High-Frequency Fluctuational Conductivity in YBa2Cu3O7−δ

We present measurements of the fluctuational excess conductivity at 48 GHz and 24 GHz in YBa₂Cu₃O_7– films above the transition temperature. The measurements depart from the gaussian prediction for finite-frequency fluctuational conductivity. We focus on the region not too close to T _c, where the real part of the excess conductivity drops much faster than the prediction of the gaussian model. We calculate the dynamic excess conductivity within a Ginzburg–Landau approach. In the calculation we insert a short-wavelength cutoff of the order of the inverse coherence length, in order to suppress high-momentum modes. The excess conductivity of all samples measured can be described very well by the modified model.     

Field Dependent Microwave Resistivity in YBa2Cu3O7−δ

We investigate the effects of directional pinning due to the layered structure and to columnar defects on the microwave response in YBa₂Cu₃O_7– films. We present measurements of the field-induced microwave resistivity at 48 GHz and 21 GHz taken in various relative orientations between the microwave currents, dc magnetic field and (a, b) planes. From measurements taken in the Lorentz-force-free configuration we experimentally show the relevance of the magnetic field induced increase of quasiparticle (qp) density. We identify the vortex motion contribution, and extract the vortex parameters. We estimate a pinning frequency of order 30 GHz when the field is aligned to the (a, b) plane. Secondly, we show that the introduction of columnar defects gives rise to a strong pinning along the columns, detectable even at 48 GHz. The pinning frequency appears to be of the same magnitude than for pinning by the layered structure.     

Analysis of deformed YBa2Cu3O7−δ

In this study, polycrystalline YBa₂Cu₃O_7– (123) was deformed under controlled conditions with a confining pressure of 1.0GPa, temperatures of 25, 500 and 800° C, and a strain rate of 10^–4 sec^–1 in order to ascertain the micromechanisms of deformation that give rise to the macroscopic plastic behaviour. The deformed material was analysed using optical microscopy, transmission electron microscopy (TEM), and a SQUID magnetometer to study the effects of deformation on the microstructure of YBa₂Cu₃O_7– and how changes in the microstructure affected the superconducting properties. The results of these preliminary experiments suggest that the 123 material will be very difficult to deform plastically as slip occurs only on the (001) plane. The lack of multiple slip systems implies that this material will show some brittle behaviour up to a very high homologous temperature. Even when plastic behaviour can be sustained for high strains it may require high annealing temperatures to remove lattice imperfections which impede the superconducting currents. Densification by high pressure deformation may make reoxygenation difficult due to the reduced diffusion rates between the grains. These factors combined suggest that traditional fabrication techniques are not applicable to the 123 material. More work needs to be carried out to determine how annealing affects the microstructures of deformed materials and how these changes in microstructure affect the superconducting properties of these materials.     

Superconductivity Enhancement in Fe3O4 Doped YBa2Cu3O7−δ

Flux pinning can lead to the enhancement of J _c in superconductor. In order to study the effect of magnetic particles on superconductivity, the composites of (YBa₂Cu₃O_7?δ )_0.98(Fe₃O₄)_0.02 and (YBa₂Cu₃O_7?δ )_0.98(α-Fe₂O₃)_0.02 are synthesized with former sintered at different temperatures. The field-cooling measurements show that the T _c of the samples has not changed. The superconducting properties are enhanced when the samples are sintered at high temperature (750 ^°C), but suppressed for the samples without sintering or sintered at low temperature (350 ^°C). Scanning electron microscopy images show that the doped Fe₃O₄ are diffused into intergrain sites. With the increase of sintering temperatures, the grain size of the sample grows up and the flux pinning, which leads to the enhancement of J _c is formed by the addition of Fe₃O₄ nanoparticles. The extended X-ray absorption fine structure and X-ray absorption near edge structure experiments prove that after sintered at high temperatures the addition of Fe₃O₄ can induce disorder of local structures of YBa₂Cu₃O_7?δ . Also, the transition from the tetragonal phase to the orthorhombic phase is observed by the X-ray diffraction spectra. By the above analysis of experimental data, the enhancement of diamagnetic signal, which represents the superconducting properties can be attributed to the disorder enhancement of local structures and structural phase transition of the cuprates induced by the addition of Fe₃O₄.     

Investigations of the copper isotope effect in oxygen-deficient YBa2Cu3O7−δ

We present data on the copper isotope effect (⁶³Cu-⁶⁵Cu), C_u =-nT_c/nm_Cu, for two isotopic pairs of oxygen-deficient YBa₂Cu₃O_7–, where varies between 0.06 and 0.52. _Cu is below 0.01 at =0.06 (fully oxygenated), it takes values between –0.14 and –0.34 in the 60 K plateau. Larger negative values of _Cu are observed away from the plateau. The dependence of _Cu is similar to that of the pressure effect dnT_c/dP.     

Long range coupling in YBa2Cu3O7−δ/Y1−xPrxBa2Cu3O7−δ multilayers

YBa₂Cu₃O_7–/(Y_1–xPr_x)Ba₂Cu₃O_7– multilayers have been used to probe coupling through (Y_1–x:Pr_x)Ba₂Cu₃O_7– alloys. We observe that the coupling between ultrathin YBa₂Cu₃O_7– layers, 12 or 24 Å thick, survives through several hundred Å of (Y_1–xPr_x)Ba₂Cu₃O_7– with x=0.4 and 0.55. T_c versus the thickness of the spacer-alloy, and activation energies for flux motion, with fields parallel and perpendicular to the c-axis, have been used to probe this long range coupling. All these experiments point to an unusually large coupling length for these two alloy compositions. In the x=0.55 case this result is particularly surprising since the alloy material display a semiconducting behaviour for this composition. T_c measurements, activation energies, and a study of the vortex dynamics in these coupled multilayers is presented along with new results obtained on a series of multilayers built with a more insulating alloy, x=0.7.     

The superconducting state of YBa2Cu3O7−δ

Many low temperature properties of high T_c superconductors deviate significantly from the detailed predictions of BCS theory. Here we discuss whether these effects could be caused by either: (a) an unconventional pairing state, or (b) local randomness in the gap function due to the intrinsic disorder. We review recent experiments pertinent to these questions: Josephson effects in (001) oriented planar junctions between YBa₂Cu₃O₇- and classic superconductors and the temperature dependence of the a-b plane electro-magnetic penetration depth at low temperatures. We also calculate the density of states of s-wave superconductors with local quenched disorder in the gap function so as to determine whether s-wave pairing could be consistent with the low energy quasiparticle excitations seen in many experiments.     

Spiral growth mechanisms in partially melted bulk YBa2Cu3O7−δ

Large domains with platelets almost parallel to each other were obtained in bulk YBa₂Cu₃O_7– by a single-step partial melting procedure. The mechanisms of nucleation and growth of platelets are discussed. The nucleation of peritectic material from the liquid phase is favoured by heterogeneities in the melt. Experimental evidence of spiral growth of the nuclei in the [0 0 1] direction is given. Furthermore, structures of growth, which could also be an indication of spiral growth in the [0 1 0]/[1 0 0] directions, are shown. The final morphology of the domains can be explained on the basis of the periodic bond chain (PBC) theory if the growth rates of flat (F) faces of the platelets are dominated by kinetic coefficients which differ between them. The morphology of the as-grown (0 0 1) surface is explained in the framework of the PBC theory with the shape of the steps of macrospirals governed by the transition from roughness to smoothness of the liquid-solid interface. An account of large step heights is given by the model of giant screw dislocations caused by an impurity-induced lattice-constant gradient. Even higher step heights are correlated to the presence of obstacles and lack of liquid phase.     

The Critical Role of the Cuo Chains in Metallizing the C-axis in YBa2Cu3O7−δ

We report measurements of the normal state c-axis magnetoresistance (MR) of untwinned single crystals of slightly overdoped YBa ₂ Cu ₃ O _7– . An orbital contribution to the transverse MR, a signature of coherent c- axis motion, arises only from the component of the magnetic field perpendicular to the CuO chains. This result implies that the CuO chains are primarily responsible for coherent transport along the c-axis in YBa ₂ Cu ₃ O _7– and that those regions of the CuO ₂ planes that do not involve the chains remain two-dimensional, even in the absence of a normal state gap.     

Scanning tunneling spectroscopy studies on YBa2Cu3O7−δ

Scanning Tunneling Spectroscopy (STS) measurements have been carried out on the high T_c superconductor (HTS) YBa₂Cu₃O_7– (YBCO). Using very high quality single crystals as well as hard tips, we obtained differential conductance spectra which showed various conductance features in agreement with other spectroscopic techniques. The reproducibility of our spectra allowed us to map for the first time the vortex lattice on a HTS using a STS technique¹. The vortices are arranged in a short range order oblique lattice, while the cores of the flux lines show an anisotropic shape. These observations can be related to the in-plane anisotropy of YBCO.     

Anisotropy of the non-equilibrium quasiparticle dynamics in single crystals of YBa2Cu3O7−δ

We report for the first time, measurements of ultrafast transients from YBCO in which the contributions from a- and b-axis response are separately identified using polarised light. A new signal is observed for Ea which has been obscured in all previous thin film experiments. Comparing the two contributions we find the new signal to be sensitive to T _c whilst the signal for Eb exhibits pseudogap behaviour.     

Novel approaches in the melt-texturing of YBa2Cu3O7−y

Electrochemically pre-textured samples have been subjected to melt-processing in order to produce dense and highly textured bulk samples of YBa₂Cu₃O_7–y (YBCO). Full oxygenation of these samples has been achieved at high processing temperatures of 720 °C by electrochemical titration, in order to increase the superconducting transition temperature to > 89 K. These samples show a large magnetic hysteresis, and the value of J _c calculated using the Bean model is in the range 4000–6000 A cm^–2 at 77 K and 1 tesla magnetic field, and is independent of the applied field in that range. In another variation of the melt-processing technique — referred to as isothermal melt-textured growth — highly textured samples have been produced by the movement of the solidification front at a constant temperature in an oxygen activity gradient.     

Disordered Vortex Phases in YBa2Cu3O7?δ

The disordered vortex phases induced by line and point pinning in YBa₂Cu₃O_7– are explored. At high defect densities, only a single disordered solid separated from the liquid phase by a melting line is observed. At low defect densities the topology of the phase diagram changes dramatically, with a vortex lattice phase adjoining disordered phases at high or low field. Critical points at the termination of the first-order melting line separate the lattice and the disordered phases. The line defect disordered phases follow the expected Bose glass behavior, while the point defect disordered phases do not exhibit the expected vortex glass behavior.     

Revisiting Heat Capacity of Bulk Polycrystalline YBa2Cu3O7−δ

In this letter, we present the superconducting property characterization of a phase pure, reasonably good quality YBa₂Cu₃O_7?δ sample. Studied compound is crystallized in orthorhombic Pmmm space group with lattice parameters a, b, and c being 3.829(2), 3.887(1) and 11.666(3) Å, respectively. Bulk superconductivity is observed below 90 K as evidenced by resistivity and dc/ac magnetization measurements. The resistivity under magnetic field (ρTH) measurements showed clearly both the intra-grain and inter-grain transitions, which are supplemented by detailed (of varying frequency and amplitude) ac susceptibility studies as well. The upper critical field at 0 K, i.e. H _c2(0) being determined from ρTH measurements with 50 % criteria of resistivity drop, is ～70 T. Studied polycrystalline YBa₂Cu₃O_7?δ is subjected to detailed heat capacity (C _P ) studies. C _P exhibited well-defined anomaly at below 90 K, which decreases with applied field. Although the C _P anomaly/peak at T _c reduces with applied field, the same is not completely suppressed in high applied fields of up to 12 T. The Sommerfeld constant (γ) and Debye temperature (Θ _D), as determined from low temperature fitting of C _P (T) data to Sommerfeld–Debye model, are 10.65 mJ/mole-K² and 312.3 K, respectively. The results are compared with existing literature on bulk polycrystalline superconducting YBa₂Cu₃O_7?δ sample.     

SmBa2Cu3O6.5 seed fabrication for seeded peritectic solidification of YBa2Cu3O7−δ

The availability of high quality SmBa2Cu3O6.5 seeds of known orientation is essential for the fabrication of a large grain YBa2Cu3O7– (YBCO) superconductor via a seeded peritectic solidification technique. The suitable seed must have a close lattice parameter match to YBCO and a relatively high melting temperature. We report a melt texturing process for the fabrication of SmBa2Cu3O7– (Sm-123) seeds using SrTiO3 to control grain nucleation. The physical and structural properties of the seeds were confirmed by differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). Seeds prepared in this study were used to grow large single grain YBCO superconductors of up to 2 cm in diameter.     

Growth and processing of single crystal superconducting YBa2Cu3O7−δ

A method for growing millimeter-sized crystals of YBa₂Cu₃O_7– is described. Processing conditions were developed to improve the superconducting transition temperature and sharpness, as observed by measurements of the dc magnetic susceptibility. Some of our crystals have been used in observations of the Raman Effect, specific heat, nuclear magnetic resonance, and nuclear quadrupole resonance, with results indicating that the samples are excellent.     

Melting of the flux line lattice observed by specific heat experiments in YBa2Cu3O7−δ

High resolution adiabatic specific heat experiments on YBa₂Cu₃O₇– (00.05) are performed in magnetic fields from 0 to 14 T (Bc and Bc). In a 0.3 gram, twinned crystal with strong pinning, a step is consistently observed at the melting temperature T_m of the vortex solid up to a critical point that depends on . The field B_m and step temperature T_m obey the relation B_m=B_mo()(1–T_m/Tc)^4/3. The anisotropy of B_m and that of the upper critical field B_c2 are found to be equal. Alternatively, in a 18 mg, twinned crystal of high purity with low pinning, first-order-like specific heat peaks are observed on the melting line from 8 to 14 T. The entropy under these peaks is 0.5 kB /vortex/bilayer. These characteristic features are attributed to the melting of a vortex glass in the former case and that of a vortex lattice in the latter case.     

Hole Content Dependent Fluctuation Diamagnetism in YBa2Cu3O7−δ: Possible Role of the Pseudogap

Journal of Superconductivity and Novel Magnetism - This study focuses on the temperature and hole content dependent fluctuation diamagnetism of hole-doped YBa2Cu3O7?δ (Y123) high-Tc...     

Sintering of YBa2Cu3O7−x compacts

Heating of YBa₂Cu₃O_7−x compacts above about 930°C is shown to induce liquid formation. Presence of the liquid phase results in excellent densification, but limited superconducting properties. Sintering below 930°C occurs primarily by solid-state diffusion. Although the density of these samples is low, the superconducting properties are similar to those of the dense materials produced via liquid-phase sintering. The highest current densities (≈ 500 A/cm²) have been obtained in these solid-state sintered samples.     

Mechanism of high-temperature superconductivity in YBa2Cu3O7−δ: Crucial role of oxygen

Oxygen atoms in the middle CuO layer of YBa₂Cu₃O₇ consisting of strings of corner-connected (CuO₄)_∞ units are shown to be crucial for superconductivity. Importance of hole-hole pairing giving rise to OO bonds is also indicated.