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We study a dilute suspension of magnetic nanoparticles in a nematic-filled channel and how the spatial magnetization M can be tailored by the nematic anisotropy. We study the spatial configurations as stable critical points of a generalized phenomenological energy for a dilute ferronematic in the absence of external magnetic fields. We show how spatial inhomogeneities in the equilibrium nematic profile, induced by confinement and boundary effects, generate nonzero spatially inhomogeneous magnetization profiles in the system. Depending on the magnetonematic coupling energy, M can either follow the nematic profile for large coupling or exhibit distinct polydomain structures separated by defect lines for weak coupling and low temperatures. Some exact solutions for prototypical situations are also obtained. [ABSTRACT FROM AUTHOR]

Magnetic and dielectric solids are well-represented by the Ising model with dipolar interactions (IM+DI). The latter are long-ranged, fluctuating in sign, and anisotropic. Equilibrium studies have revealed novel consequences of these complicated interactions, but their effect on nonequilibrium behavior is not explored. We perform a deep temperature quench to study the kinetics of domain growth in the d=3 IM+DI. Our main observations are (i) the emergence of columnar domains along the z axis (Ising axis) with a transient periodicity in the xy plane; (ii) anisotropic growth laws: ℓρ(t)∼tϕ; ℓz(t)∼tψ, where ρ=(x,y) and ℓ is the characteristic length scale; (iii) generalized dynamical scaling for the correlation function: C(ρ,z;t)=g(ρ/ℓρ,z/ℓz); and (iv) an asymptotic Porod tail in the corresponding structure factor: S(kρ,0;t)∼k-3ρ; S(0,kz;t)∼k-2z. Our results explain the experimentally observed columnar morphologies in a wide range of dipolar systems, and they have important technological implications. [ABSTRACT FROM AUTHOR]

We use a computationally efficient graph cut method to obtain ground state morphologies of the random-field Ising model (RFIM) on (i) simple cubic (SC), (ii) body-centered cubic (BCC), and (iii) face-centered cubic (FCC) lattices. We determine the critical disorder strength Δc at zero temperature with high accuracy. For the SC lattice, our estimate (Δc=2.278±0.002) is consistent with earlier reports. For the BCC and FCC lattices, Δc=3.316±0.002 and 5.160±0.002, respectively, which are the most accurate estimates in the literature to date. The small-r behavior of the correlation function exhibits a cusp regime characterized by a cusp exponent α signifying fractal interfaces. In the paramagnetic phase, α=0.5±0.01 for all three lattices. In the ferromagnetic phase, the cusp exponent shows small variations due to the lattice structure. Consequently, the interfacial energy Ei(L) for an interface of size L is significantly different for the three lattices. This has important implications for nonequilibrium properties. [ABSTRACT FROM AUTHOR]

A human pathogen, Neisseria gonorrhoeae (NG), moves on surfaces by attaching and retracting polymeric structures called Type IV pili. The tug-of-war between the pili results in a two-dimensional stochastic motion called twitching motility. In this paper, with the help of real-time NG trajectories, we develop coarse-grained models for their description. The fractal properties of these trajectories are determined and their influence on first passage time and formation of bacterial microcolonies is studied. Our main observations are as follows: (i) NG performs a fast ballistic walk on small time scales and a slow diffusive walk over long time scales with a long crossover region; (ii) there exists a characteristic persistent length l*p, which yields the fastest growth of bacterial aggregates or biofilms. Our simulations reveal that l*p~L0.6, where L×L is the surface on which the bacteria move; (iii) the morphologies have distinct fractal characteristics as a consequence of the ballistic and diffusive motion of the constituting bacteria. [ABSTRACT FROM AUTHOR]

Aggregates of magnetic nanoparticles (MNPs) exhibit unusual properties due to the interplay of small system size and long-range dipole-dipole interactions. Using the micromagnetic simulation software oommf, we study the spin morphologies and heat dissipation in micron-size spherical assemblies of MNPs. In particular, we examine the sensitivity of these properties to the dipolar strength, manipulated by the interparticle separation. As oommf is not designed for such a study, we have incorporated a novel scaling protocol for this purpose. We believe that it is essential for all studies where volume fractions are varied. Our main results are as follows: (i) Dense assemblies exhibit strong dipolar effects which yield local magnetic order in the core but not on the surface, where moments are randomly oriented. (ii) The probability distribution of ground-state energy exhibits a long high-energy tail for surface spins in contrast to small tails for the core spins. Consequently, there is a wide variation in the energy of surface spins but not the core spins. (iii) There is strong correlation between ground-state energy and heating properties on application of an oscillating magnetic field h(t)=hocos2πft: the particles in the core heat uniformly, while those on the surface exhibit a wide range from cold to intensely hot. (iv) Specific choices of ho and f yield characteristic spatial heat distributions, e.g., hot surface and cold core, or vice versa. (iv) For all values of ho and f that we consider, heating was maximum at a specific volume fraction. These results are especially relevant in the context of contemporary applications such as hyperthermia and chemotherapy, and also for novel materials such as smart polymer beads and superspin glasses. [ABSTRACT FROM AUTHOR]

While the ground-state problem for the random-field Ising model is polynomial, and can be solved using a number of well-known algorithms for maximum flow or graph cut, the analog random-field Potts model corresponds to a multiterminal flow problem that is known to be NP-hard. Hence an efficient exact algorithm is very unlikely to exist. As we show here, it is nevertheless possible to use an embedding of binary degrees of freedom into the Potts spins in combination with graph-cut methods to solve the corresponding ground-state problem approximately in polynomial time. We benchmark this heuristic algorithm using a set of quasiexact ground states found for small systems from long parallel tempering runs. For a not-too-large number q of Potts states, the method based on graph cuts finds the same solutions in a fraction of the time. We employ the new technique to analyze the breakup length of the random-field Potts model in two dimensions. [ABSTRACT FROM AUTHOR]

We study living liquid crystals (LLCs), which are an amalgam of nematic liquid crystals (LCs) and active matter (AM). These LLCs are placed in contact with surfaces which impose planar/homeotropic boundary conditions on the director field of the LC and the polarization field of the AM. The interplay of LC-AM interactions and the surface-directed conditions yield controlled pattern dynamics in the LLC, which has important technological implications. We discuss two representative examples of this pattern dynamics.

We study self-assembly in a colloidal suspension of magnetic particles by performing comprehensive molecular dynamics simulations of the Stockmayer (SM) model, which comprises spherical particles decorated by a magnetic moment. The SM potential incorporates dipole-dipole interactions along with the usual Lennard-Jones interaction and exhibits a gas-liquid phase coexistence observed experimentally in magnetic fluids. When this system is quenched from the high-temperature homogeneous phase to the coexistence region, the nonequilibrium evolution to the condensed phase proceeds with the development of spatial as well as magnetic order. We observe density-dependent coarsening mechanisms-a diffusive growth law ℓ(t)∼t^{1/3} in the nucleation regime and hydrodynamics-driven inertial growth law ℓ(t)∼t^{2/3} in the spinodal regimes. [ℓ(t) is the average size of the condensate at time t after the quench.] While the spatial growth is governed by the expected conserved order parameter dynamics, the growth of magnetic order in the spinodal regime exhibits unexpected nonconserved dynamics. The asymptotic morphologies have density-dependent shapes which typically include the isotropic sphere and spherical bubble morphologies in the nucleation region, and the anisotropic cylinder, planar slab, cylindrical bubble morphologies in the spinodal region. The structures are robust and nonvolatile, and exhibit characteristic magnetic properties. For example, the oppositely magnetized hemispheres in the spherical morphology impart the characteristics of a Janus particle to it. The observed structures have versatile applications in catalysis, drug delivery systems, memory devices, and magnetic photonic crystals, to name a few.

An amalgam of nematic liquid crystals and active matter, referred to as living liquid crystals, is a promising self-healing material with futuristic applications for targeted delivery of information and microcargo. We provide a phenomenological model to study the symbiotic pattern dynamics in this contemporary system using the Toner-Tu model for active matter (AM), the Landau-de Gennes free energy for liquid crystals (LCs), and an experimentally motivated coupling term that favours coalignment of the active and nematic components. Our extensive theoretical studies unfold two novel steady states, chimeras and solitons, with sharp regions of distinct orientational order that sweep through the coupled system in synchrony. The induced dynamics in the passive nematic is unprecedented. We show that the symbiotic dynamics of the AM and LC components can be exploited to induce and manipulate order in an otherwise disordered system.

The biaxial phase in nematic liquid crystals has been elusive for several decades after its prediction in the 1970s. A recent experimental breakthrough was achieved by Liu et al. [Proc. Natl. Acad. Sci. USA 113, 10479 (2016)0027-842410.1073/pnas.1601235113] in a liquid-crystalline medium with magnetic nanoparticles. They exploited the different length scales of dipolar and magnetonematic interactions to obtain an equilibrium state where the magnetic moments are at an angle to the nematic director. This tilt introduces a second distinguished direction for orientational ordering or biaxiality in the two-component system. Using coarse-grained Ginzburg-Landau free-energy models for the nematic and magnetic fields, we provide a theoretical framework which allows for the manipulation of morphologies and quantitative estimates of biaxial order.

The anisotropic shape of calamitic liquid crystal (LC) particles results in distinct values of energy when the nematogens are placed side by side or end to end. This anisotropy in energy which is governed by a parameter κ^{'} has deep consequences on equilibrium and nonequilibrium properties. Using the Gay-Berne (GB) model, which exhibits the nematic (Nm) as well as the low-temperature smectic (Sm) order, we undertake large-scale Monte Carlo and molecular dynamics simulations to probe the effect of κ^{'} on the equilibrium phase diagram and the nonequilibrium domain growth following a quench in the temperature T or coarsening. There are two transitions in the GB model: (i) isotropic to Nm at T_{c}^{1} and (ii) Nm to Sm at T_{c}^{2}T_{c}^{1}→T

The behavior of many magnetic and dielectric solids, and the more contemporary magnetic superlattices, is governed by dipolar interactions. They are anisotropic and long ranged, having varied consequences ranging from ground states with complicated magnetic order to the presence of glassy dynamics characterized by a plethora of relaxation times. These systems are well captured by the dipolar Ising model (DIM) with nearest-neighbor exchange interactions (J) and long-range dipolar interactions (D). Depending on the relative interaction strength Γ=J/D, there are four phases of distinct magnetic order and symmetry. Using Monte Carlo simulations, we perform deep quenches to study domain growth or coarsening in the d=3 DIM. This important nonequilibrium phenomenon has not been addressed as dipolar interactions are notoriously difficult to handle theoretically. Our study reveals that, in spite of the anisotropy in interactions and diversity in ground state configurations, we observe universality in the ordering dynamics of all phases.

We focus on a dilute uniform suspension of magnetic nanoparticles in a nematic-filled micron-sized shallow well with tangent boundary conditions as a paradigm system with two coupled order parameters. This system exhibits spontaneous magnetization without magnetic fields. We numerically obtain the stable nematic and associated magnetization morphologies, induced purely by the geometry, the boundary conditions, and the coupling between the magnetic nanoparticles and the host nematic medium. Our most striking observations pertain to domain walls in the magnetization profile, whose location can be manipulated by the coupling and material properties, and stable interior and boundary nematic defects, whose location and multiplicity can be tailored by the coupling too. These tailored morphologies are not accessible in uncoupled systems and can be used for multistable systems with singularities and stable interfaces.

We use a computationally efficient graph-cut method (GCM) to obtain the ground-state morphologies (at zero temperature) of the random-field Ising model in d=2,3. The GCM enables us to obtain comprehensive numerical results on large-scale systems. We analyze the morphologies by computing correlation functions and structure factors. These quantities enable us to precisely evaluate characteristic properties, e.g., domain sizes, scaling functions, roughness exponents, fractal dimensions, etc.

We present results from Monte Carlo simulations of hysteresis in the zero-temperature ( T=0 ) dynamics of the Sherrington-Kirkpatrick spin glass model. We study the statistics of magnetization-jumps (denoted as Deltam ) in response to a time-dependent magnetic field H (t) , which increases or decreases with constant increments Delta as H(t)-->H(t)+/-Delta . In particular, we focus on the field dependence of the Deltam -distribution function P(Deltam,H) . We formulate arguments to understand the variation of P ( Deltam,H ) along the hysteresis loop in the weak-disorder limit.

We study the effect of adsorption rate on the particle size distribution in solution-grown ternary Pb1-x Fex S nanoparticle films. Computer simulations of a stochastic lattice model with adsorption and mass dependent diffusion have been performed to mimic the underlying mechanism of particle growth. The experimental as well as numerical data exhibit identical scaling with respect to the incident flux rate. A transmission electron microscope analysis of Pb1-x Fex S nanoparticle films reveals self-similarity in the particle size distributions corresponding to different adsorption rates as a manifestation of the observed scaling.