Cholesteric liquid crystals in oscillating magnetic fields

Using the hydrodynamical equations, it is shown that an oscillating magnetic field applied perpendicular to the helical axis of a cholesteric liquid crystal causes the helical structure to oscillate at even harmonics of the field frequency. The feasibility of doing light scattering from these driven modes is also discussed.     

Effect of electric field on the dielectric permittivity of betaine phosphite crystals in the paraelectric phase

Temperature dependences of the dielectric permittivity of betaine phosphite crystals are studied both without and under application of an electric bias. It is shown that, in view of the fact that the high-temperature improper ferroelastic (antiferrodistorsive) phase transition at T _c1=355 K is nearly tricritical, the nonlinear temperature dependence of inverse dielectric permittivity in the paraelectric phase and the effect of the field on the dielectric permittivity can be described within a phenomenological model containing two coupled (polar and nonpolar) order parameters with a negative coupling coefficient. An analysis of the model revealed that, in the case where two phase transitions, a nonpolar and a ferroelectric one, can occur in the crystal, all of its dielectric properties, including the polarization response in a field, can be described by one dimensionless parameter a. For the crystal under study, we have a=?2.5. This value of the parameter corresponds to a second-order ferroelectric transition far from the tricritical point, at which a=?1. It is shown that the polarization response in the paraelectric phase in an electric field calculated within this model differs radically from that in the ferroelectric phase-transition model for which the Curie-Weiss law holds in the paraelectric phase.     

液体介电体电容率的测量方法

介绍了用电磁波谐振法测量液体相对电容率的方法,解决了可变电容器的准确取值问题,液体相对电容率的实验测量结果与实际标准值符合较好。     

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.     

Changes in the microhardness and dielectric permittivity of TGS crystals after their exposure to a static magnetic field or ultralow crossed fields in the EPR scheme

Changes are found in the microhardness and dielectric permittivity of triglycine sulfate (TGS) crystals after magnetic treatment in a permanent magnetic field or in the scheme of electron paramagnetic resonance (EPR) using the Earth’s magnetic field. The temporal kinetics of microhardness reduction and the rise in dielectric permittivity (as along with their recovery) are nearly identical. The effects disappear when the crystal’s axis of symmetry is parallel to the static magnetic field in both variants of exposure.     

A molecular theory of the dielectric permittivity of a nematic liquid crystal

We present a theory of the dielectric permittivity in which the intermolecular dipole-dipole correlations are treated explicitly. The results for the static permittivity agree with those which have previously been obtained by continuum calculations. The frequency dependence of the permittivity is related to two relaxation times which contain the effects of only short range intermolecular correlations. Approximate relationships are derived between these times and independently measurable properties.     

Deformation of NaCl crystals under combined action of magnetic and electric fields

The paper reports observation of a strong macroplastic effect of weak magnetic and electric fields (B≤0.4 T, E～1 kV/m) when applied simultaneously to NaCl samples in the course of their active deformation at constant rate ?=const. In the absence of magnetic field, electric effects in the macroplasticity of the same crystals become manifest at fields E≥10³ kV/m. Quantitative dependences of the macroplasticity on magnetic and electric fields and on the strain rate have been measured. A physical interpretation of the observed phenomena is proposed.     

Dielectric permittivity in the smectic a phase of liquid crystals

We propose a functional dependence of the dielectric constant with the smectic order parameter of liquid crystals in their smectic A phase. The proposal is verified by comparing with the published values of heptylazoxybenzene.     

Symmetry effects of electric and magnetic fields in liquid state molecular dynamics

It Is shown that ensemble averages in molecular liquids may be investigated using group theoretical methods. These my he applied both in the laboratory frame (X,Y,Z) and the frame of the principal molecular moments of inertia (x,y,z). Using these methods the effect of an external electric field () and external magnetic field ( ) may be considered in terms of second order quantities such as the single molecule cross correlation function (t) ^T(o) where is the molecular centre of mass linear velocity and the angular velocity of the same molecule. Group theory, applied in this way, confirms the recent discovery by computer simulation of non-vanishing elements of (t) ^T(o) in frame (X,Y,Z) in the presence of an electric field. Group theory predicts, however, that these elements vanish in the presence of a magnetic field, even though this produces alignment and birefringence through the Langevin function. This leads to a straightforward experimental method for the direct observation of the non-vanishing elements of (t) ^T(o).     

A study of thermal, dielectric and magnetic properties of strontium malonate crystals

Crystals of strontium malonate (SrC₃H₂O₄) were grown in silica gel by the single diffusion technique. The thermo gravimetric (TG), differential thermal analysis (DTA) and differential scanning calorimetric (DSC) studies were carried out to investigate the thermal stability of the crystal. The dielectric behavior of the title compound crystal was investigated by measuring the dielectric parameters - dielectric constant, dielectric loss and AC conductivity as a function of four frequencies −1 kHz, 10 kHz, 100 kHz and 1 MHz at temperatures ranging from 50 to 170 °C. Results indicate that the title compound is thermally stable up to about 409 °C and is a promising low ε_r-value dielectric material. The magnetic behavior of the crystal was also explored using a vibrating sample magnetometer.     

Magnetoplasticity of diamagnetic crystals in microwave fields

Physical mechanisms responsible for the effect of microwave fields on magnetoplasticity and microhardness of diamagnetic crystals are formulated. Their key elements are resonant and nonresonant interactions between microwave fields and spin-selective nanoscale reactors. In ionic and covalent crystals, a nanoscale reactor is created when an electron is transferred in a “stopper + pinned dislocation” system and when a dislocation encounters a dangling bond, respectively. The former mechanism enhances microplasticity, while the latter leads to hardening via magnetic resonance. Physical implications of the mechanisms are compared with experimental observations.     

Twinning in boron-ion-irradiated bismuth single crystals subjected to mutually orthogonal electric and magnetic fields

The effect of electric pulses (duration 10⁻⁵ s and current density up to 60 A/mm²) and an external magnetic field (0.2 T) on the behavior of wedgelike twin ensembles in boron-irradiated bismuth single crystals was studied. The irradiation energy and dose were, respectively, 25 keV and 10¹⁷ ion/cm². When the crystals were exposed to the magnetic field, both irradiation and pulse application enhanced the mobility of twinning dislocations. An equilibrium condition for twinning dislocations in the irradiated material simultaneously subjected to the electric and magnetic fields was derived.     

Effective permittivity of the system of metal nanoislands in strong electric fields

The effect of the electric field on the value and sign of the low-frequency effective permittivity of metal island films is studied. It is shown that the permittivity in island metal films can be both positive and negative. The permittivity of some samples changed sign with the polarity of the applied electric field. Film polarization mechanisms are considered.     

New microwave system to determine the complex permittivity of small dielectric and semiconducting samples

This paper describes the new microwave bridge system with dielectric antennas of very small transverse dimensions (smaller than 10 mm) convenient for measurements of the complex dielectric permittivity of slab materials. The system enables (i) to measure by nondestructive way the complex permittivity of dielectric and semiconducting samples the dimensions of which are comparable or smaller than the wavelength of probing microwaves, (ii) to detect the material inhomogeneities. This system was verified at the frequency 37·5 GHz. In this paper measurements of the resistivity of the Si samples are given. Results obtained are in good agreement with the theory.     

Fluctuation-dissipation theorem in liquid and glassy glycerol: frequency-dependent dielectric permittivity and dielectric polarization fluctuation measurements

We show frequency-dependent dielectric permittivity and dielectric polarization fluctuation measurements of liquid and glassy glycerol. This allows a direct comparison of both quantities determined independently. After cooling the glycerol sample to 179 K with a cooling rate of 0.85 K/min we studied the aging time dependence of the dielectric permittivity and the polarization fluctuations using the identical glycerol sample. A cross-correlation technique allows measurements of noise levels below the amplifier noise. In the frequency range between 0.3 and 300 Hz we find the measured data to be in agreement with the fluctuation-dissipation theorem for the liquid and glassy state not depending on the aging time.