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.     

Normal State Resistivity of YBa2Cu3O7-δ /PrBa2Cu2.8Ga0.2O7-δ c-axis Superlattices : Evidence for an Underdoping Effect of YBa2Cu3O7-δ

We measured the ab-plane resistivity of c-axis YBa ₂ Cu ₃ O _7- /PrBa ₂ Cu _2.8 Ga _0.2 O ₇ superlattices grown by pulsed laser deposition, from T _C to 325 K. The normal-state resistivity and the fluctuation regime above T _C were studied for different thickness of YBa ₂ Cu ₃ O _7- and PrBa ₂ Cu _2.8 Ga _0.2 O _7- in the superlattice period. As the concentration of PrBa ₂ Cu _2.8 Ga _0.2 O _7- is raised, the T _C decreases and the resistivity increases. Evidence is given for an underdoping effect in the YBa ₂ Cu ₃ O _7- layers and our measurements are compared to similar measurements on oxygen-deficient YBa ₂ Cu ₃ O _7- thin films, grown on the same equipment. They are also compared to measurements on YBa ₂ Cu _3-x Co _x O _7- single crystals made by other groups. We present here the different measurements and investigate the mechanisms that could be held responsible for this depletion in charge carriers.     

Multicomponent Order Parameter in the YBa2Cu3O7−δ

We have analyzed the temperature dependencies of the superfluid density in YBa₂Cu₃O₇ along a- and b- crystallographic axes using the multicomponent order parameter for the superconducting gap. Estimated values of the gap components for the d-wave and the isotropic s-wave are Δ _d =29 meV and Δ _s =5 meV, correspondingly. Band structure parameters were taken accordingly ARPES and neutron scattering data.     

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.     

Improvement of the Superconducting Transport Properties of YBa2Cu3O7?δ by BaZrO3 Doping

We have doped the YBa₂Cu₃O_7– superconducting ceramics with BaZrO₃ up to 75wt.% and studied the changes of some physical properties. The most important finding is the enhancement of the critical current density, which has a maximum at around 5 wt.% doping level. Compared with the undoped samples, the critical current density is four times higher if the doping compound is introduced prior to the calcination treatment, or two times higher if BaZrO₃ is introduced prior to the sintering treatment. The dependence of the critical current density on the doping level is consistent with the dependence of the Seebeck coefficient. We also observed a very small decrease of the critical temperature. The real density of the samples decreases with increasing doping level as expected from the ratio between the theoretical densities of YBa₂Cu₃O_7– and BaZrO₃, the effect being important for doping levels higher than 25 wt.%.     

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.     

Superconducting YBa2Cu3O7?δ Thin Film Detectors for Picosecond THz Pulses

Ultra-fast THz detectors from superconducting YBa₂Cu₃O_7?δ (YBCO) thin films were developed to monitor picosecond THz pulses. YBCO thin films were optimized by the introduction of CeO₂ and PrBaCuO buffer layers. The transition temperature of 10 nm thick films reaches 79 K. A 15 nm thick YBCO microbridge (transition temperature—83 K, critical current density at 77 K—2.4 MA/cm²) embedded in a planar log-spiral antenna was used to detect pulsed THz radiation of the ANKA storage ring. First time resolved measurements of the multi-bunch filling pattern are presented.     

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.     

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.     

Current-Temperature Diagram of Resistive States in Long Superconducting YBa2Cu3O7 Strips

Using a sequence of stepped bias currents in the nanosecond range, we have studied the destruction of superconductivity in c-axis textured YBa₂Cu₃O₇ strips of various thickness and structure (single or multi-layers). Sufficiently far from T _c, vortex flow fades out and gradually gives way to localized dissipative structures, which can be interpreted either as Phase-Slip Centers (PSC) or normal Hot Spots (HS). A plot of the corresponding threshold currents in the current–temperature (I–T) plane indicates when each of them will occur, and how to switch over from one to the other, in a manner similar to that demonstrated for metallic materials (cf. Ladan et al., J. Low Temp. Phys. 153:103, 2008). The capability of some YBCO strips to support PSC’s at arbitrarily low temperatures escapes the common picture.     

Dip-Coating of YBa2Cu3O7?δ and YBa2Cu3O7?δ-Ag Films on Polycrystalline Rare-Earth Barium Niobate Substrates

Dip-coating and partial melting technique have been used to fabricate high quality superconducting YBa₂Cu₃O_7– and YBa₂Cu₃O_7–-Ag thick films with T _c(0)=92 K on polycrystalline REBa₂NbO₆ (RE=Pr, Nd, Sm, and Eu) substrates. The superconducting films showed excellent adhesion to the REBa₂NbO₆ substrate. The effect of Ag addition in YBa₂Cu₃O_7– on the current density, microstructure, and crystal orientation of the superconducting films developed on the above substrates have been discussed in detail. Dip-coating technique was found to be one of the easiest method for obtaining good quality superconducting YBa₂Cu₃O_7– thick films with thickness as low as 3 m even on polycrystalline substrates.     

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.     

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.     

Enhanced Critical Current Density of FeF2 Added YBa2Cu3O7?δ

The effect of nanosized FeF₂ (??120?nm) addition on the critical current density, J _c, of YBa₂Cu₃O_7??? was studied. YBa₂Cu₃O_7??? superconductor was prepared by the solid-state reaction method with addition of 0 to 0.10 wt% FeF₂ additions. The magnetization critical current density, J _cm (at 77?K), was estimated from the extended Bean model by using vibrating sample magnetometer (VSM) measurements. The transport critical current density, J _ct, was measured using the four-point probe method with the 1???V/cm criterion. The onset transition temperature, $T_{\mathrm{c}\mbox{-}\mathrm{onset}}$ showed a maximal value at 92?K for 0 to 0.03?wt% additions. J _c increased with FeF₂ addition up to 0.03?wt%. Addition of FeF₂ of more than 0.03?wt% suppressed $T_{\mathrm{c}\mbox{-}\mathrm{onset}}$ and J _c. The inter-grain current density was enhanced by two orders of magnitude on 0.03?wt% FeF₂ addition. In addition to the condensation energy associated with their core, the full vortex magnetic energy by addition of nanosized FeF₂ enhanced J _c in the system.     

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.     

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.     

Charge exchange and the “Plateau” effect in YBa2Cu3O7−y

The superconducting properties of YBa₂Cu₃O_7- y, (YBCO) depend strongly on the oxygen content, whereas the transition temperatureT _cremains relatively constant for oxygen content between 6.8 and 7.0 (the “Plateau” effect) this effect has attracted considerable attention [see, e.g., D. de Fontaineet al., Nature (London) 343, 544 (1990); H. Paulsenet al., Nature (London) 349, 594 (1991); J. Jorgensen,Phys. Today 44, No. 6, 34 (1991)]. In this letter we introduce a microscopic theory of this phenomenon.     

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.     

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.     

Analysis of the spatial distribution of Y2BaCuO5 inclusions in large-grain YBa2Cu3O7-δ

High values of critical current density, Jc, in large-grain superconducting melt-processed YBa2Cu3O7- (YBCO, Y-123) have been reported to correlate closely with the distribution of Y2BaCuO5 (Y-211) particles in the Y-123 phase matrix. Extensive image analysis of the homogeneity of the Y-211 particle distribution in a large-grain sample has been performed on high-resolution, secondary scanning electron micrographs of seeded melt-processed YBCO. The variation of key parameters, such as area fraction, number density and size along the a/b axis of the Y-123 grain has been investigated in detail as a function of distance from the centre of the seed. Both area fraction and number density of Y-211 particles are observed to increase continuously with distance, whereas the mean and standard deviation of the size distribution decrease slightly towards the grain boundary. This suggests that the increase in area fraction can be attributed to an increase in number density of the Y-211 particle distribution along the a/b axis of the specimen. The implications for Y-211 particle pushing, ripening and coalescence are discussed in the light of these results. © 1998 Chapman & Hall