1,721,038 research outputs found
Scaling in the angular dependence of the critical current and temperature-dependent anisotropy ratio in Bi2Sr2CaCu2O8
No full textWe present measurements of the critical current in an epitaxial Bi2Sr2CaCu2O8 film as a function of the magnetic field, the temperature, and of the angle theta between the field and the a, b planes. The orientational study reveals an increasing anisotropy in J(c) with increasing temperature. The isothermal curves J(c)(H) taken at various angles 0-degrees less-than-or-equal-to theta less-than-or-equal-to 90-degrees are found to collapse to a single curve when the magnetic field is normalized to a scaling function f(theta): J(c)(H,theta)=J(c)[H/f(theta)]. The best fit for f(theta) is obtained using an expression originally derived for H(c2)(T,theta) by Tinkham. The increase in J(c)(0-degrees)/J(c)(90-degrees) with increasing temperature is reflected by the increase in the anisotropy ratio f(0-degrees)/f(90-degrees), consistent with the quasi-two-dimensional Tinkham expression. Consistently, the same model quantitatively describes the increasing anisotropy with increasing temperature
Fluctuation-induced resistivity in YBCO across T-c from angular measurements in magnetic field.
No full textWe present various sets of resistive transitions in an YBCO thin film, as a function of the temperature, magnetic field and field orientation theta with respect to the (a,b) planes. We show that the resistivity undergoes an angular scaling only above a characteristic field-dependent temperature T-f(H), where T-f is below the mean-field transition temperature T-C(H). The scaling function found is that expected for a strongly coupled layered superconductor. It is shown that the interacting fluctuations theory describes with details the data for T>T-f(H), with a very reduced set of parameters. Comparison with data taken on untwinned samples reveals that the upper part of the transition is identical for films and single crystals, and that the same theory works well in both cases. In addition, Bardeen-Stephen flux flow is observed in the crystal only, and in a small region of H and T. We conclude that in a wide portion of the H-T plane the dissipation is due to intrinsic properties only (order parameter fluctuations), while only at lower temperatures the pinning becomes relevant, as supported by the breakdown of the angular scaling and by the fact that pure crystals and films appear to be different at low temperatures only
Excess conductivity of overdoped Bi2Sr2CaCu2O8+x crystals well above T-c
No full textWe have used a multiterminal technique in order to measure the (a,b) plane excess conductivity Delta sigma in several Bi2Sr2CaCu2O8+x single crystals. We find that the experimental Delta sigma does not follow a simple power law Delta sigma similar to epsilon (-alpha), with epsilon =ln(T/T-c), and that it drops faster than the two-dimensional Aslamazov-Larkin law, alpha = 1, with increasing temperature. In addition, data for samples with different doping do not scale on a universal curve. We discuss our data in terms of microscopic and Ginzburg-Landau theories, where high-momentum fluctuations are either not excited, or phenomenologically cut off. The experimental Delta sigma drops even faster than the prediction of the extended microscopic theory. However, we can accurately describe all our data up to T approximate to1.3 T-c with the GL theory, assuming a sample-dependent cutoff value. We relate the cutoff parameter to the doping level of our samples
CONSISTENT EXPLANATION ON LORENTZ FORCE-FREE DISSIPATION IN H-PARALLEL-TO-L-PARALLEL-TO-C FOR VARIOUS HIGH-TC SUPERCONDUCTORS - COMMENT
No full textResistivity tensor in the normal state of BSCCO single crystals as a function of doping
No full textWe present measurements of the resistivity tensor of Bi2Sr2CaCu2O8+delta single crystals with different oxygen concentrations. Sample doping varies from underdoped to slightly overdoped. Measurements are performed through multiterminal technique, which allows for a simultaneous determination of both in-plane and out-of-plane components of the resistivity tensor. Data are analyzed in terms of a model that assumes two different mechanisms for the out-of-plane conduction, markedly thermal activation and incoherent tunneling. Within this model we are able to describe data of normal state resistivity for all samples with different doping levels. We also analyze data from the literature. In all cases, the proposed model describes very well the data in the normal state
RESISTIVE LOSSES AT MICROWAVE-FREQUENCIES IN GRANULAR HIGH-TC SUPERCONDUCTORS - FLUX-CREEP, FLUX-FLOW AND JOSEPHSON-JUNCTIONS
No full textThe resistive losses in a magnetic field H for granular superconductors can be attributed to three different processes: flux-flow, flux-creep and dephasing of Josephson junctions (JJ). The microwave absorption measurements are able to separate the three contributions to the losses, if the ranges of the magnetic field and the temperature are suitably selected. A calculation of the surface-resistance, based on the hypothesis of validity of the Bean model and which qualitatively explains the experimental results is given
Experimental study of conductivity in Bi2Sr2CaCu2O8+x crystals by means of multiterminal measurements
No full textResistivity measurements have been demonstrated to be a powerful tool for the investigation of vortex dynamics. Multiterminal measurements have been widely used to analyze the correlation of Vortices across the planes (pseudo-Giaever experiment). We present a large set of measurements in different configurations for various magnetic field strengths and orientations, both in the normal state and in the mixed state. We will discuss a large class of experimental features relating them to the correlation between the CuO planes. (C) 2000 Elsevier Science B.V. All rights reserved
MAGNETIC DISSIPATION IN HIGH-TC SUPERCONDUCTORS - EVIDENCE FOR A VORTEX-GLASS-TO-VORTEX-LIQUID TRANSITION
No full textThe experimental behaviors of the resistivity as a function of both magnetic field, rho(T)(H), and temperature, rho(H)(T), in Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O high-quality samples, are interpreted by means of a phenomenological theory based on two hypotheses: (i) the irreversibility line at zero frequency is the glass-to-liquid-vortex transition, (ii) the dissipation mechanism is that found by Ambegaokar and Halperin and applied by Tinkham to high-T(c) superconductors. The results of the fits performed on the whole range of resistivity variation (five decades) give strong support to the glass model
Angular dependence of the resistively-determined irreversibility line in YBCO
No full textWe present a wide set of resistive measurements taken in a YBCO thin film, as a function of the temperature, magnetic field and field orientation theta with respect to the (a,b) planes. We focus the attention on the low portion of the resistance curves. Contrary to what is found close to T-c, no angular scaling is possible. We examine the angular behavior of the irreversibility line, conventionally taken as the field and angle dependent temperature T-i (H, theta) at which the resistivity exceeds some sensitivity threshold. It is found that T-i does not follow a simple 2D or 3D behavior, and that its angular dependence is affected by the extended defects present in the film (twin planes) and by the layered structure. We also show that the irreversibility line as a function of the angle, taken at constant reduced field h = H/H-c2 (H-c2 is the temperature-and angle-dependent upper critical field, as determined from the fluctuation conductivity), presents a noticeable angular dependence
- …
