Nonequilibrium work fluctuations in a driven Ising model

Monte Carlo simulations of a sheared Ising model are used to study nonequilibrium fluctuations of mechanical work. The validity of the transient (starting from equilibrium) and the steady state fluctuation relations is verified. A fluctuation relation has been also shown to hold for the mechanical work done on the system, during the transition between two nonequilibrium steady states corresponding to different drivings.     

Physical model and the associative memory of a cytoskeleton microtubule as a system of dipoles

A physical model of the conformational degrees of freedom of a cytoskeleton microtubule represented as a system of interacting dipoles is elaborated, characteristic physical quantities are estimated, and a phase diagram of the ferroelectric state of the microtubule at T = 0 is constructed. The presence of frustrated couplings J _ij between the dipoles appears to be the most important feature of the disordered dipole system of the microtubule, owing to which the ground state of the dipole system splits into a large number of lower energy states. The dynamics of the dipole system is determined by the relaxation of the dipole-dipole interaction energy. After the dipole system has evolved to its final state, the input image is associated with one of reference images stored earlier; hence, the dipole system of a microtubule can be viewed as a distributed system with associative memory.     

Nonequilibrium fluctuations and mechanochemical couplings of a molecular motor

We investigate theoretically the violations of Einstein and Onsager relations and the thermodynamic efficiency for a single processive motor operating far from equilibrium using an extension of the two-state model introduced by Kafri et al. [Biophys. J. 86, 3373 (2004)10.1529/biophysj.103.036152]. With the aid of the Fluctuation Theorem, we analyze the general features of these violations and this efficiency and link them to mechanochemical couplings of motors. In particular, an analysis of the experimental data of kinesin using our framework leads to interesting predictions that may serve as a guide for future experiments.     

Microscopic model and phase diagram of the dipole system of a cytoskeleton microtubule at finite temperatures

A microscopic model of the dipole system of a cytoskeleton microtubule at finite temperatures is developed. The phase diagram of the ground state of the dipole system of the cytoskeleton microtubule is constructed and the temperature dependence of the mean square of polarization in the dipole system of the cytoskeleton microtubule is calculated.     

Nonequilibrium critical poisoning in a single-species model

A simple model is introduced, which exhibits a poisining transition similar to that observed in studies of catalyc surface reactions. Steady-state behavior and critical exponents are determined via series expansion and Monte Carlo simulations. The relation between catalytic surface models and reggeon field theory is examined.     

Nonequilibrium electromagnetic fluctuations: heat transfer and interactions

The Casimir force between arbitrary objects in equilibrium is related to scattering from individual bodies. We extend this approach to heat transfer and Casimir forces in nonequilibrium cases where each body, and the environment, is at a different temperature. The formalism tracks the radiation from each body and its scatterings by the other objects. We discuss the radiation from a cylinder, emphasizing its polarized nature, and obtain the heat transfer between a sphere and a plate, demonstrating the validity of proximity transfer approximation at close separations and arbitrary temperatures.     

Nonequilibrium phase transitions in coordinated biological motion: critical fluctuations

We report the results of experiments on biological motion demonstrating the presence of critical order parameter fluctuations as the system evolves from one coordinated state to another at a critical control parameter value. This is a key feature of nonequilibrium phase transitions.     

Nonequilibrium fluctuations from a nematic under a thermal gradient and a gravity field*

We use a fluctuating hydrodynamics (FH) approach to study the fluctuations of the hydrodynamic variables of a thermotropic nematic liquid crystal (NLC)in a nonequilibrium steady state (NESS). This NESS is produced by an externally imposed temperature gradient and a uniform gravity field. We calculate analytically the equilibrium and nonequilibrium seven modes of the NLC in this NESS. These modes consist of a pair of sound modes, one orientation mode of the director and two visco-heat modes formed by the coupling of the shear and thermal modes. We find that the nonequilibrium effects produced by the external gradients only affect the longitudinal modes. The analytic expressions for the visco-heat modes show explicitly how the heat and shear modes of the NLC are coupled. We show that they may become propagative, a feature that also occurs in the simple fluid and suggests the realization of new experiments. We show that in equilibrium and in the isotropic limit of the NLC, our modes reduce to well-known results in the literature. For the NESS considered we point out the differences between our our modes and those reported by other authors. We close the paper by proposing the calculation of other physical quantities that lend themselves to a more direct comparison with possible experiments for this system.     

Nonequilibrium fluctuations of a thin metal diffuse film exposed to microwave radiation

Nonlinear fluctuations of the electron energy distribution function in a microwave-irradiated thin diffuse metal film are calculated in the nonlinear approximation for various energy relaxation models. The spectral density of the resulting nonequilibrium fluctuations is calculated as a function of the microwave-radiation power for various ratios between the electron-phonon and electron-electron relaxation rates.     

Physical grounds and models of pattern representation and recognition in a neuron cytoskeleton microtubule

The physics of the dipole system of a neuron cytoskeleton microtubule is put forward, and a Hamiltonian of the dipole system is constructed. The previously developed microscopic model of the cytoskeleton microtubule dipole system is extended for the case of dipole-dipole bonds when the bonds are not fully ordered and are constrained (exhibit the memory property). Molecular field expressions are derived for a random polarization function and its two moments: mean polarization and rms polarization. An evolutionary equation for a random order parameter is of a relaxation character and describes the pattern recognition process. It is shown that the phase transition nonlinearly transforms (projects) one (nodal) space of higher dimension pattern features to another space of lower dimension attributes (order parameters), this transformation greatly cutting the body of data to be processed.     

Nonequilibrium fluctuations, traveling waves, and instabilities in active membranes

The stability of a flexible fluid membrane containing a distribution of mobile, active proteins (e.g., proton pumps) is shown to depend on the structure and functional asymmetry of the proteins. A stable active membrane is in a nonequilibrium steady state with height fluctuations whose statistical properties are governed by the protein activity. Disturbances are predicted to travel as waves at sufficiently long wavelength, with speed set by the normal velocity of the pumps. The unstable case involves a spontaneous, pump-driven undulation of the membrane, with clumping of the proteins in regions of high activity.     

Nonequilibrium phase transition in a model for social influence

We present extensive numerical simulations of the Axelrod's model for social influence, aimed at understanding the formation of cultural domains. This is a nonequilibrium model with short range interactions and a remarkably rich dynamical behavior. We study the phase diagram of the model and uncover a nonequilibrium phase transition separating an ordered (culturally polarized) phase from a disordered (culturally fragmented) one. The nature of the phase transition can be continuous or discontinuous depending on the model parameters. At the transition, the size of cultural regions is power-law distributed.     

Critical fluctuations in a random network model

Individual threshold behaviors generate large endogenous fluctuations in a globally interactive network. In a version of NK models, the bonds are allowed to be allocated randomly across nodes in each period. It is shown that the aggregate output asymptotically follows a branching process with martingale property on a unique globally stable state.     

Nonequilibrium wetting transition in a nonthermal 2D Ising model

Nonequilibrium wetting transitions are observed in Monte Carlo simulations of a kinetic spin system in the absence of a detailed balance condition with respect to an energy functional. A nonthermal model is proposed starting from a two-dimensional Ising spin lattice at zero temperature with two boundaries subject to opposing surface fields. Local spin excitations are only allowed by absorbing an energy quantum (photon) below a cutoff energy E _c . Local spin relaxation takes place by emitting a photon which leaves the lattice. Using Monte Carlo simulation nonequilibrium critical wetting transitions are observed as well as nonequilibrium first-order wetting phenomena, respectively in the absence or presence of absorbing states of the spin system. The transitions are identified from the behavior of the probability distribution of a suitably chosen order parameter that was proven useful for studying wetting in the (thermal) Ising model.     

Nonequilibrium model on Archimedean lattices

On (4, 6, 12) and (4, 8²) Archimedean lattices, the critical properties of the majority-vote model are considered and studied using the Glauber transition rate proposed by Kwak et al. [Kwak et al., Phys. Rev. E, 75, 061110 (2007)] rather than the traditional majority-vote with noise [Oliveira, J. Stat. Phys. 66, 273 (1992)]. We obtain T _c and the critical exponents for this Glauber rate from extensive Monte Carlo studies and finite size scaling. The calculated values of the critical temperatures and Binder cumulant are T _c = 0.651(3) and U ₄ ^* = 0.612(5), and T _c = 0.667(2) and U ₄ ^* = 0.613(5), for (4, 6, 12) and (4, 8²) lattices respectively, while the exponent (ratios) β/ν, γ/ν and 1/ν are respectively: 0.105(8), 1.48(11) and 1.16(5) for (4, 6, 12); and 0.113(2), 1.60(4) and 0.84(6) for (4, 8²) lattices. The usual Ising model and the majority-vote model on previously studied regular lattices or complex networks differ from our new results.     

Nonequilibrium dynamics of the Bose-Hubbard model: a projection-operator approach

We study the phase diagram and nonequilibrium dynamics involving ramp of the hopping amplitude J(t)=Jt/τ with ramp time τ of the Bose-Hubbard model at zero temperature using a projection-operator formalism which allows us to incorporate the effects of quantum fluctuations beyond mean-field approximations in the strong-coupling regime. Our formalism yields a phase diagram which provides a near exact match with quantum Monte Carlo results in three dimensions. We also compute the residual energy Q, the superfluid order parameter Δ(t), the equal-time order parameter correlation function C(t), and the wave function overlap F which yields the defect formation probability P during nonequilibrium dynamics of the model. We find that Q, F, and P do not exhibit the expected universal scaling. We explain this absence of universality and show that our results compare well with recent experiments.     

Motor driven microtubule shape fluctuations: force from within the lattice

We develop a general theory of microtubule (MT) deformations by molecular motors generating internal force doublets within the MT lattice. We describe two basic internal excitations, the S and V shape, and compare them with experimental observations from literature. We explain the special role of tubulin vacancies and the dramatic deformation amplifying effect observed for katanin acting at positions of defects. Experimentally observed shapes are used to determine the ratio of MT stretch and shear moduli (approximately 6 x 10(5)) and to estimate the forces induced in the MT lattice by katanin (10's of pN). We conclude that molecular motors if acting cooperatively can "animate" MTs from within the lattice and induce slack even without cross bridging to other structures, a scenario very much reminiscent of the motor driven axoneme.