Buta Singh

Effects of dichroic dye on the dielectric properties of ferroelectric liquid crystal mixture

The dielectric spectroscopy of the ferroelectric liquid crystal mixture FLC-6980 doped with a low concentration (~2% wt/wt) of anthraquinone dye has been studied. The complex permittivity has been measured in the frequency range 100 Hz to 1 MHz in SmC* and SmA phases. Dielectric increments and the relaxation frequencies at various temperatures have been evaluated. Our results indicate that the relaxation frequency and the SmC*-SmA transition temperature T(C*A) decreases whereas the dielectric increment increases about 2.5-fold on dye addition.

Dielectric relaxations in a room temperature ferroelectric liquid crystal mixture

Dielectric properties of ferroelectric liquid crystalline mixture FLC-6980 and its anthraquinone dye (2% wt/wt) doped sample have been studied in SmC* and SmA phases in the frequency range from 100 Hz to 1 MHz in planar and homeotropic aligned cells. We observed the molecular relaxation mode in the homeotropically aligned cell at 33 °C at a frequency of 36 kHz in the SmC* phase. In the planar cells, goldstone and a new relaxation mode have been observed. A soft mode appeared in the vicinity of the SmC*-SmA transition. The dielectric increment, distribution parameter and relaxation frequency of these modes have been evaluated at different temperatures and bias voltages. Our results are in agreement with the extended Landau model.

Switching Dynamics in Ferroelectric Liquid Crystal Mixture

Switching dynamics have been investigated in homogeneously aligned 5 µm thin cell filled with (FLC-6980) ferroelectric liquid crystal mixture using field reversal method and Sawyer Tower method. The effect of applied voltage and temperature on switching time has been studied. It has been observed that switching time decreases with the increase of applied voltage and also with the rise of temperature. The tortional viscosity has been calculated. It shows Arrhenius type behavior. Hysteresis loop has been studied at different temperatures and at different frequencies. It has been observed that coercive field (Ec) increases with the increase of frequency. Ec has non-zero value at zero frequency, which is important characteristics of ferroelectrics. Ec increases near the transition temperature.

Observation of relaxation modes in room temperature ferroelectric liquid crystal mixtures

The dielectric measurements in SmC* and SmA phases of a room temperature ferroelectric liquid crystal mixture FLC-6980 in the cells of different thickness in planer alignment have been carried out in the frequency range 100 Hz to 1 MHz. A relaxation mode (called NRM) whose dielectric increment is less than the Goldstone mode has been observed in the SmC* phase. This mode appears due to the surface effect. Goldstone mode and the soft mode was observable in the vicinity of SmC*-SmA transition temperature (TC*A). The dielectric parameters of the Goldstone mode, new mode and the soft mode have been studied as a function of frequency and temperature. The calculated values for fNRM, δεNRM and distribution parameter αNRM are found to be 325 kHz, 6 and 0.156 for 5μm thick planer cell at 37°C. It is seen that in the vicinity of theTC*A, soft mode obeys the Curie-Weiss law given by mean field theory. The results have been compared with materials of large spontaneous polarization.

Investigations of Angular Distribution in Two-Photon Compton Scattering

The cross-section for two-photon Compton scattering for incident photons of energy 0.662 MeV and two emitted photons having energies greater than 50 keV has been measured at different scattering angles θ 1 =30° to 150° and θ 2 =π/2; φ 2 =π. Our measured results, although of same magnitude, are consistently higher as compared to theory of Mandl and Skyrme. Some calculations of energy and direction of the recoil electron are also carried out to understand the important features of this weak process.

Experimental Study of Energy Distribution in Double Photon Compton Scattering

Abstract The energy distribution of gamma photons scattered in double photon Compton scattering is measured experimentally for 0.662-MeV incident gamma photons. The two simultaneously emitted gamma quanta are investigated using a slow-fast coincidence technique. The experimental data on energy distribution do not suffer from inherent energy resolution of the gamma detector and confirm the continuous nature of energy spectra for the emitted photons. The present results are in agreement with the current theory of this higher-order process.