Search

We present measurements of the microwave complex resistivity at 48 GHz in REBa2Cu3O7-delta thin films (RE = Y, Sm) in moderate magnetic fields (H-mu0 < 0.8 T) and above 60 K. The measured complex resistivity gives valuable information for the study of vortex dynamics. We show that the field dependence of the microwave resistivity has rather different behaviours in YBa2Cu3O7-delta and SmBa2Cu3O7-delta. By applying the conventional model of vortex motion, the data in SmBa2Cu3O7-delta would yield exotic behaviours for the vortex parameters. However, taking into account the enhanced field-induced change in quasiparticle density in a superconductor with lines of nodes in the gap, the vortex part of the response behaves similarly in YBa2Cu3O7-delta and SmBa2Cu3O7-delta. (C) 2004 Elsevier B.V. All rights reserved.

The angular and temperature dependence of the critical field H ∗ , defined as the onset magnetic field for the DC resistive dissipation, is used to test the 3D-2D crossover in Bi 2 Sr 2 CaCu 2 O 8+ x . We show that in a narrow temperature range a magnetically induced dimensional crossover is present. Information about the layer coupling energy is obtained.

Measurements of the microwave surface resistance at 23 GHz in the temperature range 20–130 K are performed in several Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O samples. Two different two-fluid models are able to describe the experimental results: a Gorter-Casimiro one for Bi-Sr-Ca-Cu-O, and an empirical one for Y-Ba-Cu-O.

We present a phenomenological model for the electrical transport in double-layered cuprates, based on the existence of two energy barriers along the c-axis. The model is applied to measurements of the de resistivity in Bi2Sr2CaCu2O8+delta single crystals. We find that the model gives a comprehensive description of the out-of-plane normal resistivity rho(e) at various doping delta. Once the normal state has been determined, it is possible to obtain the fraction eta(T, delta) of the carriers that, below the pseudogap temperature T*, do not participate to the c-axis conductivity. We find that all the curves eta(T, delta) collapse onto a single universal curve eta[T/T*(delta)]. (c) 2007 Elsevier B.V. All rights reserved.

We present measurements of the (a,b) plane complex resistivity at 24 GHz in nearly-optimally doped YBCO films grown by dc sputtering with T c ≅ 90 K. The attention is focused above T c , where we discuss the data in terms of 3D, anisotropic fluctuations. We find that at microwave frequencies simple gaussian fluctuations are unable to describe our data both well above and in the vicinity of the critical temperature. The discrepancy well above T c is recovered by including a short-wavelength cutoff in the frequency-dependent fluctuational conductivity. In order to follow the slowing down of the fluctuations at T c , we treat the region close to T c including the quartic term of the order parameter in the Ginzburg-Landau functional within the Hartree approximation. We then find excellent agreement with our data from high temperature ( T = 1.3 T c ) down to T c .

We present multi-terminal measurements on a Bi 2 Sr 2 CaCu 2 O 8+ x crystal. We show that with these measurements it is possible to reliably obtain the resistivity tensor and to study the presence of nonlocal effects.

We present measurements of the ( a, b ) plane excess conductivity Δσ in several Bi 2 Sr 2 CaCu 2 O 8+x single crystals through a multiterminal technique. We find that the measured Δσ do not show an universal behaviour. We discuss this data in terms of Gaussian fluctuations in a nearly-two-dimensional superconductor above T c , introducing a phenomenological cutoff in the fluctuative spectrum. All our data follow the extended theory up to T ≈ 1.3 T c . While the resulting cutoff differs from sample to sample, the numerical values of the cutoff are always in the expected range o (ξ −1 )

We present a wide set of resistive measurements taken in a YBCO thin film, as a function of the temperature, magnetic field and field orientation ϑ 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 , ϑ) 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 c 2 ( H c 2 is the temperature- and angle- dependent upper critical field, as determined from the fluctuation conductivity), presents a noticeable angular dependence.