Liquid-crystal diode generator of low-frequency oscillations

The excitation of rhythmic current oscillations in a diode cell containing a nematic liquid crystal is studied. The external electric field in the interelectrode gap is directed parallel to the surfaces which orient the liquid crystal molecules. The current oscillations are accompanied by the formation of an autosoliton at the cathode, which propagates and disappears at the anode. A hypothetical model is proposed to explain this current instability. Zh. Tekh. Fiz. 68, 125–127 (January 1998)     

Thermomechanical effect in a planar nematic induced by a quasistatic electric field

A thermomechanical flow of uniformly oriented nematic liquid crystal induced by a quasistatic electric field is observed. This flow occurs when the electric field strength exceeds the static Fréedericksz transition threshold. The effect is attributed to an electric-field-induced nonuniformity of the director orientation which is required for the onset of the thermomechanical effect. The experimental results show good agreement with the theoretical estimates. Zh. Tekh. Fiz. 69, 122–124 (April 1999)     

Nonchiral ferroelectric smectic-C films

Free-standing films of nonchiral liquid crystal compound are studied by optical reflectivity measurements. The existence of macroscopic ferroelectric properties and orientation of the tilt plane parallel to an electric field in the temperature range above the smectic-C-smectic-A bulk transition is demonstrated. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 10, 808–813 (25 May 1998)     

Structures and thermoelectric convection in cholesteric liquid crystals

The possibilities for excitation of electromagnetic field structures and convection cells, i.e., temperature and velocity structures, in a thermotropic cholesteric liquid crystal in the presence of flow are studied. Estimates are made and possible experiments for observing such structures are discussed. A special thermoelectric effect is investigated as the cause of these excitations — the influence of a heating-induced change in the pitch of the cholesteric helix of the molecules on the permittivity and the electric conductivity of the material. Fiz. Tverd. Tela (St. Petersburg) 41, 165–170 (January 1999)     

Chaotic behavior of triglycine sulfate crystals containing radiation defects

The temperature-amplitude region of existence of chaotic oscillations in a resonant circuit containing a triglycine sulfate crystal shifts toward higher amplitudes of the repolarization field when the crystal is irradiated with small x-ray doses. This trend is attributed to a change in the kinetics of the domain structure of the crystal in a strong electric field in the presence of radiation defects. Fiz. Tverd. Tela (St. Petersburg) 40, 716–718 (April 1998)     

Nematic LC to nematic glass phase transition in pores

The properties of a nematic liquid crystal in a porous matrix are discussed. On scales exceeding a certain characteristic scale the liquid crystal behaves as a nematic glass. Application of a weak magnetic field restores long-range orientational order. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 9, 695–698 (10 May 1997)     

Dynamics of electroconvective nematic liquid crystal structures in a nonharmonic electric field

The emergence of electroconvection in a nematic liquid crystal under the action of a nonharmonic electric field is investigated. Analysis is carried out using a 2D model. We propose new forms of the varying electric field acting on the system, for which subharmonic oscillations exist: (a) electric field of a trapezoidal form and (b) external field varying in accordance with the law of “joined cosines.” The behavior of synchronous excitations in the insulating and conducting regimes, as well as subharmonic oscillations, is analyzed. The parametric instability domains are found, and the critical frequencies of transition between different response regimes are determined. The stability maps of the nematic liquid crystal are constructed on the frequency-voltage amplitude plane.     

Nonunidirectional two-beam interaction in photorefractive bismuth silicate in an alternating electric field

The effective nonunidirectional energy exchange between like-polarized light beams is investigated experimentally on a photorefractive grating formed by diffusion drift in a bismuth silicate crystal placed in an alternating external electric field. Fiz. Tverd. Tela (St. Petersburg) 41, 632–637 (April 1999)     

Anomalous orientation of ferroelectric liquid crystal films in an electric field

An anomalous orientation of ferroelectric liquid crystals in an electric field is observed. The tilt planes of the molecules are oriented in a direction parallel to the electric field. The anomalous orientation is observed in freely suspended films above the smectic-C*-smectic-A bulk phase transition temperature. The effect is explained by the appearance of giant flexoelectric polarization, 10⁴ times greater than that in bulk liquid-crystal samples, in the film. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 6, 414–418 (25 September 1997)     

Theory of sonoluminescence in single bubbles: Flexoelectric energy contribution

An equation of motion for a cavitating gas bubble immersed in a liquid is introduced which includes a flexoelectric energy term. This energy is deduced from the electric field produced by the bubble wall acceleration (pressure gradient) in the fluid (the flexoelectric effect). We show that under conditions of sonoluminescence, this electric field reaches values typical of the electric breakdown field in water. Our theoretical results are consistent with the duration of light emission, minimum bubble radius, and energy release as measured in sonoluminescence experiments in water. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 6, 442–448 (25 September 1998)     

Magnetoelectric response of the spin-Peierls compound CuGeO3

The effect of an electric field on the magnetic susceptibility in a pure CuGeO₃ single crystal at microwave frequencies is investigated. A quadratic effect of the electric field on the magnetic susceptibility, which increases with decreasing temperature, is observed in the spin-Peierls state at temperatures below 5 K. The observed effect is tentatively attributed to residual magnetism, due to distortions of the regular dimerized structure at domain walls with different values of the dimerization phase, and to the effect of the electric field on the interchain exchange interaction. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 9, 646–651 (10 November 1996)     

On the intrinsic quadrupole electric field of a centrosymmetric dielectric

In agreement with a prediction of the theory, we have observed experimentally the intrinsic quadrupole electric field of a centrosymmetric crystal. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 1, 15–20 (10 January 1998)     

Surface flow of liquid in an electric field

An analytic solution is proposed for a model problem which demonstrates the occurrence of a surface flow of weakly conducting liquid in an electric field which was previously observed experimentally by the author. Zh. Tekh. Fiz. 69, 127–128 (June 1999)     

Does the electric Lehmann effect exist in cholesteric liquid crystals?

Experiments have shown that cholesteric droplets or cholesteric fingers may be put into motion by the action of an electric field. The former rotate whereas the latter drift perpendicularly to their axes. In all cases, the texture moves without visible material transport. The electric Lehmann effect was initially used to interpret these observations but, recently, alternative explanations were found, based on electrohydrodynamics. Another experiment in this area was that of Padmini and Madhusudana (Liq. Cryst. 14, 497 (1993)). Performed in 1993 with a compensated cholesteric liquid crystal under fixed planar boundary conditions, it was also explained in terms of electric Lehmann effect. We conducted the same experiment and extended it to a π -twisted planar geometry. Although our experimental results agree with those of Padmini and Madhusudana, we demonstrate that they are incompatible with an electric Lehmann effect. By contrast, an explanation based on flexoelectricity allows us to interpret the whole data set obtained in both geometries. The consequence is that there is at the moment no clear experimental evidence of the electric Lehmann effect.     

Accurate method for determining tilt bias angles in thin films of nematic liquid crystals

We have developed a new method for measuring tilt bias angles in spatially uniform and nonuniform thin films of nematic liquid crystals. The method employs modulation ellipsometry, based on the use of an exponentially decaying light wave to probe the boundary layer. Oscillations of the director of the liquid crystal, which are induced by the flexoelectric torque, are excited with an external periodic field. A periodic variation of the ellipticity of the light wave reflected from the interface is detected at both the first and second harmonics of the exciting electric field. When these two Fourier components of the electrooptic response are known, it is possible to calculate both the tilt bias angle θ₀ of the director and the dynamic deviation δ₀ of the tilt bias angle. The angles θ₀ and δ₀ measured by this method on the surface of an electrode (ITO) and on the surface of a ferroelectric film (a copolymer of vinylidene fluoride and trifluoroethylene), oriented in a corona discharge, were equal to θ ₀=5.1°, δθ=0.5° and θ₀=89°, δθ=0.06°, respectively. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 6, 387–392 (25 March 1998)     

Cluster self-organization of nanotubes in a nematic phase: The percolation behavior and appearance of optical singularities

The structural features, as well as the optical and electrophysical properties of a 5CB nematic liquid crystal with additions of multilayer carbon nanotubes, have been investigated in the concentration range C = 0.0025–0.1 wt %. The self-aggregation of nanotubes into clusters with a fractal structure occurs in the liquid crystal. At 0.025 wt %, the clusters are merged, initiating the percolation transition of the composite to a state with a high electric conductivity. The strong interaction of 5CB molecules with the surface of nanotube clusters is responsible for the formation of micron surface liquid crystal layers with an irregular field of elastic stresses and a complex structure of birefringence. They are easily observed in a polarization microscope and visualize directly invisible submicron nanotube aggregates. Their transverse size increases when an electric field is applied to the liquid crystal cell. Two mechanisms of the generation of optical singularities in the passing laser beam have been revealed. Optical vortices appear in the speckle fields of laser radiation scattered at the indented boundaries of the nanotube clusters, whereas the birefringence of the beam in surface liquid-crystal layers is accompanied by the appearance of polarization C points.     

Selective modulation of infrared radiation by an Al2O3 particles — liquid crystal system

We have developed a dual-frequency liquid crystal consisting of 4-n-pentyl-4′-cyanobiphenyl (5CB), 4-hexyloxyphenyl-4′-hexyloxy-3-nitrobenzoate (C2), and 4-butyl-4′-(hexyloxyphenyloxycarbonyl)phenyl carbonate (H22). We have measured the frequency dependences of the longitudinal and transverse components of the dielectric constant of the mixture and we have determined the frequency regions of positive and negative dielectric anisotropy. We show that for certain changes in the frequency of the applied electric field while the magnitude of the field remains the same, the transmission region of the Al₂O₃ particles — dual-frequency liquid crystal system switches from one wavelength to the other. The experimental results are explained by the optical homogeneity of the system in a narrow wavelength interval when the refractive indices of the particle material and the matrix are close, and also by reorientation of the liquid crystal molecules as the frequency of the applied voltage changes.     

Echo response in ferroelectric liquid crystals

It is proposed that the kinetics and dynamics of ferroelectric liquid crystals may be studied by using the echo responses to the pulsed action of a microwave electric field. For ferroelectric liquid crystals, with homogeneously oriented molecules, calculations are made of the echo signals for two pulses and it is shown that these have values which are quite easily measured. Fiz. Tverd. Tela (St. Petersburg) 40, 1370–1372 (July 1998)     

On the nature of the influence of an electric current on the magnetically stimulated microplasticity of Al single crystals

A large increase in dislocation mobility in Al single crystals in a static magnetic field in the absence of mechanical loading of the samples is observed when a dc electric current of low density (10⁵–10⁶ A/m²)is additionally passed through the samples. Apparently, the role of the current reduces to depinning of dislocations from strong pinning centers on the surface of the crystal as a result of surface electromigration of defects. This interpretation is supported by the fact that in samples whose surface is insulated by a layer of lacquer the passage of a current through the volume of the crystal does not change the ordinary dislocation mobility level in a magnetic field. It is hypothesized that surface electromigration of defects, which frees dislocations and unblocks dislocation sources, also plays a key role in the physical mechanism of the long-ago discovered macroplastification of metals upon the passage of an electric current through them. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 10, 788–793 (25 May 1998)     

Electric-field effect in a mixed-valence ion pair Cr3+−Cr2+ in a KZnF3 crystal

The influence of an external axial electric field on the absorption spectrum of a Cr³⁺−Cr²⁺ mixed-valence pair center coupled by the double-exchange mechanism in a KZnF₃ crystal is investigated. It is shown experimentally that the Cr³⁺−Cr²⁺ pair has an electric dipole moment. The migration of an electron is accompanied by local lattice strain. At the minima of the adiabatic potential, the intermediate fluorine atom is displaced from the lattice site. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 2, 147–149 (25 July 1998)