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1.
The recently reported superconducting YBa2Cu3Oy (Y123) foams are highly interesting and promising for variety of applications. In this report we present first magneto-transport measurements of the superconducting properties of these foams. The investigations reveal the superconducting properties being similar to those of bulk melt processed materials. The 123 foams reveal a Tc of 92 K and have a magnetization Jc of 40,000 A/cm2 at 77 K and 0 T. The measurements of magnetic hysteresis versus field show a high anisotropy of the critical current density up to Jcab/Jcc7.  相似文献   

2.
By using an ordinary electric balance, we are able to measure the magnetic levitation forces acting on a superconducting YBa2Cu3Oy (YBCO) single crystal. It is found that when the external magnetic field is parallel to the c-axis of the single crystal, the hysteretic levitation curve is similar to that of a melt-processed YBCO superconducting sample. However, when the external magnetic field is perpendicular to the c-axis of the YBCO single crystal, the levitation forces are too small to be measured by our equipment. Also, we have introduced a simple model with the Bean's approximations to explain the levitation forces. The critical current density derived from this model by fitting with experimental data is quite close to the value obtained from magnetization measurements.  相似文献   

3.
We have studied the microstructure and the magnetoresistivity of polycrystalline YBa2Cu3Oy (YBCO or Y-123 for brevity) embedded with nanoparticles of Y-deficient YBCO, generated by the planetary ball milling technique. Bulk samples were synthesized from a precursor YBCO powder, which was prepared from commercial high purity Y2O3, Ba2CO3 and CuO via a one-step annealing process in air at 950 °C. After planetary ball milling of the precursor, the powder was uniaxially pressed and subsequently annealed at 950 °C in air. Phase analysis by X-ray diffraction (XRD), granular structure examination by scanning electron microscopy (SEM), microstructure investigation by transmission electron microscopy (TEM) coupled with energy dispersive X-ray spectroscopy (EDXS) were carried out. TEM analyses show that nanoparticles of Y-deficient YBCO, generated by ball milling, are embedded in the superconducting matrix. Electrical resistance as a function of temperature, ρ(T), revealed that the zero resistance temperature, Tco, is 84.5 and 90 K for the milled and unmilled samples respectively. The milled ceramics exhibit a large magnetoresistance in weak magnetic fields at liquid nitrogen temperature. This attractive effect is of high significance as it makes these materials promising candidates for practical application in magnetic field sensor devices.  相似文献   

4.
Reflectance measurements with E c on a YBa2Cu3Ox single crystal where the oxygen content was varied between x = 6.8 and X = 6.98 are presented in the far infrared range (5 rmmeV < ω < 100 meV) at temperatures between 10 K and 300 K. Performing a Kramers-Kronig transformation we derived the optical conductivity δ c ( ω). An analysis of the spectral weight shows that the main contribution of the electronic transport is due to localized carriers. With decreasing the oxygen content, the spectral weight of free carriers decreases whereas the spectral weight of localized carriers is enhanced.  相似文献   

5.
The Ag diffusion in superconducting YBa2Cu3O7 (YBaCuO) ceramic has been studied over the duration of the diffusion range 5-24 h in the temperature range 700-850 °C by the energy-dispersive X-ray fluorescence (EDXRF) technique. For the excitation of silver atoms, an annular Am-241 radioisotope source (50 mCi) emitting 59.543 keV photons was used. The temperature dependences of silver diffusion coefficients in grains (D1) and over the grain boundaries in the range 700-850 °C (D2) are described by the relations D1=1.4×10−2 exp[−(1.18±0.10)/kT] and D2=3.1×10−4 exp[−(0.87±0.10)/kT].  相似文献   

6.
In access to optical spectroscopy of heavily underdoped detwinned YBa2Cu3Oy (YBCO) crystals, we re-examine the doping dependent evolution of infrared spectra in the CuO2 plane of underdoped YBCO in terms of two-component optical conductivity. The extended Drude model analysis is applied to the two-component conductivity, and the results are compared with experimental data in the pseudogap state. We demonstrate that a model consisting of a Drude and Lorentz oscillator components reproduces characteristics of infrared spectra in underdoped YBCO.  相似文献   

7.
在1064 nm波长脉冲激光(脉宽25 ps)的照射下,钙钛矿氧化物薄膜La0.67Ca0.33MnO3/SrTiO3具有超快光电效应,对激光脉冲显示ps量级的响应时间,上升沿响应时间300 ps,半高宽700 ps,同时,对激光能量的响应灵敏度为500 mV/mJ。  相似文献   

8.
In the present study, the effect of heat treatment conditions on the structural/microstructural, electrical and transport properties, such as thermoelectric power and thermal conductivity, of the Y-358 system was investigated. The Y-123 phase instead of Y-358 were obtained with heat treatments. XRD and SEM analyses showed that the best structural formation was obtained for the sample prepared at 900 °C for 24 h. All the samples exhibited the Tc value around 91 K. Positive S(T) value was obtained in all the samples. S(T) data was analyzed by “Two band model with linear T-term”. In thermal conductivity of the samples, small peak with broad maximum was found just below Tc, unlike that of the HTc cuprates.  相似文献   

9.
Both oxygen and calcium play important roles in inducing superconductivity in Y Ba2Cu3Oy (YBCO), which is an antiferromagnetic insulator at low O and Ca content. O induces superconductivity in Ca-free YBCO, while Ca does similarly in oxygen-deficient YBCO. For doping oxygen HgO was used as it decomposes at 476 °C into Hg, which escapes from the matrix leaving the crystal unaltered, and O, which provide a way to dope O in YBCO. Considering these facts, polycrystalline samples of Y 1−xCaxBa2Cu3Oy with x=0, 0.1 and 0.2 with and without HgO addition were prepared through a solid-state reaction method. The samples were sintered at 950 °C in open atmosphere. These synthesized samples were characterized through using the X-ray diffraction technique (XRD) for phase evaluation, scanning electron microscopy (SEM) for grain morphology, energy dispersive X-ray analysis (EDX) for compositional analysis and the four-contact measurement technique for determining the superconducting transition temperature.  相似文献   

10.
We report the results of a wide experimental study of the irreversible modulated microwave absorption as a function of temperature, applied magnetic field and modulation amplitude in the YBa2Cu3Ox single crystals. To analyze the experimental data the model of the microwave power dissipation by the flux lines has been developed taking into account thermal fluctuations, the distributions of currents and vortices over a sample. We have obtained the information about regimes of vortex motion in the different areas of the HT phase diagram and estimated the values of flux flow viscosity and critical current density.  相似文献   

11.
The magnetic moments parallel and perpendicular to thec-axis (measured simultaneously) have been studied as a function of direction of applied magnetic fieldH in twinned and de-twinned YBa2Cu3O7−δ single crystals. They show pronounced anomalies when the field direction approaches thec-axis. These allow clear identification of the angle ϕL at which vortices are locked into twin planes. Complete shielding of theH ab field component (transverse Meissner effect) was observed in the locked state. For larger angles, up to ϕT, the vortices continue to be non-collinear with the applied field, but their direction deviates from the trapping plane. ϕL exhibits a 1/H field-dependence, whereas ϕT shows only weak logarithmic variations withH.  相似文献   

12.
An investigation into the superconducting order parameter thermodynamic fluctuations and their manifestations on paraconductivity in cuprate superconductors is done using a renormalized Gaussian approach based on the Ginzburg–Landau theory. The temperature dependence of paraconductivity is affected by repulsive interactions between Cooper pairs and does not follow the universal power laws predicted by the conventional Aslamazov–Larkin theory. In addition to the well known Lawrence–Doniach crossover from three to two dimensions, we also highlight the crossover from one-dimensional to two-dimensional behavior and the crossover from weak two-dimensional to strong two-dimensional critical behavior in the vicinity of the critical temperature. These dimensional crossovers result from the resistance between Cooper pairs due to critical and thermal fluctuations which cause a transition from a metastable state to one with a smaller current. Two illustrative examples (the cases of YBa2Cu3O6.9 and Bi2Sr2CaCu30x compounds) are provided in support of the analysis, so as to demonstrate the usefulness of the approach.  相似文献   

13.
In a further study of the field-broadening of the resistive transition in high-Tc superconductors, resistivity measurements have been performed as a function of the direction of the applied field with respect to the crystallographic axes on a YBa2Cu3O7 single-crystalline sample for field and current directions within the ab-plane over the full range of field orientations. The angular modulation in resistivity is compared wit predictions of flux-creep and flux-flow theories. Magneto-resistivity measurements have been performed at specific angles of the applied field with respect to the crystallographic directions. Saturation of the Lorentz-force effect for perpendicular field and current directions is found for fields larger than a critical value that is strongly temperature dependent. A complex resistivity anomaly is observed for fields parallel to the twin boundary.  相似文献   

14.
XRD and residual surface stress (sin2 ψ) measurements were carried out on YBa2Cu3Ox superconductors with varying oxygen stoichiometry (6.3 < x < 7.0). Slopes of the surface strain versus sin2 ψ were plotted against oxygen content for certain reflections. Compressional surface stress has been found along the c-axis, while a tensile surface stress has been observed along the ab-plane. Both surface stresses were found to vary slightly with oxygen content. These findings qualitatively agree with a very small hydrostatic pressure effect on Tc for fully oxygenated YBa2Cu3Ox (x = 7) compared to oxygen deficient material at the surface.  相似文献   

15.
We have studied experimentally the in-plane fluctuation conductivity near the superconducting transition in single crystal samples of YBa2Cu3O7, Y0.98Ca0.02Ba2Cu3O7, YBa1.9Sr0.1Cu3O7 and YBa2Cu2.97Zn0.03O7. In order to test the stability of the observed fluctuation regimes, low magnetic fields were applied perpendicular to the Cu-O2 atomic planes. When the transition is approached from above we first observe a three-dimensional (3D) Gaussian regime then a crossover to a genuine critical region where the exponent is consistent with the predictions of the 3D-XY-E universality class. Decreasing further the temperature towards Tc, our results systematically reveal the occurrence of a regime beyond 3D-XY characterized by a very small critical exponent. We propose that this regime is precursory to a weak first-order superconducting transition driven by antiferromagnetic excitations related to the pseudogap phenomenon. The dilution of divalent impurities in YBa2Cu3O7 does not affect the stability of the fluctuation regime beyond 3D-XY and in the case of Ca doping a further approach towards the first-order behaviour is observed.  相似文献   

16.
To investigate the effect of grain boundaries on paraconductivity of YBa2Cu3Ox, melt-textured and c-axis oriented thin films with controlled grain boundaries (superconducting transition width, ΔT, varying between 0.54 and 2.85 K) were prepared, and dc-conductivity has been measured as a function of temperature. In the logarithmic plots of excess-conductivity (Δσ) and reduced temperature (?), starting from low values of ?, we have observed three different regions namely critical region, mean field region and short wave fluctuation region. A correlation is observed between the range of critical region and ΔT, which is found to increase with ΔT. While for ΔT values smaller than 2.5 K only static critical region is observed, for higher ΔTs both static and dynamic critical regions are observed. In the mean field region a crossover from 3D to 2D was observed for all the samples. At ? values larger than 0.24, the excess-conductivity decreased sharply as ?−3, which suggested the existence of the short wave fluctuations.  相似文献   

17.
We study the electric-field-driven transport of electrons in superconductors (CuxBi2Se3) by using the borrowed approach which has been successfully adopted to study the critical transport of other superconductors as well as supersolid helium in very low temperature environment. The critical temperatures related to the nearly frictionless transport of electrons were found to be directly relevant to the superconducting temperature of superconductors (CuxBi2Se3) after selecting specific activation energies.  相似文献   

18.
The even Hall effect and guided vortex motion were observed for the first time under the action of the unidirectional twins array in YBa2Cu3O7−δ single crystals. The twins act as planar barriers.  相似文献   

19.
Incorporation of Ag in the crystal lattice of Sb2Te3 creates structural defects that have a strong influence on the transport properties. Single crystals of Sb2−xAgxTe3 (x=0.0; 0.014; 0.018 and 0.022) were characterized by measurements of the temperature dependence of the electrical resistivity, Hall coefficient, Seebeck coefficient and thermal conductivity in the temperature range of 5-300 K. With an increasing content of Ag the electrical resistance, the Hall coefficient and the Seebeck coefficient all decrease. This implies that the incorporation of Ag atoms in the Sb2Te3 crystal structure results in an increasing concentration of holes. However, the doping efficiency of Ag appears to be only about 50% of the expected value. We explain this discrepancy by a model based on the interaction of Ag impurity with the native defects in the Sb2−xAgxTe3 crystal lattice. Defects have a particularly strong influence on the thermal conductivity. We analyze the temperature dependence of the lattice thermal conductivity in the context of the Debye model. Of the various phonon scattering contributions, the dominant influence of Ag incorporation in the crystal lattice of Sb2Te3 is revealed to be point-defect scattering where both the mass defect and elastic strain play a pivotal role.  相似文献   

20.
We prepared in-situ Au contacts on high-quality epitaxial YBa2Cu3O7 (YBCO) films. Very high specific contact resistivity values up to ∼10−2 Ω cm2 at 4.2 K were obtained on 12×5 μm2 contact areas. This resistivity value decreased by two orders of magnitude as the temperature was raised to room temperature. In the temperature range T<200 K, the contacts showed non-ohmic behavior suggesting the presence of a well-defined insulating native Y-Ba-Cu-O barrier between the two electrodes. The electrical transport in this barrier layer was analyzed in the limit of high temperatures and high voltages to follow Mott's variable-range hopping conduction mechanism with physically reasonable parameters describing the localized states in the barrier. The high-resistivity contacts were tested successfully in quasiparticles injection experiments where the critical current Ic of the YBCO microbridge could be strongly suppressed on injection of an additional current through the contact into the superconducting channel.  相似文献   

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