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ABSTRACT: We focus on the statistics of word occurrences and of the waiting times between such occurrences in Blogs. Due to the heterogeneity of words’ frequencies, the empirical analysis is performed by studying classes of “frequently-equivalent” words, i.e. by grouping words depending on their frequencies. Two limiting cases are considered: the dilute limit, i.e. for those words that are used less than once a day, and the dense limit for frequent words. In both cases, extreme events occur more frequently than expected from the Poisson hypothesis. These deviations from Poisson statistics reveal non-trivial time correlations between events that are associated with bursts of activities. The distribution of waiting times is shown to behave like a stretched exponential and to have the same shape for different sets of words sharing a common frequency, thereby revealing universal features.
Journal of Informetrics 03/2013; · 4.23 Impact Factor
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ABSTRACT: Co-authorship networks of neighbouring scientific disciplines, i.e. granular (G) media and networks (N) are studied in order
to observe drastic structural changes in evolving networks. The data is taken from arXives. The system is described as coupled
networks. By considering the 1995–2005time interval and scanning the author-article network evolution with a mobile time
window, we focus on the properties of the links, as well as on the time evolution of the nodes. They can be in three states,
N, G or multi-disciplinary (M).
This leads to drastic jumps in a so-called order parameter, i.e. the link proportion of a given type, forming the main island,
that reminds of features appearing at percolation and during metastable (aggregation-desaggregation) processes. The data analysis
also focuses on the way different kinds (N, G or M) of authors collaborate, and on the kind of the resulting collaboration.
Physics of Condensed Matter 04/2012; 57(1):89-94. · 1.53 Impact Factor
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ABSTRACT: A home made experimental set-up allows us to measure the thermal conductivity, the thermopower and the thermal diffusivity
simultaneously in the temperature range (20-300 K). Therefore the specific heat can be deduced. The role of a radiation shield
is shown to be relevant. Tests of the system are made on a 99.9% pure Cu sample and two polycrystalline cuprate ceramics for
illustration. Without any complicated optimisation, the technique indicates much promise already due to its efficiency and
rapidity.
PACS. 06.30.-k Measurements common to several branches of physics and astronomy - 72.15.Jf Thermoelectric and thermomagnetic
effects
Physics of Condensed Matter 04/2012; 13(3):437-443. · 1.53 Impact Factor
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ABSTRACT: The chemical synthesis of dysprosium doped Bi-based 2223 high-Tc superconducting materials prepared by either a “two-powder process” or a “crystalline-glassy matrix precursor” method has
been examined. In both cases, the 2223 phase is seen to be unstable. The reason for the Dy-doped 2223 instability is discussed
and found to originate in electronically mediated bond-mismatch, resulting in a strain energy minimization by phase decomposition.
06/2011: pages 153-162;
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ABSTRACT: We report on microwave measurements on DyBa$_2$Cu$_3$O$_{7-\rm\delta}$
monodomains grown by the top-seeded melt-textured technique. We measured the
field increase of the surface resistance $R_{\rm s}(H)$ in the a-b plane at
48.3 GHz. Measurements were performed at fixed temperatures in the range 70 K -
$T_{\rm c}$ with a static magnetic field $\mu_0H<0.8$ T parallel to the c-axis.
Low field steep increase of the dissipation, typical signature of the presence
of weak links, is absent, thus indicating the single-domain behaviour of the
sample under study. The magnetic field dependence of $R_{\rm s}(H)$ is ascribed
to the dissipation caused by vortex motion. The analysis of $X_{\rm s}(H)$
points to a free-flow regime, thus allowing to obtain the vortex viscosity as a
function of temperature. We compare the results with those obtained on RE-BCO
systems. In particular, we consider strongly pinned films of
YBa$_2$Cu$_3$O$_{7-\rm\delta}$ with nanometric BaZrO$_3$ inclusions.
03/2011;
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ABSTRACT: The trapped magnetic field is examined in bulk high-temperature superconductors that are artificially drilled along their c-axis. The influence of the hole pattern on the magnetization is studied and compared by means of numerical models and Hall probe mapping techniques. To this aim, we consider two bulk YBCO samples with a rectangular cross-section that are drilled each by six holes arranged either on a rectangular lattice (sample I) or on a centered rectangular lattice (sample II). For the numerical analysis, three different models are considered for calculating the trapped flux: (i), a two-dimensional (2D) Bean model neglecting demagnetizing effects and flux creep, (ii), a 2D finite-element model neglecting demagnetizing effects but incorporating magnetic relaxation in the form of an E-J power law, and, (iii), a 3D finite element analysis that takes into account both the finite height of the sample and flux creep effects. For the experimental analysis, the trapped magnetic flux density is measured above the sample surface by Hall probe mapping performed before and after the drilling process. The maximum trapped flux density in the drilled samples is found to be smaller than that in the plain samples. The smallest magnetization drop is found for sample II, with the centered rectangular lattice. This result is confirmed by the numerical models. In each sample, the relative drops that are calculated independently with the three different models are in good agreement. As observed experimentally, the magnetization drop calculated in the sample II is the smallest one and its relative value is comparable to the measured one. By contrast, the measured magnetization drop in sample (1) is much larger than that predicted by the simulations, most likely because of a change of the microstructure during the drilling process. Comment: Proceedings of EUCAS 09 conference
05/2010;
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ABSTRACT: In the present work we study, both theoretically and experimentally, the temperature increase in a bulk high-temperature superconductor subjected to applied AC magnetic fields of large amplitude. We calculate analytically the equilibrium temperatures of the bulk sample as a function of the experimental parameters using a simple critical-state model for an infinitely long type-II superconducting slab or cylinder. The results show the existence of a limit heat transfer coefficient (AUlim) separating two thermal regimes with different characteristics. The theoretical analysis predicts a "forbidden" temperature window within which the temperature of the superconductor can never stabilize when the heat transfer coefficient is small. In addition, we determine an analytical expression of two threshold fields Htr1 and Htr2 characterizing the importance of magneto-thermal effects and show that a thermal runaway always occurs when the field amplitude is larger than Htr2. The theoretical predictions of the temperature evolution of the bulk sample during a self-heating process agree well with the experimental data. The simple analytical study presented in this paper enables order of magnitude thermal effects to be estimated for simple superconductor geometries under applied AC magnetic fields and can be used to predict the influence of experimental parameters on the self-heating characteristics of bulk type-II superconductors. Comment: 32 pages, 6 figures
05/2010;
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ABSTRACT: We study the magnetic shielding properties of hybrid ferromagnetic/superconductor (F/S) structures consisting of two coaxial cylinders, with one of each material. We use an axisymmetric finite-element model in which the electrical properties of the superconducting tube are modeled by a nonlinear E - J power law with a magnetic-field-dependent critical current density whereas the magnetic properties of the ferromagnetic material take saturation into account. We study and compare the penetration of a uniform axial magnetic field in two cases: 1) a ferromagnetic tube placed inside a larger superconducting tube ( Ferro-In configuration) and 2) a ferromagnetic tube placed outside the superconducting one ( Ferro-Out configuration). In both cases, we assess how the ferromagnetic tube improves the shielding properties of the sole superconducting tube. The influence of the geometrical parameters of the ferromagnetic tube is also studied: It is shown that, upon an optimal choice of the geometrical parameters, the range of magnetic fields that are efficiently shielded by the high-temperature superconductor tube alone can be increased by a factor of up to 7 (2) in a Ferro-Out ( Ferro-In ) configuration. The optimal configuration uses a 1020 carbon steel with a thickness of 2 mm and a height that is half that of the superconducting cylinder (80 mm).
IEEE Transactions on Appiled Superconductivity 03/2010; · 1.04 Impact Factor
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ABSTRACT: In the first part of this paper, we discuss the effects of grain boundaries on the properties of bulk colossal magnetoresistive (CMR) manganites. We compare the electrical resistivity and AC magnetic susceptibility of perovskite La-Ca-Mn-O samples with the same nominal stoichiometry but differing in their microstructure: (i) a single grain sample, (ii) a sample containing two grains and (iii) a polycrystalline sample. Emphasis is placed on information that can be deduced from the measurements in each case. In the second part of the paper, we report the data measured on composite samples containing a CMR phase (La-Ca-Mn-O) and an insulating phase (Mn3O4). The results are discussed in the framework of percolation theory. We show how the grain boundaries affect the electrical properties of these materials, and we highlight the crucial role of geometric (demagnetization) effects on the resistance vs. magnetic field measurements.
Journal of Optoelectronics and Advanced Materials 10/2009; 11:1115-1121. · 0.46 Impact Factor
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ABSTRACT: In the first part of this paper, we discuss the effects of grain boundaries on the properties of bulk colossal magnetoresistive
(CMR) manganites. We compare the electrical resistivity and AC magnetic susceptibility of perovskite La-Ca-Mn-O samples
with the same nominal stoichiometry but differing in their microstructure: (i) a single grain sample, (ii) a sample containing
two grains and (iii) a polycrystalline sample. Emphasis is placed on information that can be deduced from the
measurements in each case. In the second part of the paper, we report the data measured on composite samples
containing a CMR phase (La-Ca-Mn-O) and an insulating phase (Mn3O4). The results are discussed in the framework of
percolation theory. We show how the grain boundaries affect the electrical properties of these materials, and we highlight
the crucial role of geometric (demagnetization) effects on the resistance vs. magnetic field measurements.
Journal of Optoelectronics and Advanced Materials 09/2009; 11(9):1115-1121. · 0.46 Impact Factor
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ABSTRACT: Magnetic shielding efficiency was measured on high- Tc superconducting hollow cylinders subjected to either an axial or a transverse magnetic field in a large range of field sweep rates, dBapp/dt. The behaviour of the superconductor was modelled in order to reproduce the main features of the field penetration curves by using a minimum number of free parameters suitable for both magnetic field orientations. The field penetration measurements were carried out on Pb-doped Bi-2223 tubes at 77 K by applying linearly increasing magnetic fields with a constant sweep rate ranging between 10 µT s−1 and 10 mT s−1 for both directions of the applied magnetic field. The experimental curves of the internal field versus the applied field, Bin(Bapp), show that, at a given sweep rate, the magnetic field for which the penetration occurs, Blim, is lower for the transverse configuration than for the axial configuration. A power law dependence with large exponent, n', is found between Blim and dBapp/dt. The values of n' are nearly the same for both configurations. We show that the main features of the curves Bin(Bapp) can be reproduced using a simple 2D model, based on the method of Brandt, involving a E(J) power law with an n-exponent and a field-dependent critical current density, Jc(B), (following the Kim model: Jc = Jc0(1+B/B1)−1). In particular, a linear relationship between the measured n'-exponents and the n-exponent of the E(J) power law is suggested by taking into account the field dependence of the critical current density. Differences between the axial and the transverse shielding properties can be simply attributed to demagnetizing fields.
Superconductor Science and Technology 08/2009; 22(10):105002. · 2.66 Impact Factor
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ABSTRACT: We present a method for characterizing the propagation of the magnetic flux in an artificially drilled bulk high-temperature superconductor (HTS) during a pulsed-field magnetization. As the magnetic pulse penetrates the cylindrical sample, the magnetic flux density is measured simultaneously in 16 holes by means of microcoils that are placed across the median plane, i.e. at an equal distance from the top and bottom surfaces, and close to the surface of the sample. We discuss the time evolution of the magnetic flux density in the holes during a pulse and measure the time taken by the external magnetic flux to reach each hole. Our data show that the flux front moves faster in the median plane than on the surface when penetrating the sample edge; it then proceeds faster along the surface than in the bulk as it penetrates the sample further. Once the pulse is over, the trapped flux density inside the central hole is found to be about twice as large in the median plane than on the surface. This ratio is confirmed by modelling.
08/2009;
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ABSTRACT: We have studied numerically and experimentally the magnetic flux penetration in high- Tc superconducting tube subjected to a uniform magnetic field parallel to its long axis. This study is carried in view of designing low-frequency magnetic shields by exploiting the diamagnetic properties of high- Tc superconducting ceramics. We have measured the field attenuation for applied magnetic fields in the frequency range 5 mHz-0.1 Hz by Hall probe measurements and at audio frequencies using a sensing coil. A simple 1D analysis using the Kim critical state model was found to be able to reproduce the experimental data satisfactorily. We have also determined the phase shift between the internal and the applied field both experimentally and numerically. Finally, we have studied the sweep rate dependence of the magnetic shielding properties, using data recorded either at several constant sweep rates d B /d t or at several AC fields of various amplitudes and frequencies. Both methods agree with each other and lead to a n -value of the E ~ J <sup>n</sup> law equal to ~ 40 at 77 K.
IEEE Transactions on Appiled Superconductivity 07/2009; · 1.04 Impact Factor
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ABSTRACT: Physical properties of polycrystalline La <sub>0.5</sub> Ba <sub>0.5</sub> MnO <sub>3</sub> are reported from low temperature (10 K) up to above room temperature. An aim has been to obtain microscopic parameters and to search for the characteristic lengths in terms of which one can discuss the interplay between magnetic, electric, and phonon excitations. The structural and magnetotransport measurements reveal a set of relatively high transition temperatures (near 300 K) between ferromagnetic/metallic and paramagnetic/semiconducting phases. It is found, in particular, that the so-called localization length increases from 0.085 to 0.24 nm when the magnetic field varies from 0 to 8 T. Moreover a “special field value” ∼0.03 T is observed in the description of the electrical resistance. It cannot be presently distinguished whether it is the signature of a spin reorientation transition in the canted phase or a mere saturation field for aligning magnetic domains. The relatively high magnetoresistance effect ( ≃55% at 8 T and 10 K) makes the La <sub>0.5</sub> Ba <sub>0.5</sub> MnO <sub>3</sub> a very interesting material for among others sensor applications.
Journal of Applied Physics 02/2009; · 2.17 Impact Factor
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ABSTRACT: Drilling holes in a bulk high-Tc superconductor enhances the oxygen annealing and the heat exchange with the cooling liquid. However, drilling holes also reduces the amount of magnetic flux that can be trapped in the sample. In this paper, we use the Bean model to study the magnetization and the current line distribution in drilled samples, as a function of the hole positions. A single hole perturbs the critical current flow over an extended region that is bounded by a discontinuity line, where the direction of the current density changes abruptly. We demonstrate that the trapped magnetic flux is maximized if the center of each hole is positioned on one of the discontinuity lines produced by the neighbouring holes. For a cylindrical sample, we construct a polar triangular hole pattern that exploits this principle; in such a lattice, the trapped field is ~20% higher than in a squared lattice, for which the holes do not lie on discontinuity lines. This result indicates that one can simultaneously enhance the oxygen annealing, the heat transfer, and maximize the trapped field.
11/2008;
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ABSTRACT: We present a microwave characterization of a DyBa <sub>2</sub> Cu <sub>3</sub> O <sub>7-x</sub> single domain, grown by the top-seeded melt-textured technique. We report the (a,b) plane field-induced surface resistance, ΔR<sub>s</sub>(H) , at 48.3 GHz, measured by means of a cylindrical metal cavity in the end-wall-replacement configuration. Changes in the cavity quality factor Q against the applied magnetic field yield ΔR<sub>s</sub>(H) at fixed temperatures. The temperature range (70 K; T<sub>c</sub> ) was explored. The magnetic field μ<sub>0</sub>H≪0.8 T was applied along the c axis. The field dependence of ΔR<sub>s</sub>(H) does not exhibit the steep, steplike increase at low fields typical of weak links. This result indicates the single-domain character of the sample under investigation. ΔR<sub>s</sub>(H) exhibits a nearly square-root dependence on H , as expected for fluxon motion. From the analysis of the data in terms of motion of Abrikosov vortices, we estimate the temperature dependences of the London penetration depth λ and the vortex viscosity η , and their zero-temperature values λ(0)=165 nm and η(0)=3×10<sup>-7</sup> N s m <sup>-2</sup> , which are found in excellent agreement with reported data in YBa <sub>2</sub> Cu <sub>3</sub> O <sub>7-x</sub> single crystals. Comparison of microwave properties with those of related samples indicates the need for reporting data as a function of T/T-
<sub>c</sub> in order to obtain universal laws.
Journal of Applied Physics 06/2008; · 2.17 Impact Factor
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ABSTRACT: The main purpose of this work was to design, develop and construct a simple, low-cost AC susceptometer to measure large, bulk superconducting samples (up to 32 mm in diameter) in the temperature range 78-120 K. The design incorporates a double heating system that enables a high heating rate (25 K/hour) while maintaining a small temperature gradient (< 0.2 K) across the sample. The apparatus can be calibrated precisely using a copper coil connected in series with the primary coil. The system has been used successfully to measure the temperature dependence of the AC magnetic properties of entire RE-Ba-Cu-O [(RE)BCO] bulk superconducting domains. A typical AC susceptibility measurement run from 78 K to 95 K takes about 2 hours, with excellent temperature resolution (temperature step ~ 4 mK) around the critical temperature, in particular. Comment: 25 pages, 7 figures. Accepted for publication in Measurement Science and Technology
05/2008;
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ABSTRACT: We present a microwave characterization of a DyBa$_{2}$Cu$_{3}$O$_{7-x}$ single domain, grown by the top-seeded melt-textured technique. We report the (a,b) plane field-induced surface resistance, $\Delta R_s(H)$, at 48.3 GHz, measured by means of a cylindrical metal cavity in the end-wall-replacement configuration. Changes in the cavity quality factor Q against the applied magnetic field yield $\Delta R_s(H)$ at fixed temperatures. The temperature range [70 K ; T_c] was explored. The magnetic field $\mu_0 H <$ 0.8 T was applied along the c axis. The field dependence of $\Delta R_s(H)$ does not exhibit the steep, step-like increase at low fields typical of weak-links. This result indicates the single-domain character of the sample under investigation. $\Delta R_s(H)$ exhibits a nearly square-root dependence on H, as expected for fluxon motion. From the analysis of the data in terms of motion of Abrikosov vortices we estimate the temperature dependences of the London penetration depth $\lambda$ and the vortex viscosity $\eta$, and their zero-temperature values $\lambda(0)=$165 nm and $\eta(0)=$ 3 10$^{-7}$ Nsm$^{-2}$, which are found in excellent agreement with reported data in YBa$_{2}$Cu$_{3}$O$_{7-x}$ single crystals. Comparison of microwave properties with those of related samples indicate the need for reporting data as a function of T/T_c in order to obtain universal laws.
04/2008;
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ABSTRACT: We consider a three dimensional (3D) finite element method to model high temperature su-perconductors (HTS) subjected to a uniform magnetic field varying linearly with time. HTS are characterized by a non linear conductivity described by a power law with a critical exponent, n. We implement an A-φ weak formulation into the open source solver GetDP. Backward Euler method is used for the temporal resolution and is coupled to a non linear Picard iteration scheme for dealing with the non-linear resistivity of HTS. We consider single and multiple time step methods and show that the single time step method produces an accurate result in a drastically reduced calculation time. The finite element formulation is validated by comparing the calculated magnetization of a HTS tube with large pinning strength (or similarly, with large values of n) to the results obtained with other well-known techniques [1, 2]. The numerical method is then applied to the study of the magnetiza-tion of cylinders that are drilled by different periodic arrangements of columnar holes parallel to the cylinder axis.
01/2008;
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ABSTRACT: Some possible connection between spin and charge degrees of freedom in magnetoresistive manganites is investigated through a thorough experimental study of the magnetic [alternating current susceptibility and direct current (dc) magnetization] and transport (resistivity and thermal conductivity) properties. Measurements are reported in the case of well characterized polycrystalline La0.8Sr0.2MnO3 samples. The experimental results suggest rather strong field-induced polarization effects in our material, clearly indicating the presence of ordered ferromagnetic regions inside the semiconducting phase. Using an analytical expression which fits the spontaneous dc magnetization, the temperature and magnetic field dependences of both electrical resistivity and thermal conductivity data are found to be well reproduced through a universal scenario based on two mechanisms: (i) a magnetization dependent spin polaron hopping influenced by a Zeeman splitting effect and (ii) properly defined thermally excited polaron states which have to be taken into account in order to correctly describe the behavior of the less conducting region. Using the experimentally found values of the magnetic and electron localization temperatures, we obtain L = 0.5 nm and mp = 3.2me for estimates of the localization length (size of the spin polaron) and effective polaron mass, respectively.
Journal of Applied Physics 10/2007; 102(8):083916-083916-9. · 2.17 Impact Factor
