Search

In this study, a chemical stabilization method, that preserves the tensile strength of artificially aged paper pretreated with an aqueous solution of ionic liquid, is presented. Pure cotton cellulose paper samples were soaked with cholinium glycinate ionic-liquid solution either before or after the artificial aging process, which was based on thermal degradation in dry air. The tensile strength of artificially aged paper was measured by using the double-folding technique. The role of ionic liquids was investigated by mid-infrared and terahertz time-domain absorption spectroscopy. It was found that the tensile strength of pretreatedsamples is higher than that of other aged samples. A model for changes in the cellulose structure caused by oxidation and by the binding of ionic liquid molecules is proposed, based on the analysis of the mid-infrared absorption bands in the carbonyl/carboxyl region at 1590–1750 cm–1. Terahertz spectroscopy data indicate that the ionic liquid molecules penetrate in the larger size porosity, acting as binders among cellulose fibers to maintain the tensile strength of paper.

Abstract: We report measurements of the magnetic field induced microwave complex resistivity in REBa2Cu3O7−δ thin films, with RE = Y, Sm. Measurements are performed at 48 GHz by means of a resonant cavity in the end-wall-replacement configuration. The magnetic field dependence is investigated by applying a moderate (0.8 T) magnetic field along the c-axis. The measured vortex-state complex resistivity in YBa2Cu3O7−δ and SmBa2Cu37−δ is analyzed within the well-known models for vortex dynamics. It is shown that attributing the observed response to vortex motion alone leads to inconsistencies in the as-determined vortex parameters (such as the vortex viscosity and the pinning constant). By contrast, attributing the entire response to field-induced pair breaking leads to a nearly quantitative description of the data.

We investigate the effects of directional pinning due to the layered structure and to columnar defects on the microwave response in YBa2Cu3O7−δfilms. We present measurements of the field-induced microwave resistivity at 48 GHz and 21 GHz taken in various relative orientations between the microwave currents, dc magnetic field and (a, b) planes. From measurements taken in the Lorentz-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 fluctuational excess conductivity at 48 GHz and 24 GHz in YBa2Cu3O7−δfilms 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 Tc, 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.

We present measurements of the resistivity tensor of Bi2Sr2CaCu2O8+δ 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 present measurements of the real part of the microwave resistivity at 48 GHz in YBa2Cu3O7−δand Bi2Sr2CaCu2O8+xfilms, 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 Tc, the resistivity in YBa2Cu3O7−δcan be fully described by a simple flux-flow model, while in Bi2Sr2CaCu2O8+xthis model applies only partially. We discuss possible alternative models.

Measurements of the real part of the resistivity at 23 and 48 GHz are performed in Bi2Sr2CaCu2O8+xthin films as a function of the temperature. The experimental behavior above Tcis analyzed in the frame of the time dependent Ginzburg-Landau theory for the excess conductivity. We show that the observed resistivity can be interpreted by the 2D gaussian theory due to the high anisotropy of this compound. The validity of the theory is extended up to the high temperature region with the introduction of a phenomenogical cut-off on the order parameter spectrum. Approaching Tc, data depart from the predicted behavior; this departure can be ascribed either to a dimensional crossover or to the effects of interacting fluctuations.

We present measurements of the (a,b) plane complex resistivity at 24 GHz in nearly-optimally doped YBCO films grown by dc sputtering with Tc ≅ 90 K. The attention is focused above Tc, 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 Tc 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 Tc, we treat the region close to Tc 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 Tc) down to Tc.

We report on resisitive transitions in YBCO films as a function of strength and orientation of the applied magnetic field. Angular behaviour is found to be markedly different in the upper and lower part of the transition. At high dissipation level, and angular scaling behaviour is observed, following the anisotropic Lawerence-Doniach expression for the upper critical field. The scaling breaks down at the midpoint of the transition. No scaling is observed at lower temperatures, even if different angular scaling functions are used. The results are very well interpreted in terms of superconducting fluctuations above ρ/ρn=0.5 and pinning phenomena below.

We report on measurement of the microwave surface resistance Rsat 48 GHz on a BSCCO (2212 phase) crystal. We describe the procedure developed to perform measurements on crystals by the use of a resonant cavity with end-wall-replacement technique. The measurements are taken as a function of the temperature, magnetic field and angle ν between the field orientation and the (a,b) planes. The measured Rsis strongly anisotropic. The overall angular behavior reasonably follows the sinν scaling rule, but deviation from this simple behavior appears approaching the parallel orientation. When the field is parallel to the (a,b) planes, a local maximum is observed in Rs, instead of a minimum. Finally, we show that the magnetic field dependence of Rscannot be easily reconciled with existing theories for the motion of rigid flux lines.

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 Tc, no angular scaling is possible. We examine the angular behavior of the irreversibility line, conventionally taken as the field and angle dependent temperature Ti(H, ϑ) at which the resistivity exceeds some sensitivity threshold. It is found that Tidoes 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/Hc2(Hc2is the temperature- and angle- dependent upper critical field, as determined from the fluctuation conductivity), presents a noticeable angular dependence.

We present a model of the density of state for non parabolic electron dispersion law ?(\vecp) in 2D conductors with a weak random potential. In particular the model is developed assuming the existence of a critical point of saddle type in a narrow zone of the p-space. The application to the Quantum Hall Effect is given. The analysis of a series of experimental results concerning the measures of density of state and Hall resistivity in 2D conductors in an external magnetic field is presented. In particular the non gaussian tails of the Landau levels density of states as found by Klitzing's group (1985) are justified.