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arXiv2009

Casimir-like force between intruders in granular gases

We numerically study a two-dimensional granular gas of rigid disks where an external driving force is applied to each particle in such a way that the system is driven into a steady state by balancing the energy input and the dissipation due to inelastic collisions. Two intruder particles embedded in this correlated medium experience a fluctuation-induced force -- that is itself a fluctuating quantity -- due to the confinement of the hydrodynamic fluctuations between them. We find that the probability distribution of this force is a Gaussian centered on a value that is proportional to the steady-state temperature and grows logarithmically with system size. We investigate the effect of the other relevant parameters and estimate the force using the Fourier transform of the fluctuating hydrodynamic fields.

cond-mat.stat-mech↗
arXiv2008

Giant and negative dielectric tunability induced by interfacial polarization in Pb(Fe1/2Nb1/2)1-xTixO3 single crystals

The giant and negative dielectric tunability of Pb(Fe1/2Nb1/2)1-xTixO3 single crystals is reported. A low field of 120 V/cm can induce a great reduction of the capacitance, and the tunability is larger than 80% in low frequency range (<1 MHz) at room temperature. This giant tunability is ascribed to the interfacial polarization at the interface of electrode/sample. A negative dielectric tunability detected only in the tetragonal sample can be also attributed to the interfacial polarization. The origin of the giant and negative tunabilities is discussed with the multipolarization-mechanism model and equivalent circuit model, respectively.

cond-mat.stat-mech↗
arXiv2009

Hydrophobic Interactions and Dewetting between Plates with Hydrophobic and Hydrophilic Domains

We study by molecular dynamics simulations the wetting/dewetting transition and the dependence of the free energy on distance between plates that contain both hydrophobic and hydrophilic particles. We show that dewetting and strength of hydrophobic interaction is very sensitive to the distribution of hydrophobic and hydrophilic domains. In particular, we find that plates characterized by a large domain of hydrophobic sites induce a dewetting transition and an attractive solvent-induced interaction. On the other hand, a homogeneous distribution of the hydrophobic and hydrophilic particles on the plates prevents the dewetting transition and produces a repulsive solvent-induced interaction. We also present results for a kind of Janus interface in which one plate consists of hydrophobic particles and the other of hydrophilic particles showing that the inter-plate gap remains wet until steric constraints at small separations eject the water molecules. Our results indicate that the Cassie equation, for the contact angle of a heterogeneous plate, can not be used to predict the critical distance of dewetting. These results indicate that hydrophobic interactions between nanoscale surfaces with strong large length-scale hydrophobicity can be highly cooperative and thus they argue against additivity of the hydrophobic interactions between different surface domains in these cases. These findings are pertinent to certain protein-protein interactions where additivity is commonly assumed.

cond-mat.soft↗
arXiv2009

Nonperturbative Renormalization of the Heisenberg Spin-1/2 Antiferromagnet on the Square Lattice

We investigate the critical scaling of the spin-1/2 antiferromagnet on the square lattice in the easy-plane (XXZ) regime, via numerical measurements of the entanglement entropy constructed from the zeroes of a polynomial ring. We relate these results to conformal field theory predictions for the area law scaling of entanglement entropyin the vicinity of quantum critical points in gapless conformally invariant 2d quantum systems, and in gapless systems with finite Fermi surfaces. Our measurements are focused in three low-temperature regions at fixed XXZ anisotropies of $Δ=1.01$, $Δ=1.78$ and $Δ=2.0$ which probe the quantum regime between the Néel point and Ising phase.

cond-mat.stat-mech↗
arXiv2008

Projection-operator formalism and coarse-graining

A careful derivation of the generalized Langevin equation using "Zwanzig flavor" projection operator formalism is presented. We provide arguments why this formalism has better properties compared to alternative projection-operator formalisms for deriving non-equilibrium, non-thermodynamic-limit, equations. The two main ingredients in the derivation are Liouville's theorem and optimal prediction theory. As a result we find that equations for non-equilibrium thermodynamics are dictated by the formalism once the choice of coarse-grained variables is made. This includes a microcanonical entropy definition dependent on the coarse-grained variables. Based on this framework we provide a methodology for succesive coarse-graining. As two special cases, the case of linear coefficients and coarse-graining in the thermodynamic limit are treated in detail. In the linear limit the formulas found are equivalent with those of homogenization theory. In this framework there are no restrictions with respect to the thermodynamic-limit or nearness to equilibrium. We believe the presented approach is very suitable for the development of computational methods by means of coarse-graining from a more detailed level of description.

cond-mat.stat-mech↗
arXiv2010

The Enlightened Game of Life

We investigate a special class of cellular automata (CA) evolving in a environment filled by an electromagnetic wave. The rules of the Conway's Game of Life are modified to account for the ability to retrieve life-sustenance from the field energy. Light-induced self-structuring and self-healing abilities and various dynamic phases are displayed by the CA. Photo-driven genetic selection and the nonlinear feedback of the CA on the electromagnetic field are included in the model, and there are evidences of self-organized light-localization processes. The evolution of the electromagnetic field is based on the Finite Difference Time Domain (FDTD) approach. Applications are envisaged in evolutionary biology, artificial life, DNA replication, swarming, optical tweezing and field-driven soft-matter.

physics.optics↗
arXiv2008

Critical Behavior of Liquid 3He

We investigate the liquid-gas second-order phase transition in liquid $^3He$ using the variational calculations based on the cluster expansion of the energy functional. We also compute the critical point exponents of liquid $^3He$ which are in agreement with experimental data.

cond-mat.stat-mech↗
arXiv2008

Quantum phases of light coupled to a periodic Bose-Einstein condensate in a cavity

We study a composite photon-Bose condensed atoms (BEC) system in an optical lattice confined in a cavity. We show that the system is able to produce an effective photon-photon repulsion in the presence of an external pump. We predict for the first time zero-temperature quantum phases of light coupled to BEC in the photon blockade regime in the presence of an external pump.

cond-mat.stat-mech↗
arXiv2008

Coherent Exciton Dynamics and Trapping in Topologically Disordered Systems

We analyze the coherent dynamics of excitons in three dimensional topologically disordered networks with traps. If the interactions between the nodes of the network are long ranged, i.e., algebraically decaying as a function of the distance between the nodes, the average survival probability of an exciton surprisingly shows a characteristic decay with features similar to the decay found for regular one-dimensional systems. We further show how this decay can be related to the eigenstates of the same system without a trap.

quant-ph↗
arXiv2009

Statistical Characterizers of Transport in a Communication Network

We identify the statistical characterizers of congestion and decongestion for message transport in model communication lattices. These turn out to be the travel time distributions, which are Gaussian in the congested phase, and log-normal in the decongested phase. Our results are demonstrated for two dimensional lattices, such the Waxman graph, and for lattices with local clustering and geographic separations, gradient connections, as well as for a 1-d ring lattice with random assortative connections. The behavior of the distribution identifies the congested and decongested phase correctly for these distinct network topologies and decongestion strategies. The waiting time distributions of the systems also show identical signatures of the congested and decongested phases.

physics.soc-ph↗
arXiv2008

Facilitated oriented spin models:some non equilibrium results

We analyze the relaxation to equilibrium for kinetically constrained spin models (KCSM) when the initial distribution $ν$ is different from the reversible one, $μ$. This setting has been intensively studied in the physics literature to analyze the slow dynamics which follows a sudden quench from the liquid to the glass phase. We concentrate on two basic oriented KCSM: the East model on $\bbZ$, for which the constraint requires that the East neighbor of the to-be-update vertex is vacant and the model on the binary tree introduced in \cite{Aldous:2002p1074}, for which the constraint requires the two children to be vacant. While the former model is ergodic at any $p\neq 1$, the latter displays an ergodicity breaking transition at $p_c=1/2$. For the East we prove exponential convergence to equilibrium with rate depending on the spectral gap if $ν$ is concentrated on any configuration which does not contain a forever blocked site or if $ν$ is a Bernoulli($p'$) product measure for any $p'\neq 1$. For the model on the binary tree we prove similar results in the regime $p,p' p_c$ and $p\leq p_c$ convergence to equilibrium cannot occur for all local functions. Finally we present a very simple argument (different from the one in \cite{Aldous:2002p1074}) based on a combination of combinatorial results and ``energy barrier'' considerations, which yields the sharp upper bound for the spectral gap of East when $p\uparrow 1$.

math.PR↗
arXiv2013

Green's Function Approach to the Bose-Hubbard Model

We use a diagrammatic hopping expansion to calculate finite-temperature Green functions of the Bose-Hubbard model which describes bosons in an optical lattice. This technique allows for a summation of subsets of diagrams, so the divergence of the Green function leads to non-perturbative results for the boundary between the superfluid and the Mott phase for finite temperatures. Whereas the first-order calculation reproduces the seminal mean-field result, the second order goes beyond and shifts the phase boundary in the immediate vicinity of the critical parameters determined by the latest high-precision Monte-Carlo simulations of the Bose-Hubbard model. In addition, our Green's function approach allows for calculating the excitation spectrum at finite temperature and for determining the effective masses of particles and holes.

cond-mat.stat-mech↗
arXiv2008

The exact probability distribution of saturating states in random sequential adsorption

We consider the non-overlapping irreversible random sequential adsorption (RSA) process on one-dimensional finite line, which is known also as the car parking process. The probability of each coverage in saturating states is analytically and exactly obtained. In the derivation, a new representation of states in RSA process is introduced, which effectively works to make the calculation clear and simple.

cond-mat.stat-mech↗
arXiv2008

Nonlinear dynamics of surface steps

This is a review of recent theoretical work on complex dynamical behavior of steps on crystal surfaces. The first part of the article is devoted to the electromigration-driven motion of islands and voids, and the second part addresses step bunching on vicinal surfaces.

cond-mat.mtrl-sci↗
arXiv2009

Effective Hamiltonians for state selection in Heisenberg antiferromagnets

In frustrated antiferromagnets with isotropic exchange interactions, there is typically a manifold of degenerate classical ground states. This degeneracy is broken by the (free) energy of quantum or thermal fluctuations, or the uniform effects of bond disorder. We derive effective Hamiltonians to express each kind of selection effect, in both exact forms and convenient approximate forms. It is argued that biquadratic terms, representing the collinear-selecting effects of quantum fluctuations, should be included in classical simulations of large-$S$ frustrated magnets at low temperatures

cond-mat.stat-mech↗
arXiv2008

Nonlocal-local multimode bifurcation in turbulence

It is shown that a mechanism of energy redistribution and dissipation by the inertial waves can be effectively utilized in isotropic turbulence at small Reynolds numbers (when the nonlocal interactions are the dominating ones). This mechanism totally suppresses the local interactions (cascades) in isotropic turbulence at the Taylor-scale based Reynolds number $R_λ < 10^2$. This value of $R_λ$ (that can be considered as a bifurcation value at which the local regime emerges from the nonlocal one in isotropic turbulence) is in agreement with recent direct numerical simulations data. Applicability of this approach to channel flows is also briefly discussed. A theory of multimode bifurcations has been developed in order to explain anomalous (in comparison with the Landau-Hopf bifurcations) properties of the nonlocal-local bifurcation in isotropic turbulence.

nlin.CD↗