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2Department of Physics,
3School of Mathematics and Physics,
Received:2019-11-27Revised:2020-01-21Accepted:2020-02-26Online:2020-06-24
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Abstract
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Xiao-Ping Yuan, Li-Yan Qiao, Xia Huang. A dynamical transition of a chain of charged particles in a 2D substrate potential. Communications in Theoretical Physics, 2020, 72(7): 075602- doi:10.1088/1572-9494/ab7ecc
1. Introduction
The problem of dynamical transitions in a periodic substrate potential influences several fields in physics, chemistry and biology. The problem is especially applicable to solid friction [1–4], charge-density wave condensates [5, 6] and the Josephson junction [7, 8], to name a few. As a means of gaining insight into the physics of complex macroscopic multibody systems with competing interactions, the driven Frenkel–Kontorova (FK)-type model has been studied to represent a chain of harmonically interacting particles in a sinusoidal substrate potential that is driven by an external force [9–13]. Although the model is simple, the two inherent locking frequencies that come from the external force and from the particles’ motion in the periodic potential continue to exhibit very rich dynamics, and they account for many nonlinear problems [14–16]. This model has recently received increasing interest because it may useful for understanding solid friction phenomena and staircase macroscopic response [17, 18]. In both theoretical and experimental studies, system parameters such as winding number, external driving force, interaction between atoms, damping, pinning and geometry of the substrate play a crucial role in the transition phenomena and the properties of the dynamical phase [19–22].The dynamics of the driven FK model is characterized by the collective phenomenon induced by multiparticle coupling [23–26]. This interaction-induced transport is based on the collective behavior of interacting particles that yield a large amount of new spatiotemporal nonequilibrium. The breaking behavior related to parity and time symmetries and system nonlinearity become the new mechanisms for inducing directional transport, e.g., controlling the interaction potential by time-dependent length modulation between particles [27] and an asymmetrically coupled lattice in a symmetric potential without external force [25, 28]. Inspired by how molecular motors convert chemical energy into directional motion [29, 30], interest in these studies has now focused on driving energy that can be converted into directional motion in another direction. In Hamiltonian systems, current can be induced dynamically from diffusive particle motion in one direction into ballistic motion in the opposite direction [31]. Zheng et al explored the ratchet motion of interacting particles with the aid of external rocking forces applied transversely in single- and double-channel potentials [32, 33]. This cooperative ratchet effect may open ways to investigate 2D collaborative ratchet dynamics that can be implemented in many different experiments, such as on optical lattices [34, 35], superlattices [36] and some biological locomotion processes [37].
The staircase macroscopic response, also called Shapiro steps, as a response function of a system, appears to be another main characteristic dynamics of the driven FK model and thus is of greater significance for technical applications [38, 39]. These steps are the result of mode-locking solutions. References [40, 41], which presented a detailed analysis of the amplitude dependence of the Shapiro steps, revealed that the relative sizes of the steps follow a Farey sequence only for a purely sinusoidal substrate. In nonsinusoidal potentials, the symmetry of the Stern–Brocot tree was increasingly broken. New subharmonic steps, which became separated by chaotic windows, appeared in the overdamped regime, even in the commensurate structure [42].
In contrast to most previous works on the driven FK model, which were performed to determine the impact of linear or short-range interactions on noise, we study it in the statistical context, with particular focus on the particles’ cooperation. Moreover, the cooperative directed transport among particles remains a significant topic. In this study, we examine simultaneously the motion of charged particles with multiple symmetry breaks to describe a chain of charged atoms subject to a 2D potential that is periodic in one direction and parabolic in the transverse direction. The hopping process is made possible with the aid of repulsive interaction among adjacent charged particles in the electric field. In the previous work, the ground state of the model transitioned from a 2D configuration to a 1D configuration when system parameters varied, which indicates that the particles are more likely to be moving over the potential with less depinning force [43]. We concentrate on how the ac electric driving force can be shifted to the vertical direction. The coupling of particles may enhance the transport current and the resonance steps by manifesting in the response function
2. Modified Frenkel–Kontorova model
We consider N charged particles in two dimensions with real coordinates ri=(xi, yi) and unit mass m=1 that are exposed to a spatially periodic and laterally parabolic substrate potential. As shown in figure 1(a), the 2D substrate potential Vs(ri) is assumed to beFigure 1.
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Figure 1.(a) Schematic diagram of the two-dimensional substrate potential. (b) Schematic structure of the effective tilted ratchet potential. The tilted ratchet potential is represent by the black solid line. The red dashed sinusoid is the effective potential field force.
The interaction among particles plays a significant role in the dynamics of collective transport. For simplicity, the interatomic interaction is assumed to be a generalized elastic coupling, in which the nearest-neighbor interaction is denoted by Wint:
Without loss of generality, ϵ=2 and as=2π are always chosen in this work. In this manner, the longitudinal vibration frequency
Each atom configuration is characterized by its total potential energy V(ri), which is the sum of substrate potential Vs and interatomic interaction Wint, namely,
The driving force can be written as
3. Numerical simulation
In the present work, we examine how to control direct transport in a ratchet potential Veff. We briefly initially recall the structure of the ground states (GS) of the chain, as described in our previous work [43]. Then, the GS configuration experiences a transition from a 2D pattern to a 1D pattern with the changing of the system parameters, such asStarting from the different initial configurations is significant in the evolutionary dynamics of the chain. A critical depinning force Fc≈0.38 exists for the GS configuration in an incommensurate structure for a given set of parameters, such as when atoms cannot overcome the potential barrier and pin. With the help of an appropriate periodic driving electrical field E(t) in the y direction, the average velocity becomes greater than zero at Fd=0.28<Fc. As shown by the whole parameter plane in figure 2, depinning regions alternate with pinning ones. For a very low frequency ω, the role of the periodic ac force is equivalent to that of a constant force due to the period length. As such, the system can easily convert horizontal energy into movements in the x direction, as shown by the thin strip-like region in the left part of figure 2. Furthermore, the amplitude of ac driving must be large enough to help the particles cross the barrier, i.e.,
Figure 2.
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Figure 2.Dependence of the average velocity of the system on amplitude b and frequency ω of the ac force. The grayscale indicates the value of the average velocity. The parameters are Fd=0.28, K=0.51,
We analyze in detail the dynamics of the two points at ω =1.56π, which are marked in figure 2, to gain insight into the different dynamical regimes. Figure 3 shows the time evolution of the average velocity of the chain and the corresponding center-of-mass displacement. This exhibits the transition process from stick–slip motion to periodic oscillation. The left panel with b=7 clearly shows a typical regular stick–slip motion and step dependence of x(t). In this case, each particle jumps from one well to the next, stays in the well at a short time, and then jumps again to the next well. Meanwhile, with b=19, the stick–slip motion is replaced by periodic oscillation in a potential well with zero displacements in the x direction. The particles oscillate in the potential well and fail to pass over successfully. This scenario indicates that the lateral periodic force should not be as large as possible, and the focus should be on how much energy can be converted into mutual cooperation between particles.
Figure 3.
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Figure 3.The time evolution of average velocity and displacement of the chain separately for (a) ac amplitude b=7, ω=1.56π, (b) ac amplitude b=19, ω=1.56π. The other parameters are the same as in figure
Our next concern is the situation in which particles can overcome the barriers and produce direct current under the dc driving force Fd>Fc. What role will the ac force play in the transverse direction? Figure 4 presents the dependence of the average velocity of the system on amplitude b and frequency ω of the ac force with dc driving force Fd=0.5. In this case with Fd>Fc, the energy of the lateral driver cannot be effectively converted into a directional current in the x direction. As shown in figure 4, when the frequency ω is low, the velocity of the direct transport alternates as amplitude b increases. For larger ω, the velocity decreases instead as the amplitude b increases. The chain velocity can be easily controlled to a lower value or zero by properly adjusting the values of the amplitude and frequency to the gray or blank area in figure 4. This finding indicates that the critical forces that transform a locked state into a sliding one in the chain vary with the ac driving force, and an optimal electric field exists to control the particles. This scenario is very different from our intuition, which suggests that increasing the ac force would help the chain to collectively overcome the substrate. If the external force Fd increases continually, then it becomes difficult to keep system transport under control.
Figure 4.
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Figure 4.Dependence of the average velocity of the system on amplitude b and frequency ω of the ac force for Fd=0.5. The grayscale indicates the value of the average velocity. The other parameters are the same as in figure
The mutual couplings among particles describe the relative strength of the two interaction types. When coupling strength K is below a critical value, the system is in a floating state with zero static friction. However, for K>Kc, the system enters a stick–slip state. This transition that involves the breaking of analyticity is known as an Aubry transition. Here, we are interested in the transport against the coupling strength under an ac force field. The parameter plane between coupling strength and amplitude of the ac driving is shown in figure 5. For weaker couplings in which K<Kc=0.1, no current exists regardless of whether the horizontal ac electric forcing is opposing. This finding can be explained by the loose connection between particles that may introduce phonon dispersion, which brings additional energy dissipation to the system. Only when the coupling strength exceeds the threshold Kc will the transport be enhanced. In this case, the particles cooperate with one another to produce high-efficiency directed transport. When the amplitude of the ac force becomes sufficiently strong, a very large resonance platform of velocity appears on the K−b parameter plane, where
Figure 5.
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Figure 5.(a) Dependence of the average velocity of the system on the coupling strength K and the amplitude of the ac force b at ω=0.02π. (b) The variation of the current with the coupling strength for different amplitudes of the ac force for b=0.5, 1, 1.3, 3.5. The other parameters are Fd=0.8,
Incommensurability is a popular topic in the field of nonlinear dynamical systems with competing length scales. The critical force changes with the configuration of a system θ, which is in good agreement with an Aubry transition. In commensurate situations, because atoms always lie at the bottom of a potential well to obtain the lowest total potential, the critical force Fc reaches the maximum to depin the chain. Meanwhile, in incommensurate situations, the critical force may be much lower for the higher total potential of the system. As shown in figure 6, the response function
Figure 6.
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Figure 6.Dependence of the average velocity of the system on the atoms’ configuration 1/θ for three different ac forces. (a) b=0, (b) b=0.5, (c) b=5. The other parameters are
4. Conclusion
In this work, we present the overdamped dynamics of interacting charged particles subject to a 2D substrate potential in a dc- and ac-driven Frenkel–Kontorova model. The focus of our attention is the action of the ac force on the transport of the whole system. The mutually repulsive interaction between the nearest particles enhances the particles’ cooperation. When Fd<Fc, directional transport can be generated with the help of an appropriate driving energy of the periodic force in the transverse direction. The transition from stick–slip motion to periodic oscillation occurs in the process of modulation by the ac force. When Fd>Fc, as the energy of the ac field increases, the transport velocity decreases. In the examination of the dependence of system transport on coupling strength, resonance platforms are found under a moderate periodic force. The most striking and interesting behavior is the appearance of a whole staircase structure in the response functionAcknowledgments
This work was supported by Zhejiang Province Commonweal Projects (Grant No. LGF18A050001), China Scholarship Council (Grant No. 201 708 330 401), National Natural Science Foundation of China (Grant No. 11 605 055) and Fundamental Research Funds for the Central Universities of China (Grant No. 2017MS054).Reference By original order
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