V. I. Zadorozhnii

Magnetic sensitivity of a dispersion of aggregated ferromagnetic carbon nanotubes in liquid crystals

Using carbon nanotubes filled with α-Fe, we have shown that aggregated ferronematic colloids demonstrate reliable and very effective response to a weak (<5 mT) magnetic field. The magnetic field realigns the aggregates of the particles which results in a non-threshold reorientation of the LC nearby, leading to the optically observed director distortions. The distortion regions expand with the increase of the magnetic field and achieve maximum size of several micrometres, comparable with the size of the agglomerates. In the non-distorted regions the reorientation of the director begins at the magnetic field reaching the Freedericksz transition value. Taking into account the extreme sensitivity of aggregated ferronematics to magnetic field, the following experimental and theoretical studies of the individual response of the aggregated nanoparticles to magnetic field may became the topical task of the physics and applications of ferronematics.

Penetration of an electrostatic field from the lithosphere into the ionosphere and its effect on the D-region before earthquakes

Abstract The penetration of an electrostatic field, from a source located in the lithosphere into the ionosphere is investigated. The electrostatic problem is solved numerically for a medium with an inhomogeneous anisotropic conductivity coupled to an “effective upper boundary condition”. The results show that the electric field in the ionosphere D-layer can effectively change the parameters of the lower ionosphere. The kinetics in the D-region are considered along with calculations of the atmospheric conductivity at an altitude of 60 km . It is concluded that (i) the atmospheric conductivity at 60 km can change by 30–70% as a result of electrostatic perturbations, created before the onset of earthquakes, and (ii) the penetration of an electrostatic field from the lithosphere into the ionosphere above 60 km is much better at night-time than during the day.

The Frederiks effect and related phenomena in ferronematic materials

Using continuum and statistical mechanical theories, we study the switching properties of a ferronematic in a nematic liquid crystal cell subject to homeotropic boundary conditions at the cell and particle walls. An external magnetic field normal to the cell plane is also imposed. At low fields we find thresholdless switching of the nematic director, consistent with experimental data. At higher fields, there are three regimes, depending on the strength of the anchoring interaction between the director and the ferroparticle orientation. For low anchoring strengths, there is an inverse Frederiks effect, and the nematic reorientation reduces and then disappears continuously at a critical magnetic field. At intermediate fields, the degree of reorientation reduces at high fields but remains finite. For high fields, however, the director switching saturates. The dimensionless temperature scale in the problem involves the temperature, the mean nematic elastic constant, the colloidal density, and the cell dimensi...

Weak anchoring effects in ferronematic systems

We study the magnetically induced behaviour of a ferronematic cell with finite anchoring energy at the cell surfaces. We calculate the dependence of orientational and concentration profiles on the magnetic field, the director anchoring energy, and the cell thickness. We find a new intermediate thickness high field state, in which there is no ferroparticle segregation and a highly ordered director field. This contrasts with previous work in which only a restricted set of system parameters and strong anchoring at the cell surfaces were considered.

Inverse Frederiks Effect and Bistability in Ferronematic Cells

In recent work we have examined switching properties of a ferronematic in an external magnetic field in a cell with homeotropic boundary conditions, and subject also to a bias field in the plane of the cell. There are three regimes, depending on the strength of the director-ferroparticle coupling. For low coupling, there is a high field inverse Frederiks transition to an undistorted phase. At low non-dimensional temperatures, high magnetic fields can cause the ferroparticles to segregate. Segregation-director distortion coupling can drive the inverse Frederiks transition first order, causing bistability. This article considers homogeneous planar, rather than homeotropic anchoring at the cell walls and particle surfaces. The bias field is unnecessary, but the basic physical picture is retained, with the same set of regimes. The lack of bias field means that this case is a more suitable model for basic studies.

Improved analytical method for calculating the parameters of phase-matched nonlinear-optical interactions in biaxial crystals

Abstract A new analytical method for calculating the polarization directions of an electric displacement and electric field, walk-off angles, the effective nonlinear coefficient, and other parameters describing the process of nonlinear-optical frequency conversion in orthorhombic biaxial crystals is presented. This method is based on introduction of a new polarization parameter and allows to simplify the known exact formulas for calculating the parameters of three-wave interactions at propagation of waves out of the principal planes of the optical indicatrix of a crystal. On its basis a method of obtaining the approximate formulas for calculation of these parameters is devised. The convenient for practical purposes exact and approximate formulas, which take into account the dielectric dispersion, for calculation of walk-off angles and the effective nonlinear coefficient are obtained at all possible combinations of the crystallophysical (optical) and crystallographic frames.

Noncritical vectorial phase matchings in nonlinear optics of crystals and infrared up-conversion

The exact analytic expressions for the vectorial group phase matching (PM) conditions have been obtained for sum and difference frequency generation processes ω3 = ω1 ± ω2 (ω3= ω2 − ω1) in nonlinear anisotropy crystals, which ensure a wide-band conversion of signals (ω2) and images. The tangential PM conditions for all possible interaction types in the negative and positive uniaxial crystals have been studied. Scanning of the frequency of vectorial group PM has been studied by examples of LiNbO3, LiIO, and HgGa2S4 crystals when changing the pump direction or pump frequency ω1. The possibility of tuning the wide-band operation all over the transparency region of the LiIO3, crystal has been shown. The double noncriticality conditions of PM in frequency cd; and acceptance angle have been studied and the possibility of their realization in a near and middle infrared region (IR) has been shown. A generalization of consideration to the other types of noncritical PMs (including multiples ones), has been made, including at an angle 90° between the propagation directions of waves with frequencies ω1 (1) and ω2 (2). The method of definition of the spectral bandwidths and acceptance angles of critical and noncritical PM taking into account the concordant change in the propagation angles of waves 1 and 2 has been proposed.

Magnetic field induced orientational bistability in a ferronematic cell

The equilibrium states of a suspension of single-domain ferroparticles in a nematic liquid-crystalline homeotropic cell subject to an external magnetic field are studied. We predict the existence of magnetic field induced orientational bistability in such a system in a magnetic field ∼10 2 Oe. The existence of the bistability phenomenon is governed by conditions on the cell thickness and on the director anchoring energy. The effect can be controlled using a small bias magnetic field normal to the unperturbed director. The director reorientation in a magnetic field causes an effective change in refraction index, which enables the orientational bistability to be exploited in optical devices.

Electrical Properties of Composite Materials with Electric Field-Assisted Alignment of Nanocarbon Fillers

The article reports about electric field-induced alignment of the carbon nanoparticles embedded in epoxy matrix. Optical microscopy was performed to consider the effect of the electric field magnitude and configuration, filler morphology, and aspect ratio on alignment process. Characteristic time of aligned network formation was compared with modeling predictions. Carbon nanotube and graphite nanoplatelet rotation time was estimated using an analytical model based on effective medium approach. Different depolarization factor was applied according to the geometries of the particle and electric field.Solid nanocomposites were fabricated by using AC electric field. We have investigated concentration dependence of electrical conductivity of graphite nanoplatelets/epoxy composites using two-probe technique. It was established that the electrical properties of composites with random and aligned filler distribution are differ by conductivity value at certain filler content and distinguish by a form of concentration dependence of conductivity for fillers with different morphology. These differences were explained in terms of the dynamic percolation and formation of various conductive networks: chained in case of graphite nanoplatelets and crossed framework in case of carbon nanotubes filler.

Magneto-Optical Response of Twisted Ferronematic Cells

We construct a theoretical model for a thermotropic ferronematic (magnetic liquid crystal colloid) in an asymmetric twisted nematic cell, subject to in-plane magnetic fields. At low colloidal concentrations, the nematic director ṋ is unchanged by strong fields, the magnetic director [mcirc] principally responds to its coupling to ṋ, and the suspension density peaks sharply where [mcirc] and H are co-aligned. At higher concentrations feedback between [mcirc] and ṋ aligns ṋ over a significant part of the cell, and the suspension divides into a region where ṋ is parallel to H , and a region where it is not. We also make calculations of the optical properties of this cell.