Thermal conductivity of high-Tc crystals: Dependence on magnitude and orientation of magnetic field, and effect of Zn doping

We present precise measurements of in-plane thermal conductivity for superconducting single crystals of YBa₂Cu₃O_7–x (YBCO) withT _c =92 and 60 K, Bi₂Sr₂CaCu₂O₈ (BSCCO), and of Zn-doped YBCO. Magnetization and thermal conductivity data obtained with the same 90-K YBCO crystal demonstrate a close relationship between the magnetic thermal resistivity and the internal magnetic fieldB in a superconductor in the mixed state. For all superconductors studied here, the magnetic thermal resistivity is a sublinear function of magnetic field. The origins of the nonlinearity are discussed.Angular dependences of the magnetic thermal resistivity have been shown to depart from the anisotropic 3D superconductor model and are in quantitative agreement with a quasi-2D model. Implications for spatial modulation of the order parameter are made.     

Excess Conductivity Studies in Zn0.95Mn0.05O and ZnO Added YBa2Cu3O y Superconductors

We report electrical conductivity fluctuation analyses on YBa₂Cu₃O _y (denoted as YBCO) granular samples added with nanosize ZnMnO (ZnMnO for brevity) and ZnO (30 nm) particles. Nanoparticles are added to the precursor powders during the final sintering cycle of a two-step preparation process. Phase analysis by X-ray diffraction and granular structure examination by transmission electron microscopy (TEM) were carried out. When ZnMnO and ZnO are added to the YBCO, the orthorhombic structure is maintained. TEM and energy dispersive X-ray spectroscopy analysis show the presence of inhomogeneities embedded in the superconducting matrix. The temperature dependence of electrical resistivity in zero magnetic field has been measured on free, 1 wt.% ZnMnO and 1 wt.% ZnO added samples and the effect of microscopic inhomogeneities in the paraconductivity region has been reported. Data about the dimensionality of the thermodynamic fluctuation are obtained by analyzing the excess of conductivity ???? as a function of the reduced temperature $\varepsilon =\ln(\frac{T}{T_{c}^{mf}}-1)$ on the basis of the Aslamazov?CLarkin theory. In the mean-field region a crossover from 3D to 2D was observed for each sample. 1D behavior of fluctuation conductivity was found at high temperatures (above the 2D regime) for nanoparticle added samples. The zero-temperature coherence length, the effective layer thickness of the two-dimensional system, the wire cross-sectional area for one-dimensional system, critical magnetic fields and critical current density are estimated. Superconducting parameters are affected by the nanoparticle additions.     

Inhomogeneity Induced Conductivity Fluctuation in YBa2Cu3O7−y /Cr2O3 Composite

Dimensional fluctuations of superconducting order parameters in YBa₂Cu₃O_7?y +xCr₂O₃ (x=0.0, 2.3, 4.4, and 6.1 wt.%) has been analyzed. SEM micrographs reveal the reduced grain size with the incorporation of magnetic Cr₂O₃ particles in the YBCO matrix. XRD graphs show the unchanged orthorhombic structure and improved oxygen ordering in the composite samples. With the increase of Cr₂O₃ wt.%, it is found that the superconducting transition temperatures determined from standard four-probe method decreases gradually. Excess conductivity fluctuation analysis using Aslamazov–Larkin model fitting reveals transition of two dominant regions (2D and 3D) above T _c . 2D to 3D crossover temperature, i.e., Lawerence–Doniach temperature that demarcates dimensional nature of fluctuation inside the grains is influenced by Cr₂O₃ incorporation in YBCO matrix. The decrease in Lawerence–Doniach temperature in the mean field region has been observed as a consequent dominance of the 3D region with an increase in wt.% of Cr₂O₃ in the composite.     

Simulated Hysteretic Loops for YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7</Subscript>

This paper presents modification for a model to retrace magnetic hysteresis loop and calculate hysteresis energy loss (HEL) in YBa₂Cu₃O₇ (YBCO). In comparison with reported experimental results, it is revealed that the modified model could retrace successfully magnetic hysteresis loop for YBCO. As an application of this model, the calculation formula of HEL has been included in this article. The results show also that HEL in YBCO increases by 14% with the addition of the magnetic NiW substrate. This leads to conclude that the type of substrate has an effect on HEL of YBCO. This modified model can help us to explain experimental results and expect the functioning of superconductor tapes.     

Intermittent Flux Penetration at Different Temperatures in?YBa2Cu3O7?x on NdGaO3 Substrates

In type-II superconductors, increasing applied magnetic field penetrates gradually in the form of magnetic vortices. It is of great interest to understand the dynamics of magnetic flux in different superconducting materials, as this phenomenon can severely limit the performance of superconductors in applications. YBa₂Cu₃O_7?x (YBCO) is an important high-temperature superconductor, but until recently, it has been hard to make wires from it due to misalignment of superconducting grains. A solution to this problem is to deposit YBCO on vicinal substrates to better align the grains. Some of these samples show a strongly intermittent flux penetration at low temperatures. In this work, we have studied flux penetration in YBCO deposited on a 14° vicinal substrate of NdGaO₃ (NGO) at different temperatures.     

Fluctuation Hall Conductivity Beyond Linear Response in Layered Superconductors Under a Magnetic Field

We calculate fluctuation Hall conductivity of a strongly type-II superconductor in strong electric fields by using the time dependent Ginzburg–Landau approach. Thermal fluctuations, represented by the Langevin white noise, are assumed to be strong enough to melt the Abrikosov vortex lattice created by the magnetic field into a moving vortex liquid. The layered structure of the superconductor is accounted for by means of the Lawrence–Doniach model. The nonlinear interaction term in dynamics is treated within self-consistent Gaussian approximation and we go beyond the often used lowest Landau level approximation to treat arbitrary magnetic fields. The results are compared to experimental data on high-T _c superconductor YBa₂Cu₃O_7?δ .     

Investigation and Comparing the Effects of CNTs and Ag Nanoparticles Doping on YBCO Superconductor Properties

In this work, we investigate and compare doping effects of Ag nanoparticles and carbon nanotubes (CNTs) on the properties of Y₁Ba₂Cu₃ O _7?δ (YBCO) high-temperature superconductor. The YBCO samples were prepared using sol-gel method and characterized by resistivity versus temperature (ρ–T), the electrical field versus current density (E–J), x-ray diffraction (XRD) and scanning electron microscope (SEM) analysis. The results show that the orthorhombic phase of superconductivity was formed for all the prepared samples. Also, we found that the crystalline size of the YBCO samples decreases from 62 to 33 nm by adding CNTs and Ag nanoparticles to the compound. The pinning energy, critical current density and critical temperature of the samples increase by adding CNTs and Ag nanoparticles to YBCO compound, but CNTs play a more effective role than Ag nanoparticles in this compound.     

AC Susceptibility Study of Superconducting YBa2Cu3O7:Ag x Bulk Composites (x=0.0–0.20): The Role of Intra and Intergranular Coupling

We report the effect of silver addition on superconducting performance of bulk YBCO (YBa₂Cu₃O₇) superconductor. All the studied samples are prepared by conventional solid-state reaction method. Rietveld fitted X-ray diffraction data confirmed the single phase formation for all the studied samples. Detailed AC susceptibility measurements as a function of driven AC amplitude (1 Oe–17 Oe) of these samples revealed the enhancement of grains coupling with increasing Ag content in YBCO + Ag _x composite system. 10 wt% Ag added YBCO superconductors exhibited the optimal intergranular coupling. The Scanning Electron Microscopy (SEM) observations indicate an increase in the grains connectivity in terms of narrow grain boundaries for doped samples. The average grain size is found to increase with Ag doping. It is concluded that limited addition of Ag in bulk YBCO superconductor significantly improves the grains coupling and, as a result, the optimal superconducting performance. YBCO + Ag composites could prove to be potential candidates for bulk superconducting applications of the studied high-T _c system.     

Reel-to-Reel PLD Fabrication of YBCO Coated Conductor by Single and Multi-coating Processes

YBa₂Cu₃O_7?δ (YBCO) coated conductors have been fabricated on CeO₂/YSZ/Y₂O₃ buffered Ni-5at%W tapes by pulsed laser deposition (PLD) using the reel-to-reel process. A multi-coating process for YBCO film was employed, and single and multi-coating methods for YBCO films are compared. X-ray diffraction texture measurements showed good in-plane and out-of-plane crystalline orientations for the YBCO films. Magnetic measurements were carried out on the samples, and the critical current density as a function of the magnetic field was investigated. The results showed that the superconducting properties of YBCO films fabricated by the multi-coating process were better than those prepared by the single coating process.     

Peculiarities of the current-voltage characteristics of a Josephson medium in a YBCO high-temperature superconductor

The influence of a weak magnetic field (H < 150 Oe) on the current-voltage (I-U) characteristic of a YBa₂Cu₃O_{7 ? x} (YBCO) high-temperature superconductor (HTSC) near the superconducting transition temperature has been studied. It is established that there exist narrow (<0.2 K) temperature regions where the I-U curve exhibits sharp bending for H < 30 Oe and the ohmic behavior changes to a quadratic dependence of the voltage on current in a region of several milliamperes. At higher temperatures, the I-U curve bending exhibits smearing. This behavior is observed at a temperature below that corresponding to a zero critical current. Above a certain current, the temperature and magnetic field exhibit equivalent effects on the I-U curve of YBCO. Experimental results are explained by a sharp decrease in the critical currents of intergranular Josephson junctions under the action of magnetic field and by the current-induced formation of uncoupled (with respect to the order parameter) superconducting grains. Characteristic currents for the transition of the intergranular Josephson medium into an incoherent state are determined and the first critical fields in YBCO are evaluated.     

Study of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">δ</Emphasis></Subscript> Superconductor/Graphene Oxide Composite

The aim of this research was to fabricate YBa₂Cu₃O_7?δ (YBCO) superconductor composite with graphene oxide nanosheets and to study the effect of the graphene oxide nanosheets on YBCO superconductor properties. For this purpose, the samples of pure superconductor and superconductor composite with 0.001, 0.01, and 0.1 wt.% graphene oxide were synthesized. First, graphite oxide was made by Hummer’s chemical method; after that, graphene oxide nanosheets were produced by bath-keeper ultrasonic. Then, different amounts of graphene oxide were added to the process of superconductor fabrication, which was made by solid-state reaction method. The samples were characterized and studied by Meissner effect test, XRD analysis, FESEM imaging, EDX measurement, and ac magnetic susceptibility. The critical current density (J_c) of samples was measured by four probes method. The results showed that by increasing the weight ratio of graphene oxide, J_c and T_c decrease.     

Effect of Ti Doping on Structural and Superconducting Property of YBa2Cu3O7−y High T c Superconductor

Dimensional fluctuations of superconducting order parameters in YBa₂(Cu_1?x Ti _x )₃O_7?y (x=0.01, 0.02, 0.04, 0.05) have been analyzed. SEM micrographs reveal the reduced grain size and the formation of TiO₂ nanowires covering over the grains of YBCO matrix. XRD graphs show the unchanged orthorhombic structure. With the increase of TiO₂%, it is found that the superconducting transition temperatures determined from standard four-probe method decrease gradually. Excess conductivity fluctuation analysis using the Aslamazov–Larkin model fitting reveals transition of two dominant regions (2D and 3D) above T _c . 2D to 3D crossover temperature, i.e., the Lawerence–Doniach temperature that demarcates dimensional nature of fluctuation inside the grains is influenced by Ti incorporation in YBCO matrix. The decrease in the Lawerence–Doniach temperature in the mean field region has been observed as a consequent dominance of 3D region with increase in Ti%.     

Temperature Response of BaZrO3-Inclusion to YBa2Cu3O7−δ Probed by Magneto-Transport Measurements

This paper reflects a simple and practically scalable technique, in which attempts have been made for preparation of fine BaZrO₃ (BZO) as defects in YBa₂Cu₃O_7?δ (YBCO) superconductor, to enhance superconducting properties. The texture growth of YBCO + xBZO (x=1.0,2.5,5.0 and 10.0 wt.%) composites has been made through solid-state reaction route by employing calcined YBCO and sub-micron sized powders of BZO. The phase formation, texture and grain alignment were analyzed using XRD and SEM. The magnetoresistivity data obtained as a function of temperature (T) in the tail region shows two key features: first an anomalous secondary peak at $T_{c_{\mathrm{on}2}}$ well below 92 K as a function of BZO content with varying magnetic fields, and secondly a drop in global resistivity transition temperature (T _c0) following the incorporation of excess BZO to the grain boundaries. Impurities appear to reduce $T_{c_{\mathrm{on}2}}$ greatly to the lower-temperature values as a function of BZO content and to influence the onset of $T_{c_{\mathrm{on}1}}$ in presence of varying magnetic fields. The findings of this investigation suggest the presence of multiple transition temperatures in the composites.     

A Novel All-Conductive Architecture on Biaxially Textured Metal Substrates for YBCO Coated Conductors

In order to avoid thermal destruction of the superconductor in case of overcurrent, and improve the engineering current density J _E of the tapes, a novel buffer architecture, i.e., conductive SrRuO₃/TiN, has been developed on biaxially textured Ni-5 at.%W substrate for YBa₂Cu₃O_7?x (YBCO) coated conductors using pulsed laser deposition. The conductive TiN served as the effective seed and barrier layer because of low diffusion of metal atoms in the TiN layer, and the epitaxial conductive SrRuO₃ with good lattice match with the YBCO material, and was used as the cap layer. The X-ray diffraction showed the TiN and SrRuO₃ films had a perfect c-axis orientation and very good out-of-plane and in-plane texture. Atomic force microscopy indicated a very flat and dense TiN film with a layer-by-layer growth mode, and the root mean square surface roughness (R _rms) of the SrRuO₃ was less than 5 nm over a 2 μm × 2 μm scan area. The current–voltage curves of SRO/TiN/NiW showed a good conductivity of 16.7 Ω at the applied voltage of zero.     

Direct Peritectic Growth of c-Axis Textured YBa2Cu3O x on a Flexible Metallic Substrate

Previous work in the development of YBa₂Cu₃O _x (YBCO) superconducting wires and tapes has been focused on the deposition of YBCO on buffered metallic substrates. Although such an approach has proved successful in terms of achieving grain texturing and high transport current density, critical issues involving continuous processing of long-length conductors and stabilization of the superconductor have not yet been entirely settled. We have developed a novel process, the so-called direct peritectic growth (DPG), in which textured YBCO thick films have been successfully deposited directly onto a silver alloy substrate. No buffer layer is employed in the film deposition process. The textured YBCO grains have been obtained through peritectic solidification over a wide range of temperatures and times. The substrate materials have not demonstrated any observable reaction with the YBCO melt at the maximum processing temperature near 1010°C. The transport J _c has reached a respectable value of 10⁴ A/cm² at 77 K and zero magnetic field. Based on the experimental results in this work, we show that the DPG method offers an effective alternative for the fabrication of long-length YBCO conductors. Also reported is a physical explanation of the texturing mechanism on the metal substrate.     

Coexistence of Superconductivity and Ferromagnetic Phases in YBa2Cu3O7−δ Nanoparticles

High-temperature superconductor YBa₂Cu₃O_7?δ (YBCO) nanopowders were synthesized by the citrate-gel route, which is a modification of the sol-gel method. The fine powders were calcinated at 860 and 900 °C. They were of small size, in the range of 30–35 nm. X-ray diffraction (XRD) patterns verified production of the orthorhombic superconducting phase in all samples. Measuring the magnetic properties of these nanoparticles at room temperature, via a vibrating sample magnetometer (VSM), indicated ferromagnetism behavior in the YBa₂Cu₃O_7?δ nanoparticles. As the size of the nanoparticles decreased, the magnetic saturation of all samples increased. The development of the ferromagnetism effect was attributed to the presence of surface oxygen vacancies that lead to electron redistribution on the different ions at the surface. Thus, in an innovative work, the produced samples were annealed at 700 °C for 5 h under 0.8–0.9 bar of air atmosphere. The results showed that a small increase in the nanoparticle size provided a dramatic increase of magnetic saturation in all samples. Thus, we can say that the annealing process at vacuum improves the ferromagnetic properties of YBCO nanoparticles.     

Analytical modeling of residual stresses in multilayered superconductor systems

Residual stresses-induced damages in multilayered films grown on technical substrates present a reliability issue for the fabrication and applications of multilayered superconductor systems. Using closed-form solutions for residual stresses in multilayered systems, specific results were calculated for residual stresses induced by the lattice and the thermal mismatches in the system of YBCO/CeO₂/YSZ/Y₂O₃ films on a Ni-5 W substrate. It was concluded that lattice mismatch-induced residual stresses must be relaxed by forming interfacial defects. Studies of residual thermal stresses showed the following. When the thickness of a film is negligible compared to the substrate, the changes of its properties modify the residual stresses in this film layer but have negligible effects on the residual stresses in other layers in the system. On the other hand, when the thickness of certain film layer is not negligible compared to the substrate, residual stresses in each layer can be controlled by adjusting the properties and thickness of this film layer. Finally, the effects of buffer layers on thermal stresses in YBa₂Cu₃O_7–x (YBCO) were addressed by using YBCO/LaMnO₃/homo-epi MgO/IBAD MgO/Y₂O₃/Al₂O₃ films on Hastelloy substrate as an example.     

The Effect of Sb and Pb Doping on the Critical Temperature of the Bi1.6Pb x Sb y Sr2Ca2Cu3O z Superconductor

In this paper, the effect of Sb and Pb elements doping on the Bi_1.6Pb _x Sb _y Sr₂Ca₂Cu₃O _z (BPSSCCO) superconductor was studied. The BPSSCCO superconductor was synthesized by a solid state reaction method. The superconductivity properties such as the critical temperature, magnetic susceptibility, and room temperature resistivity have been investigated. In order to study the effect of Sb and Pb elements on the room temperature resistivity of the samples, the V–I curves were measured. The resistivity of the Sb doped samples at room temperature increases when the content of Sb is increased. It is also decreased by increasing the annealing time. The results showed that the optimum condition was x=0.35, y=0.05, with an annealing time of 270 h.     

Anisotropic transport properties in a layered d+s-wave superconductor

Recent measurements of the penetration depth and optical conductivity in YBa₂Cu₃O_x (YBCO) single crystals suggest the possibility of an order parameter with d+s-wave symmetry. We present results from calculations of both the anisotropic low temperature penetration depth and the frequency-dependent conductivity for a layered superconductor with such a pairing symmetry. The system is modelled as a stack of weakly coupled, two dimensional superconducting planes, and the effect of non-magnetic impurity scattering in both Born and unitarity limits is examined.     

Optical study of localization in theab-plane conductivity of single crystals of YBa2Cu3O6.95 induced by ion damage

We studied the changes in the dc resistivity, Raman scattering, and IR conductivity of single crystals of YBa₂Cu₃O_6.95 induced by damage from low-energy He⁺ ion bombardment. It appears thatT _c , transport properties, and optical conductivity are strongly affected by modest irradiation doses whereas the chemical composition of the sample is not modified. Carrier localization is evidenced by the frequency dependence of the optical conductivity in the strongly damaged crystal and is used to explain the suppression of both the superfluid density andT _c upon irradiation. The temperature dependence of the superfluid density is in agreement with the theoretical predictions for ad-wave superconductor. We also show that the intrinsic residual losses in the FIR are dramatically reduced in the disordered crystal.