We report that the diamagnetism of as-grown YBa₂Cu₃O_7−δ powders can be enhanced by treatment with RF-generated hydrogen plasma. The field-cooling susceptibility at 5 K is found to increase by up to 24%. The enhancement is reproducible for all five samples studied. The lattice parameter c decreases after hydrogenation. For a sample treated for the shortest period, its pinning potential,obtained from magnetic relaxation measurements, is reduced following H-treatment.     

Critical current density of polycrystalline YBa2Cu3O7−δ consisting of tin-coated grains

Tin and its oxides have been introduced into the intergrain areas of polycrystalline YBa₂Cu₃O_7−δ by coating the crystalline grains with a thin layer of tin and sintering the ceramics in flowing oxygen and argon. The transport critical current density J_c at 77 K in a magnetic field of 0−1.5 T is enhanced as a result of the coating. A probable improvement of the intergrain weak links is suggested.     

Realization and properties of ramp-type YBa2Cu3O7−δ/Au/Nb junctions

All-thin-film ramp type Josephson junctions between YBa₂Cu₃O_7−δ and Nb have been fabricated. This procedure allows connections between high-T_c and low-T_c superconductors at different crystal sides of the high-T_c superconductor on one chip, which is of great interest for novel phase devices. A thin Au layer is incorporated as a chemical barrier to avoid oxygen transfer from the YBa₂Cu₃O_7−δ to the Nb. Critical current densities up to 600 A/cm² are obtained at T=4.2 K, with typical R_nA values of 0.8 μΩ cm². The variation of the magnetic field dependence of the critical current with the angle between the junction barrier and the YBa₂Cu₃O_7−δ crystal axes is explained by considering a predominant d_x²−y² order parameter symmetry of the YBa₂Cu₃O_7−δ. The successful fabrication of these junctions allows the implementation of novel superconducting electronics, such as complementary Josephson circuitry or proposed qubit concepts, using the unconventional order parameter symmetry of the high-T_c superconductor.     

Properties of YBa2Cu3O7−δ/YBa2CoxCu3−xOy/YBa2Cu3O7−δ Josephson edge junctions with 0.1 x 0.3 and the effect of flux flow on their normal resistance

YBa₂Cu₃O_7−δ (YBCO) based SNS edge junctions with cobalt doped YBCO barriers were prepared and characterized. At 77 K, good junctions had RSJ-like I–V curves with excess current, magnetic suppression of I_c of about 50% or more, and clear microwave steps. The conductance values 1/R_N at 77 K of junctions with different barrier thickness and composition, were proportional to the junction areas A, but show little correlation with the thickness of the barriers t_B in the range of 15t_B100nm. The corresponding I_cR_N products were observed to scale as J^0.66±0.09_c, similar to what was found by others. At the same time, the measured values of R_N are much smaller than what is expected based on the dimensions of the junction and the resistivity of the barrier material. To explain all of this, we propose a model in which at high supercurrent densities, flux flow of Josephson vortices in the junction leads to R_N values which are lower than expected. This model predicts

Full-size image (1K)

, which fits the observed results very well.     

Thermally activated flux dissipation in c-axis-oriented YBa2Cu3O7−δ ultrathin films

The resistive transitions of ultrathin YBa₂Cu₃O_7−δ (YBCO) films with thicknesses 75 and 200 Å were studied under magnetic fields. For the 75 Å film under a 5 T parallel magnetic field (Hbab-plane), no broadening of the resistive transition occurred. In the perpendicular magnetic field (H ab-plane), the broadening of the resistive transition of the 75 Å film is larger than that of the 200 Å thick film. The flux activation energy U was found to be linearly dependent on the temperature and logarithmically dependent on the magnetic field for both 75 and 200 Å films, which means the two samples have a two-dimensional vortex lattice. Furthermore, the activation energy U also increased with the film thickness, indicating that the magnetic correlation length in the c-axis direction l_c is larger than the 200 Å for bulk YBCO.     

Temperature pulse induced thermal instability in Y1Ba2Cu3O7−δ superconductors

As a common effect of a large and variable temperature gradient and a temperature pulse, a thermal instability of a new type has been observed in high-T_c superconductors. The primary effect is the temporal change of local energy density, that causes a dynamic state of simultaneously coexisting normal and superconducting domains, establishing a spatial and temporal variation of superconducting and normal states. The fluctuation of the ratio of the superconducting and normal volume fractions can be detected by the oscillation of DC and AC voltages measured on the samples. Supposing that every jump of the AC voltage represents the transformation of a single coherent domain, it leads to the average value of the linear dimension of domains as 2.1 mm, characteristic time of the transformation as 7.5 s and velocity of the propagation of this effect as 0.3 mm/s in the case of Y₁Ba₂Cu₃O_7−δ specimens. The macroscopic consequence of the thermal disturbance introduced into the sample is the appearance of a giant flux creep. Its existence is confirmed by comparison of our measurements to others, by analytical and numerical calculations of the forces acting on the vortices and by evaluation of the effect of the huge fluctuation created in the samples by our experiments.     

Low field microwave absorption of YBa2Cu3O7−δ thin films

The modulated microwave absorption in YBa₂Cu₃O_7−δ thin films was studied as a function of temperature, modulation amplitude, and microwave power. The comparative nature of weak links in YBaCuO thin films, ceramics, and powders is discussed.