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Motivated by the peculiar features observed through intrinsic tunneling spectroscopy of Bi(2)Sr(2)CaCu(2)O(8+delta) mesas in the normal state, we have extended the normal-state two-barrier model for the c-axis transport [M. Giura , Phys. Rev. B 68, 134505 (2003)] to the analysis of dI/dV curves. We have found that the purely normal-state model reproduces all the following experimental features: (a) the parabolic V dependence of dI/dV in the high-T region (above the conventional pseudogap temperature), (b) the emergence and the nearly voltage-independent position of the "humps" from this parabolic behavior by lowering the temperature, and (c) the crossing of the absolute dI/dV curves at a characteristic voltage V(x). Our findings indicate that conventional tunneling can be at the origin of most of the uncommon features of the c-axis transport in Bi(2)Sr(2)CaCu(2)O(8+delta). We have compared our calculations to experimental data taken in severely underdoped and slightly underdoped Bi(2)Sr(2)CaCu(2)O(8+delta) small mesas. We have found good agreement between the data and the calculations, without any shift of the calculated dI/dV on the vertical scale. In particular, in the normal state (above T(*)) simple tunneling reproduces the experimental dI/dV quantitatively. Below T(*) quantitative discrepancies are limited to a simple rescaling of the voltage in the theoretical curves by a factor similar to 2. The need for such modifications remains an open question, that might be connected to a change of the charge of a fraction of the carriers across the pseudogap opening.

We report swept frequency measurements (2-20 GHz) of the microwave resistivity in MgB2, in the presence of a static magnetic field. Through these data, we experimentally determine the region of the {H,T} plane in which the pi band does not significantly contribute to the superfluid density. Within that region, we show that data can be interpreted through standard models for vortex motion and quasiparticle resistivity. We obtain the temperature-dependent sigma band superfluid density and the upper critical field. We find excellent agreement between the dc and microwave estimates of the upper critical field. The temperature-dependent superfluid density agrees with a BCS calculation based on independently obtained data for the large gap. We also measure and discuss the vortex characteristic frequency due to pinning effects.

We measure and analyze the resistivity of Bi2Sr2CaCu2O8+delta crystals for different doping delta. We obtain the fraction of carriers eta(T,delta)=n(g)/n(tot) that do not participate in the c-axis conductivity. All the curves eta(T,delta) collapse onto a universal curve when plotted against a reduced temperature x=[T-Theta(delta)]/Delta(*)(delta). We find that at the superconducting transition n(g) is doping independent. We also show that a magnetic field up to 14 T does not affect the degree of localization in the (a,b) planes but widens the temperature range of the x scaling by suppressing the superconducting phase coherence.

We systematically investigate the effect of doping on the temperature dependence of the c-axis resistivity in Bi2Sr2CaCu2O8+delta. We present simultaneous measurements of the resistivity tensor components rho(c) and rho(ab) at different doping from the underdoped to the overdoped regime. The c-axis resistivity behaviors as a function of temperature are interpreted, in the normal state (T>T*), through a single phenomenological model based on the existence of two energy barriers with different heights and widths. Two complementary processes are considered for each barrier: incoherent tunneling and thermal activation. The resistive measurements at different doping are well fitted with a small number of free parameters with well defined physical meanings. The analysis of the behaviors of the fitting parameters as a function of doping gives support and consistency to the two-barrier model.

We investigate the effects of directional pinning due to the layered structure and to columnar defects on the microwave response in YBa(2)Cu(3)O(7-delta) films. We present measurements of the field-induced microwave resistivity at 48 GHz and 21 GHz taken in various relative orientations between the microwave currents, do magnetic field and (a, b) planes. From measurements taken in the Loventz-force-free configuration we experimentally show the relevance of the magnetic field induced increase of quasiparticle (qp) density. We identify the vortex motion contribution, and extract the vortex parameters. We estimate a pinning frequency of order 30 GHz when the field is aligned to the (a, b) plane. Secondly, we show that the introduction of columnar defects gives rise to a strong pinning along the columns, detectable even at 48 GHz. The pinning frequency appears to be of the same magnitude than for pinning by the layered structure.

We present measurements of the microwave complex conductivity at 23.9 and 48.2 GHz in YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ films, in the fluctuational region above $T_{c}$. With increasing temperature, the fluctuational excess conductivity drops much faster than the well-known calculations within the time-dependent Ginzburg-Landau theory [H. Schmidt {\em Z. Phys.} {\bf 216}, 336]. Approaching the transition temperature, slowing down of the fluctuations is observed. We discuss the results in terms of a modified gaussian theory for finite-frequency fluctuational conductivity, where renormalization is introduced in order to account for the $T\to T_{c}$ regime, and a spectral cutoff is inserted in order to discard high-momentum modes. The data are in excellent agreement with the modified theory, when formulated for three-dimensional, anisotropic superconductors, in the whole experimentally accessible temperature range, and for both frequencies., Comment: RevTeX, 12 printed pages with 5 figures, revised version of cond-mat/0208343, to be published in European Physical Journal B

We present measurements of the fluctuational excess conductivity at 48 GHz and 24 GHz in YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ films above the transition temperature. The measurements depart from the gaussian prediction for finite-frequency fluctuational conductivity. We focus on the region not too close to $T_{c}$, where the real part of the excess conductivity drops much faster than the prediction of the gaussian model. We calculate the dynamic excess conductivity within a Ginzburg-Landau approach. In the calculation we insert a short-wavelength cutoff of the order of the inverse coherence length, in order to suppress high-momentum modes. The excess conductivity of all samples measured can be described very well by the modified model., Comment: RevTeX, 4 pages, 2 figures. To appear in J. Low Temp. Phys. (Proceedings of MOS 02 Conference, Hsinchu, Taiwan, 13-17/8/2002)

We 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.

We 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.

We present measurements of the real part of the microwave restitivity at 48 GHz in YBa2Cu3O7-delta and Bi2Sr2CaCu2O8+x films, as a function of the temperature and of the static magnetic field. In zero field, the resistivity above the critical temperature is well described by the frequency-dependent fluctuational excess conductivity. In a static magnetic field and below T-c, the resistivity in YBa2Cu3O7-delta can Be fully described by a simple flux-flow: model, while in Bi2Sr2CaCu2O8+x this model applies only partially. We discuss possible alternative models.

We discuss the excess conductivity above T-c due to renormalized order-parameter fluctuations in YBa2Cu3O7-delta (YBCO) at microwave frequencies. We calculate the effects of the uniaxial anisotropy on the renormalized fluctuations in the Hartree approximation, extending the isotropic theory developed by Dorsey [Phys. Rev. B 43, 7575 (1991)]. Measurements of the real part of the microwave resistivity at 24 and 48 GHz and of the de resistivity are performed on different YBCO films. The onset of the superconducting transition and the deviation from the linear temperature behavior above T-c can be fully accounted for by the extended theory. According to the theoretical calculation here presented, a departure from Gaussian toward renormalized fluctuations is observed. Very consistent values of the fundamental parameters (critical temperature, coherence lengths, penetration depth) of the superconducting state are obtained. [S0163-1829(98)01342-3].

We measure resistive transitions in YBa2Cu3O7-delta for several orientations and strength of the applied magnetic field. Experiments show that an angular scaling takes place in the upper part of the transition, for all angles, down to a field-dependent temperature T-f(H). The fluctuations theory is shown to fully describe the experimental findings for T>T-f, including the angular scaling and the detailed temperature behavior of the resistivity. The scaling breakdown at T-f(H) is argued to be due to pinning effects on vortex lines that become relevant at low temperatures.

We analyze measurements of the anisotropy in several Bi(2)Sr2CaCu(2)OB(8+x) films of different magnetotransport properties: critical-current density, microwave dissipation, and resistively determined irreversibility line. It is shown that all those measurements present the same angular scaling property, irrespective of the different measured quantities. The anisotropy depends on the temperature, accordingly to the quasi-two-dimensional (2D), decoupled-layer picture. In the same picture, a single parameter epsilon(T) (anisotropy ratio) is sufficient to describe the evolution of the angular scaling functions. All the data for all the samples follow the same identical curve of epsilon versus the reduced temperature. Departures from the quasi-2D model are observed very close to T-c.