M. Reza Rahimi Tabar

Sharif University of Technology, Teheran, Tehrān, Iran

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Publications (105)250.96 Total impact

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    ABSTRACT: Although fluctuations in the waiting time series have been studied for a long time, some important issues such as its long-range memory and its stochastic features in the presence of nonstationarity have so far remained unstudied. Here we find that the “waiting times” series for a given increment level have long-range correlations with Hurst exponents belonging to the interval 1/2 < H < 1. We also study positive-negative level asymmetry of the waiting time distribution. We find that the logarithmic difference of waiting times series has a short-range correlation, and then we study its stochastic nature using the Markovian method and determine the corresponding Kramers-Moyal coefficients. As an example, we analyze the velocity fluctuations in high Reynolds number turbulence and determine the level dependence ofMarkov time scales, as well as the drift and diffusion coefficients. We show that the waiting time distributions exhibit power law tails, and we were able to model the distribution with a continuous time random walk.
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    E. Daryaei, M. Reza Rahimi Tabar, A. Z. Moshfegh
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    ABSTRACT: The effect of etching time on the statistical properties of the hydrophilic surface of SiO_2/TiO_2/Glass nano bi-layer has been studied using Atomic Force Microscopy (AFM) and stochastic approach based on the level crossing analysis. We have created a rough surface of the hydrophilic SiO_2/TiO_2 nano bi-layer system by using 26% Potassium Hydroxide (KOH) solution. Measuring the average apparent contact angle assessed the degree of hydrophilicity and the optimum condition was determined at 10 min etching time. Level crossing analysis based on AF images provided deeper insight into the microscopic details of the surface topography. For different etching time, it has been shown that the average frequency of visiting a height with positive slope behaves Gaussian for heights near the mean value and obeys power law for the heights far away from the mean value. Finally, by applying the generalized total number of crossings with positive slope, it was found that the both high heights and deep valleys of the surface are extremely effective in hydrophilic degree of the SiO_2/TiO_2/Glass nano bi-layer investigated system.
    08/2013;
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    Reza Sepehrinia, M. Reza Rahimi Tabar, Muhammad Sahimi
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    ABSTRACT: Localization of elastic waves in two-dimensional (2D) and three-dimensional (3D) media with random distributions of the Lamé coefficients (the shear and bulk moduli) is studied, using extensive numerical simulations. We compute the frequency dependence of the minimum positive Lyapunov exponent γ (the inverse of the localization length) using the transfer-matrix method, the density of states utilizing the force oscillator method, and the energy-level statistics of the media. The results indicate that all the states may be localized in the 2D media, up to the disorder width and the smallest frequencies considered, although the numerical results also hint at the possibility that there might be a small range of the allowed frequencies over which a mobility edge might exist. In the 3D media, however, most of the states are extended (with only a small part of the spectrum in the upper band tail that contains localized states) even if the Lamé coefficients are randomly distributed. Thus, the 3D heterogeneous media still possess a mobility edge. If both the Lamé coefficients vary spatially in the 3D medium, the localization length Λ follows a power law near the mobility edge, Λ∼(Ω−Ωc)−ν, where Ωc is the critical frequency. The numerical estimate, ν≃1.89±0.17, is significantly larger than the numerical estimate, ν≃1.57±0.01, and ν=3/2 (which was recently derived by a semiclassical theory for the 3D Anderson model of electron localization). If the shear modulus is constant but the bulk modulus varies spatially, the plane waves with transverse polarization propagate without any scattering—leading to a band of completely extended states, even in the 2D media. At the mobility edge of such media the localization length follows the same type of power law as Λ but with an exponent νT≃1/2 for both 2D and 3D media.
    Physical review. B, Condensed matter 08/2013; 78(2). · 3.77 Impact Factor
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    ABSTRACT: Although fluctuations in the waiting time series have been studied for a long time, some important issues such as its long-range memory and its stochastic features in the presence of nonstationarity have so far remained unstudied. Here we find that the "waiting times" series for a given increment level have long-range correlations with Hurst exponents belonging to the interval 1/2<H<1. We also study positive-negative level asymmetry of the waiting time distribution. We find that the logarithmic difference of waiting times series has a short-range correlation, and then we study its stochastic nature using the Markovian method and determine the corresponding Kramers-Moyal coefficients. As an example, we analyze the velocity fluctuations in high Reynolds number turbulence and determine the level dependence of Markov time scales, as well as the drift and diffusion coefficients. We show that the waiting time distributions exhibit power law tails, and we were able to model the distribution with a continuous time random walk.
    Physical Review E 06/2013; 87(6-1):062139. · 2.31 Impact Factor
  • E. Daryaei, M. Reza Rahimi Tabar, A.Z. Moshfegh
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    ABSTRACT: The effect of etching time on the statistical properties of hydrophilic surfaces of SiO2/TiO2/glass nano bilayers has been studied using atomic force microscopy (AFM) and a stochastic approach based on a level crossing analysis. We have created rough surfaces of the hydrophilic SiO2/TiO2 nano bilayer system by using 26% potassium hydroxide (KOH) solution. Measuring the average apparent contact angle allowed us to assess the degree of hydrophilicity, and the optimum condition was determined to be 10 min etching time. A level crossing analysis based on AFM images provided deeper insight into the microscopic details of the surface topography. With different etching times, it has been shown that the average frequency of visiting a height with positive slope behaves in a Gaussian manner for heights near the mean value and obeys a power law for heights far away from the mean value. Finally, by applying the generalized total number of crossings with positive slope, it was found that the both high heights and deep valleys of the surface have a great effect on the hydrophilic degree of the SiO2/TiO2/glass nano bilayer investigated system.
    Physica A: Statistical Mechanics and its Applications 05/2013; 392(9):2175–2181. · 1.68 Impact Factor
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    ABSTRACT: Large-scale ab initio molecular-dynamics simulations have been carried out to compute, at human-body temperature, the vibrational modes and lifetimes of pure and hydrated dipalmitoylphosphatidylcholine (DPPC) lipids. The projected atomic vibrations calculated from the spectral energy density are used to compute the vibrational modes and the lifetimes. All the normal modes of the pure and hydrated DPPC and their frequencies are identified. The computed lifetimes incorporate the full anharmonicity of the atomic interactions. The vibrational modes of the water molecules close to the head group of DPPC are active (possess large projected spectrum amplitudes) in the frequency range 0.5–55 THz, with a peak at 2.80THz in the energy spectrum. The computed lifetimes for the high-frequency modes agree well with the recent data measured at room temperature where high-order phonon scattering is not negligible. The computed lifetimes of the low-frequency modes can be tested using the current experimental capabilities. Moreover, the approach may be applied to other lipids and biomolecules, in order to predict their vibrational dispersion relations, and to study the dynamics of vibrational energy transfer.
    EPL (Europhysics Letters) 04/2013; 102:28008. · 2.26 Impact Factor
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    ABSTRACT: Using the cross-correlation of the wavelet transformation, we propose a general method of studying the scale dependence of the direction of coupling for coupled time series. The method is first demonstrated by applying it to coupled van der Pol forced oscillators and coupled nonlinear stochastic equations. We then apply the method to the analysis of the log-return time series of the stock values of the IBM and General Electric (GE) companies. Our analysis indicates that, on average, IBM stocks react earlier to possible common sector price movements than those of GE.
    Journal of Statistical Mechanics Theory and Experiment 02/2013; 2013(02):P02042. · 1.87 Impact Factor
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    ABSTRACT: We study the electronic properties of superlattice with rough interfaces in two and three dimensions using the transfer-matrix method and direct diagonalization of the Anderson Hamiltonian. The system consists of layers with an average constant width, but with stochastic roughness added to the interfaces between the layers. The numerical results indicate that, in the thermodynamic limit, the two-dimensional superlattice is an insulator in the presence of even small roughness. In three-dimensional systems, however, the superlattice exhibits a metal-insulator transition with a well-defined mobility edge located at an energy Ec that we compute numerically. For three-dimensional superlattice, the localization length follows a power law near the mobility edge ξ(E)∼(Ec−E)−ν, where the exponent is ν≃1.6. We also show that the existence of the extended states in three-dimensional superlattices gives rise to a finite conductivity in the limit M/L→∞, where L is the length and M the width of the bar.
    Physical review. B, Condensed matter 06/2012; 85(22). · 3.77 Impact Factor
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    ABSTRACT: We report the complete assignment of the vibrational spectrum of dipalmitoylphosphatidylcholine (DPPC), which belongs to the most ubiquitous membrane phospholipid family, phosphatidylcholine. We find that water hydrating the lipid headgroups enables efficient energy transfer across membrane leaflets on sub-picosecond time scales. The emergence of spatially extended vibrational modes upon hydration, underlies this phenomenon. Our findings illustrate the importance of collective molecular behavior of biomembranes and reveal that hydrated lipid membranes can act as efficient media for the transfer of vibrational energy.
    The Journal of Physical Chemistry B 05/2012; 116(22):6455-60. · 3.61 Impact Factor
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    ABSTRACT: We investigate the structure and electronic properties of phosphatidylcholine (PC) under different degrees of hydration at the single-molecule and monolayer type level by linear scaling ab initio calculations. Upon hydration, the phospholipid undergoes drastic long-range conformational rearrangements which lead to a sickle-like ground-state shape. The structural unit of the tilted gel-phase PC appears to be a water-bridged PC dimer. We find that hydration dramatically alters the surface potential, dipole and quadrupole moments of the lipids and consequently guides the interactions of the lipids with other molecules and the communication between cells.
    The Journal of Chemical Physics 03/2012; 136(11):114709. · 3.12 Impact Factor
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    A. Sheikhan, P. Maass, M. Reza Rahimi Tabar
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    ABSTRACT: The single and multiple scattering regimes of electromagnetic waves in a disordered system with fluctuating permittivity are studied by numerical simulations of Maxwell's equations. For an array of emitters and receivers in front of a medium with randomly varying dielectric constant, we calculate the backscattering matrix from the signal responses at all receiver points j to electromagnetic pulses generated at each emitter point i. We show that the statistical properties of the backscattering matrix are in agreement with the recent experimental results for ultrasonic waves (Aubry A. and Derode A., Phys. Rev. Lett., 102 (2009) 084301) and light (Popoff S. M. et al., Phys. Rev. Lett., 104 (2010) 100601). In the multiple scattering regime the singular value distribution of the backscattering matrix obeys the quarter-circle law.
    EPL (Europhysics Letters) 01/2012; 98(1). · 2.26 Impact Factor
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    ABSTRACT: Analysis of flow in fluidized beds, a common chemical reactor, is of much current interest due to its fundamental as well as industrial importance. Experimental data for the successive increments of the pressure fluctuations time series in a fluidized bed are analyzed by computing a multiscale probability density function (PDF) of the increments. The results demonstrate the evolution of the shape of the PDF from the short to long time scales. The deformation of the PDF across time scales may be modeled by the log-normal cascade model. The results are also in contrast to the previously proposed PDFs for the pressure fluctuations that include a Gaussian distribution and a PDF with a power-law tail. To understand better the properties of the pressure fluctuations, we also construct the shuffled and surrogate time series for the data and analyze them with the same method. It turns out that long-range correlations play an important role in the structure of the time series that represent the pressure fluctuation.
    Journal of Statistical Mechanics Theory and Experiment 01/2012; 2012(07). · 1.87 Impact Factor
  • M. Sadegh Movahed, F. Ghasemi, Sohrab Rahvar, M. Reza Rahimi Tabar
    Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics 09/2011; 84(3).
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    M Sadegh Movahed, F Ghasemi, Sohrab Rahvar, M Reza Rahimi Tabar
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    ABSTRACT: We investigate the statistical anisotropy and gaussianity of temperature fluctuations of Cosmic Microwave Background (CMB) radiation data from the Wilkinson Microwave Anisotropy Probe survey, using the Multifractal Detrended Fluctuation Analysis, Rescaled Range, and Scaled Windowed Variance methods. Multifractal Detrended Fluctuation Analysis shows that CMB fluctuations has a long-range correlation function with a multifractal behavior. By comparing the shuffled and surrogate series of CMB data, we conclude that the multifractality nature of the temperature fluctuation of CMB radiation is mainly due to the long-range correlations, and the map is consistent with a gaussian distribution.
    Physical Review E 08/2011; 84(2 Pt 1):021103. · 2.31 Impact Factor
  • N. Abedpour, Reza Asgari, M. Reza Rahimi Tabar
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    ABSTRACT: A Reply to the Comment by Mehdi Neek-Amal.
    Physical Review Letters 05/2011; 106(20):209702. · 7.73 Impact Factor
  • P. Partovi-Azar, N. Nafari, M. Reza Rahimi Tabar
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    ABSTRACT: It has been argued that the electron-hole puddles formed on graphene are mostly due to substrate-induced charged impurities [ J. Martin et al. Nature Phys. 4 144 (2008), Y. Zhang et al. Nature Phys. 5 722 (2009)]. Here, using first-principles ab initio calculations, we show that the existence of ripples and electron-hole puddles is indeed an intrinsic property of graphene at finite temperatures. We found a relatively large correlation between the electronic charge density distribution on the surface of graphene and its local geometrical properties, such as local mean curvature and average bond length. We show that the electron and hole puddles appear in places where curvatures are large and small, respectively. We also determined the average sizes of the observed electron-hole puddles and have reported their percolating nature.
    Physical review. B, Condensed matter 04/2011; 83(16). · 3.77 Impact Factor
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    ABSTRACT: Several methods have been developed in the past for analyzing the porosity and other types of well logs for large-scale porous media, such as oil reservoirs, as well as their permeability distributions. We developed a method for analyzing the porosity logs ϕ(h) (where h is the depth) and similar data that are often nonstationary stochastic series. In this method one first generates a new stationary series based on the original data, and then analyzes the resulting series. It is shown that the series based on the successive increments of the log y(h)=ϕ(h+δh)-ϕ(h) is a stationary and Markov process, characterized by a Markov length scale h(M). The coefficients of the Kramers-Moyal expansion for the conditional probability density function (PDF) P(y,h|y(0),h(0)) are then computed. The resulting PDFs satisfy a Fokker-Planck (FP) equation, which is equivalent to a Langevin equation for y(h) that provides probabilistic predictions for the porosity logs. We also show that the Hurst exponent H of the self-affine distributions, which have been used in the past to describe the porosity logs, is directly linked to the drift and diffusion coefficients that we compute for the FP equation. Also computed are the level-crossing probabilities that provide insight into identifying the high or low values of the porosity beyond the depth interval in which the data have been measured.
    Physical Review E 02/2011; 83(2 Pt 2):026309. · 2.31 Impact Factor
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    Journal of Statistical Mechanics Theory and Experiment 01/2011; 2011(09). · 1.87 Impact Factor
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    ABSTRACT: This review addresses a central question in the field of complex systems: given a fluctuating (in time or space), sequentially measured set of experimental data, how should one analyze the data, assess their underlying trends, and discover the characteristics of the fluctuations that generate the experimental traces? In recent years, significant progress has been made in addressing this question for a class of stochastic processes that can be modeled by Langevin equations, including additive as well as multiplicative fluctuations or noise. Important results have emerged from the analysis of temporal data for such diverse fields as neuroscience, cardiology, finance, economy, surface science, turbulence, seismic time series and epileptic brain dynamics, to name but a few. Furthermore, it has been recognized that a similar approach can be applied to the data that depend on a length scale, such as velocity increments in fully developed turbulent flow, or height increments that characterize rough surfaces. A basic ingredient of the approach to the analysis of fluctuating data is the presence of a Markovian property, which can be detected in real systems above a certain time or length scale. This scale is referred to as the Markov–Einstein (ME) scale, and has turned out to be a useful characteristic of complex systems. We provide a review of the operational methods that have been developed for analyzing stochastic data in time and scale. We address in detail the following issues: (i) reconstruction of stochastic evolution equations from data in terms of the Langevin equations or the corresponding Fokker–Planck equations and (ii) intermittency, cascades, and multiscale correlation functions.
    Physics Reports 01/2011; 506(5):87-162. · 22.93 Impact Factor
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    ABSTRACT: High-quality measurements of seismic activities around the world provide a wealth of data and information that are relevant to understanding of when earthquakes may occur. If viewed as complex stochastic time series, such data may be analyzed by methods that provide deeper insights into their nature, hence leading to better understanding of the data and their possible implications for earthquakes. In this paper, we provide further evidence for our recent proposal [P. Mansour, Phys. Rev. Lett. 102, 014101 (2009)10.1103/PhysRevLett.102.014101] for the existence of a transition in the shape of the probability density function (PDF) of the successive detrended increments of the stochastic fluctuations of Earth's vertical velocity V_{z} , collected by broadband stations before moderate and large earthquakes. To demonstrate the transition, we carried out extensive analysis of the data for V_{z} for 12 earthquakes in several regions around the world, including the recent catasrophic one in Haiti. The analysis supports the hypothesis that before and near the time of an earthquake, the shape of the PDF undergoes significant and discernable changes, which can be characterized quantitatively. The typical time over which the PDF undergoes the transition is about 5-10 h prior to a moderate or large earthquake.
    Physical Review E 09/2010; 82(3 Pt 2):036105. · 2.31 Impact Factor

Publication Stats

919 Citations
250.96 Total Impact Points

Institutions

  • 1996–2013
    • Sharif University of Technology
      • Department of Physics
      Teheran, Tehrān, Iran
  • 2010–2012
    • Universität Osnabrück
      Osnabrück, Lower Saxony, Germany
  • 2011
    • Shahid Beheshti University
      • Department of Physics
      Tehrān, Ostan-e Tehran, Iran
  • 2009
    • University of Southern California
      • Department of Chemical Engineering and Materials Science
      Los Angeles, CA, United States
    • Carl von Ossietzky Universität Oldenburg
      • Department of Physics
      Oldenburg, Lower Saxony, Germany
  • 2002–2009
    • Institute for Research in Fundamental Sciences (IPM)
      • Physics
      Tehrān, Ostan-e Tehran, Iran
  • 2002–2008
    • Collège de France
      Lutetia Parisorum, Île-de-France, France
  • 2007
    • Max Planck Institute for the Physics of Complex Systems
      Dresden, Saxony, Germany
  • 2006
    • Institute For Trade Studies and Research, Iran
      Dārāb, Fārs, Iran
  • 2005–2006
    • Observatoire de la Côte d'Azur
      Grasse, Provence-Alpes-Côte d'Azur, France
    • Isfahan University of Technology
      • Department of Physics
      Eşfahān, Ostan-e Esfahan, Iran
  • 1996–2000
    • Iran University of Science and Technology
      • School of Physics
      Teheran, Tehrān, Iran