Temperature dependence of the critical current density in Y-Ba-Cu-O thin films

We present a study of the temperature dependence of the critical currentJ _c of several dc magnetron sputtered thin Y-Ba-Cu-O films on single crystalline SrTiO₃, ZrO₂ and Al₂O₃ substrates. Near the critical temperature T_c it is found thatJ _c(1–T/T_c)ⁿ withn=3 for the SrTiO₃ and ZrO₂ substrates, whilen=1·3 for the Al₂O₃ substrate. The temperature dependence in our samples approximately agrees with standard theories for weak links or with the Ambegaokar-Baratoff equation.     

Angular dependence of the critical current density and the temperature scaling of the flux pinning force in YBCO thin film

In this paper, the critical current J_c(Θ) have been investigated as a function of magnetic-field angle Θ. Θ is the angle between the c-axis and the applied magnetic field direction. This investigation concerned three temperature values (60?K, 78?K and 81?K). The normalized pinning force f_p versus the normalized magnetic field h was also studied (f_p?=?F_p / F_pmax and h?=?H / H_max). The F_p expression was determined based on the Kramer model.The studied sample was a single crystal of YBaCuO thin film deposited by the ablation laser method on the surface (100) of a SrTiO₃ substrate.The results of this work show the existence of point core pinning of the normal centers in the low field regime and the occurrence of the flux creep in high field regime.     

Critical current measurement on Y-Ba-Cu-O

We report critical current measurements on sintered Y_0.35Ba_0.65CuO_y. The sample, in the perovskite phase, shows zero resistance at 87 K. The critical current transition is seen, in zero field and at 77 K, at a current densityJ _c of 50 A/cm².     

Anomalous temperature dependence of resistance observed for high TC Y-Ba-Cu-O thin films

Anomalies in the resistance versus temperature plots of some Y-Ba-Cu-O sputtered thin films have been observed. Compositional variations over some portion of the film are found responsible for the appearance of the anomalies. The need for exercising caution in the interpretation of such anomalies is emphasized.     

Transient creep of Cd single crystals at room temperature

Czechoslovak Journal of Physics -     

Demonstration of the density dependence of x-ray flux in a laser-driven hohlraum

Experiments have been conducted using laser-driven cylindrical hohlraums whose walls are machined from Ta2O5 foams of 100 mg/cc and 4 g/cc densities. Measurements of the radiation temperature demonstrate that the lower density walls produce higher radiation temperatures than the high density walls. This is the first experimental demonstration of the prediction that this would occur [M. D. Rosen and J. H. Hammer, Phys. Rev. E 72, 056403 (2005)10.1103/PhysRevE.72.056403]. For high density walls, the radiation front propagates subsonically, and part of the absorbed energy is wasted by the flow kinetic energy. For the lower wall density, the front velocity is supersonic and can devote almost all of the absorbed energy to heating the wall.     

The dependence of current density on the electric field strength in homopolar semi-conductors

The distribution function is derived for conduction electrons in a semi-conductor under the effect of a homogeneous electric field, on the basis of which one can explain the deviations from the validity of Ohm's law in strong electric fields or at low temperatures in homopolar semiconductors as well as the temperature dependence of the mobility.     

Effect of short- and long-range forces on the structure of water: temperature and density dependence

The individual effects of short-range and long-range forces on the structure of water, a prerequisite for developing a perturbation theory, are assessed using a decomposition of realistic water—water potential models into trial potentials. Computer simulations for one typical liquid density and a number of temperatures ranging from the freezing temperature up to supercritical ones, and for several densities on a supercritical isotherm were performed. The trial potentials were constructed from the ST2 and TIP4P potentials and it is shown that for both potentials the results are practically identical. It is shown that (i) regardless of the thermo-dynamic conditions and potential models used, the structure of water and the mutual orientational arrangement of water molecules, given by a set of site—site correlation functions, are determined nearly exclusively only by the short range forces, and (ii) for high density states the effect of the short range electrostatic part of the intermolecular potential on the spatial arrangement of the water molecules rapidly decreases with increasing temperature but does not become negligible.     

Critical current density and flux pinning in an unconventional superconductor

The functional dependence of the critical current density on magnetic field, Jc(H), observed at fixed temperatures in the unconventional type-II superconductor, LaAg1−cMnc (c=0.1,0.2,0.3) alloys, but not the relative magnitude of Jc in different alloy compositions at any given temperature and field, is adequately described by the exponential-decay critical state model. In accordance with the predictions of the Kramer's flux-pinning model, the peak value of the pinning force density with the exponent 1.7?m?2.8 and scales with h=H/Hc₂, where Hc₂ is the upper critical field. Irrespective of sample composition and temperature in the superconducting state, the pinning of the flux line lattice (FLL) dominates over the plastic FLL shear.     

Influence of a non-uniform current density profile on quasiparticle excitations and thermally activated flux creep in YBa2Cu3O7−σ

The transport current density, flowing radially from the center of a superconducting disk to its perimeter, in a so-called Corbino geometry, results in a double action on the vortex motion when the applied magnetic field is perpendicular to the disk’s plane. First, the depinned vortices are set into a nearly circular motion in the plane of the disk. Second, the non-uniform current density profile activates the intrinsic weak links, resulting in a non negligible proximity dominated quasiparticle contribution. In turn, these intrinsic junctions impede the circular motion of vortices giving rise to a proximity influenced thermally activated flux creep. This provides a simple technique to deconvolute the flux motion and the quasiparticle induced components of the total dissipative resistivity broadening below T _c . The case for a YBaCuO disk is hereby examined and measured in this context. It is shown that such a deconvolution can be made and theoretical laws for the field dependences are also obtained and confirm the data. Furthermore, the paraconductivity region just above T _c (B) appears to be dominated by the macroscopic fluctuations accompanying the vortex core motion.     

Macroscopic states induced in superconducting media by a transport current under flux creep

The physical features of the formation of macroscopic states of superconducting composites consisting of a superconductor and a coating under flux creep are discussed. It is demonstrated that there exist characteristic electric field strengths depending on the properties of the superconductor, cooling conditions, and characteristics of the stabilizing coating, which affect the intensity of the E-I characteristics of the superconducting composites. Analysis shows that the measurements of the critical properties of superconductors can be accompanied by a nonuniform electric field distribution over the composite cross section and high stable superheating of the superconductor, which do not lead to superconductivity breaking.     

Allowable overheat and limiting current in a superconducting composite with magnetic flux creep

Instability arising in a superconducting composite when the current is injected with an ultimately low rate is analyzed. Under these conditions, the nonuniformity of the temperature and electric field distributions over the composite cross section is negligibly small. Equations that allow one to estimate the effect of the magnetic flux creep on the maximum current and temperature of the superconductor before the onset of instability are derived. It is shown that the allowable overheat of the composite depends on conditions of its thermal stabilization especially near the steady-state stabilization range. It is noted that conditions for the existence of stable current states may differ when the magnetic flux creep is described by power and exponential I-V characteristics.     

High current density in light-emitting transistors of organic single crystals

We measured the external electroluminescence quantum efficiency (eta(ext)) in light-emitting field-effect transistors (LETs) made of organic single crystals and found that, in the ambipolar transport region, eta(ext) is not degraded up to several hundreds A/cm(2) current-density range, which is 2 orders of magnitude larger than that achieved in conventional organic light-emitting diodes. The present result indicates the single-crystal organic LET is a promising device structure that is free from various kinds of nonradiative losses such as exciton dissociation near electrodes and exciton annihilations.     

The anomalous temperature dependence of the protonic E mode in kdp-type crystals

The protonic E mode frequency in KDP type crystals is calculated employing the RPA method for an extended version of the Blinc-de Gennes model. Below T_c the frequency is found to have a strong temperature dependence.     

Excitation and temperature dependence of the blue luminescence of ZnS crystals

The blue luminescence of pure and Al-, Au-, Ag-, Cu-, and Mn-doped ZnS crystals has been studied in the temperature range between LNT and RT and for different wavelengths of the exciting UV-light. All investigated crystals emit two blue bands. The energies of the maxima of these emission bands are independent of the temperature. The temperature dependence of the intensity ratio of the two blue luminescence bands can simulate a temperature shift of the blue emission. The experimental results support the assumption of an essentially uniform structure of the luminescence complex responsible for the visible luminescence of pure and doped ZnS phosphors.