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Partner: UNT College of Arts and Sciences
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₁¹H+ - and ₂⁴He+ - induced M-shell x-ray-production cross sections for selected elements in the rare-earth region
This article discusses ₁¹H+ and ₂⁴He+ -induced M-shell x-ray-production cross sections for selected elements in the rare-earth region. digital.library.unt.edu/ark:/67531/metadc139491/
Ab initio transport properties of nanostructures from maximally localized Wannier functions
Article on ab initio transport properties of nanostructures from maximally localized Wannier functions. Abstract: We present a comprehensive first-principles study of the ballistic transport properties of low-dimensional nanostructures such as linear chains of atoms (Al, C) and carbon nanotubes in the presence of defects. An approach is introduced where quantum conductance is computed from the combination of accurate plane-wave electronic structure calculations, the evaluation of the corresponding maximally localized Wannier functions, and the calculation of transport properties by a real-space Green's function method based on the Landauer formalism. This approach is computationally very efficient, can be straightforwardly implemented as a post-processing step in a standard electronic-structure calculation, and allows us to directly link the electronic transport properties of a device to the nature of the chemical bonds, providing insight onto the mechanisms that govern electron flow at the nanoscale. digital.library.unt.edu/ark:/67531/metadc234911/
Absorption and Emission in the Non-Poissonian Case
This article discusses absorption and emission in the Non-Poissonian Case. Abstract: This Letter addresses the challenging problems posed to the Kubo-Anderson (KA) theory by the discovery of intermittent resonant fluorescence with a nonexponential distribution of waiting times. We show how to extend the KA theory from aged to aging systems, aging for a very extended time period or even forever, being a crucial consequence of non-Poisson statistics. digital.library.unt.edu/ark:/67531/metadc67641/
Activation of water on the TiO2 (110) surface: The case of Ti adatoms
This article discusses the activation of water on the TiO2 (110) surface. Abstract: Using first-principles calculations we have studied the reactions of water over Ti adatoms on the (110) surface of rutile TiO2. Our results provide fundamental insights into the microscopic mechanisms that drive this reaction at the atomic level and assess the possibility of using this system to activate the water dissociation reaction. In particular, we show that a single water molecule dissociates exothermically with a small energy barrier of 0.17 eV. After dissociation, both H⁺ and OH⁻ ions bind strongly to the Ti adatom, which serves as an effective reactive center on the TiO2 surface. Finally, clustering of Ti adatoms does not improve the redox activity of the system and results in a slightly higher energy barrier for water dissociation. digital.library.unt.edu/ark:/67531/metadc132989/
Aging and Rejuvenation with Fractional Derivatives
This article discusses aging rejuvenation with fractional derivatives. Abstract: We discuss a dynamic procedure that makes a fractional derivatives emerge in the time asymptotic limit of non-Poisson processes. We find that two-state fluctuations, with an inverse power-law distribution of waiting times, finite first moment, and divergent second moment, namely, with the power index μ in the interval 2<μ<3, yield a generalized master equation equivalent to the sum of an ordinary Markov contribution and a fractional derivative term. We show that the order of the fractional derivative depends on the age of the process under study. If the system is infinitely old, the order of the fractional derivative, o, is given by o=3-μ. A brand new system is characterized by the degree o=μ-2. If the system is prepared at time -tₐ<0 and the observation begins at time t=0, we derive the following scenario. For times 0<t«tₐ the system is satisfactorily described by the fractional derivative with o=3-μ. Upon time increase the system undergoes a rejuvenation process that in the time limit t⪢tₐ yields o=μ-2. The intermediate time regime is probably incompatible with a picture based on fractional derivatives, or, at least, with a mono-order fractional derivative. digital.library.unt.edu/ark:/67531/metadc67638/
Aging and Rejuvenation with Fractional Derivatives
Paper discussing aging and rejuvenation with fractional derivatives. Abstract: We discuss a dynamic procedure that makes the fractional derivative emerge in the time asymptotic limit of non-Poisson processes. We find that two-state fluctuations, with an inverse power-law distribution of waiting times, finite first moment and divergent second moment, namely with the power index μ in the interval 2 < μ < 3, yields a generalized master equation equivalent to the sum of an ordinary Markov contribution and of a fractional derivative term. We show that the order of the fractional derivative depends on the age of the process under study. If the system is infinitely old, the order of the fractional derivative, ord = μ - 2. If the system is prepared at time -tₐ < 0 and the observation begins at time t = 0, we derive the following scenario. For times 0 < t << tₐ the system is satisfactorily described by the fractional derivative with ord = 3 - μ. Upon time increase the system undergoes a rejuvenation process that in the time limit t >> tₐ yields ord = μ - 2. The intermediate time regime is probably incompatible with a picture based on fractional derivatives, or, at least, with a mono-order fractional derivative. digital.library.unt.edu/ark:/67531/metadc174699/
Aging in financial market
Article discussing aging in the financial market. Abstract: We analyze the data of the Italian and U.S. futures on the stock markets and we test the validity of the Continuous Time Random Walk assumption for the survival probability of the returns time series via a renewal aging experiment. We also study the survival probability of returns sign and apply a coarse graining procedure to reveal the renewal aspects of the process underlying its dynamics. digital.library.unt.edu/ark:/67531/metadc174703/
Analytical solutions for a two-level system driven by a class of chirped pulses
In this article, the authors present analytical solutions for the problem of a two-level atom driven by a class of chirped pulses. The solutions are given in terms of Heun functions. By use of the appropriate chirping parameters, an enhancement of four orders of magnitude in the population transfer is obtained. digital.library.unt.edu/ark:/67531/metadc103257/
L- and M-shell x-ray production cross sections of Nd, Gd, Ho, Yb, Au, and Pb by 25-MeV carbon and 32-MeV oxygen ions
This article discusses research on L- and M-shell x-ray production cross sections. For this research, L- and M-shell x-ray production cross sections have been measured for thin solid targets of neodymium, gadolinium, holmium, ytterbium, gold and lead by 25-MeV ₆¹²C(q)+ (q=4,5,6) and by 32-MeV ₈¹⁶O(q)+ (q=5,7,8). digital.library.unt.edu/ark:/67531/metadc139494/
Anomalous diffusion and ballistic peaks: A quantum perspective
This article discusses anomalous diffusion and ballistic peaks. Abstract: The quantum kicked rotor and the classical kicked rotor are both shown to have truncated Lévy distributions in momentum space, when the classical phase space has accelerator modes embedded in a chaotic sea. The survival probability for classical particles at the interface of an accelerator mode and the chaotic sea has an inverse power-law structure, whereas that for quantum particles has a periodically modulated inverse power law, with the period of oscillation being dependent on Planck's constant. These logarithmic oscillations are a renormalization group property that disappears as ħ → 0 in agreement with the correspondence principle. digital.library.unt.edu/ark:/67531/metadc75417/
Anomalous diffusion and environment-induced quantum decoherence
This article discusses anomalous diffusion and environment-induced quantum decoherence. Abstract: We study the anomalous diffusion resulting from the standard map in the so-called accelerating state, and we observe that it is determined by unusually large times of sojourn of the classical trajectories in the fractal region at the border between the chaotic sea and the acceleration island. The quantum-mechanical breakdown of this property implies a coherence among so slightly different values of momentum as to become much more robust against environment fluctuations than the quantum localization corresponding to normal diffusion. digital.library.unt.edu/ark:/67531/metadc139477/
Anomalous diffusion associated with nonlinear fractional derivative Fokker-Planck-like equation: Exact time-dependent solutions
This article discusses anomalous diffusion associated with nonlinear fractional derivative Fokker-Planck-like equation. Abstract: We consider the d=1 nonlinear Fokker-Planck-like equation with fractional derivatives (∂/∂t)P(x,t) = D(∂ƴ/∂xƴ)[P(x,t]v. Exact time-dependent solutions are found for v = (2 - y)/(1 + y)(-∞ < y ⩽ 2). By considering the long-distance asymptotic behavior of these solutions, a connection is established, namely, q = (y + 3)/(Y + 1)(0 < y ⩽ 2), with the solutions optimizing the nonextensive entropy characterized by index q. Interestingly enough, this relation coincides with the only already known for Lévy-like superdiffusion (i.e., v = 1 and 0 < y ⩽ 2). Finally, for (y,v) = (2,0) the authors obtain q=5/3, which differs from the value q = 2 corresponding to the y = 2 solutions available in the literature (v < 1 porous medium equation), thus exhibiting nonuniform convergence. digital.library.unt.edu/ark:/67531/metadc77162/
Atomic resolution ultrahigh vacuum scanning tunneling microscopy of epitaxial diamond (100) films
In this article, the authors report atomic resolution images of chemical vapor deposition grown epitaxial diamond (100) films obtained in ultrahigh vacuum (UHV) with a scanning tunneling microscope. A (2X1) dimer surface reconstruction and amorphous atomic regions are observed. The (2X1) unit cell is measured to be 0.51±0.01X0.25±0.01 nm2. The amorphous regions are identified as carbon. A radial structure 1.5 nm in diameter is observed on a plane at a 20° slope to the (2X1) surface. Tunneling current versus voltage spectra in UHV and Raman spectra are also obtained. digital.library.unt.edu/ark:/67531/metadc84147/
Atomic structure of steps and defects on the clean diamond (100)-2 X 1 surface studied using ultrahigh vacuum scanning tunneling microscopy
In this article, the authors report ultrahigh vacuum scanning tunneling microscopy studies of the clean nonhydrogen-terminated diamond (100)-2 X 1 surface showing single- and double-layer steps that are rebonded. The main defects observed are single, multiple, and row dimer vacancies, and antiphase boundaries. Buckling of dimers is not observed, consistent with symmetric dimers. digital.library.unt.edu/ark:/67531/metadc83794/
Beyond the Death of Linear Response: 1/f Optimal Information Transport
This article discusses linear response and 1/f optimal information transport. Article: Nonergodic renewal processes have recently been shown by several authors to be insensitive to periodic perturbations, thereby apparently sanctioning the death of linear response, a building block of nonequilibrium statistical physics. The authors show that it is possible to go beyond the "death of linear response" and establish a permanent correlation between an external stimulus and the response of a complex network generating nonergodic renewal processes, by taking as stimulus a similar nonergodic process. The ideal condition of 1/f noise corresponds to a singularity that is expected to be relevant in several experimental conditions. digital.library.unt.edu/ark:/67531/metadc40407/
This article is a reply to a comment by Massimo Falcioni and Angelo Vulpiani. In a previous letter, the authors have discussed the linear response theory (LRT) and shown that the breakdown of this theory occurring at intermediate times, observed in an earlier paper [2] as well as in [1], disappears upon an increase of the number of degrees of freedom. In a comment to [1] Falcioni and Vulpiani [3] claim that this breakdown is rather a consequence of the lack of mixing: according to them, regardless of the number of degrees of freedom, mixing is the key ingredient behind the LRT. digital.library.unt.edu/ark:/67531/metadc77166/
Brain, Music, and Non-Poisson Renewal Processes
Breakdown of the Onsager Principle as a Sign of Aging
Article discussing the breakdown of the Onsager principle as a sign of aging. Abstract: We discuss the problem of the equivalence between Continuous Time Random Walk (CTRW) and Generalized Master Equation (GME). The walker, making instantaneous jumps from one site of the lattice to another, resides in each site or extended time. The sojourn times have a distribution ψ (t) that is assumed to be an inverse power law with the power index μ. We assume that the Onsager principle is fulfilled, and we use this assumption to establish a complete equivalence between GME and the Montroll-Weiss CTRW. We prove that this equivalence is confined to the case when ψ (t) is an exponential. We argue that is so because the Montroll-Weiss CTRW, as recently proved by Barkai [E. Barkai, Phys. Rev. Lett. 90, 104101 (2003)], is non-stationary, thereby implying aging, while the Onsager principle, is valid only in the case of fully aged systems. The case of Poissonian distribution of sojourn times is the only one with no aging associated to it, and consequently with no need to establish special initial conditions to fulfill the Onsager principle. We consider the case of a dichotomous fluctuation, and we prove that the Onsager principle is fulfilled for any form of regression to equilibrium provided that the stationary condition hold true. We set the stationary condition on both the CTRW and the GME, thereby creating a condition of total equivalence, regardless the nature of the waiting time distribution. As a consequence of this procedure we create a GME that is a bona fide master equation, in spite of being non-Markovian. We note that the memory kernel of the GME affords information on the interaction between system of interest and its bath. The Poisson case yields a bath with infinitely fast fluctuations. We argue that departing from the Poisson form has the effect of creating a condition of infinite memory and that these results might be useful to shed light into the problem of how to unravel non-Markovian master equations. digital.library.unt.edu/ark:/67531/metadc174692/
Canonical and noncanonical equilibrium distribution
This article discusses canonical and noncanonical equilibrium distribution. Abstract: We address the problem of the dynamical foundation of noncanonical equilibrium. We consider, as a source of divergence from ordinary statistical mechanics, the breakdown of the condition of time scale separation between microscopic and macroscopic dynamics. We show that this breakdown has the effect of producing a significant deviation from the canonical prescription. We also show that, while the canonical equilibrium can be reached with no apparent dependence on dynamics, the specific form of noncanonical equilibrium is, in fact, determined by dynamics. We consider the special case where the thermal reservoir driving the system of interest to equilibrium is a generator of intermittent fluctuations. We assess the form of the noncanonical equilibrium reached by the system in this case. Using both theoretical and numerical arguments we demonstrate that Lévy statistics are the best description of the dynamics and that the Lévy distribution is the correct basin of attraction. We show that the correct path to noncanonical equilibrium by means of strictly thermodynamic arguments has not yet been found, and that further research has to be done to establish a connection between dynamics and thermodynamics. digital.library.unt.edu/ark:/67531/metadc77164/
Carbon K-shell x-ray and Auger-electron production in hydrocarbons and carbon oxides by 0.6-2.0-MeV protons
This article discusses carbon K-shell x-ray and Auger-electron production in hydrocarbons and carbon oxides by 0.6-2.0-MeV protons. digital.library.unt.edu/ark:/67531/metadc139493/
Carbon Nanotube-Metal Cluster Composites: A New Road to Chemical Sensors?
Article on carbon nanotube-metal cluster composites. Abstract: Novel carbon nanotube-metal cluster structures are proposed as prototype systems for molecular recognition at the nanoscale. Ab initio calculations show that already the bare nanotube cluster system displays some specificity because the adsorption of ammonia on a carbon nanotube-Al cluster system is easily detected electrically, while diborane adsorption does not provide an electrical signature. Since there are well-established procedures for attaching molecular receptors to metal clusters, these results provide a "proof-of-principle" for the development of novel, high-specificity molecular sensors. digital.library.unt.edu/ark:/67531/metadc228297/
Carrier-Envelope Phase Effect on Atomic Excitation by Few-Cycle rf Pulses
This article discusses carrier-envelope phase effect on atomic excitation by few-cycle rf pulses. Abstract: We present an experimental and theoretical study of the carrier-envelope phase effects on population transfer between two bound atomic states interacting with intense ultrashort pulses. Radio frequency pulses are used to transfer population among the ground state hyperfine levels in rubidium atoms. These pulses are only a few cycles in duration and have Rabi frequencies of the order of the carrier frequency. The phase difference between the carrier and the envelope of the pulses has a significant effect on the excitation of atomic coherence and population transfer. The authors provide a theoretical description of this phenomenon using density matrix equations. The authors discuss the implications and possible applications of their results. digital.library.unt.edu/ark:/67531/metadc103274/
Chaos and thermal conductivity
This article discusses chaos and thermal conductivity. Abstract: We argue that the condition of local thermal equilibrium realized several years ago by Rich and Visscher [Phys. Rev. B 11, 2164 (1975)] through a process of mathematical convergence can be obtained dynamically by adopting the prescription of a recent paper [M. Bianucci, R. Mannella, B.J. West, and P. Grigolini, Phys. Rev. E 51, 3002 (1995)]. This should contribute to shedding light on the still unsolved problem fo the microscopic derivation of the heat Fourier law. digital.library.unt.edu/ark:/67531/metadc139502/
Charge-state dependence of K-shell x-ray production in aluminum by 2-12-MeV carbon ions
This article discusses charge-state dependence of K-shell x-ray production in aluminum by 2-12-MeV carbon ions. Abstract: Charge-state dependence for K-shell x-ray production cross sections in 13Al bombarded by 2-12-MeV 6C ions with charge states from 2+ to 6+ was measured using a Si(Li) detector. A thin Al target was used to ensure single collision conditions. Contributions of the electron capture as well as direct ionization to the inner-shell ionization were determined by an analysis of the charge-state dependence of the target x-ray production. The measurements are compared with the prediction of the ECPSSR theory using a single-hole fluorescence yield. The ECPSSR theory is based on the perturbed stationary state (PSS) formalism and relativistic efforts (R) for the target electrons, and energy loss (E) and Coulomb deflection (C) of the projectile. In general, this theory gives reasonable agreement with the data for carbon ions without K vacancies while it overpredicts the data for carbon ions with K vacancies. The significant underprediction of the data at the lowest energy is likely associated with the molecular-orbital effect that is not accounted for in the ECPSSR theory. digital.library.unt.edu/ark:/67531/metadc139497/
Charge-state dependence of M-shell x-ray production in 67Ho by 2-12-MeV carbon ions
This article discusses charge-state dependence of M-shell x-ray production in 67Ho by 2-12-MeV carbon ions. Abstract: Charge-state dependence of M-shell x-ray production cross sections of 67Ho bombarded by 2-12-MeV carbon ions, with and without K-shell vacancies, were measured using a windowless Si(Li) x-ray detector with a full-width-at-half-maximum resolution of 135 eV at 5.9 keV. Carbon ions of different charge states were produced using a postacceleration, nitrogen gas stripping cell. The carbon ions were then magnetically analyzed to select the desired charge state and energy before entering the target chamber. The total M-shell and Mζ, Mα,β, and Mγ x-ray cross sections were measured. The electron-capture (EC) contributions as well as the direct-ionization (DI) contributions can be determined by making a comprehensive study of the projectile-charge-state dependence of the target x-ray production cross sections for targets in which the single-collision realm is maintained. In this paper, both EC and DI contributions and the total M-shell x-ray production cross sections are compared to both the first Born theory and to the perturbed-stationary-state theory with energy-loss, Coulomb-deflection, and relativistic corrections. digital.library.unt.edu/ark:/67531/metadc139496/
Charge transfer equilibria in ambient-exposed epitaxial graphene on (0001) 6 H-SiC
This article discusses charge transfer equilibria in ambient-exposed epitaxial graphene on (0001) 6 H-SiC. Abstract: The transport properties of electronic materials have been long interpreted independently from both the underlying bulk-like behavior of the substrate or the influence of ambient gases. This is no longer the case for ultra-thin graphene whose properties are dominated by the interfaces between the active material and its surroundings. Here, the authors show that the graphene interactions with its environments are critical for the electrostatic and electrochemical equilibrium of the active device layers and their transport properties. Based on the prototypical case of epitaxial graphene on (0001) 6 H-SiC and using a combination of 'in-situ' thermoelectric power and resistance measurements and simulations from first principles, the authors demonstrate that the cooperative occurrence of an electrochemically mediated charge transfer from the graphene to air, combined with the peculiar electronic structure of the graphene/SiC interface, explains the wide variation of measured conductivity and charge carrier type found in prior reports. digital.library.unt.edu/ark:/67531/metadc132974/
Chiral Steering of Molecular Organization in the Limit of Weak Adsorbate-Substrate Interactions: Enantiopure and Racemic Tartaric Acid Domains on Ag(111)
Article on chiral steering of molecular organization in the limit of weak adsorbate-substrate interactions. Abstract: The influence of intermolecular interactions involving molecular chiral centers on two-dimensional organization in the limit of a weak adsorbate-surface interaction has been studied with low-temperature scanning tunneling microscopy (STM) and density functional theory (DFT). A model system composed of a chiral organic molecule, tartaric acid, and an inert metallic surface, Ag(111), was employed. Dual component films formed from the serial deposition of (S,S)- and (R,R)-tartaric acid enantiomers onto this surface exhibit homochiral domain formation as revealed by molecularly resolved STM images. In contrast, a unique tartaric acid enantiomeric heteropair is experimentally and computationally verified as the basis unit of films formed via the deposition of both enantiomers simultaneously from a racemic (1:1) mixture. The molecular adsorption geometry relative to the Ag(111) lattice in both enantiomerically pure and racemic domains is determined primarily by the interaction of chiral centers between nearest neighbors. digital.library.unt.edu/ark:/67531/metadc228317/
CO Adsorption on Noble Metal Clusters: Local Environment Effects
Cognitive Scale-Free Networks as a Model for Intermittency in Human Natural Language
Paper discussing cognitive scale-free networks as a model for intermittency in human natural language. Abstract: We model certain features of human language complexity by means of advanced concepts borrowed from statistical mechanics. Using a time series approach, the diffusion entropy method (DE), we compute the complexity of an Italian corpus of newspapers and magazines. We find that the anomalous scaling index is compatible with a simple dynamical model, a random walk on a complex scale-free network, which is linguistically related to Saussurre's paradigms. The model yields the famous Zipf's law in terms of the generalized central limit theorem. digital.library.unt.edu/ark:/67531/metadc174698/
Coherent control of atomic excitation using off-resonant strong few-cycle pulses
This article discusses coherent control of atomic excitation using off-resonant strong few-cycle pulses. Abstract: We study the dynamics of a two-level system driven by an off-resonance few-cycle pulse which has a phase jump ø at t = t₀, in contrast to many-cycle pulses, under the nonrotating-wave approximation (NRWA). We give a closed form analytical solution for the evolution of the probability amplitude |Cₐ(t)| for the upper level. Using the appropriate pulse parameters like the phase jump ø, jump time t₀, pulse width Շ, frequency ν, and Rabi frequency Ώ₀ the population transfer after the pulse is gone can be optimized and, for the pulse considered here, an enhancement factor of 10⁶-10⁸ was obtained. digital.library.unt.edu/ark:/67531/metadc103266/
Coherent control of refractive index in far-detuned Λ systems
This article discusses the coherent control of refraction index in far-detuned Λ systems. Abstract: Enhancement and control of the index of refraction in a mixture of two three-level atomic species that form a pair of far-detuned Λ schemes under two-photon resonance and has been studied. We employ the density-matrix approach to properly take population relaxation into account and to describe the interaction of each Λ system with the electromagnetic fields. Both Λ systems are driven by a corresponding far-detuned coherent field at one atomic transition and are probed by the same weak field. In the dressed-state basis, it represents a superposition of effective two-level subsystems with the positions, widths, and amplitudes of the resonances controlled by the driving fields and allows for efficient control of the susceptibility of the total system; leading to refractive index (RI) enhancement with vanishing absorption in the absence of amplification. We analyze the experimental implementation of such a system in a cell of Rb atoms with a natural abundance of isotopes. An upper limit estimate of the RI enhancement is obtained. digital.library.unt.edu/ark:/67531/metadc103270/
Coherent excitation of a two-level atom driven by a far-off-resonant classical field: Analytical solutions
This article discusses the coherent excitation of a two-level atom driven by a far-off-resonant classical field. Abstract: We present an analytical treatment of coherent excitation of a two-level atom driven by a far-off-resonant classical field. A class of pulse envelope is obtained for which this problem is exactly solvable. The solutions are given in terms of the Heun function, which is a generalization of the hypergeometric function. Degeneracy of the Heun to a hypergeometric equation can give all the exactly solvable pulse shapes of Gauss hypergeometric form from the generalized pulse shape obtained here. We discuss the application of the results obtained to the generation of soft x-ray and ultraviolet radiations. digital.library.unt.edu/ark:/67531/metadc103262/
Collective behavior and evolutionary games - An introduction
Article on collective behaviors and evolutionary games. Abstract: This is an introduction to the special issue titled "Collective behavior and evolutionary games" that is in the making at Chaos, Solitons & Fractals. The term collective behavior covers many different phenomena in nature and society. From bird flocks and fish swarms to social movements and herding effects, it is the lack of a central planner that makes the spontaneous emergence of sometimes beautifully ordered and seemingly meticulously designed behavior all the more sensational and intriguing. The goal of the special issue is to attract submissions that identify unifying principles that describe the essential aspects of collective behavior, and which thus allow for a better interpretation and foster the understanding of the complexity arising in such systems. As the title of the special issue suggests, the later may come from the realm of evolutionary games, but this is certainly not a necessity, neither for this special issue, and certainly not in general. Interdisciplinary work on all aspects of collective behavior, regardless of background and motivation, and including synchronization and human cognition, is very welcome. digital.library.unt.edu/ark:/67531/metadc174707/
Collective polarization effects in β-polyvinylidene fluoride and its copolymers with tri- and tetrafluoroethylene
Article on collective polarization effects in β-polyvinylidene fluoride and its copolymers with tri- and tetrafluoroethylene. Abstract: The polar properties of the β phase of polyvinylidene fluoride (PVDF) and its copolymers with tri- and tetrafluoroethylene are investigated theoretically at different VDF-to-copolymer ratios. The calculations show that polarization in such polymers is described by cooperative, quantum-mechanical interactions between polymer chains, which cannot be viewed as a superposition of rigid dipoles. For β-PVDF, the monomer dipole moment is increased by 50% (from 2 to 3 D) as the isolated chains are brought together to form a crystal. In PVDF crystals containing copolymers, we observe a weakly parabolic dependence of monomer dipole moments on copolymer concentration. digital.library.unt.edu/ark:/67531/metadc234928/
Comment on "Modification of graphene properties due to electron-beam irradiation"
Complex Materials for Molecular Spintronics Applications: Cobalt Bis(dioxolene) Valence Tautomers, from Molecules to Polymers
This article discusses complex materials for molecular spintronics applications. Abstract: Using first principles calculations, the authors predict a complex multifunctional behavior in cobalt bis(dioxolene) valence tautomeric compounds. Molecular spin-state switching is shown to dramatically alter electronic properties and corresponding transport properties. This spin state dependence has been demonstrated for technologically relevant coordination polymers of valence tautomers as well as for novel conjugated polymers with valence tautomeric functionalization. As a result, these materials are proposed as promising candidates for spintronic devices that can couple magnetic bistability with novel electrical and spin conduction properties. The authors' findings pave the way to the fundamental understanding and future design of active multifunctional organic materials for spintronics applications. digital.library.unt.edu/ark:/67531/metadc132994/
Complexity and Synchronization
Complexity and the Fractional Calculus
Paper discussing complexity and fractional calculus. Abstract: We study complex processes whose evolution in time rests on the occurrence of a large and random number of events. The mean time interval between two consecutive critical events is infinite, thereby violating the ergodic condition and activating at the same time a stochastic central limit theorem that supports the hypothesis that the Mittag-Leffler function is a universal property of nature. The time evolution of these complex systems is properly generated by means of fractional differential equations, thus leading to the interpretation of fractional trajectories as the average over many random trajectories each of which satisfies the stochastic central limit theorem and the condition for the Mittag-Leffler universality. digital.library.unt.edu/ark:/67531/metadc174709/
Compression and Diffusion: A Joint Approach to Detect Complexity
Article discussing a joint approach to detect complexity by combining the Compression Algorithm Sensitive To Regularity (CASToRe) and Complex Analysis of Sequences via Scaling AND Randomness Assessment (CASSANDRA) procedures. digital.library.unt.edu/ark:/67531/metadc139462/
Conflict between trajectories and density description: the statistical source of disagreement
Paper discussing the statistical source of disagreement between trajectories and density description. Abstract: We study an idealized version of intermittent process leading the fluctuations of a stochastic dichotomous variable. It consists of an overdamped and symmetric potential well with a cusp-like minimum. The right-hand and left-hand portions of the potential corresponds to = W and = W, respectively. When the particle reaches this minimum is injected back to a different and randomly chosen position, still within the potential well. We build up the corresponding Frobenius-Perron equation and we evaluate the correlation function of the stochastic variable, called (t). We assign the potential well a form yielding (t) = (T = (t=T)), with > 0. Thanks to the symmetry of potential, there are no biases, and we limit ourselves to considering correlation functions with an even number of times, indicated for concision, by h12i, h1234i, and more, in general, by h1:::2ni. The adoption of a formal treatment, based on density, and thus of the operator driving the density time evolution, establishes a prescription for the evaluation of the correlation functions, yielding h1::2ni - h12i:::h(2n 1)2ni. We study the same dynamic problem using trajectories, and we establish that the resulting two-time correlation function coincides with that ordered by the density picture, as it should. We then study the four-times correlation function and we prove that in the non-Poisson case it departs from the density prescription, namely, from h1234i=h12ih34i. We conclude that this is the main reason why the two pictures yield two different diffusion processes, as noticed in an earlier work. [M. Bologna, P. Grigolini, B. J. West, Chem. Phys. 284, (1-2) 115-128 (2002)]. digital.library.unt.edu/ark:/67531/metadc174689/
Control of chaos in a CO2 laser
This article discusses the control of chaos in a CO2 laser. Abstract: We report the experimental control of chaos in an optically modulated CO2 laser. The CO2 laser was driven into chaos by injecting a feedback beam modulated by an electro-optical modulator. Control of chaos was achieved using a modified proportional feedback technique in which the control pulses were delayed by approximately one relaxation period. Using this technique, it was possible to control unstable periodic orbits up to period 6. digital.library.unt.edu/ark:/67531/metadc84149/
Controllable enhanced dragging of light in ultradispersive media
This article discusses controllable enhanced dragging of light in ultradispersive media. Abstract: We have theoretically demonstrated an enhanced Fizeau effect due to dragging the light that occurs when the group velocity of light is ultraslow. The proposed experiment can be done in a cell of atomic Rb vapor under conditions such that the group velocity of light is of the order of a few hundred meters per second. We show theoretically that higher-order dispersion can influence the Fizeau effect and can be observed experimentally. It has been shown that the change of phase is sensitive to the motion of the cell with the speed of the order of 10⁻³ cm/s and for possible displacements as small as 10 Å. The enhanced dragging effect can be applied for position control, detection of slow mechanical motion, and efficient modulators of light. digital.library.unt.edu/ark:/67531/metadc103255/
Controlled terahertz frequency response and transparency of Josephson chains and superconducting multilayers
This article discusses controlled terahertz frequency response and transparency of Josephson chains and superconducting multilayers. Abstract: A fundamental property of wave propagation is Anderson localization, which affects the transfer of information, energy, mass, and charge in disordered media. This localization can manifest itself via, e.g., the metal-insulator transition. We exactly map the behavior of a quantum particle moving in a potential with correlated disorder to teh sub-terahertz wave propagation in either Josephson chaines or superconducting multilayers. When the Josephson junction parameters vary randomly, the sub-THz electromagnetic waves cannot propagate through these Josephson structures due to localization. For parameter variations with long-range correlations, we predict sharp transitions from transparent to reflective frequency regions for Josephson plasma waves. With appropriate choices of the correlation function, frequency windows with targeted or designed transparencies for THz or sub-THz electromagnetic waves could be achieved. This could be useful for tailoring the electromagnetic wave spectrum of Josephson arrays within the THz frequency range, which is important for applications in physics, astronomy, chemistry, biology, and medicine. digital.library.unt.edu/ark:/67531/metadc103256/
Cooperation in neural systems: Bridging complexity and periodicity
This article discusses cooperation in neural systems. Abstract: Inverse power law distributions are generally interpreted as a manifestation of complexity, and waiting time distributions with power index μ < 2 reflect the occurrence of ergodicity-breaking renewal events. In this paper we show how to combine these properties with the apparently foreign clocklike nature of biological processes. We use a two-dimensional regular network of leaky integrate-and-fire neurons, each of which is linked to its four nearest neighbors, to show that both complexity and periodicity are generated by locality breakdown: Links of increasing strength have the effect of turning local interactions into long-range interactions, thereby generating time complexity followed by time periodicity. Increasing the density of neuron firings reduces the influence of periodicity, thus creating a cooperation-induced renewal condition that is distinctly non-Poissonian. digital.library.unt.edu/ark:/67531/metadc132986/
Cooperation-induced topological complexity: a promising road to fault tolerance and Hebbian learning
Article discussing decoherence, wave function collapses, and non-ordinary statistical mechanics. Abstract: We consider a toy model of pointer interacting with a 1/2-spin system, whose $\sigma_{x}$ variable is \emph{measured} by the environment, according to the prescription of decoherence theory. If the environment measuring the variable $\sigma_{x}$ yields ordinary statistical mechanics, the pointer sensitive to the 1/2-spin system undergoes the same, exponential, relaxation regardless of whether real collapses or an entanglement with the environment, mimicking the effect of real collapses, occur. In the case of non-ordinary statistical mechanics the occurrence of real collapses make the pointer still relax exponentially in time, while the equivalent picture in terms of reduced density matrix generates an inverse power law relaxation. digital.library.unt.edu/ark:/67531/metadc174684/