Field theory of Ising percolating clusters

The clusters of up spins of a two-dimensional Ising ferromagnet undergo a second order percolative transition at temperatures above the Curie point. We show that in the scaling limit the percolation threshold is described by an integrable field theory and identify the non-perturbative mechanism which allows the percolative transition in absence of thermodynamic singularities. The analysis is extended to the Kertész line along which the Coniglio–Klein droplets percolate in a positive magnetic field.     

Percolative phase separation in La2CuO4+δ and La2−x Sr x CuO4

We report on the formation of conducting phases in slightly doped La₂CuO₄ samples by the existence of a percolative phase separation. Phase separation can be quenched by rapid cooling and can be restored by the application of a 3 T magnetic field. Magnetically polarizable quasiparticles are shown to be formed by hole doping which fuse to form percolative conducting and below 37 K superconducting phases.     

Inhomogenous electronic structure, transport gap, and percolation threshold in disordered bilayer graphene

The inhomogenous real-space electronic structure of gapless and gapped disordered bilayer graphene is calculated in the presence of quenched charge impurities. For gapped bilayer graphene, we find that for current experimental conditions the amplitude of the fluctuations of the screened disorder potential is of the order of (or often larger than) the intrinsic gap Δ induced by the application of a perpendicular electric field. We calculate the crossover chemical potential Δ(cr), separating the insulating regime from a percolative regime in which less than half of the area of the bilayer graphene sample is insulating. We find that most of the current experiments are in the percolative regime with Δ(cr)?Δ. The huge suppression of Δ(cr) compared with Δ provides a possible explanation for the large difference between the theoretical band gap Δ and the experimentally extracted transport gap.     

Locally ordered regions in the phase transition in the systems with a finite-range correlated quenched disorder

The locally ordered regions (LOR) in the phase transition in disordered systems are studied. There are two parts in this paper. One part is to report our numerical results on the one-dimensional saddle point equation of the Ginzburg-Landau Hamiltonian with random temperature in the presence of an ordering field. The disordered system is modelled as a lattice, on which each cell has a local reduced temperature. The random part of the local reduced temperature is distributed in the Gaussian form. The one-dimensional saddle point equation is solved numerically. The average, the fluctuation and the correlation length of the solution are calculated. The scaling relations for these quantities with the temperature, the ordering field and the disorder strength are derived. The numerical data are fitted with the scaling relations well. Another part is to discuss qualitatively the phase diagram of the finite-range correlated disordered systems. There are two proposed classes for the phase transition in connection with the LOR. One class is described by the percolative scenario, in which the phase transition is inhomogeneous. In the percolative scenario the percolation of the LOR dominates the phase transition. In another class, the phase transition is homogeneous, and can be described by the renormalization group (RG) with replica symmetry breaking (RSB). In the RG with RSB, there is nothing to do with the percolation of LOR. We shall show that these two theories, which seem contradictory, may describe two parts of the whole phase diagram. Whether the phase transition is homogeneous or inhomogeneous depends on the interaction between the LOR. If the interaction between the LOR is strong enough, the phase transition is percolative and inhomogeneous. If the interaction between the LOR is weak, the phase transition is homogeneous. The interaction between the LOR is discussed with the numerical solution on the saddle point equation.     

Proton diffusion mechanism in amorphous SiO2

We study proton diffusion in amorphous SiO2 from the atomic scale to the long-range percolative regime. Ab initio molecular dynamics suggest that the dominant atomic process consists in cross-ring interoxygen hopping assisted by network vibrations. A statistical analysis accounting for the disorder in amorphous SiO2 yields relations between transition energies and interoxygen distances for both cross-ring and nearest-neighbor hopping. The percolative regime is then addressed through large-size model systems reproducing these relations. Cross-ring hopping is confirmed as the dominant diffusion mechanism and supported by a good agreement with experiment for the activation energy.     

Electric field induced by magnetic flux motion in superconductor containing fractal clusters of a normal phase

The influence of the fractal clusters of a normal phase, which act as pinning centers, on the dynamics of magnetic flux in percolative type-II superconductor is considered. The voltage–current characteristics of such a superconductor are obtained taking into account the effect of fractal properties of cluster boundaries on the magnetic flux trapping. It is revealed that the fractality reduces the electric field arising from magnetic flux motion and thereby raises the critical current of a superconductor.     

Giant discrete steps in metal-insulator transition in perovskite manganite wires

Optical lithography is used to fabricate LPCMO wires starting from a single (La(5/8-0.3)Pr(0.3))Ca3/8MnO3 (LPCMO) film epitaxially grown on a LaAlO3(100) substrate. As the width of the wires is decreased, the resistivity of the LPCMO wires exhibits giant and ultrasharp steps upon varying temperature and magnetic field in the vicinity of the metal-insulator transition. The origin of the ultrasharp transitions is attributed to the effect of spatial confinement on the percolative transport in manganites.     

Spin-flip induced magnetoresistance in positionally disordered organic solids

A model for magnetoresistance in positionally disordered organic materials is presented and solved using percolation theory. The model describes the effects of spin dynamics on hopping transport by considering changes in the effective density of hopping sites, a key quantity determining the properties of percolative transport. Faster spin-flip transitions open up "spin-blocked" pathways to become viable conduction channels and hence produce magnetoresistance. Features of this percolative magnetoresistance can be found analytically in several regimes, and agree with previous measurements, including the sensitive dependence of the magnetic-field dependence of the magnetoresistance on the ratio of the carrier hopping time to the hyperfine-induced carrier spin precession time. Studies of magnetoresistance in known systems with controllable positional disorder would provide an additional stringent test of this theory.     

An electrosprayed coating process for fabricating hemispherical PMMA droplets for an optical diffuser

A modified electrospraying process is proposed for fabricating uniform microsized poly(methyl methacrylate) (PMMA) hemispherical droplets on poly(ethylene terephthalate) films for use as optical diffusers. In this process, an electrode controlled by an electric field is attached to a rotating collector, and various electric field conditions are applied to the droplets ejected from a positively charged nozzle. The distribution of small hemispherical droplets and the surface roughness resulting from this modified electrospraying process are more uniform than those achieved in the normal electrospraying process. The frequency of the field applied to the electrode is not critical to the production of stable PMMA droplets. The optical diffusivity of the PMMA film fabricated using this process is greater than that of normally fabricated film, demonstrating the feasibility of using this process to create newly designed optical diffusers.     

Experiments on breakups of a magnetic fluid drop through a micro-orifice

We experimentally study the breakups of a ferrofluid drop passing through a narrow passage under the attraction of an external field. After passing through the orifice, the fluid thread starts to neck down significantly and eventually breaks to form new droplets. The dynamics of the ferrofluid breakup are analyzed parametrically, including the diameter of the orifice and the local field strength. The patterns of fluid breakups can be characterized by two measurements of the breaking droplets, such as their sizes and stretching lengths. These two characteristic measurements mainly depend on the diameter of the orifice. Breaking droplets with less stretching and smaller sizes are resulted from a narrower orifice. On the other hand, the number of total breaking droplets that represents the transport effectiveness of ferrofluids significantly depends on both the diameter of the orifice and the local field strength. While a stronger field generates more breaking droplets, a maximum number of breaking droplets occurs at an intermediate orifice's diameter.     

Phase transition and critical dynamics in site-diluted Josephson-junction arrays

Measurements of the IV characteristics of site-diluted Josephson-junction arrays have revealed intriguing effects of percolative disorder on the phase transition and the vortex dynamics in a two-dimensional XY system. Different from other types of phase transitions, the Kosterlitz-Thouless transition was eliminated with the introduction of percolative disorder far below the percolation threshold. Even after the Kosterlitz-Thouless order had been removed, the system remained superconducting at low temperatures by establishing a different type of order. Near the percolation threshold, evidence was found that, as a consequence of the underlying fractal structure, the critical dynamics of the phase degrees of freedom persisted down to zero temperature.     

Heating and non-ohmic transport in phase-separated manganites

We investigate the features of non-ohmic transport in phase-separated manganites by Monte Carlo simulations in a random network with clusters of ferromagnetic metal and antiferromagnetic insulator. It is found that, by taking into account the heating process and the temperature dependence of resistivity, the current-voltage characteristics are highly non-ohmic and exhibit hysteresis in sweeping the voltage. We calculate the temperature dependence of this behavior, and investigate the changes of the percolative paths at different currents. The breakdown of most percolative paths by the heating effect of the current results in an interrupt transition from the metallic behavior to the insulator behavior of the system. The obtained results are in good agreement with related experiments.     

Fréedericksz transition in confined liquid crystals: concentration and microgeometry effects

The Fréedericksz transition in dispersions of liquid-crystal droplets is studied analytically by balancing the electrostatic energy of the droplets with a strong anchoring elastic energy. Explicit dependence of the transition threshold field on the liquid-crystal volume fraction and the spatial arrangement of the droplets is obtained for the first time. As a result of the confined geometry, this threshold field does not depend on the thickness of the sample and splitting of the transition occurs in some situations.     

Dynamics of a dielectric droplet suspended in a magnetic fluid in electric and magnetic fields

The behavior of a microdrop of dielectric liquid suspended in a magnetic fluid and exposed to the action of electric and magnetic fields is studied experimentally. With increasing electric field, the deformation of droplets into oblate ellipsoid, toroid and curved toroid was observed. At the further increase in the electric field, the bursting of droplets was also revealed. The electrorotation of deformed droplets was observed and investigated. The influence of an additional magnetic field on the droplet dynamics was studied. The main features of the droplet dynamics were interpreted and theoretically examined.     

A simplified model of electrocoalescence of two close water droplets in oil

The conditions of electrocoalescence of close droplets of conducting liquid suspended in insulating oil under the action of an electric field are investigated. Previous results are first recalled and discussed. A physical picture allows justifying the asymptotic laws obtained for coalescence conditions of very close drops under a given potential difference. An extrapolation to the coalescence conditions of close droplets subjected to a uniform electric field is proposed and shortly discussed.     

格子Boltzmann方法模拟双液滴撞击液膜的流动过程

采用单相自由面格子Boltzmann方法中流场计算,自由面条件和表面张力处理方法,研究双液滴撞击液膜的流动过程,实现对垂直间隔的两液滴相继撞击液膜流动过程的模拟.数值结果表明,液滴相继冲击液膜时,液滴间上下距离对液膜状态有很大的影响,液滴间距不同,液膜形状会出现很大差异.数值结果与实验结论定性一致.     

Phase transition in ellipsoidal droplets of nematic liquid crystals

We developed a simple method for the calculation of the director field distribution in the droplets of nematic liquid crystals (LCs) of any shape, allowing for the interaction of LCs with the droplet surface, as well as the influence of constant electric field. In contrast to different approaches, the approach that is developed in the present paper does not require any simplifying suppositions about the structure of the LC director field. The elastic-continuum theory is used, complemented with the possibility of consideration of point and linear defects. Calculations are performed using the Monte Carlo method on a simple grid. The triangulation technique is used to take the boundary conditions of droplets of a complex shape into account. The developed approach can be used for investigation of the properties of polymer-dispersed liquid crystals (PDLCs). The topological phase transitions in the nematic LC 4-cyano-4′-pentyl-biphenyl (5CB) in spherical and ellipsoidal droplets are investigated in this paper.     

Magnetohydrodynamic mechanism of destabilization of the electron-hole liquid droplets

The emission spectra and photogalvanomagnetic phenomena are studied in nonhomogeneously compressed germanium. It is shown that into the electron-hole liquid (EHL) droplets moving in the field of nonhomogeneous deformation perpendicular to a magnetic field and surrounded with the dense gas of excitons and free carriers, the Hall current is arised which results to heating and destabilization of the EHL droplets.