arXiv subjects

M. Ausloos

Publications and source records attributed to M. Ausloos.

103 records · Page 6Linked to original sources

arXiv2001

Power law correlations in the Southern Oscillation Index fluctuations characterizing El Nino

The southern oscillation index (SOI) is a characteristic of the El Nino phenomenon. SOI monthly averaged data is analyzed for the time interval 1866-2000. The tail of the cumulative distribution of the fluctuations of SOI signal is studied in order to characterize the amplitude scaling of the fluctuations and the occurrence of extreme events. Large fluctuations are more likely to occur than the Gaussian distribution would predict. The time scaling of fluctuations is studied by applying the energy spectrum and the Detrended Fluctuation Analysis (DFA) statistical method. Self-affine properties are found to be pertinent to the SOI signal and therefore suggest power law correlations of fluctuations of the signal. Antipersistent type of correlations exist for a time interval ranging from about 4 months to about 6 years. This leads to favor specific physical models for El Nino description.

cond-mat.stat-mech↗

arXiv2001

Magnetic properties of magnetically textured Bi-2212 ceramics

This paper aims at reporting magnetic properties of bulk polycrystalline Bi2Sr2Ca0.8Dy0.2Cu2O8-y samples textured under a magnetic field. The microstructure of these materials is highly anisotropic and exhibits particular features needed to be taken into account in order to interpret their magnetic and electrical properties. First the AC magnetic susceptibility c = c ' - j c" has been measured for several magnetic fields (H // ab and H // c) and compared to the electrical resistivity data. The structure of the c" peak is shown to be related to the chemical content distribution of the superconducting grains. Next, the magnetic flux profiles have been extracted from the magnetic measurements using the Campbell - Rollins procedure. The anisotropy of the flux profiles and their peculiar curvature behaviour for H // c point out the role of both grain platelet structure and the presence of secondary phases. From these results, we conclude that the magnetic properties of such magnetically textured materials do not allow for a reliable extraction of the critical current density Jc but essentially probe geometric effects. Such effects have to be taken into account for improving the manufacture of attractive high-Tc materials.

cond-mat.supr-con↗

arXiv2001

Electrical and thermal transport properties in high T_c superconductors : effects of a magnetic field

Experimental studies of the electric and heat currents in the normal, superconducting and mixed states of high T$_c$ superconductors (HTcS) lead to characterization, complementary to data obtained from equilibrium property based techniques. A magnetic field superimposed on the superconducting sample generates {\it magneto-transport phenomena}, from which an excess electrical resistivity, an excess thermoelectric power, the Hall or the Nernst effect. Different behavioral effects allow one to distinguish various dissipation mechanisms, like quasi particle scattering, vortex motion dissipation and superconductivity fluctuations, in particular when the Corbino geometry is used. Moreover bulk measurements of the thermal conductivity and the electrothermal conductivity in a magnetic field give us sure indications of the order parameter symmetry. The location of the mixed state phase transition lines in the technological phase diagram of HTcS are briefly pointed out through precise measurements performed over broad temperature and magnetic field ranges. The results are mainly reviewed with the aim of defining further investigation lines.

cond-mat.supr-con↗

arXiv2001

Coherent description of electrical and thermal impurity-and-phonon limited transport in simple metals

The electrical resistivity, thermoelectric power and electronic thermal conductivity of simple (isotropic) metals are studied in a uniform way. Starting from results of a variational solution of the Boltzmann equation, a generalized Matthiessen rule is used in order to superpose the inelastic (or not) electron-phonon and elastic electron-impurity scattering cross sections ("matrix elements"). The temperature dependence relative to these processes is given through simple functions and physical parameters over the usually investigated range of temperature for each transport coefficient. The coherence of such results is emphasized.

cond-mat.mtrl-sci↗

arXiv2001

Financial Time Series and Statistical Mechanics

A few characteristic exponents describing power law behaviors of roughness, coherence and persistence in stochastic time series are compared to each other. Relevant techniques for analyzing such time series are recalled in order to distinguish how the various exponents are measured, and what basic differences exist between each one. Financial time series, like the JPY/DEM and USD/DEM exchange rates are used for illustration, but mathematical ones, like (fractional or not) Brownian walks can be used also as indicated.

cond-mat.stat-mech↗

arXiv2001

False EUR exchange rates vs. DKK, CHF, JPY and USD. What is a strong currency?

The Euro (EUR) has been a currency introduced by the European Community on Jan. 01, 1999. This implies eleven countries of the European Union which have been found to meet the five requirements of the Maastricht convergence criteria. In order to test EUR behavior and understand various features, we have extrapolated the EUR backwards and therefore have obtained a {\it false euro} (FEUR) dating back to 1993. We have derived the exchange rates of the FEUR with respect to several currencies of interest not belonging to the EUR, i.e., Danish Kroner (DKK), Swiss Franc (CHF), Japanese Yen (JPY) and U.S. Dollar (USD). We have first observed the distribution of fluctuations of the exchange rates. Within the {\it Detrended Fluctuation Analysis} (DFA) statistical method, we have calculated the power law behavior describing the root-mean-square deviation of these exchange rate fluctuations as a function of time, displaying in particular the JPY exchange rate case. In order to estimate the role of each currency making the EUR and therefore in view of identifying whether some of them mostly influences its behavior, we have compared the time-dependent exponent of the exchange rate fluctuations for EUR with that for the currencies that form the EUR. We have found that the German Mark (DEM) has been leading the fluctuations of EUR/JPY exchange rates, and Portuguese Escudo (PTE) is the farthest away currency from this point of view.

cond-mat.stat-mech↗

arXiv1999

Anomalous temperature behavior of resistivity in lightly doped manganites around a metal-insulator phase transition

An unusual temperature and concentration behavior of resistivity in $La_{0.7}Ca_{0.3}Mn_{1-x}Cu_xO_3$ has been observed at slight $Cu$ doping ($0\leq x \leq 0.05$). Namely, introduction of copper results in a splitting of the resistivity maximum around a metal-insulator transition temperature $T_0(x)$ into two differently evolving peaks. Unlike the original $Cu$-free maximum which steadily increases with doping, the second (satellite) peak remains virtually unchanged for $x<x_c$, increases for $x\ge x_c$ and finally disappears at $x_m\simeq 2x_c$ with $x_c\simeq 0.03$. The observed phenomenon is thought to arise from competition between substitution induced strengthening of potential barriers (which hamper the charge hopping between neighboring $Mn$ sites) and weakening of carrier's kinetic energy. The data are well fitted assuming a nonthermal tunneling conductivity theory with randomly distributed hopping sites.

cond-mat.str-el↗

arXiv1999

Using Corbino disk sample geometry to separate quasiparticle and vortex motion contributions to the mixed state dissipation of YBCO

A method, using a Corbino disk sample geometry, is described and applied to study of resistive tails of sintered $YBCO$. When the transport current passes radially from the rim of the disk sample to its center, the two component potential drop signal is detected below $T_c$, which is due to (i) quasiparticles $W_q$ and (ii) vortex-core-motion related contribution $W_{ϕ}$. When the contact pairs for $W$ are placed radially, $W_q(r)$ and $W_{ϕ}(r)$ are found to follow the markedly distinctive functional dependences, providing a unique possibility to deconvolute the relative strengths of both contributions. The results obtained suggest that the mixed state dissipation of high-$T_c$ superconductors is strongly influenced by the quasiparticle excitations.

cond-mat.supr-con↗

arXiv1999

Probing the field-induced variation of the chemical potential in Bi(2)Sr(2)CaCu(2)O(y) via the magneto-thermopower measurements

Approximating the shape of the measured in $Bi_2Sr_2CaCu_2O_y$ magneto-thermopower (TEP) $ΔS(T,H)$ by asymmetric linear triangle of the form $ΔS(T,H)\simeq S_p(H)\pm B^{\pm}(H)(T_c-T)$ with positive $B ^{-}(H)$ and $B ^{+}(H)$ defined below and above $T_c$, we observe that $B ^{+}(H)\simeq 2B ^{-}(H)$. In order to account for this asymmetry, we explicitly introduce the field-dependent chemical potential of holes $μ(H)$ into the Ginzburg-Landau theory and calculate both an average $ΔS_{av}(T,H)$ and fluctuation $ΔS_{fl}(T,H)$ contributions to the total magneto-TEP $ΔS(T,H)$. As a result, we find a rather simple relationship between the field-induced variation of the chemical potential in this material and the above-mentioned magneto-TEP data around $T_c$, viz. $Δμ(H)\propto S_p(H)$.

cond-mat.supr-con↗

arXiv1999

Influence of Cu on spin-polaron tunneling in the ferromagnetic state of La(2/3)Ca(1/3)Mn(1-x)Cu(x)O(3) from the resistivity data

Nearly a 50% decrease of resistivity ρ(T,x) due to just 4% Cu doping on the Mn site of La(2/3)Ca(1/3)Mn(1-x)Cu(x)O(3) is observed. Attributing the observed phenomenon to the substitution induced decrease of the spin polaron energy E_s(x) below the Curie point T_C(x)=T_C(0)(1-x), all data are found to be well fitted by the nonthermal coherent tunneling expression ρ(T,x) = ρ_0*exp(-γM^2(T,x)) assuming M(T,x)=M_R(x)+M_0(x)*tanh{\sqrt{[T_C(x)/T]^2-1}} for the magnetization in the ferromagnetic state. The best fits through all the data points yield M_0(x)= \sqrt(1-x)M_0(0), M_R(x)=\sqrt(x)M_0(0), and E_s(x)=E_s(0)(1-x)^4 for the Cu induced modifications of the Mn spins dominated zero-temperature spontaneous magnetization, the residual paramagnetic contribution, and spin-polaron energy, respectively, with E_s(0)=0.12 eV.

cond-mat.str-el↗

arXiv1999

Negative magnetoresistance in La(0.6)Y(0.1)Ca(0.3)MnO(3): Evidence for charge localization governed by the Curie-Weiss law

Colossal negative magnetoresistance Δρ(T,B) observed in La(0.6)Y(0.1)Ca(0.3)MnO(3) at B=1T shows a nearly perfect symmetry around T_0=160K suggesting a universal field-induced transport mechanism in this material. Attributing this symmetry to strong magnetic fluctuations (triggered by the Y substitution and further enhanced by magnetic field, both above and below the field-dependent Curie temperature T_C(B)=T_0), the data are interpreted in terms of the nonthermal spin hopping and magnetization M dependent charge carrier localization scenario leading to Δρ(T,B)= -ρ_s(1-exp(-γM^2)) with M(T,B)=CB/|T-T_C|^n. The separate fits through all the data points above and below T_C yield C^{+}\simeq C^{-} and n^{+}\simeq n^{-}\simeq 1. The obtained results corroborate the importance of fluctuation effects in this material recently found (cond-mat/9812219) to dominate its magneto-thermopower behavior far beyond T_C.

cond-mat.str-el↗

arXiv1998

The Kronig-Penney-Ising picture of the colossal magnetoresistance

From general arguments, it is shown that a magnetic Kronig-Penney model based on the thermodynamics of an Ising model can be used for describing the Colossal Magnetoresistance (CMR) phenomenon. The model considers a tunneling-like transmission process of hopping electrons through a dynamic lattice characterized by evolving magnetic clusters. In this model, correlations between the magnetic states are considered to be more relevant than the lattice strain effects for obtaining the CMR features. Physical arguments lead to the theoretical description of the intrinsic temperature and field dependences of the CMR observed in typical manganite materials.

cond-mat.stat-mech↗

arXiv1998

Estimation of the charge carrier localization length from Gaussian fluctuations in the magneto-thermopower of La_{0.6}Y_{0.1}Ca_{0.3}MnO_3

The magneto-thermoelectric power (TEP) $ΔS(T,H)$ of perovskite type manganise oxide $La_{0.6}Y_{0.1}Ca_{0.3}MnO_3$ is found to exhibit a sharp peak at some temperature $T^{*}=170K$. By approximating the true shape of the measured magneto-TEP in the vicinity of $T^{*}$ by a linear triangle of the form $ΔS(T,H)\simeq S_p(H)\pm B^{\pm}(H)(T^{*}-T)$, we observe that $B ^{-}(H)\simeq 2B ^{+}(H)$. We adopt the electron localization scenario and introduce a Ginzburg-Landau (GL) type theory which incorporates the two concurrent phase transitions, viz., the paramagnetic-ferromagnetic transition at the Curie point $T_C$ and the "metal-insulator" (M-I) transition at $T_{MI}$. The latter is characterized by the divergence of the field-dependent charge carrier localization length $ξ(T,H)$ at some characteristic field $H_0$. Calculating the average and fluctuation contributions to the total magnetization and the transport entropy related magneto-TEP $ΔS(T,H)$ within the GL theory, we obtain a simple relationship between $T^{*}$ and the above two critical temperatures ($T_{C}$ and $T_{MI}$). The observed slope ratio $B ^{-}(H)/B ^{+}(H)$ is found to be governed by the competition between the electron-spin exchange $JS$ and the induced magnetic energy $M_sH_0$. The comparison of our data with the model predictions produce $T_{C}=195K$, $JS=40meV$, $M_0=0.4M_s$, $ξ_0=10Å$, and $n_e/n_i=2/3$ for the estimates of the Curie temperature, the exchange coupling constant, the critical magnetization, the localization length, and the free-to-localized carrier number density ratio, respectively.

cond-mat.str-el↗