Barrier Grossing Induced by Fractional Gaussian Noise

A problem of the rate of escape of a particle under the influence of the external fractional Gaussian noise is studied by using the method of numerical integration of an overdamped Langevin equation. Considering a truncated harmonic potential, the dependences of the mean escape time on the noise int...

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Опубліковано в: :Український фізичний журнал
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Автори: Sliusarenko, O.Yu., Gonchar, V.Yu., Chechkin, A.V.
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Опубліковано: Відділення фізики і астрономії НАН України 2010
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Цитувати:Barrier Grossing Induced by Fractional Gaussian Noise / O.Yu. Sliusarenko, V.Yu. Gonchar, A.V. Chechkin // Український фізичний журнал. — 2010. — Т. 55, № 5. — С. 579-585. — Бібліогр.: 51 назв. — англ.

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Digital Library of Periodicals of National Academy of Sciences of Ukraine
_version_ 1860121912698470400
author Sliusarenko, O.Yu.
Gonchar, V.Yu.
Chechkin, A.V.
author_facet Sliusarenko, O.Yu.
Gonchar, V.Yu.
Chechkin, A.V.
citation_txt Barrier Grossing Induced by Fractional Gaussian Noise / O.Yu. Sliusarenko, V.Yu. Gonchar, A.V. Chechkin // Український фізичний журнал. — 2010. — Т. 55, № 5. — С. 579-585. — Бібліогр.: 51 назв. — англ.
collection DSpace DC
container_title Український фізичний журнал
description A problem of the rate of escape of a particle under the influence of the external fractional Gaussian noise is studied by using the method of numerical integration of an overdamped Langevin equation. Considering a truncated harmonic potential, the dependences of the mean escape time on the noise intensity and Hurst index are evaluated, together with the probability density functions for the escape times. It is found that, like the corresponding classical problem with white Gaussian noise, they both obey an exponential law. За допомогою чисельного iнтегрування передемпфованого рiвняння Ланжевена дослiджено задачу про швидкiсть вильоту частинки iз потенцiальної ями пiд дiєю дробового гаусового шуму. На прикладi обрiзаного гармонiчного потенцiалу отримано залежностi середнього часу вильоту вiд iнтенсивностi шуму та показника Херста, а також обчислено функцiї розподiлу часiв вильоту. Зроблено висновок, що, як i у випадку класичної задачi з бiлим гаусовим шумом, цi величини є експоненцiальними функцiями вiдповiдних параметрiв.
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fulltext BARRIER CROSSING INDUCED BY FRACTIONAL GAUSSIAN NOISE BARRIER CROSSING INDUCED BY FRACTIONAL GAUSSIAN NOISE O.YU. SLIUSARENKO,1 V.YU. GONCHAR,1 A.V. CHECHKIN1, 2 1National Science Center “Kharkiv Institute of Physics and Technology”, Akhiezer Institute for Theoretical Physics (1, Akademichna Str., Kharkiv 61108, Ukraine) 2School of Chemistry, Tel Aviv University (Ramat Aviv 69978, Tel Aviv, Israel) PACS 05.40.Fb, 02.50.Ey, 82.20.-w c©2010 A problem of the rate of escape of a particle under the influ- ence of the external fractional Gaussian noise is studied by using the method of numerical integration of an overdamped Langevin equation. Considering a truncated harmonic potential, the depen- dences of the mean escape time on the noise intensity and Hurst index are evaluated, together with the probability density func- tions for the escape times. It is found that, like the corresponding classical problem with white Gaussian noise, they both obey an exponential law. 1. Introduction The problem of a Brownian particle’s escape rate arises in a lot of natural processes in physics, chemistry, and biology, such as diffusion in solids, homogeneous nucle- ation, electrical transport theory, chemical kinetics, un- folding of proteins etc. (see, e.g., [1, 2] and references therein). The first attempt to describe the process was made by S. Arrhenius [3], who introduced the rate coef- ficient k and noticed the dependence k = ν exp(−βEb), (1) where ν and β are constants, and Eb is the activation energy. Since is was realized that the escapes could hap- pen due to thermal noise, the further development of the problem awaited the creation of a consistent fluctu- ation theory, attaining the second birth after the works of Smoluchowski [4]. First, the problem was studied in [5], the seminal papers on the rate theory were written by H. Kramers [6] (now the problem of the escape rate is also known as the Kramers problem) and S. Chan- drasekhar [7], nowadays being included in almost every textbook on statistical physics in that “classical” form. Later on, it was re-considered using different approaches and in more details (see e.g. [2, 8]). However, the theory of Brownian motion cannot ex- plain anomalous diffusion phenomena which are widely observed in various physical (Sinai diffusion [9], turbu- lent Richardson flow [10,11], motion of charge carriers in amorphous semiconductors [12, 13]), biological (motion in biological cells [14, 15]), biochemical (the spreading of tracer molecules in subsurface hydrology [16]), chem- ical, and geophysical systems [17]. In such systems, the mean squared displacement of a particle does not obey a regular diffusion law〈 x2(t) 〉 = 2DHt 2H , (2) where DH is a generalized diffusion coefficient, H is Hurst exponent varying between 0 and 1. The case where H > 1/2 (the mean squared displacement grows faster than t1) is called superdiffusion; when H < 1/2, we have a subdiffusion phenomenon. Despite the “symptoms” of systems may be the same, the anomalous diffusion has several mechanisms. The most discussed ones nowadays are the continuous time random walks and the fractional Brownian motion mod- els. The former implies either subdiffusion (while walk- ing, the particle experiences long periods of rest, so that the waiting times have an infinite characteristic time) or superdiffusion (e.g., Lévy flights, when the mean squared displacement diverges, but the waiting times are finite). The Kramers problem for Lévy flights was considered in [17–20]. The second model (fractional Brownian motion) was suggested by Kolmogorov in 1940 [21] and later reconsid- ered by Mandelbrot and van Ness [22]. They defined the fractional Brownian motion as a self-similar stochastic process, whose formal derivative ξH(t) called the frac- tional Gaussian noise is a stationary random process with long memory effects. Namely, its autocorrelation function in the discrete time approximation has the form 〈ξH(0)ξH(j)〉=DH ( |j+1|2H−2|j|2H +|j−1|2H , ) . (3) where j is an integer. At large j corresponding to a long-time asymptotics, the autocorrelation function de- ISSN 2071-0194. Ukr. J. Phys. 2010. Vol. 55, No. 5 579 O.YU. SLIUSARENKO, V.YU. GONCHAR, A.V. CHECHKIN cays as 〈ξH(0)ξH(t)〉 ' 2DHH(2H − 1)t2H−2, H 6= 1/2, thus showing a long power-law memory. The “ordi- nary” Brownian limit corresponds to H = 1/2, and〈 ξ1/2(t1)ξ1/2(t2) 〉 = 2Dδ(t1 − t2). Despite its wide use, the fractional Brownian motion is not completely understood, since still there are no con- sistent analytical methods. However, the development of modern computational devices allows one to investi- gate the stochastic systems with such diffusion mecha- nisms, by using various numerical simulation techniques. Thus, the fractional Brownian motion is used to model a variety of processes including the monomer diffusion in a polymer chain [23], single file diffusion [24], diffu- sion of biopolymers in the crowded environment inside biological cells [25], long-term storage capacity in reser- voirs [26], climate fluctuations [27], econophysics [28], and teletraffic [29]. Due to such substantial quantity of applications, the further development of both analytical and simulation approaches is promising. In this paper, we consider a generalization of the bar- rier crossing problem by using a numerical method of integrating the overdamped Langevin equation with a fractional Gaussian random source ξH(t): dx dt = −dU dx +D1/2ξH (t) , (4) where x(t) is particle’s coordinate, U(x) is the potential, ξH(t) is the fractional Gaussian noise with intensity D, and H stands for the Hurst index. We also stress that, in our Langevin description, the fluctuation–dissipation theorem does not hold, as it will be seen below. Therefore, our model is different from that analyzed in [30] and [31]. On the other hand, our approach is similar to that of paper [32]; however, the autocorrelation function of the long-correlated Gaussian noise used there is different from that for the fractional Gaussian noise. 2. Simulation Details For a simulation, we will need a reliable fast generator of random fractional Gaussian numbers ξH(t) (see Eq. (4)). Since the generators provide usually good results either for H < 1/2 (antipersistent case) or for H > 1/2 (per- sistent case), we choose two separate ones for each of cases. The fastest, precise enough (see the tests below), and free of edge effects fractional Gaussian noise generator for the antipersistent case is described in [33]. In brief, the idea is as follows. First, we define a function Rx(n) = { 2−1 [ 1− (n/N)2H ] , for 0 ≤ n ≤ N, Rx(2N − n), forN < n < 2N, (5) where H is the Hurst parameter, 0 < H < 1/2; n is the step number, and N is the random sample length. Second, we perform the Fourier transformation of Eq. (5): Sx(k) = F {Rx(n)} . Next, we define X(k) =  0, for k = 0, exp(iθk)ξ(k) √ Sx(k), for 0 < k < N, ξ(k) √ Sx(k), for k = N, X∗(2N − k), forN < k < 2N, (6) where ∗ stands for the complex conjugation, θk are uni- form random numbers from [0, 2π), ξ(k) are Gaussian random variables with the zero mean and the variance equal to 2, and all random variables are independent of one another. Finally, y(n) = x(n) − x(0), where x(n) = F−1X(k) is the inverse Fourier transformation of Eq. (6), repre- sents a free fractional Brownian trajectory which is to be differentiated with respect to the time in order to get fractional Gaussian random numbers. Since the vari- ance 〈 ξ2 〉 depends on N, it should be normalized so that〈 ξ2 〉 = 2. For the persistent case, we use a generator exploit- ing the spectral properties of a fractional Gaussian noise [34]. – First, take a white Gaussian noise ξ(t), t is an integer. – Take the Fourier transformation of it: S(k) = F{ξ(t)}. – Multiply it by 1/ωH−1/2, 1/2 < H < 1. – Make the inverse Fourier transformation, so that ξH(t) = F−1{S(k)/ωH−1/2} is supposed to approximate a fractional Gaussian noise with index H. – Normalize it. A set of tests of the generators was performed to ver- ify the correctness of the program and to determine the validity limits of the algorithm itself at various values of parameters. The first and the most natural is the verification of the autocovariance function of the noise (Eq. (3)) with D = 1 and C(j) ≡ 〈ξH(0)ξH(j)〉: The second test is the calculation of the mean squared displacement of a free fractional Brownian particle, whose results are omitted in the present paper as trivial. More spectacular is the test of the mean squared dis- placement of a particle in an infinite harmonic potential well. We start from the overdamped Langevin equation 580 ISSN 2071-0194. Ukr. J. Phys. 2010. Vol. 55, No. 5 BARRIER CROSSING INDUCED BY FRACTIONAL GAUSSIAN NOISE Fig. 1. Autocovariance function of a fractional Gaussian noise, persistent and antipersistent cases, a log-log scale. Squares, circles, triangles, and rhombi with error bars are the simulation data for H=0.8, 0.6, 0.45, and 0.1, respectively. Solid lines provide the linear fitting with Eq. (3) (4) with the potential U(x) = ax2/2. In dimensionless variables, we get xn+1 − xn = −xnδt+D1/2δtHξH(n). (7) The results of simulations and their comparison with analytical asymptotes are shown in Fig. 2. Finally, we verify whether the particle’s mean escape time from a semiaxis matches the analytical scaling sug- gested in [35]: p(t) ∝ t−2+H . (8) Setting U(x) ≡ 0 in Eq. (4), we come to the following discrete-time dimensionless Langevin equation: xn+1 − xn = δtHξH(n). (9) Now, the simulation procedure for the mean escape time is as follows (see the sketch in the inset in Fig. 3): – Place a “particle” into the starting point (x = 0). – Begin the iterations of Eq. (9). – Stop the iterations when the “particle” reaches the ab- sorbing boundary at x0 = 1. – Remember the time of this escape event. – Re-execute these steps for 100,000 times and average the escape times. The results demonstrate a good coincidence with Eq. (8) (see Fig. 3). Here, the time step δt= 0.01. Thus, after ascertaining the work of the algorithm and the generators properly, we pass directly to the main Fig. 2. Mean squared displacement of a particle inside a har- monic potential well. Main graph: antipersistent case, H = 0.25 and D=0.1, 0.25, 0.5, and 1.0 (upward). Inset: persistent case, D=1.0 and H=0.8, 0.7, and 0.6 (upward). Points are the simula- tion data; dashed lines are the asymptotes limt→0 〈 x2(t) 〉 ∝ t2H and limt→∞ 〈 x2(t) 〉 = DΓ(1 + 2H) Fig. 3. Verification of the analytical scaling of the free semiaxis mean escape time for a fractional Brownian motion suggested in [35] (solid lines). Circles and triangles represent the simulation results forH = 0.25 andH = 0.75, respectively. The inset explains the simulation algorithm problem. Due to the presence of two different random fractional Gaussian noise generators and different typical time scales for the persistent and antipersistent cases, it is natural to subdivide the further description into two parts. ISSN 2071-0194. Ukr. J. Phys. 2010. Vol. 55, No. 5 581 O.YU. SLIUSARENKO, V.YU. GONCHAR, A.V. CHECHKIN Fig. 4. Mean escape time for a fractional Gaussian noise in the antipersistent case as a function of the noise intensity, a log plot (main panel) and a linear plot (inset). The points show the sim- ulation data (the inaccuracies are of points’ size magnitude), the solid lines demonstrate the fitting with Eq. (10) 3. Antipersistent Case When dealing with a simulation of the escape problem, two moments are of major importance: the proper se- lection of a time step (δt) and a set capacity (N). The latter is the generator’s parameter that determines the number of random variables in a sample, within which they are correlated with one another. The only way to evaluate them correctly is to simulate some sample paths with arbitrary δt (small enough compared to time scales) and N (large enough). Varying them, we should achieve a satisfactory relationship between the time needed for the simulations and the accuracy. Certainly, they may be different for each pair of the noise intensity and the Hurst parameter. In the following simulations, we take the time step δt=0.001, and the set capacity varies from N = 213 to N = 220. Again, starting from Eq. (7), we perform the common procedure of evaluating the mean escape time: – Place a “particle” into the starting point (x = 0). – Begin the iterations of Eq. (7). – Stop the iterations, when the “particle” reaches the edge of the potential x0 = √ 2. – Remember the time of this escape event. – Re-execute these steps for 100,000 times and average the escape times. The results are shown in Figs. 4 and 5 below. Fig. 5. Mean escape time for a fractional Gaussian noise in the antipersistent case as a function of the Hurst index, a log plot. The points show the simulation data (the inaccuracies are of points’ size magnitude), the solid lines demonstrate the fitting with Eq. (10) As clearly seen from Fig. 4, the data points of the mean escape time dependence on the noise intensity may be nicely fitted with an exponential function, so we in- troduce the coefficients aA(H) and bA(H) (the indices A here indicate the antipersistent case): Tesc = exp(aA + bA/D), or lnTesc = aA(H) + bA(H) 1 D . (10) The quantity aA(H) may be fitted well with a linear dependence aA(H) = a′A + a′′AH, (11) while bA(H) is better fitted with a function bA(H) = b′A + b′′AH + b′′′AH 2, (12) where a′A = −3.019, a′′A = 7.296, b′A = 0.705, b′′A = 1.489 and b′′′A = −2.281. The escape times probability density function is sim- ulated almost in the same way as the mean escape time. But, instead of averaging the escape times at the last step, we handle them with a routine that constructs the probability density function. Again, like the classical Kramers problem with a white Gaussian noise source, the escape times probability density function obeys an exponential law (see Fig. 6): p(t) = 1 Tesc exp(−t/Tesc). (13) 582 ISSN 2071-0194. Ukr. J. Phys. 2010. Vol. 55, No. 5 BARRIER CROSSING INDUCED BY FRACTIONAL GAUSSIAN NOISE Fig. 6. Escape times probability density function as a function of the walking time, the antipersistent case. The points show the simulation data, the solid lines demonstrate the fitting with Eq. (13) Fig. 7. Mean escape time for a fractional Gaussian noise in the persistent case as a function of the noise intensity, a log plot. The points show the simulation data (the inaccuracies are of points’ size magnitude), the solid lines demonstrate the fitting with Eq. (14) 4. Persistent Case Here, the procedures are completely the same, with only slight differences in fittings. Figure 7 shows the mean escape time dependence on the noise intensity, which is again exponential: lnTesc = aP (H) + bP (H) 1 D . (14) Now, both aP (H) and bP (H) are, with a good preci- sion, linear functions of H (the subscript “P ” indicates Fig. 8. Mean escape time for a fractional Gaussian noise in the persistent case as a function of the Hurst index, a log plot. The points show the simulation data (the inaccuracies are of points’ size magnitude), the solid lines demonstrate the fitting with Eq. (14) Fig. 9. Escape times probability density function as a function of the walking time, the persistent case. The points show the simu- lation data, the solid lines demonstrate the fitting with Eq. (13) the relation to the persistent case): aP (H) = a′P + a′′PH, bP (H) = b′P + b′′PH, where a′P = −1.680, a′′P = 4.869, b′P = 1.051 and b′′P = −0.399. The mean escape time dependence on the Hurst pa- rameter is shown in Fig. 8. As expected, the escape times probability density function in the persistent case is also exponential of the form of Eq (13) (see Fig. 9). ISSN 2071-0194. Ukr. J. Phys. 2010. Vol. 55, No. 5 583 O.YU. SLIUSARENKO, V.YU. GONCHAR, A.V. CHECHKIN Fig. 10. Relative escape rate k (H) /k (H = 1/2) as a function of µ = |H − 1/2| in the persistent case (squares) and the antipersis- tent case (circles). The solid parabolic and straight lines stand for fitting the results with Eqs. (10) and (14), respectively 5. Conclusions We have considered the Kramers problem for the frac- tional Brownian motion. Using the method of numerical integration of the overdamped Langevin equation with a fractional Gaussian random source, we have shown that, similarly to the classical result presented, e.g., in [7], the dependence of the mean escape time from a trun- cated harmonic potential on the noise intensity is expo- nential, both for the persistent and antipersistent cases. This contradicts the conclusion made in [32], where the stretched exponential behavior of the escape time prob- ability density function was reported. The escape time probability density function also behaves qualitatively in the same manner as that of the classical Brownian particle. An important phenomenon is revealed when consid- ering the diffusion rate. Let us introduce the escape rate k = 1/Tesc and make a comparison of the escape rate with the classical Kramers one. Figure 10 indicates that, in the persistent case, the escape rate is smaller than the classical one, and we have a subdiffusion phe- nomenon. On the contrary, when dealing with the free fractional Brownian motion, the mean squared displace- ment is 〈 x2(t) 〉 = 2D|t|2H (the larger the value of H, the faster the particle) and thus, the persistent noise gives birth to the superdiffusivity. The same picture arises in the antipersistent case, but vice versa: for the free frac- tional Brownian motion, we have the subdiffusion, while the walks inside a harmonic potential are superdiffusive. However, such an event is in accordance with Molchan’s analytics [35] discussed in Section 2. At the end, we would like to mention some very promising applications of the numerical results obtained here. 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Received 08.10.09 ПРОХОДЖЕННЯ ЧЕРЕЗ БАР’ЄР, ЗУМОВЛЕНЕ ДРОБОВИМ ГАУСОВИМ ШУМОМ О.Ю. Слюсаренко, В.Ю. Гончар, О.В. Чечкiн Р е з ю м е За допомогою чисельного iнтегрування передемпфованого рiв- няння Ланжевена дослiджено задачу про швидкiсть вильоту частинки iз потенцiальної ями пiд дiєю дробового гаусового шуму. На прикладi обрiзаного гармонiчного потенцiалу отри- мано залежностi середнього часу вильоту вiд iнтенсивностi шу- му та показника Херста, а також обчислено функцiї розподiлу часiв вильоту. Зроблено висновок, що, як i у випадку класичної задачi з бiлим гаусовим шумом, цi величини є експоненцiаль- ними функцiями вiдповiдних параметрiв. ISSN 2071-0194. Ukr. J. Phys. 2010. Vol. 55, No. 5 585
id nasplib_isofts_kiev_ua-123456789-56201
institution Digital Library of Periodicals of National Academy of Sciences of Ukraine
issn 2071-0194
language English
last_indexed 2025-12-07T17:39:28Z
publishDate 2010
publisher Відділення фізики і астрономії НАН України
record_format dspace
spelling Sliusarenko, O.Yu.
Gonchar, V.Yu.
Chechkin, A.V.
2014-02-13T20:34:52Z
2014-02-13T20:34:52Z
2010
Barrier Grossing Induced by Fractional Gaussian Noise / O.Yu. Sliusarenko, V.Yu. Gonchar, A.V. Chechkin // Український фізичний журнал. — 2010. — Т. 55, № 5. — С. 579-585. — Бібліогр.: 51 назв. — англ.
2071-0194
PACS 05.40.Fb, 02.50.Ey, 82.20.-w
https://nasplib.isofts.kiev.ua/handle/123456789/56201
A problem of the rate of escape of a particle under the influence of the external fractional Gaussian noise is studied by using the method of numerical integration of an overdamped Langevin equation. Considering a truncated harmonic potential, the dependences of the mean escape time on the noise intensity and Hurst index are evaluated, together with the probability density functions for the escape times. It is found that, like the corresponding classical problem with white Gaussian noise, they both obey an exponential law.
За допомогою чисельного iнтегрування передемпфованого рiвняння Ланжевена дослiджено задачу про швидкiсть вильоту частинки iз потенцiальної ями пiд дiєю дробового гаусового шуму. На прикладi обрiзаного гармонiчного потенцiалу отримано залежностi середнього часу вильоту вiд iнтенсивностi шуму та показника Херста, а також обчислено функцiї розподiлу часiв вильоту. Зроблено висновок, що, як i у випадку класичної задачi з бiлим гаусовим шумом, цi величини є експоненцiальними функцiями вiдповiдних параметрiв.
The authors acknowledge the discussions with J. Klafter, R. Metzler, and I. Sokolov. AVC acknowledges the financial support from the MC IIF Programme, grant “LeFrac”.
en
Відділення фізики і астрономії НАН України
Український фізичний журнал
Загальні питання теоретичної фізики
Barrier Grossing Induced by Fractional Gaussian Noise
Проходження через бар’єр, зумовлене дробовим гаусовим шумом
Article
published earlier
spellingShingle Barrier Grossing Induced by Fractional Gaussian Noise
Sliusarenko, O.Yu.
Gonchar, V.Yu.
Chechkin, A.V.
Загальні питання теоретичної фізики
title Barrier Grossing Induced by Fractional Gaussian Noise
title_alt Проходження через бар’єр, зумовлене дробовим гаусовим шумом
title_full Barrier Grossing Induced by Fractional Gaussian Noise
title_fullStr Barrier Grossing Induced by Fractional Gaussian Noise
title_full_unstemmed Barrier Grossing Induced by Fractional Gaussian Noise
title_short Barrier Grossing Induced by Fractional Gaussian Noise
title_sort barrier grossing induced by fractional gaussian noise
topic Загальні питання теоретичної фізики
topic_facet Загальні питання теоретичної фізики
url https://nasplib.isofts.kiev.ua/handle/123456789/56201
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