Barrier Fluctuations Research Articles

Diffusion over a fluctuating barrier in underdamped dynamics

We apply a Langevin model by imposing additive and multiplicative noises to study thermally activated diffusion over a fluctuating barrier in underdamped dynamics. The barrier fluctuation is characterized by Gaussian colored noise with exponential correlation. We present the exact solutions for the first and second moments. Furthermore, we use direct simulations to calculate the asymptotic probability for a Brownian particle passing over the fluctuating barrier. The results indicate that the correlation of the fluctuating barrier is crucial for barrier crossing dynamics.

Amplitude Spectrum of Structural Fluctuations in Proteins from the Internal Diffusion of Solutes of Increasing Molecular Size: A Trp Phosphorescence Quenching Study

The accessibility of O(2), acrylamide, and four acrylamide derivatives of increasing molecular size {N-(hydroxymethyl)acrylamide, N,N'-methylene-bisacrylamide, N-[tris(hydroxymethyl)methyl]acrylamide, and 2-acrylamido-2-methyl-1-propanesulfonic acid} to buried Trp residues in four proteins, as determined by dynamic quenching of their phosphorescence emission, was utilized for probing the amplitude range of structural fluctuations in these macromolecules. The quenching rate constant of each solute, k(q), was determined (at 25 and -5 °C) for liver alcohol dehydrogenase, glyceraldehyde-3-phosphate dehydrogenase, azurin, and alkaline phosphatase. The results show that high-frequency small amplitude motions pervade the protein globular fold, permitting relatively unhindered diffusion of small diatomic molecules all the way to compact cores of the macromolecule. For larger solutes, the access to deep regions drops sharply with molecular size, with acrylamide probably representing a threshold for diffusion of a solute through homogeneous compact domains, on the long second time scale. The results emphasize the variability in the amplitude of protein motions between deep cores and more superficial regions of the globular fold and unveil the existence of unexpectedly large amplitude low-activation barrier fluctuations permitting the penetration of solutes with comparatively large M(w) values.

Noise induced escape through an unstable limit cycle in the presence of a fluctuatingbarrier

The escape of Brownian particles through an unstable limit cycle in the presence ofdeterministic barrier fluctuations has been studied. It has been observed that withincreasing non-Gaussian character of the noise, resonant activation gets suppressed andperiodically forced resonant activation becomes robust. Secondly, the ratio of the meanlifetimes in the presence and in the absence of barrier fluctuation increases very rapidlywith the damping constant for Gaussian noise unlike that for the non-Gaussian case. Thisratio decreases sharply for deviation of the noise behavior from Gaussian characteristics.The Arrhenius plot, on the other hand, exhibits deviation from linearity particularly atlarge noise strength for both Gaussian and non-Gaussian cases. The deviation is large fornon-Gaussian noise. We have also shown that additive colored non-Gaussian noise caninduce RA-like phenomena even in a system having harmonic potential energy.

Relaxation through an asymmetric fluctuating potential barrier

We revise the problem of thermally activated crossing of a fluctuating potential barrier, laying stress on the asymmetry of the barrier. Considering as a working model a paradigmatic triangular dichotomously varying potential landscape we find an atypical dependence of the mean first-passage time on the correlation time tau of fluctuations. Namely, in the range of small tau an additional maximum appears. We propose a qualitative explanation of this feature emphasizing the relevance of dynamics in the vicinity of the barrier top, i.e., recrossing, which generally has not been recognized by this time in this particular context. Moreover, we observe that addition of fast barrier fluctuations of some finite intensity needs not to increase the relaxation rate, as has been indicated many times in extensive studies related to the resonant activation phenomenon. Our findings are confirmed numerically for some other systems.

Replica theory for fluctuations of the activation barriers in glassy systems

We consider the problem of slow activation dynamics in glassy systems undergoing a random first-order phase transition. Using an effective potential approach to supercooled liquids, we determine the spectrum of activation barriers for entropic droplets. We demonstrate that fluctuations of the configurational entropy and of the liquid glass surface tension are crucial to achieve an understanding of the barrier fluctuations in glassy systems and thus are ultimately responsible for the broad spectrum of excitations and heterogeneous dynamics in glasses. In particular we derive a relation between the length scale for dynamic heterogeneity and the related barrier fluctuations. Diluted entropic droplets are shown to have a Gaussian distribution of barriers, strongly suggesting that non-Gaussian behavior results from droplet-droplet interactions.

A pseudopotential method for investigating the surface roughness effect in ultrathin bodytransistors

An atomistic method based on the diffraction pseudopotential model is established, forinvestigating the surface roughness (SR) effect in ultrathin body double-gatemetal–oxide–semiconductor field effect transistors. The scattering of electrons due to atomsand vacancies responsible for roughness results from a three-dimensional effective field, andits planar components provide essentially roughness scattering, while a vertical effectivefield is the source of scattering in the method developed in which roughness is treated as asemiclassical barrier fluctuation. The present model involves a stronger effect on mobilitythan the previously developed one and results in an excellent fit, as regards mobility, to thereported experimental data. The extracted SR parameter also matches the observed value.

ESCAPE THROUGH A FLUCTUATING ENERGY BARRIER IN THE PRESENCE OF NON-GAUSSIAN NOISE

In this paper we have studied how barrier crossing dynamics is affected by colored additive non-Gaussian noise if the barrier fluctuates deterministically. Our investigation indicates that resonant activation(RA) is either enhanced or it becomes robust if noise characteristic is deviated from the Gaussian behavior. We find that additive colored non Gaussian noise can induce the RA-like phenomenon. Another interesting observation is that the turnover behavior persists even in presence of barrier fluctuations at finite rate. Finally, it is observed that mean first passage time(MFPT) decreases with increase of non-Gaussian characteristic of the additive colored noise for a given noise strength and noise correlation time and ultimately reaches to a limiting value. The limiting value remains almost the same if the barrier fluctuating frequency is zero or far from the resonant condition. But near the resonant condition the mean first passage time initially decreases and then increases passing through a minimum as the non Gaussian parameter grows.

Effects of barrier fluctuation on the tunneling dynamics in the presence of classical chaos in a mixed quantum-classical system

We present a numerical investigation of the tunneling dynamics of a particle moving in a bistable potential with fluctuating barrier which is coupled to a non-integrable classical system and study the interplay between classical chaos and barrier fluctuation in the tunneling dynamics. We found that the coupling of the quantum system with the classical subsystem decreases the tunneling rate irrespective of whether the classical subsystem is regular or chaotic and also irrespective of the fact that whether the barrier fluctuates or not. Presence of classical chaos always enhances the tunneling rate constant. The effect of barrier fluctuation on the tunneling rate in a mixed quantum-classical system is to suppress the tunneling rate. In contrast to the case of regular subsystem, the suppression arising due to barrier fluctuation is more visible when the subsystem is chaotic.

Transport barrier fluctuations governed by SOL turbulence spreading

Turbulence spreading, namely turbulent transport extending into a stable region is reported both for the flat density profiles in the far SOL and into a modeled H-mode barrier. It is shown that due to turbulence penetration, the pedestal width fluctuates and that its effective width is a factor 2 smaller than the linear predicted width. Turbulence overshooting throughout the pedestal leads to a non-vanishing turbulent transport within the barrier and provides a coupling of core and SOL turbulence despite the transport barrier.

Yield in inhomogeneous PtSi–n-Si Schottky photodetectors

The electrical characteristics (current–voltage and capacitance–voltage) of PtSi–Si Schottky detectors annealed at different temperatures are found to be qualitatively similar. They all show deviations from the ideal behavior predicted by thermionic emission theory. The variations in the barrier height, which is significantly temperature dependent, are well fitted to a single Gaussian distribution function. The estimated efficiency of these detectors is shown to be only slightly sensitive to the presence of such barrier fluctuations except for very low temperatures and at wavelengths near the cut-off detection edge.

Catalytic Mechanism and Product Specificity of the Histone Lysine Methyltransferase SET7/9: An ab Initio QM/MM-FE Study with Multiple Initial Structures

Histone lysine methylation is emerging as an important mechanism to regulate chromatin structure and gene activity. To provide theoretical understanding of its reaction mechanism and product specificity, ab initio quantum mechanical/molecular mechanical free energy (QM/MM-FE) calculations and molecular dynamics simulations have been carried out to investigate the histone lysine methyltransferase SET7/9. It is found that the methyl-transfer reaction catalyzed by SET7/9 is a typical in-line S(N)2 nucleophilic substitution reaction with a transition state of 70% dissociative character. The calculated average free energy barrier at the MP2(6-31+G) QM/MM level is 20.4 +/- 1.1 kcal/mol, consistent with the activation barrier of 20.9 kcal/mol estimated from the experimental reaction rate. The barrier fluctuation has a strong correlation with the nucleophilic attack distance and angle in the reactant complex. The calculation results show that the product specificity of SET7/9 as a monomethyltransferase is achieved by disrupting the formation of near-attack conformations for the dimethylation reaction.

Effects of thermal modulation on tunneling through a fluctuating barrier in presence of bichromatic electromagnetic field

AbstractThe effects of thermal modulation on the tunneling dynamics of a particle moving in a bistable potential with a fluctuating barrier in the presence of bichromatic electromagnetic field are examined. This study brings to light various interesting features. While rate constant increases steadily with increasing fluctuation strength, a stochastic resonance‐like behavior is exhibited when tunneling rate is plotted as a function of barrier fluctuation frequency. The tunneling rate constant decreases with the increase in the differences of the strength of the two electric fields; on the other hand, it displays a maxima when plotted as a function of the ratio of the frequencies of the two fields. When the rate constant is plotted as a function of temperature, it increases and eventually settles down to a saturation value. We have also studied the effect of the phase difference between the two electric fields on the tunneling behavior in the presence of thermal modulation. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Activated Hopping, Barrier Fluctuations, and Heterogeneity in Glassy Suspensions and Liquids

Our entropic barrier hopping theory of glassy hard sphere colloidal suspensions is extended to include heterogeneity within a simple trap model framework. The origin of local domains, their size, and the corresponding static barrier fluctuations are attributed to mesoscopic density fluctuations of an amplitude controlled by the bulk compressibility. Based on typical values of the density fluctuation correlation length in dense liquids, the domain size on which correlated hopping occurs is estimated to be 3−4 particle or molecular diameters. Consequences of barrier fluctuations include an increased average relaxation time, faster diffusion, stretched exponential relaxation, diffusion-viscosity decoupling, and a fractional Stokes−Einstein relation. The common origin of the fluctuation effects is the heterogeneity-induced component of the barrier. For colloidal suspensions in the typically studied volume fraction regime the barrier fluctuations have modest consequences, but significantly larger effects are predicted in the putative glassy regime. A statistical dynamical analysis of domain lifetime suggests that for suspensions the relaxation time of mesoscopic collective density fluctuations is at least as long as the single particle hopping time. A general, model-independent analysis of the single molecule incoherent dynamic structure factor for suspensions and thermal liquids has also been performed in the long time and intermediate wavevector regime. The coupling of single particle density and longitudinal stress fluctuations results in a wavevector-dependent apparent diffusion constant and a dynamic correlation length scale which is strongly temperature dependent and directly related to the translation-rotation decoupling factor. This dynamic length is estimated to be 10 times larger than a molecular diameter for tris-naphthyl benzene near the glass transition temperature but shrinks to a molecular size above the crossover temperature that signals the emergence of collective barriers.

Effects of the potential barrier fluctuations in non-adiabatic long-distance electron transfer

Equations for the transition probability of non-adiabatic electron transfer are derived which take into account both the effect of reorganization of “non-symmetric” classical or quantum vibrational modes and the effects which are due to the dependence of the electron matrix element on the normal coordinates of “promoting” vibrational modes. Any number of vibrational modes with an arbitrary frequency spectrum are incorporated in a simple manner. The results are represented in a simple form convenient for the analysis of the dependence of the transition probability on the temperature, transfer distance, and driving force.

Theory of dynamic barriers, activated hopping, and the glass transition in polymer melts.

A statistical mechanical theory of collective dynamic barriers, slow segmental relaxation, and the glass transition of polymer melts is developed by combining, and in some aspects extending, methods of mode coupling, density functional, and activated hopping transport theories. A coarse-grained description of polymer chains is adopted and the melt is treated as a liquid of segments. The theory is built on the idea that collective density fluctuations on length scales considerably longer than the local cage scale are of primary importance in the deeply supercooled regime. The barrier hopping or segmental relaxation time is predicted to be a function primarily of a single parameter that is chemical structure, temperature, and pressure dependent. This parameter depends on the material-specific dimensionless amplitude of thermal density fluctuations (compressibility) and a reduced segmental density determined by the packing length and backbone characteristic ratio. Analytic results are derived for a crossover temperature T(c), collective barrier, and glass transition temperature T(g). The relation of these quantities to structural and thermodynamic properties of the polymer melt is established. A universal power-law scaling behavior of the relaxation time below T(c) is predicted based on identification of a reduced temperature variable that quantifies the breadth of the supercooled regime. Connections between the ratio T(c)/T(g), two measures of dynamic fragility, and the magnitude of the local relaxation time at T(g) logically follow. Excellent agreement with experiment is found for these generic aspects, and the crucial importance of the experimentally observed near universality of the dynamic crossover time is established. Extensions of the theory to treat the full chain dynamics, heterogeneity, barrier fluctuations, and nonpolymeric thermal glass forming liquids are briefly discussed.

Fusion and breakup in reactions involving weakly bound nuclei

Fusion and breakup in reactions involving weakly bound nuclei A. Szanto de Toledo”, F.A. Souza”, C. Beckb, S.J. Sandersc, M.G. Munhoza, J Takahashi”, N. Carlin”, A.A.P. Suaide”, M.M. de Moura” and E.M. Szanto” “Instituto de Fisica - Universidade de Slo Paulo, Deptartamento de Fisica Nuclear, Laboratorio Pelletron, CP 66318, 05315-970, Sao Paulo - SP, Brazil bCentre National de la Recherche Scientifique Universitd Louis Pasteur, 23 rue du Loess, B.P. 28, F-67037 Strasbourg Cedex 2, France ‘University of Kansas, Lawrence, KS 66045, USA The effect of collective degrees of freedom on the fusion process has been extensively investigated over the past few years [l]. A significant enhancement of the sub-barrier fu- sion cross section is often found as compared to the predictions of one-dimensional barrier penetration models. This enhancement is understood in terms of dynamical processes involving couplings to collective inelastic excitations of the target and/or projectile. A precise determination of the “barrier distributions” leading to the enhancement requires an understanding of the dominant channels that couple to the fusion channel[2]. However, in the case of reactions where at least one of the colliding nuclei has a sufficiently low binding energy so that breakup becomes an important process, conflicting experimental and theoretical results have been reported [3-111. The many questions regarding the influence of breakup become more relevant with the recent availability of radioactive beams and the renewed interest in super-heavy element formation. Radioactive ion beams are likely to produce intense breakup yields. In the fusion processes, and more specifically in the fusion of weakly bound nuclei, that can be used in super-heavy element studies, two different and independent processes can be distinguished both experimentally and theoretically. One, denoted as “Complete Fusion” (CF), is associated with the capture of all of the projectile constituents by the target. The other, denoted as “Incomplete Fusion” (ICF) or Partial Fusion, occurs when part of the projectile is captured by the target and the remaining part escapes. “Total Fusion” is understood as the sum of these two processes (CF + ICF). In order to avoid misinterpre- tations, a clear definition of the experimental and theoretical quantities being compared is essential. This is the main difficulty in comparing data and/or calculations from different authors. Depending on the theoretical approach, different results are achieved. If fusion occurs incoherently, i.e., with possible breakup of the incoming particle [4,5], the survival probability of this particle prior to fusion, is lower than unity, resulting in a decrease in the effective entrance-channel flux and a corresponding reduction of the fusion cross section. On the other hand, if the breakup channel is coupled coherently to the fusion channel, as expected in references [3,6], entrance barrier fluctuations will lead to an effective lowering

Tunneling through a fluctuating barrier in presence of bichromatic electromagnetic field

The effect of bichromatic electromagnetic field on the tunneling dynamics of a particle moving in a bistable potential with a fluctuating barrier is examined. In presence of bichromatic electromagnetic field we observed stochastic resonance like behaviour, i.e., tunneling rate passes through a maximum when plotted as a function of barrier fluctuation frequency and it increases with the increase in the ratio of the frequencies of two fields. We have also investigated the effect of the phase difference of the two electric fields on the tunneling rate constant both in absence and presence of barrier fluctuation.

Tunneling through a fluctuating barrier in the presence of a periodically driving field

The zero and finite temperature tunneling dynamics of a periodically driven particle moving in a bistable potential with a fluctuating barrier is studied. We have focused on the influence of barrier fluctuation and thermal modulation on the tunneling processes in the presence of a driving field. At zero temperature, for a fixed strength of the driving field, both the tunneling probability and rate passes through a well-defined minimum when plotted as a function of fluctuation frequency while it reveals a clear maximum as a function of driving frequency. However, at T > 0 the tunneling probability and rate show two maxima as a function of both fluctuation frequency and driving frequency. In both zero and finite temperature, the tunneling rate constant decreases with increasing fluctuation strength. So, the barrier fluctuation may enhance the stability of a periodically driven system. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004

Mean escape time over a fluctuating barrier.

An approximate method for studying activation over a fluctuating barrier of potential is proposed. It involves considering separately the slow and fast components of barrier fluctuations, and it applies for any value of their correlation time tau. It gives exact results for the limiting values tau-->0 and tau--> infinity, and the agreement with numerics in between is also excellent, both for dichotomic and Gaussian barrier perturbations.

Tunneling dynamics of a double‐well oscillator: Effects of barrier fluctuation and thermal modulation

AbstractThe tunneling dynamics of a particle moving in a bistable potential with fluctuating barrier is studied. For barriers fluctuating randomly in time we show by exact numerical calculation the significant effect of barrier fluctuation on the tunneling behavior of the particle. At nonzero temperatures the computed tunneling rate constant passes through a maximum when plotted against fluctuation frequency. The resonant frequency (at which the maximum appears) slowly decreases with increase in temperature and attains a constant value at higher temperature and it increases linearly with increase in barrier height of the potential. Another important observation is that in presence of barrier fluctuation the dependence of tunneling rate constant on temperature is strongly guided by the barrier fluctuation frequency. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 93: 280–285, 2003