YBa2Cu3Ox Single Crystals Research Articles

Intergrowth Structures of YBA2Cu3Ox Single Crystals with Oxygen Inhomogeneity

AbstractThe diffraction pattern simulation of four YBa2Cu3Ox single crystals with inhomogeneous distribution was successfully carried out on the base of the theory of X‐ray diffraction by disordered interstratified structures. The specific feature of their two‐component irregular mixed‐layered structures is the total segregation of layers with various oxygen content that led to the formation of so‐called intergrowth structures. The concentration of admixture layers varied from 10 to 40% while oxygen content difference within the crystal ranged from 0.5 to 0.3. In one of the crystals the fluctutations of layer heights corresponded to oxygen variation from 6.2 to 7.0.

Influence of synthesis and postgrowth treatment conditions on the structural homogeneity of YBa2Cu3Ox single crystals

AbstractThe effect of synthesis and postgrowth treatment (by annealing at various temperatures and partial oxygen pressures) conditions on structural homogeneity of YBa2Cu3Ox single crystals has been studied. The obtaining of single crystals with both high oxygen content and high critical temperatures Tc has been optimized. The wide range of postgrowth treatment conditions proposed allows to receive single crystals with given oxygen content from 6.2 to 7.0.

Homogeneity of Superconductivity in YBa2Cu3Ox Single Crystals

We investigated the homogeneity of high-purity YBa2Cu3Ox (123) single crystals at different oxygen concentrations by means of the higher-harmonic susceptibility. The superconductivity of the onset temperature at 93.8 K was observed in the samples of the hypostoichiometric composition (x=6.87). Also, we succeeded in obtaining the 123 single crystals of the optimum stoichiometry (x=7.00) which showed a sharp single transition at 92.5 K corresponding to occurrence of the homogeneous superconductivity.

Reversible magnetization of radiation-disordered YBa2Cu3O7-x single crystals.

Single crystals of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ were exposed to fast neutron irradiation with fluences up to ${10}^{19}$ n/${\mathrm{cm}}^{2}$ (E>0.1 MeV). Ac and dc magnetization measurements were performed to study the influence of irradiation-induced defects on the intrinsic and extrinsic parameters of superconductors. It has been shown that the observed increase of the anisotropy of the lower critical field in irradiated samples is due to the suppression of the Bean-Livingston surface barrier. Drastic changes of intrinsic parameters of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ single crystals are interpreted within the framework of existing theoretical models.

Lock-in oscillations in magnetic hysteresis curves of YBa2Cu3O7-x single crystals.

We report the first direct experimental evidence of intrinsic pinning in hysteresis loops of untwinned YBa2Cu3O7-x single crystals, for magnetic fields perpendicular to the c axis. We have observed oscillations in the M-H loops that are well explained when one considers the layered structure of this compound. The oscillations are found to be periodic in H-1/2a and are induced by a succession of transitions between states for which the vortex lattice is commensurate with the crystal lattice periodicity in the direction perpendicular to the CuO2 planes. The following anisotropy parameters were extracted from the experimental results: Gamma cb=(mc/mb)1/2=6.3, Gamma ca=(mc/ma)1/2=5.6, and Gamma ab=(ma/mb)1/2=1.13.

Dissipation behaviour of YBa2Cu3Ox single crystals studied through magnetization and magnetization decay measurements

Hysteresis loop and magnetization decay measurements performed on a YBCO single crystal in the low temperature range (T<or=45 K) are presented. The hysteresis loops show the typical behaviour observed in such kind of sample with the so called 'fishtail effect'. The magnetization decays are analysed in terms of the collective creep theory and, in the explored temperature range, they indicate a very different behaviour of the sample at two relatively near magnetic field values ( mu 0 H=1.6 T and mu 0 H=3.4 T) where the measurements were performed. The results support the hypothesis of a structural variation of the density and kind of pinning centres induced by field and temperature. This hypothesis could also explain the behaviour of the electric field E versus the current density J measured by 'flux creep annealing'.

Growth mechanism and crystal habit of RBa2Cu3O7-x (R = Y, Tm, Gd) single crystals

High Tc superconducting single crystals of RBa2Cu3O7-x (R = Y, Tm, Gd) have been grown by flux method. The surface morphology of these crystals has been investigated by scanning electron microscopy (SEM). Spiral growth hillocks, about 106 cm-2, have been observed for the first time on (100)/(010) surfaces of YBa2Cu3O7-x (YBCO) crystals grown in Al2O3 crucibles. This indicates a spiral growth mechanism for (100/(010) faces, whereas a two-dimensional, layer-by-layer growth mechanism was observed on the (001) surfaces of YBa2Cu3O7-x and TmBa2Cu3O7-x crystals grown in a Pt crucible. The crystal habit, i.e., the ratio of width versus thickness, ranges from(10–12):1 to (4–5):1; this results is consistent with the PBC (Periodic bond chain) prediction. The density of defects in YBCO crystals is much lower than that in sputtered thin films and this may be a clue to why YBCO single crystals carry less current than thin films.

Growth of Large Isometric YBa2Cu3Ox Single Crystals from Coexisting Region of Solid with Melt in Y2O3 Crucibles

Large isometric single crystals of oxide superconductor YBa2Cu3Ox (YBCO) have been grown from a mixed region of solid Y2BaCuO5 and melt. The crystals possess sharp, shiny habits with a maximum thickness of about 7 mm along the c-axis. The best growth condition is that wherein the solvent composition is 7BaCuO2·11CuO which is close to the binary eutectic of BaCuO2-CuO, and the optimum concentration of the growth system is that wherein the ratio of YBCO to 7BaCuO2·11CuO is 64:36. The use of a Y2O3-crucible prevents contamination from containers during growth, and facilitates the synthesis of high-purity crystals. Temperature gradient and seed crystals are both effective for controlling nucleation. The annealed crystals show a sharp superconductive transition at 91 K. The temperature dependence of the resistivities is found to be metallic in the ab- as well as the c-direction. Sufficient diamagnetism is manifested as a sharp transition at Tc and a complete shielding effect below Tc.

Uniaxial pressure dependence of Tc of untwinned YBa2Cu3Ox single crystals for x=6.5–7

Abstract The dependence of Tc on uniaxial pressure dTc/dpi(i= a, b, c) is determined for untwinned YBa2Cu3Ox single crystals as a function of O-content using ultrahigh-resolution thermal-expansion data and the Ehrenfest relationship. The dTc/dpi values are found to be highly anisotropic and are likely due to several different effects. For uniaxial-pressure applied parallel to the c-axis, we find a strong O-content (or carrier density) dependence of the dTc/dpb values, which have opposite signs charge transfer model. This model does not, however, explain the large dTc/dpa and dTc/dpb values, which have opposite signs and do not show a strong O-content dependence except at x=6.7, where both dTc/dpa and dTc/dpb values increase sharply. The a, b effects are discussed in terms of the orthorhombic distortion.

Pinning in proton-irradiated and annealed YBa2Cu3O7-x single crystals.

Pinning in flux-grown ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{x}}$ single crystals is characterized with superconducting-quantum-interference-device magnetometry at 10 and 77 K. A wide variety of magnetic hysteresis behaviors are observed in crystals with different growth and oxygen annealing treatments, indicative of variable native pinning populations. These will be discussed in terms of the pinning forces (${\mathit{F}}_{\mathit{p}}$) for the native pins. Irradiation at a dose of ${10}^{16}$/${\mathrm{cm}}^{2}$ with 3.4-MeV protons was used to increase the population of pinning defects. Changes in the flux-pinning parameters after postirradiation thermal anneals are used to determine the thermal stability of the defects responsible for the enhanced pinning. Isochronal anneals, t=10 min, were able to achieve total recovery to the preirradiated state by 600 \ifmmode^\circ\else\textdegree\fi{}C. This annealing experiment revealed two stages of recovery for the enhanced pinning at 100 and 470 \ifmmode^\circ\else\textdegree\fi{}C, respectively. By calculating theoretical Frenkel pair recovery curves, we found that oxygen point defects recombine between 100 and 200 \ifmmode^\circ\else\textdegree\fi{}C, copper between 450 and 600 \ifmmode^\circ\else\textdegree\fi{}C, and barium between 800 and 900 \ifmmode^\circ\else\textdegree\fi{}C. The slowly diffusing yttrium was not considered. Since the annealing range of point defects is coincident with the experimental results, we conclude that oxygen and copper point defects are responsible for the excess pinning. Isothermal anneals, T=450 \ifmmode^\circ\else\textdegree\fi{}C, verified that two species enhance the pinning properties which correlated to the O and Cu point defect model.

Magneto-Seebeck and Nernst-effect measurements on YBa2Cu3O7-x single crystals.

We report on a detailed investigation of the magneto-Seebeck and Nernst effects in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{x}}$ single crystals. Measurements were performed by means of a sensitive ac technique for fields up to 5 T and temperatures varying from 70 to 100 K. Our experimental results are consistent with those obtained in epitaxial films. The data are interpreted using Caroli and Maki's microscopic theory. The transport line energy ${\mathit{U}}_{\mathrm{\ensuremath{\varphi}}}$ carried by each vortex is possibly dependent upon oxygen concentration.

High critical currents in peritectically grown YBa2Cu3O7-x single crystals

Large single crystals of YBa2Cu3O7-x are grown from ceramic pellets by a peritectic growth mechanism. SEM and microprobe analysis show a high defect density of Y2BaCuO5 precipitates and microcracks. The anisotropic magnetic properties of peritectic grown crystals are compared with those of more perfect eutectic crystals grown from the flux. Magnetization loops measured with a vibrating sample magnetometer are used for the estimation of the anisotropic critical current densities. Peritectic grown crystals show higher critical current densities with smaller anisotropy than eutectic grown crystals. These results are confirmed by torque magnetometer measurements.

A−b anisotropy properties of millimeter-sized monodomain YBa2Cu3O7-x single crystals

Millimeter-sized monodomain YBa2Cu3O7-x single crystals have been obtained both by growth from self-flux of the oxides (under the naturally occurring uniaxial strain in the solidifie d flux) as well as with a strain-annealing procedure. In the a−b plane, these crystals show a small Bcl-anisotropy and large pinning-anisotropies. At low temperatures the Bcl values are enhanced by surface pinning effects.

Specific heat of an untwinned YBa2Cu3O7-x crystal: crossover to critical fluctuations

The authors present results for the specific heat of an untwinned YBa2Cu3O7-x single crystal. The specific heat was measured with a resolution better than 50 mK between 85 K and 110 K. The divergence in the specific heat at Tc is extremely sharp with no indication of rounding. It is not possible to fit the divergence by simply adding Gaussian fluctuations to a BCS-like step, however, they find that the divergence does fit a logarithmic divergence very well over nearly three decades between 10-4< mod t mod <0.1. This can be interpreted either as providing evidence for local pair superconductivity or for a wide crossover region from Gaussian to critical fluctuations.

Influence of crystal defects on the irreversibility line of YBa2Cu3O7-x single crystals

High quality flux grown and defect enriched peritectically grown YBa2Cu3O7−x single crystals have been investigated by an ac-susceptibility technique. This method allows to determine an irreversibility line from the temperature and field dependence of the peaked imaginary part of the susceptibility, which is due to magnetic losses. For magnetic fieldsHac perpendicular to thec-axis of the crystal, the irreversibility line of the defect enriched crystal shows a shift to higher field values as compared to the perfect crystal, a sign that crystal defects like Y2BaCuO5(211)-precipitates and microcracks act as strong pinning centers. ForHac parallel to thec-axis no clear evidence for a stronger pinning is found. From these results we conclude that different pinning mechanisms are dominating at different field orientations.

Elastic constant and internal friction in YBa2Cu3Ox single crystal

The internal friction and Young's modulus in yttrium superconducting monocrystal and ceramics have been measured. There are no significant differences in internal friction and Young's modulus spectra of both specimens.

Microwave loss and oxygen annealing in YBa2Cu3Ox single crystals.

Reversible thermal annealing of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathrm{x}}$ single crystals in air has been studied by microwave loss measurements. Among the advantages of this contactless technique are the following: Crystals are kept in sealed capillaries throughout cycles of annealing and low-temperature measurement; features are resolved in loss versus temperature measurements that are not revealed by other techniques. Microwave loss is interpreted in terms of the Gittleman-Rosenblum model of fluxon motion in a viscous medium. Narrow temperature ranges are found for the formation of the 60- and 90-K phases. Optimal microwave response is obtained after but a few hours of annealing, indicating the importance of c-axis diffusion.

INTRINSIC GEOMETRICAL BOUNDARIES FOR THE CRITICAL CURRENTS INSIDE YBa2Cu3O7-x SINGLE CRYSTALS

Magnetization curves have been measured on YBa2Cu3O7-x single crystals of various size, and compared with other results published in the literature. The irreversible magnetization does not follow the scaling law predicted by the Bean model. We conclude that even the superconducting crystals contain normal barriers or weak links which limit the spatial scale of supercurrents.

Superstructure along the c axis in YBa2Cu3O7-x single crystals having Tc=92 K.

We have grown by flux methods $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ single crystals, having critical temperature ${T}_{c}=92$ K, and orthorhombic distortion not detectable either by x-ray or by electron diffraction (difference between $a$ and $b$ lattice constants smaller than 7 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}3}$ \AA{}). In such crystals, transmission-electron-microscopy investigations have shown the presence of a domain structure, and electron-diffraction measurements have revealed the occurrence of a new superstructure along the $c$ axis. This superstructure consists in a systematic crossing of the Cu-O chains in alternate (001) planes, which causes a doubling of the $c$ lattice parameter and a strong decrease of the orthorhombic distortion. The occurrence of this superstructure is probably induced by tiny dopant impurities, and it can explain the discrepancies existing among the reports by several authors about the amount of orthorhombic distortion of $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ single crystals.

Direct observation of the lattice of Abrikosov vortices in high-Tc superconductor YBa2Cu3Ox single crystals

Abstract The magnetic structure of the YBa2Cu3Ox single crystal has been investigated using the technique of decorating the sample with small ferromagnetic particles. The lattice of the Abrikosov vortices and the effects caused by pinning on the twin boundaries have been observed in both the electron and optical microscopes. The twin structure of the single crystals has been investigated using the polarized light in the optical microscope. The penetration depth for the magnetic field parallel to the c axis has been evaluated as ≲ 0.3 μm at 4.2 K for the YBa2Cu3Ox single crystal.