Transverse rectification of disorder-induced fluctuations in a driven system

Alejandro B. Kolton*Université de Genève, DPMC, 24 Quai Ernest Ansermet, CH-1211 Genève 4, Switzerland

�Received 23 October 2006; revised manuscript received 12 December 2006; published 3 January 2007�

We study numerically the overdamped motion of particles driven in a two-dimensional ratchet potential. Inthe proposed design, of the so-called geometrical-ratchet type, the mean velocity of a single particle inresponse to a constant force has a transverse component that can be induced by the presence of thermal or otherunbiased fluctuations. We find that additional quenched disorder can strongly enhance the transverse drift atlow temperatures, in spite of reducing the transverse mobility. We show that, under general conditions, therectified transverse velocity of a driven particle fluid is equivalent to the response of a one-dimensional flashingratchet working at a drive-dependent effective temperature, defined through generalized Einstein relations.

DOI: 10.1103/PhysRevB.75.020201 PACS number�s�: 05.40.�a, 05.45.�a, 05.60.Cd, 74.25.Qt

The idea of generating a directed dissipative transport in asystem kept out of thermal equilibrium only by unbiasedperturbations has motivated an outburst of experimental andtheoretical works in the last years. The ratchet effect is in-deed of interest, for both applications and modeling, in verydiverse systems, ranging from biological motors,1 colloidalmatter,2 granular matter,3 vortex matter in superconductors,4

Josephson junction arrays,5 atoms in optical traps,6 and elec-trons in semiconductor heterostructures7 to gamblinggames.8 One of the simplest models is the “flashing ratchet,”where a directed motion of a Brownian particle �i.e., break-ing of the detailed balance condition� is obtained by couplingit to a pulsating asymmetric-periodic potential. The identifi-cation of the essential physical ingredients for the effectshows that a large variety of ratchets and rectification mecha-nisms can be realized.9

Recently, there has been a growing interest in the so-called geometrical ratchets since they can be used as con-tinuous “molecular sieves” to separate particles experimen-tally �such as macromolecules or mesoscopic objects�,according to their physical properties. These devices aretypically two-dimensional systems containing a periodic ar-ray of asymmetric obstacles. By driving the particles throughthe array an average lateral drift appears, as transverse diffu-sive motion is rectified by collisions with the asymmetricobstacles. Different types of geometrical ratchets have beenanalyzed in the literature, both experimentally10 andtheoretically.11 The effect of additional quenched disorder inthese two-dimensional systems has not been discussed yet,though interesting anomalous transport properties of one-dimensional disordered ratchet systems were reported.12

Such a study is not only relevant for applications where dis-order cannot be avoided, but it is also an interesting andchallenging issue. The driven motion of particles in a disor-dered substrate yields a nontrivial hydrodynamics. Thecurrent-driven motion of vortices in type-II superconductorsis a prominent example, where disorder, apart from reducingdissipation, is responsible for marked nonequilibrium trans-port and magnetic properties.13 On the other hand, alreadythe simpler case of driven noninteracting Brownian particlesin two dimensions has displayed complex phenomena. Whilediffusion is anomalous at equilibrium,14 under a finite drivediffusion becomes normal in the comoving frame, with an-isotropic and velocity-dependent diffusion constants and

mobilities.15 Moreover, a disordered substrate can providealone a local or global ratchet effect, such as the generationof large-scale vorticity in the probability current by drivingparticles with a uniform alternate drive16 and the net directedmotion produced by driving the particles with crossed acdrives.17

In this paper we investigate the effect of quenched disor-der in a simple geometrical ratchet design under a uniformand constant driving force. We find that disorder can stronglyenhance the transverse drift at low temperatures both fornoninteracting and interacting particles, thus improving theperformance of the device for applications. We show that thetransverse velocity of a driven fluid is equivalent to the re-sponse of a one-dimensional flashing ratchet working at adrive-dependent effective temperature, defined through gen-eralized fluctuation-dissipation relations.

Let us consider the overdamped motion of particles in atwo-dimensional �2D� potential like the one depicted in Fig.1. The equation of motion of a particle in position Ri is

�dRi

dt= − �i��

j�i

V�Rij� + U�Ri�� + F + �i�t� , �1�

where Rij = �Ri−R j� is the distance between particles iand j, Rip is the distance between the particle i and a site atRp, � is the friction, and F=Fyy is the driving force. Theeffect of a thermal bath at temperature T is given by the

stochastic force �i�t�, satisfying ��i��t��=0 and ��i

��t�� j���t���

=2�kBT��t− t���ij����, where �¯� denotes the averageover the ensemble of �i. For concreteness we consider alogarithmic repulsive particle-particle interactionV�r�=−Av ln�r� which corresponds, for instance, to thevortex-vortex interaction in 2D thin-film superconductors.19

Particles interact with the quenched potential U�R�=UR�R�+Up�R�. UR is a ratchet potential with theform UR�R�= a

2�FR�Y�GR�X�, where XR · x, Y R · y,GR�X�=sin�2�X /a�+0.25 sin�4�X /a�, and FR�y�=U0 cos�2�y /b��cos�2�y /b��, with � the Heaviside func-tion. This ratchet potential is similar to a periodic array ofobstacles, asymmetrical around the x axis but symmetricalaround the y axis, as the ones considered in Ref. 11. Disorderis short-range correlated, and it is modeled as a random dis-

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tribution of centers such that Up�Ri�=�pApe−�Rip / rp�2, where

Rip= �Ri−Rp� is the distance between particle i and a centerat Rp. Centers can be either attractive Ap�0 �wells� or re-pulsive Ap0 �humps� or a combination of both. We solveEq. �1� numerically by using the method of Ref. 18. Lengthis normalized by rp, energy by 2Ap, and time by =�rp

2 / �2Ap�. We consider N=60 particles and Np pinningcenters in a rectangular box of size Lx�Ly and periodicboundary conditions, with Ly =100, Lx=20�3Ly, b=33, anda=20. We average calculated properties over 500 disorderrealizations.

We start by discussing the simplest case of noninteractingparticles, Av=0, without disorder, Np=0, and with a ratchetpotential of amplitude U0=1. The dashed lines of Fig. 2�a�show the transverse drift rate V� 1

N�idXi

dt� at T=0.05 as a

function of the longitudinal velocity Vy � 1N�i

dYi

dt�. We see

that the transverse velocity V increases from zero, has amaximun V 0.0075 at Vy 0.5, and decays to zero at largelongitudinal velocity. Since V=0 at T=0, the average di-rected transverse motion observed is induced by the thermalnoise. This rectification effect is easy to understand, and Fig.1�b� illustrates the mechanism, where the transverse diffu-sion constant is D=2T if there is no disorder or interparticleinteractions. For our discussion it is useful to make explicitthe connection between the type of response shown inFig. 2�a� and the one of a flashing ratchet. If Fy is large,the mean velocity in the driven direction, Vy � dY

dt�, is

Vy Fy −O�Fy−1� and longitudinal fluctuations are much

smaller by a factor of O�Fy−1�� than transverse fluctuations.15

At T=0, the equation of motion for the coordinate X of aparticle located at R=Xx+Yx can be thus written as

dX

dt� − FR�Vyt�GR��X� . �2�

Since FR�Vyt�=U0 cos�2�Vyt /b��cos�2�Vyt /b��, X feelsthe ratchet potential GR�X� switching on and off periodicallywith time periods on=off=b /2Vy. At small drives themechanism is the same although on becomes increasinglylarger than off since the wells of UR�X ,Y� delay the motionin the y direction �see Fig. 1�. The mapping to a flashingratchet explains the observed directed transverse motion withV0 when T0 and can be thus used as an effective modelto explain all the features of the response shown in Fig. 2�a�.

The effect discussed so far is similar to the one describedin Ref. 11. Let us now add disorder by putting Np=2000randomly located pinning sites. The resulting response isshown in Fig. 2�a� �symbols�. As we can see, disorderstrongly enhances the rectification at intermediate and largelongitudinal velocities and also broadens the range of Vywhere the response is appreciable with respect to the cleancase. In addition, we find that at intermediate and large Vy, Vis finite even in the T=0 limit, since disorder induces trans-verse diffusion when Vy 0, even in the absence of thermalfluctuations.

Finally, let us now turn on the repulsive interaction be-tween particles. In Fig. 2�b� we show V as a function of Vyfor different values of the repulsion strength Av. In the insetof Fig. 2�b� we see that the maximun response Vmax is almost

FIG. 1. �Color online� �a� Ratchet potential with disorder. �b� Schematics of the transverse rectification mechanism �top view�. Particlesmove with an average velocity Vy in the direction of the applied force Fy. In the white regions the interaction with the �attractive orrepulsive� centers and with the thermal bath induces diffusion in the nondriven direction. In the shaded regions a periodic-asymmetricpotential tends to localize particles at its minima. An average transverse shift �see circles� is produced at a rate V.

FIG. 2. Transverse velocity V vs longitudinal velocity Vy. �a�Symbols correspond to the disordered system at T=0.05, the dashedline to the clean system at T=0.05, and the solid line to the cleansystem at an effective temperature Teff�Vy ,T�. �b� V vs Vy for dif-ferent interaction strengths Av: Av=0.05 ���, Av=0.2 ���, Av=0.5���, Av=1 ���, Av=2 ���, and Av=5 ���. ��� symbols correspondto purely attractive pinning centers, with Ap=−0.5, at T=0.05. In-set: maximun rectification as a function of Av.

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constant with Av up to values Av 0.2 where a slow decaystarts, but it is larger than the response of the noninteractingclean system up to Av=2. The decay of the response at largeAv is explained by the decrease of transverse wandering dueto increasingly correlated collective motion.19 This effect is,however, stronger in the absence of disorder. In Fig. 2�b� weshow that the response for purely attractive pinning centers,Ap=−1, is smaller than for repulsive centers, Ap1, forsmall values of Vy, but indistinguishable for larger values ofVy. This is due to the fact that, at the density of centersconsidered, attractive centers are more effective to pin par-ticles than humps, since the latter can provide two-dimensional pinning only by forming rare geometric traps.However, at a density Np /LxLy 1/rp

2 all these differencesdisappear completely.

In order to understand the rectification characteristics de-scribed above it is instructive to study, separately, the motionof particles in the purely disordered case without the ratchetpotential �i.e., U0=0�. For simplicity we consider only thecase of noninteracting particles, but we expect similar resultsfor interacting particles in the dynamical regimes wheretransverse diffusion is nonzero.18,20 We analyze in detail thenonequilibrium transverse fluctuations as a function of Fy,since they affect directly the rectification in the presence ofthe ratchet potential. Following Ref. 18 we define the

observables O�t�= 1Nv

�i=1Nv siXi�t� and O�t�=�i=1

Nv siXi�t�, wheresi=−1,1 are random numbers with si=0 and sisj =�ij. Thequadratic mean displacement can be written as �t , t0� 1

Nv�i=1

Nv ��Xi�t�−Xi�t0��2�=C�t , t�+C�t0 , t0�−2C�t , t0�, with

C�t , t0�= �O�t�O�t0��. The integrated response function � forthe observable O is obtained by applying a perturbative forcefi=�six at time t0 and keeping it constant for all subsequenttimes on each particle, ��t , t0�=lim�→0

1� �O�t���− �O�t���=0�.

In the steady state �t , t0�= �t− t0� and ��t , t0�=��t− t0� andin particular at equilibrium the fluctuation-dissipation theo-rem �FDT� imposes ��t�= �t� /2T. When Fy 0 the systemis out of equilibrium and the FDT does not hold. We willshow, however, that generalized fluctuation-dissipation rela-tions can still be defined for our system. In the long-timelimit we find �t� Dt and ��t� �t, thus allowing us todefine the transverse diffusion constant D and the transversemobility �. These two quantities depend on the longitudinaldriving force as shown in Fig. 3�b�. D is nonmonotonic, hasa peak at Fy 1.5, and decays approximately as a power lawtowards the equilibrium value without disorder, 2T, for largeforces. This behavior can be understood by considering theeffective transverse random walk induced only by collisionswith the pinning centers at T=0, and by simple heuristicarguments it is possible to find the asymptotic formsD nprp

3Vy at small Vy and D nprpAp2 /Vy at large Vy,

15 in-dicated in Fig. 3�b�. At large Fy the transverse mobility �approaches the equilibrium value without disorder, �=1 �in-dependent of T�. At small Fy, � decreases due to trappingand its value at the limit Fy→0 is controlled by T. In theinset of Fig. 3�c� we show the parametric plot of ��t� vs �t� /2T for Fy =0 �equilibrium� and Fy =4.0 �out of equilib-rium�. We see that the equilibrium FDT holds for Fy =0 asexpected. For Fy =4.0 �and in general for Fy 0� we observe

instead that the FDT holds only at very short time scales,t�rp /Vy, but it is violated at long times. The type of viola-tion observed can be quantified using the notion of time-scale-dependent “effective temperatures” introduced byCugliandolo, Kurchan, and Peliti.21 Following Ref. 18 wedefine a velocity-dependent transverse effective temperatureTeff from the slope shown in the inset of Fig. 3�c�. At longtimes this implies the generalized Einstein relationTeff D /2� in the nondriven direction. In Fig. 3�c� we seethat Teff follows closely D except at low forces where �decreases towards the value � 0.5 at very low forces. Forinteracting particles similar results for Teff were obtained atlow and intermediate forces in the plastic and smectic re-gimes of motion.19 At large forces and small temperatures,however, the formation of pinned rough channels for particlemotion13 leads to a transverse freezing of the moving vortexfluid19 at T=0. At this dynamical transition D and � arestrongly reduced and vanish at T=0 strictly.15,18

The fluctuating motion in a purely disordered substrateand the connection between the response of a geometricaland a flashing ratchet discussed above motivate a simplemodel that can be used to describe the rectification responseof the disordered geometrical ratchet. We propose the follow-ing equation for the transverse motion of noninteracting par-ticles:

1

��Vy�dXi

dt� − FR�Vyt�GR��Xi� + �eff

i �t� , �3�

where we have replaced Yi Vyt as before, the pinning forceby an effective thermal noise �eff

i �t�, and the bare mobility by

FIG. 3. Motion in a purely disordered potential. �a� Transversediffusion constant D and mobility �. Dashed and solid lines indi-cate asymptotic forms of D. �b� Effective temperature Teff D /2�.The dash-dotted line indicates the bath temperature T=0.05. Theinset shows that Teff satisfies a generalized fluctuation-dissipationrelation between the integrated response ��t� and the quadraticmean displacement �t�. The dashed line corresponds to the equi-librium fluctuation-dissipation relation, valid only at short timest�� /Vy. Upper symbols correspond to Fy =0.0 and lowersymbols to Fy =4.0.

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��Vy�. We use that ��effi �t��=0 and ��eff

i �t��effj �t���

= 2Teff�Vy� /��Vy����t− t���ij where Teff�T ,Vy� and ��T ,Vy�are the ones shown in Fig. 3�a�. Equation �3� therefore mod-els transverse motion in a coarse-grained way �in time andspace�, satisfying the generalized fluctuation-dissipation re-lation shown in the inset of Fig. 3�b� �in the absence of theratchet potential�. By construction, the assumptions of themodel are that �i� transverse forces are small compared withthe longitudinal drive Fy and �ii� the particle motion is inco-herent at the length scales of the ratchet potential. In Fig.2�a� we see that the transverse drift generated by this modelis close to the one of the full model for the parameters ana-lyzed in this paper, which assure that the particle has manyindependent collisions with the pinning centers between therectifying regions. The rectification characteristics of thetwo-dimensional geometrical ratchet are therefore well de-scribed by the one-dimensional flashing ratchet described byEq. �3� working at the effective temperature Teff�T ,Vy� andfriction �−1�T ,Vy�, determined by the disorder and the lon-gitudinal velocity. Using this model the enhancement of therectification observed in Fig. 2�a� can be simply attributed tothe fact that TeffT, but with Teff still smaller than the opti-mal temperature for rectification of the effective pulsating

ratchet. Equation �3� is also expected to work for interactingparticles by using the respective Teff�T ,Vy� and ��T ,Vy� ex-cept at low T and large Fy where condition �ii� can be vio-lated since particles become correlated over long times anddistances.20

In conclusion, we have studied numerically the effect ofquenched disorder in a geometrical ratchet. We find that dis-order enhances the transverse-rectified velocity of a drivenfluid. If particle motion is incoherent at the scale of theratchet potential, the response can be simply described by aone-dimensional flashing ratchet working at a disorder-induced, drive-dependent effective mobility and temperatureTeff, satisfying generalized Einstein relations. This effect canbe used experimentally to enhance and control the perfor-mance of geometrical ratchets at low temperatures, and con-versely, it can be used as a “thermometer” to access Teff inthis kind of driven disordered systems.

We acknowledge discussions with L.F. Cugliandolo, D.Domínguez, T. Giamarchi, V.I. Marconi, and A. Rosso. Thiswork was supported in part by the Swiss National Fund un-der Division II.

*Present address: Dept. de Física Atómica, Molecular y Nuclear,Universidad Complutense de Madrid, 28040 Madrid, Spain.

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