Emil S. Barna

Polarity sensitive electrooptical response in a nematic liquid crystal–polymer mixture

A polarity sensitive electrooptical effect has been observed in nematic liquid crystal–polymer mixtures where a gradient of the polymer network density is created by strongly absorbed ultraviolet light. The effect is accounted for by a model based upon consideration of the space charge distribution caused by the polymerization process. The effect may be of interest for practical applications in optical shutters and electrically controlled windows.

Fast electro-optic switching in nematic liquid crystals

The switching between on and off states of nematic liquid crystal pixels can be controlled with the well-known electro-optic effect. This effect however presents a fast response at switching on the electric field but a slow response at switching it off. Here we show a suitable choice of materials, cell geometries, surface preparations, and time dependence of the applied voltage that leads to a switch off response as fast as that at switch on. This is due to the particular conductivity mechanism in polymers with aromatic rings leading to rectifying properties when deposited on top of indium tin oxide surface. A transient negative charge at polymer–liquid crystal interface favors a faster and stronger planar orientation of the nematic molecules diminishing drastically the switch off time and increasing the contrast ratio. These facts, interesting enough from a fundamental point of view, could also lead to important technological consequences.

Pyroelectric properties of polymer dispersed ferroelectric liquid crystals

A polymer dispersed ferroelectric liquid crystal (PDFLC) is a novel material combining properties of a polymer (flexibility, elasticity) and ferroelectric liquid crystal (low voltage, fast electro-optical switching). Another interesting field of application of PDFLC is the detection of low intensity radiation using the pyroelectric response of the material. A PDFLC material containing a ferroelectric liquid crystal (FLC) with very high spontaneous polarization (close to 200 nC/cm2 at room temperature) was prepared to study its pyroelectric properties. The matrix was pure polymer poly-butadiene-styrene which can be mixed with liquid crystals in volatile solvents. The pyroelectric response and the spontaneous polarization were measured over a wide temperature range of the smectic C* phase and compared with the corresponding quantities of the pure FLC. Figures of merit of pyroelectric PDFLC films were estimated and compared with those known for conventional ferroelectrics.

Control of the spin Hall current in two dimensional electronic gas

The intrinsic spin Hall conductivity is obtained for a two-dimensional electronic gas (2DEG) in the presence of strain, Rashba coupling, and an external in-plane applied magnetic field. The conduction electrons of [001] oriented quantum well are used to model the 2DEG. The spin current value is dependent on the stress applied in direction [111].

Influence of an elastic deformation on the scalar nematic order parameter

Abstract The influence of an elastic deformation on the scalar nematic order parameter is considered. The analysis is performed starting directly from the definition of the scalar order parameter, without any use of phenomenological model. Our calculations show that the deformation imposed on the nematic director reduces the scalar order parameter. The reduction of the order is proportional to the square of the deformation, in agreement with the result of Landau–de Gennes-like models.

Model for trap-assisted electron tunneling in thin insulators

The trap-assisted electron tunneling in thin insulators has been modeled to explain the apparition of plateaus in the current-voltage characteristics of some existing experimental data. As a consequence of the electron tight confinement in the trap, the charge therein has a steplike variation with the voltage increase, a feature that further translates into wide plateaus of the current-voltage diagrams, in accordance with some experimental data. The temperature dependence of these diagrams has been included into the model and compares well with the experimental evidence. The possibility of quantitatively comparing the model computations with the measured data opens useful insight into trap’s physical properties such as spatial extent or energy depth.