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Ancient paper yellowing, inducing major esthetic damage, is mainly due to cellulose oxidation. The complexity of cellulose is an obstacle to the interpretation of optical spectra for the characterization and quantification of chemical damage induced by ageing in ancient paper. Here we report optical absorption measurements of paper samples obtained in a temperature range from 14 to 300 K. Their interpretation by abinitio theoretical computational simulations revealed a dominant role of static disorder on oxidized cellulose optical response, compared to temperature effect. Our findings are of crucial importance to understand ancient paper degradation processes and to conceive new diagnostic methods.

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.

We have measured the orientational dependence of the magnetic-field induced microwave dissipation at 21 GHz in a Bi2Sr2CaCu2O8+x epitaxial film, by means of a cavity technique. The dissipation is measured as a function of the external magnetic field H and of the angle (curly theta) between H and the (a, b) planes, at fixed temperatures. Measurements were taken in the range 49-81 K. Apart from a narrow (Delta(curly theta)approximate to 0.5 degrees) angular range close to the parallel orientation, the strongly anisotropic dissipation can be successfully scaled over the orthogonal-field curve with a simple rescaling of the magnetic field: H --> H/f(curly theta). Best scaling is obtained with the quasi-two-dimensional (quasi-2D), thin-film expression for f(curly theta). One single parameter, the anisotropy ratio epsilon, is used in the scaling. It is found that epsilon increases with temperature, consistent with the quasi-2D result, which gives a quantitative fitting for the data. The anisotropy ratios so obtained are compared with those previously obtained by the angular scaling of the critical currents in a sample from the same batch. The quantitative agreement between the two sets is excellent, strongly indicating that the same scaling properties apply in Bi2Sr2CaCu2O8 for very different dissipative properties.

We develop a phenomenological model for the magnetoresistivity in high-T-c superconductors that includes the contribution of the fluctuation excess conductivity and the effects of the phase slip due to thermal motion of vortices above the irreversibility line over local depressions of the order parameter. The fluctuation conductivity in the proximity of the mean-field transition is inserted into the final expression for the resistivity through a scaling function, obtained theoretically by Ullah and Dorsey. The behavior of the system of vortices is taken into account assuming that below the irreversibility line the solid phase is a glass phase. Crossing the irreversibility line, the vortex system becomes a viscous fluid and, finally, a liquid. It is possible to fully describe the resistivity by recalling some of the main concepts of the conventional glass transitions. We obtain a compact expression for the resistivity that we compare to previously reported experimental data in twinned and untwinned Y-Ba-Cu-O single crystals. With very few parameters we can fit extremely well the resistive transitions in the full temperature and field range. Also, the transitions in very pure, untwinned crystals can be entirely fitted, including the ''kink' at the so-called melting transition. Moreover, the resistivity is shown to be heavily influenced by fluctuations.

We 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.

We present an experimental study of the dimensional crossover present in the extremely anisotropic superconductor Bi2Sr2CaCu2O8+x by means of measurements of the onset field for the dissipation H* as a function of the temperature and of the orientation theta between the field direction and the a, b plane. The crossover is experimentally shown to depend on the strength of the applied magnetic field in a narrow temperature range below T(c). The entire set of experimental data is described by a coupled-layers model, in which a decoupling can be induced by a sufficiently high magnetic field. The anisotropy ratio is found to depend strongly on the temperature and the model of coupled slabs of finite thickness quantitatively describes the data.

Magneto-resistive measurements at microwave frequencies have been carried out in c-oriented Bi2Sr2CaCu2O8+x films. When the angle curly theta between the magnetic-field direction and the (a, b)-plane is varied a large anisotropy is observed in the microwave absorption. In the range 4-degrees < curly theta less-than-or-equal-to 90-degrees, the dissipation scales as H sin curly theta, but below 4-degrees an anomalous enhancement of dissipation is present. The behaviour of the enhancement is measured as a function of the temperature in the range (70 divided-by 85) K.

The 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.

At nonzero frequencies both the normal and superfluid fractions, as well as moving vortices, contribute to the electrical transport, resulting in intricate expressions for the resistivity. We present an extended study of mi- crowave resistivity data measured on YBa2 Cu3 O7−δ, SmBa2 Cu3 O7−δ, and MgB2 by means of a broad-band technique between 2 and 20 GHz and of a resonant system at 50 GHz. We discuss the main experimental fingerprints that allow one to identify the relevance of the different contributions from the measured microwave response as a function of frequency, temperature, and magnetic field. We show that the field-dependent superfluid/quasiparticles dynamics cannot be in general neglected with respect to the vortex motion, albeit its relative importance is different in different materials: in high tem- perature superconductors vortex motion prevails at high fields, but at low fields a significant superfluid/quasiparticle contribution exists (ascribed to the presence of lines of nodes). In MgB 2 the two contributions are compa- rable even at high fields. In this general frame, we derive from the mea- surements vortex and superfluid parameters in agreement with theoretical predictions and independent measurements.

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.

We present measurements of the complex resistivity of YBCO thin filmsas a function of frequency, in the microwave range. Data arecollected through a Corbino disc geometry, allowing for a continuous frequency spectrum in the range 1-20 GHz. We focus on the behaviour of the imaginary part of the resistivity as a function of temperature. We show that it presents a broad peak justbelow Tc, whose amplitude and width depend on the frequency. We also discuss to what extent the experimental findings can be explained using the standard two fluid model, and which are the possible responsibles for the deviations from these predictions.

We present microwave resistivity measurements at 21 GHz in YBa 2 Cu 3 O 7-δ thin film as a function of the temperature and the magnetic field. Measurements are taken in various relative orientations of the microwave current, dc magnetic field and c axis. Attention is focussed on measurements taken with the dc field parallel to the (a, b) planes. In moderate magnetic field, we show that the microwave magnetic response is made up of a vortex motion contribution, which can be described by conventional models, and a noticeable magnetic field induced increase of the quasiparticle density, which exhibits features typical of the existence of lines of nodes in the superconducting gap. We estimate the effective vortex viscosity for vortex motion across the (a, b) planes.

We present measurements of the field-induced change of the microwave surface impedance in optimally doped Bi 2 Sr 2 CaCu 2 O 8-x crystals. Measurements are taken with microwave fields in the (a,b) planes. A moderate (< 0.7 T) dc magnetic field is applied normal to the (a,b) planes. Measurements are taken for temperatures above 70 K. At fixed temperature, with increasing field the surface resistance presents a sharp onset at low fields, a steep rise then a plateau or a peak, followed by a second, slower rise with the field. The surface reactance presents similarly a steep raise at low fields above the onset, then a sudden decrease. We discuss the applicability of commonly established vortex motion models to the observed response, and we find that substantial inconsistencies emerge. Possible influences of the layered nature on the vortex response are investigated.

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.

The temperature-induced crossover from a two-dimensional (2D) to 3D behavior found in 2:2:1:2 Bi-Sr-Ca-Cu-O is studied by means of a large set of measurements of the magnetic field ${\mathit{H}}^{\mathrm{*}}$ for the onset of dissipation. Some interesting features of the angular dependence near the crossover temperature are found; in particular, the ${\mathit{H}}^{\mathrm{*}}$ values perpendicular to a-b planes lie on the irreversibility line. The angular behavior of ${\mathit{H}}^{\mathrm{*}}$ is analogous to that of the upper critical field in conventional layered superconductors. A tentative explanation is given.

We show that in a highly c-oriented 2:2:1:2 Bi-Sr-Ca-Cu-O film the angular-dependent magnetic field ${\mathit{H}}^{\mathrm{*}}$ at the onset of the resistivity behaves experimentally in a way completely analogous to the critical field found in synthetic multilayered superconductors. In particular, on varying the temperature, the field ${\mathit{H}}^{\mathrm{*}}$ exhibits a crossover from a 2D to an anisotropic 3D behavior as a function of the angle \ensuremath{\vartheta} between the external magnetic field and the a-b plane.

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 large set of experimental results of the microwave magnetoabsorption at various temperatures for granular samples of Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O. In the frame in which the absorption, as a function of the magnetic field, is due to the decoupling of the Josephson junctions (present in large number in these granular superconductors), we show that the array of Josephson junctions is driven in a frozen state by the magnetic field at low temperature. The transition to the frozen state occurs in a narrow range of temperature (15\char21{}20 K for Y-Ba-Cu-O and 20\char21{}30 K for Bi-Sr-Ca-Cu-O). The paper clarifies the role of the magnetic and thermal history, and adds experimental evidence to the superconducting-glass model for high-${\mathit{T}}_{\mathit{c}}$ granular superconductors.

The electrical transport in superconductors at nonzero frequencies is affected by the normal and superfiuid fractions, as well as moving vortices, resulting in intricate expressions for the complex resistivity. In MgB2 , additional complications arise from the two-band nature of this material. We present an accurate study of microwave resistivity data measured in MgB2 thin films by means of the Corbino disk broadband technique between 2 and 20 GHz. We show that a two-fluid model applies well in a relatively large region of the H–T phase diagram. Excellent agreement is found between the derived superfluid parameters (superfluid density, upper critical field) and theoretical predictions. In the same H–T region we isolate and discuss the vortex motion complex resistivity. To this end, we make use of the expressions given by the model by Coffey and Clem (CC). We show that the frequency dependence of the complex vortex resistivity recovers the CC model. However, the temperature and field dependence of the obtained parameters are at odds with the assumptions of the model. We discuss possible explanations of these oddities by considering collective pinning of vortices.

We present a compared experimental investigation of the (a,b) plane vortex-state complex resistivity at 48 GHz in YBaCuO, SmBaCuO and BiSrCaCuO (2212). In YBaCuO and SmBaCuO the field dependence of the response can be consistently described by a combination of flux flow and strong pair breaking due to the presence of lines of nodes in the gap. In BiSrCaCuO (2212), by contrast, the data might be described by the pair breaking alone., 4 printed pages, 3 eps figures, uses elsart.cls and elsart3.cls styles, presented at SNS2004, accepted for publication on J. Phys. Chem. Solids

We consider the problem of c-axis transport in double-layered cuprates, in particular with reference to Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ compounds. We exploit the effect of the two barriers on the thermal and tunnel transport. The resulting model is able to describe accurately the normal state c-axis resistivity in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$, from the underdoped side up to the strongly overdoped. We extend the model, without introducing additional parameters, in order to allow for the decrease of the barrier when an external voltage bias is applied. The extended model is found to describe properly the c-axis resistivity for small voltage bias above the pseudogap temperature $T^{*}$, the c-axis resistivity for large voltage bias even below $T_c$, and the differential $dI/dV$ curves taken in mesa structures., Comment: 12 pages, 6 figures. Submitted to Superconductor Science and Technology

We report measurements of the magnetic-field induced microwave complex resistivity in REBa$_{2}$Cu$_{3}$O$_{7-\delta}$ 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 YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ and SmBa$_{2}$Cu$_{3}$O$_{7-\delta}$ 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., Comment: 6 pages, 4 figures, to be published in J. Supercond. as proceedings of 8th HTSHFF (May 26th-29th, 2004, Begur, Spain)

We report measurements of the microwave resistivity in YBa2Cu3O7-delta (YBCO), in the presence of an applied magnetic field. Measurements are performed as a function of frequency, over a continuum spectrum between 6 and 20 GHz, by means of a Corbino disc geometry. These data allow for a direct identification of different dynamical regimes in the dissipation of YBCO in the presence of an applied magnetic field. While at high temperatures a frequency independent resistivity is observed, at lower temperatures we find a marked frequency dependence. The line in the (H, T) plane at which this change in the dynamical regime is observed is clearly identified and discussed in terms of vortex motion and fluctuational resistivity.

We present measurements of the dc and microwave excess conductivity above T-c in anisotropic superconducting cuprates and MgB2. When temperature raises well above T-c, both the dc and microwave excess conductivities drop much faster than predicted by the well-established Gaussian theory. We introduce a spectral cutoff in the calculation of the finite-frequency fluctuational conductivity in order to suppress the contribution of high-momentum modes at high temperatures. We find that our data are well described by the model for appropriate dimensionalities. (C) 2003 Elsevier Science B.V. All rights reserved.

We present dc resistivity multi-terminal measurements performed in Bi2Sr2CaCu2O8+delta single crystal. Sample doping was varied from underdoped to slightly overdoped. Data are analyzed in term of a model which 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. We also analyze data from the literature. In all cases, the proposed model describes very well the data in the normal state. (C) 2003 Elsevier Science B.V. All rights reserved.

Using an eight terminal geometry in standard current/voltage contact configurations we have determined both the out-of-plane and the in-plane d.c. resistivity in BSCCO(2212) samples at different doping level. Comparison of the temperature behaviors for these transport quantities highlights that use of these configurations results in incorrect determinations for the in-plane components. Exploiting the possibility of crossed comparisons when using a multi-terminal technique we are able to ascribe this effect at presence on the sample of surface layer with oxygen contents different from the bulk.