Magnetization Relaxation of YBa2Cu3O7−x Single Crystals in High Magnetic and Electric Fields

Magnetic properties of high-T _c superconductor Yba ₂ Cu ₃ O _7–x and their dependencies on the magnetic field variation rate (from 2·10² up to 3·10⁴ T/s) have been investigated in pulsed magnetic fields up to 35 T, parallel to the ab plane of the crystals. A dependence of fast magnetization relaxation on the high electric field (from 10^–1 to 10 V/m) has been revealed and studied for the case of electric current directed along the c axis and for a vortex movement parallel to the ab plane. A pronounced peak effect accompanying such movement has been observed over a wide range of magnetic and electric fields including the flux creep to flux flow transition region. The peak effect origin is discussed in connection with this observation.     

Phase Transition of Josephson Vortices Under High Magnetic Fields up to 30 T in Heavily Overdoped YBa2Cu3O7−δ Single Crystals

We have measured the temperature and the magnetic-field dependences of the resistivity, ρ(T) and ρ(H), under magnetic fields parallel to the ab planes up to 30 T for fully oxidized YBa₂Cu₃O_7?δ single crystals. Above 11 T, ρ(T) showed a kink structure and successively a gradual transition toward the zero-resistivity. Below 9 T, the resistive transition was continuous. These results indicate the appearance of a slush regime.     

Fabrication of Micro-pipes of YBa2Cu3O7−δ Superconductor

Synthesis of micro-pipes of YBa₂Cu₃O_7−δ superconductor has been carried out for the first time by using chemical method. The shape, size and length of these micro-pipes depends upon dimensions of growth matrix and structure of geometry, i.e., circular, square, rectangular and hexagonal, etc., which could be uniformly coated with superconductor. The onset temperature of diamagnetism of the material is found to be 93 K, which is not shifted to lower values by the application of DC magnetic field; this is most likely due to large surface area provided by micro-pipes to the shielding currents. These micro-pipes behave as pinning centers, hence they can be used for the fabrication of devices that are capable of working in high magnetic fields.      

Electromagnetic properties of electrophoretic YBa2Cu3O7−δ films

In order to apply highT _c superconductivity to general microwave cavities and to shielding devices for quasistationary magnetic fields, we have developed an electrophoretic coating technique. This work reports about the continuation of our experiments. More than 60 samples of electrophoretic layers deposited on silver substrates were fabricated by systematically varying the processing parameters. The r.f. surface resistance measured at 21.5 GHz and 77 K at low excitation fields is found to be a sensitive measure of the quality of the samples and falls rapidly with increasing average grain size. Textured electrophoretic layers of large grain size show the smallest sensitivity ofR _s against the r.f. surface magnetic fieldH _s. The magnetic sensitivitydR _s /dH _s is found to be in direct proportion toR _s (77 K). After optimizing the sintering procedure the electrodes of the prototype cavity for a compact hydrogen maser were coated. The cavity was successfully operated at 1.42 GHz, and a surface resistance of 1 mΩ was achieved at 77 K. This compares to 4 mΩ for copper at the same temperature.     

An Approach of the Sintering YBa2Cu3O7−δ System

The abnormal glow discharge applied to the sintering process is a recent technique used for processing both metallic and ceramic materials. In this paper, we use the abnormal glow discharge as an alternative method for the sintering step of the YBa₂Cu₃O_7?δ superconductor material. The physical properties of the sintered samples by glow discharge were compared with the properties of sintered samples in resistive furnace, which is commonly used for production of high temperature superconductor ceramics. The structural analysis of the YBa₂Cu₃O_7?δ samples was carried out by the X-ray diffraction technique, microstructural analysis by Scanning Electron Microscopy and electrical resistivity by the four probes method (resistance measurements as a function of temperature). The experimental results permitted to establish the similar structure and morphology for all samples: produced by plasma and resistive furnace sintering. The superconducting behavior was corroborated for both sintering processes.     

Enhancement of Flux Pinning in YBa2Cu3O7−δ + Ag Nanocomposites

YBa₂Cu₃O_7?δ /Ag thin film is prepared through pulsed laser deposition. The presence of Ag brings about significant inter- and intragranular modification in the microstructure of the composites. The resistive broadening in the tail part is suggested to be associated with the link between the grains and to be extremely sensitive to magnetic fields. It is assumed that the zero resistance state at the T _c0 region of the above high temperature superconductor is governed by the excitations in the weak link network. Invoking Beans critical state model, enhancement in critical current density J _c is observed.     

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.     

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.     

Effect of Artificial Defects on Electric and Magnetic Transport Properties in YBa2Cu3O7−δ Thick Film

The effect of 200 MeV Ag ions on YBa₂Cu₃O_7?δ /5 wt.% Y₂O₃ composite thick films is studied. The structural deformity is analysed with X-ray diffraction showing reduced peak intensity. The decrease of transition temperature as a function of ion fluence has been observed from temperature-dependent resistivity and magnetization measurement. Fluctuation conductivity studied within the framework of Aslamazov–Larkin and Lawrence–Doniach theories fits well for 3D and 2D regimes with the appearance of critical region beyond 3D regime. Pseudogap temperature estimated above 100 K shifts to lower temperature zone as a function of ion doses. We report an enhancement of critical current density and flux pinning due to dual impact of swift heavy ion and Y₂O₃ inclusions at isothermal temperatures 40 K and 60 K.     

“Raman-Forbidden” Phonons in YBa2Cu3O7−x Crystals

We have thoroughly studied the additional modes in the phonon spectra of YBa ₂ Cu ₃ O _7–x single crystals in resonant conditions. We have investigated the fine structure of the forbidden Raman spectrum, its dependence on the oxygen concentration and temperature as well as its temporal behavior which reflects the photoinduced oxygen redistribution.     

Instability of the Critical State in NdBa2Cu3O7−δ Single Crystals

Instabilities of the critical state are observed in various NdBa ₂ Cu ₃ O _7– single crystals by means of direct magneto-optic (MO) imaging while warming up flux states containing vortices of opposite polarity. Using the same samples as in the MO investigations, we observe for the first time characteristic steps in the integral magnetization measured by means of a SQUID system, which allows a constant temperature sweep. The integral measurement technique enables the study of the instability phenomenon (turbulence) in a much wider range of temperatures and fields than the MO investigations, and also in the orientations H _a c and H _a c. As a result, we find turbulence not only in a narrow window of temperatures (65 T 80 K), but also in a window of applied negative fields ranging between 30 and 80 mT.     

Thermally Assisted Flux Creep of a Driven Vortex Lattice in Untwinned YBa2Cu3O7−δ Single Crystals

We have studied the driven vortex lattice in untwinned, clean YBa ₂ Cu ₃ O _7– single crystals, showing the first order (melting) transition T _m . At high enough driving currents (j 10 ³ A/cm ² , j j _c ) and temperatures T < T _m , a clear distinction is found between two different behaviours of the moving vortex lattice. The onset of dissipation is characterized by a noisy flux creep with a temperature independent activation energy U(j). At higher temperatures, the creep regime crosses over into a flux flow regime with linear resistivity. Apart from the dip in resistivity at T _m , associated with the peak effect and usually assigned to a softening of the shear modulus, the resistivity of the flowing flux lattice continuously extends into the vortex-liquid.     

Magnetoresistance of Superconducting Granules and Grain Boundaries in Ceramic YBa2Cu3O7−δ High-Temperature Superconductors in Weak Magnetic Fields

The objective of the work is to establish the contribution of superconductive granules and grain boundaries (weak links) in magnetoresistance ρ value of YBa₂Cu₃O_7?δ granular high-temperature superconductor (HTS) at T<T _c. The current-voltage characteristics (CVCs) $E(j)_{H_{\mathrm{ext}} = \mathrm{const}}$ of YBa₂Cu₃O_6.95 ceramic samples were measured in H _ext (0≤H _ext≤≈500 Oe) magnetic fields. The CVCs $E(j)_{H_{\mathrm{trap}} = \mathrm{const}}$ of the samples magnetized in H _treat magnetic fields were measured at H _ext=0. Based on the CVCs, $\rho(j)_{H_{\mathrm{ext}} = \mathrm{const}}$ , ρ(H _ext) _j=const and $\rho(j)_{H_{\mathrm{treat}} = \mathrm{const}}$ dependences were reestablished. The comparative analysis of $\rho(j)_{H_{\mathrm{ext}} = \mathrm{const}}$ and ρ(H _ext) _j=const dependences indicates the magnetic field redistribution between grain boundaries and superconductive granules influence on transport and galvanomagnetic properties of granular HTS. The superconductive grain magnetoresistance ρ _g was established to be significantly lower than ρ _J value of Josephson medium.     

Mutual inductance critical current measurements in oxygen-deficient YBa2Cu3O7−δ thin films

Comparative studies of a standard four-contact DC method and a contactless mutual inductance setup have been carried out on the temperature dependence of the critical current of YBa₂Cu₃O_7– (YBCO) thin films. The DC measurements are restricted to low currents due to contact heating, whereas the mutual inductance method works well at higher currents. The temperature dependence of the values obtained by the two methods agrees fairly well in the temperature window accessible by both of them. Hence the DC values can be used to calibrate the mutual inductance ones. Thus, the critical current can be determined in a larger temperature range as compared to simple DC measurements. We have applied both techniques to two films with oxygen deficiencies of 0.08 and 0.31. The obtained calibration factor allowed us to determine the critical current density of a third sample with =0.24 inductively. The critical current density drops dramatically with the oxygen content. At T/T_C=0.80, the critical current of the film with =0.08 is more than 40 times larger than that of the film with =0.31.     

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.     

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.     

Enhancement of Vortex Pinning in YBa2Cu3O7?δ -BaHfO3 Superconductor-Insulator System

A systematic study on phase evolution, bulk density, resistivity, critical transition temperature (T _C), critical current density (J _C) and in-field transport critical current density (J _C?CB) of pure and nano particles of BaHfO₃ added YBa₂Cu₃O_7??? (YBCO) samples of different weight percentages has been carried out. The result shows a significant improvement in the electrical transport properties of the superconductor, namely the J _C, J _C?CB characteristics and flux pinning force (F _p) of the BaHfO₃ added samples. The transition temperature also was found to increase marginally. A maximum transport J _C of 742?A/cm² at 77?K (for x=2?wt%) was observed for BaHfO₃ added samples against 147?A/cm² for pure YBCO sample. The enhancement of the critical current density in the YBCO?CBaHfO₃ sample is attributed to the formation of an insulating and non-reacting YBa₂HfO_5.5 phase, acting as artificial pinning centers in the matrix. The introduction of nano particles of BaHfO₃ in bulk YBCO increases the pinning force density from 30×10³?N/m³ to 424×10³?N/m³ at 77 K. The improvement in sintering density due to an optimum value 2?wt% BaHfO₃ addition in bulk YBCO indicates its potential use in electrical devices and technology. The enhancement in the superconducting properties, particularly in the J _C?CB characteristics due to BaHfO₃ addition, seems to have great technological significance.     

Autocorrelation study of quasiparticle relaxation in a YBa2Cu3O7-δ film

Using an autocorrelation method we studied the voltage photoresponse of a current-carrying superconducting YBa₂Cu₃O_7– film excited by femtosecond near infrared radiation pulses. Near the transition temperature we found a picosecond response time consistent with the energy relaxation of nonequilibrium quasiparticles. We also observed a subpicosecond component of the voltage response which we attributed to the current redistribution between excited quasiparticles and the superconducting condensate.     

Effect of Firing Temperature on the Structure and Properties of YBa2Cu3O7−x Films by TFA-MOD Method

YBa₂Cu₃O_7−x (YBCO) films were fabricated on LaAlO₃ (LAO) substrate under various firing temperatures (760–870 °C) in the crystallization process by metalorganic deposition (MOD) method using trifluoroacetates. The effect of firing temperature on the structure and properties of YBCO films was systematically investigated. According to the XRD and SEM images, the films fired at low temperature (760–800 °C) showed poor electrical performance due to rough surfaces and impurity phases. However, the films fired at 850 °C showed the highest critical temperature of 90 K and the highest J _c of 3.1 MA/cm² which attribute to the formation of a purer YBCO phase, fewer pores, and stronger biaxial texture.