New type of high field electrooptic response in nematics

We have made concurrent measurements of ionic current and optical transmission between crossed polarisers on several nematics with positive dielectric anisotropy under the action of applied low frequency (< 1KHz) square wave voltages. When the field E is low, the measured current is linear in E and there is no electrooptic response. Beyond some value of the field (E(0)similar to 100 esu), the current becomes independent of the field (phenomenon of limiting current). Further an electrooptic signal is measured at twice the frequency of the applied voltage, which exhibits a peak as a function of the field. The width of the peak is 3 to 4 times the value of E-0, and the signal level at the peak decreases as the frequency is increased. These measurements have been made on three highly polar compounds with cyano end groups. Careful observations do not show any evidence of electrohydrodynamic instabilities in the sample. It is argued that the observations can be understood if at the onset of the phenomenon of the limiting current, a strong electric field gradient is established near one of the electrodes due to the sweeping of an ionic species with high mobility. The field gradient produces a flexoelectric deformation of the director field, which in turn gives rise to the electrooptic effect. At higher fields, the stabilising dielectric torque takes over to suppress this instability.

Influence of crossed electric and quantizing magnetic fields on the Einstein relation in nonlinear optical, optoelectronic and related materials: Simplified theory, relative comparison and suggestion for experimental determination

An attempt is made to study the Einstein relation for the diffusivity-to-mobility ratio (DMR) under crossed fields' configuration in nonlinear optical materials on the basis of a newly formulated electron dispersion law by incorporating the crystal field in the Hamiltonian and including the anisotropies of the effective electron mass and the spin-orbit splitting constants within the framework of kp formalisms. The corresponding results for III-V, ternary and quaternary compounds form a special case of our generalized analysis. The DMR has also been investigated for II-VI and stressed materials on the basis of various appropriate dispersion relations. We have considered n-CdGeAs2, n-Hg1-xCdxTe, n-In1-xGaxAsyP1-y lattice matched to InP, p-CdS and stressed n-InSb materials as examples. The DMR also increases with increasing electric field and the natures of oscillations are totally band structure dependent with different numerical values. It has been observed that the DMR exhibits oscillatory dependences with inverse quantizing magnetic field and carrier degeneracy due to the Subhnikov-de Haas effect. An experimental method of determining the DMR for degenerate materials in the present case has been suggested. (C) 2010 Elsevier B.V. All rights reserved.

Microstructure related influence on the electrical properties of pulsed laser ablated (Ba, Sr)TiO3 thin films

The microstructural dependence of electrical properties of (Ba, Sr)TiO3(BST) thin films were studied from the viewpoint of dc and ac electrical properties. The films were grown using a pulsed laser deposition technique in a temperature range of 300 to 600 degrees C, inducing changes in grain size, structure, and morphology. Consequently, two different types of films were realized, of which type I, was polycrystalline, multigrained, while type II was [100] oriented possessing a densely packed fibrous microstructure. Leakage current measurements were done at elevated temperatures to provide evidence of the conduction mechanism present in these films. The results revealed a contribution from both electronic and ionic conduction. In the case of type I films, two trapping levels were identified with energies around 0.5 and 2.73 eV, which possibly originate from oxygen vacancies V-O and Ti3+ centers, respectively. These levels act as shallow and deep traps and are reflected in the current-voltage characteristics of the BST thin films. The activation energy associated with oxygen vacancy motion in this case was obtained as 1.28 eV. On the contrary, type II films showed no evidence of deep trap energy levels, while the identified activation energy associated with shallow traps was obtained as 0.38 eV. The activation energy obtained for oxygen vacancy motion in type II films was around 1.02 eV. The dc measurement results were further elucidated through ac impedance analysis, which revealed a grain boundary dominated response in type I in comparison to type II films where grain response is highlighted. A comparison of the mean relaxation time of the two films revealed three orders of magnitude higher relaxation time in the case of type I films. Due to smaller grain size in type I films the grains were considered to be completely depleted giving rise to only grain boundary response for the bulk of the film. The activation energy obtained from conductivity plots agree very well with that of dc measurements giving values 1.3 and 1.07 eV for type I and type II films, respectively. Since oxygen vacancy transport have been identified as the origin of resistance degradation in BST thin films, type I films with their higher value of activation energy for oxygen ion mobility explains the improvement in breakdown characteristics under constant high dc field stress. The role of microstructure in controlling the rate of degradation is found useful in this instance to enhance the film properties under high electric field stresses. (C) 2000 American Institute of Physics. [S0021-8979(00)00418-7].

Capacitive-Resistive Transients in terms of field quantities

Capacitive-resistive transients in extended media are discussed in tenns of electric field quantities. Obviously, in rhese problems, the contribution of the magnetlc field to the electric field is deemed negligible. For a simple lllusfratlve example, the field solution is compared with the circuit-theoretical resuit for the voltage and current. An algorithm for solving such transients in space and time doman with the help of a Laplace solver is presented. Any other Laplace solver can also be used far this purpose. Its applicability is demonstrated with three examples, one of which is chosen to have a circuit-theoretical solution.

Analytical Study of Low-Field Diffusive Transport in Highly Asymmetric Bilayer Graphene Nanoribbon

We present a simplified theory of carrier backscattering coefficient in a twofold degenerate asymmetric bilayer graphene nanoribbon (BGN) under the application of a low static electric field. We show that for a highly asymmetric BGN(Delta = gamma), the density of states in the lower subband increases more that of the upper, in which Delta and gamma are the gap and the interlayer coupling constant, respectively. We also demonstrate that under the acoustic phonon scattering regime, the formation of two distinct sets of energy subbands signatures a quantized transmission coefficient as a function of ribbon width and provides an extremely low carrier reflection coefficient for a better Landauer conductance even at room temperature. The well-known result for the ballistic condition has been obtained as a special case of the present analysis under certain limiting conditions which forms an indirect validation of our theoretical formalism.

Field Induced Dielectric Properties of 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 Thin Films

DC electric field induced dielectric properties of 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT) thin films were studied as a function of frequency at different temperatures. It was observed that the dielectric constant (ε) and dissipation factor (tanδ) were decreased in presence of bias field. The temperature of dielectric maxima was found to increase with increasing bias level. The low temperature (electric permittivity was suppressed with the application of dc bias. After a certain bias voltage the relaxor property of films was disappeared i.e. the films exhibited normal ferroelectric behavior. Since the absence of long range interaction among the nanopolar clusters in PMN and its family is believed to be the origin of relaxor behavior, disappearance of relaxor nature in PMN-PT (70/30) films could be attributed to manifestation of long-range order at higher bias voltage. This was observed in the temperature dependence of dielectric constant i.e. the films neither exhibited any frequency dispersion in the temperature of dielectric maximum (Tm) nor showed any diffused phase transition. The relaxor property of PMN-PT thin films was studied in terms of diffused phase transition together with frequency dispersion of the temperature of dielectric maximum (Tm). Vogel-Fulcher relation was used to analyze the frequency dependence of temperature of dielectric maximum.

Stabilizing a pulsed field emission from an array of carbon nanotubes

In this paper, we propose a new design configuration for a carbon nanotube (CNT) array based pulsed field emission device to stabilize the field emission current. In the new design, we consider a pointed height distribution of the carbon nanotube array under a diode configuration with two side gates maintained at a negative potential to obtain a highly intense beam of electrons localized at the center of the array. The randomly oriented CNTs are assumed to be grown on a metallic substrate in the form of a thin film. A model of field emission from an array of CNTs under diode configuration was proposed and validated by experiments. Despite high output, the current in such a thin film device often decays drastically. The present paper is focused on understanding this problem. The random orientation of the CNTs and the electromechanical interaction are modeled to explain the self-assembly. The degraded state of the CNTs and the electromechanical force are employed to update the orientation of the CNTs. Pulsed field emission current at the device scale is finally obtained by using the Fowler-Nordheim equation by considering a dynamic electric field across the cathode and the anode and integration of current densities over the computational cell surfaces on the anode side. Furthermore we compare the subsequent performance of the pointed array with the conventionally used random and uniform arrays and show that the proposed design outperforms the conventional designs by several orders of magnitude. Based on the developed model, numerical simulations aimed at understanding the effects of various geometric parameters and their statistical features on the device current history are reported.

Multi-Mode Phonon-Controlled Field Emission from Carbon Nanotubes: Modeling and Experiments

The main idea proposed in this paper is that in a vertically aligned array of short carbon nanotubes (CNTs) grown on a metal substrate, we consider a frequency dependent electric field, so that the mode-specific propagation of phonons, in correspondence with the strained band structure and the dispersion curves, take place. We perform theoretical calculations to validate this idea with a view of optimizing the field emission behavior of the CNT array. This is the first approach of its kind, and is in contrast to the the conventional approach where a DC bias voltage is applied in order to observe field emission. A first set of experimental results presented in this paper gives a clear indication that phonon-assisted control of field emission current in CNT based thin film diode is possible.

Investigation of dielectric, piezoelectric and ferroelectric properties of b-axis grown triglycine sulphate single crystal

Large single crystal of triglycine sulphate (dimension 100 mm along monoclinic b-axis and 15 mm in diameter) was grown using the unidirectional solution growth technique. The X-ray diffraction studies confirmed the growth/long axis to be b-axis (polar axis). The dielectric studies were carried out at various temperatures to establish the phase transition temperature. The frequency response of the dielectric constant, dielectric loss and impedance of the crystal along the growth axis, was monitored. These are typically characterized by strong resonance peaks in the kHz region. The piezoelectric coefficients like stiffness constant (C), elastic coefficient (S), electromechanical coupling coefficient (k) and d (31) were calculated using the resonance-antiresonance method. Polarization (P)-Electric field (E) hysteresis loops were recorded at various temperatures to find the temperature-dependent spontaneous polarization of the grown crystal. The pyroelectric coefficients were determined from the pyroelectric current measurement by the Byer and Roundy method. The ferroelectric domain patterns were recorded on (010) plane using scanning electron microscopy and optical microscopy.

Structure of the Pyridine-Chloranil Complex in Solution: A Surprise from Depolarized Hyper-Rayleigh Scattering Measurements

In this article, we report the structure of a 1:1 charge transfer complex between pyridine (PYR) and chloranil (CHL) in solution (CHCl(3)) from the measurement of hyperpolarizability (beta(HRS)) and linear and circular depolarization ratios, D and D', respectively, by the hyper-Rayleigh scattering technique and state-of-the-art quantum chemical calculations. Using linearly (electric field vector along X) and circularly polarized incident light, respectively, we have measured two macroscopic depolarization ratios D = I(X,X)(2 omega)/I(X,Z)(2 omega) and D' = I(X,C)(2 omega)/I(Z,C)(2 omega) in the laboratory fixed XYZ frame by detecting the second harmonic (SH) scattered light in a polarization resolved fashion. The stabilization energy and the optical gap calculated through the MP2/cc-pVDZ method using Gaussian09 were not significantly different to distinguish between the cofacial and T-shape structures. Only when the experimentally obtained beta(HRS) and the depolarization ratios, D and D', were matched with the theoretically computed values from single and double configuration interaction (SDCI) calculations performed using the ZINDO-SCRF technique, we concluded that the room temperature equilibrium structure of the complex is cofacial. This is in sharp contrast to an earlier theoretical prediction of the T-shape structure of the complex.

Backward switching phenomenon from field forced ferroelectric to antiferroelectric phases in antiferroelectric PbZrO3 thin films

Antiferroelectric materials (example: lead zirconate and modified lead zirconate stannate), in which a field-induced ferroelectric phase transition is feasible due to a small free energy difference between the ferroelectric and the antiferroelectric phases, are proven to be very good candidates for applications involving actuation and high charge storage devices. The property of reverse switching from the field-induced ferroelectric to antiferroelectric phases is studied as a function of temperature, applied electric field, and sample thickness in antiferroelectric lead zirconate thin films deposited by pulsed excimer laser ablation. The maximum released charge density was 22 μC/cm2 from a stored charge density of 36 μC/cm2 in a 0.55 μ thick lead zirconate thin film. This indicated that more than 60% of the stored charge could be released in less than 7 ns at room temperature for a field of 200 kV/cm. The content of net released charge was found to increase with increasing field strength, whereas with increasing temperature the released charge was found to decrease. Thickness-dependent studies on lead zirconate thin films showed that size effects relating to extrinsic and intrinsic pinning mechanisms controlled the released and induced charges through the intrinsic switching time. These results proved that antiferroelectric PZ thin films could be utilized in high-speed charge decoupling capacitors in microelectronics applications.

Field-induced dielectric properties of laser ablated antiferroelectric (Pb0.99Nb0.02)(Zr0.57Sn0.38Ti0.05)0.98O3 thin films

Niobium-modified lead zirconate stannate titanate antiferroelectric thin films with the chemical composition of (Pb0.99Nb0.02)(Zr0.57Sn0.38Ti0.05)0.98O3 were deposited by pulsed excimer laser ablation technique on Pt-coated Si substrates. Field-induced phase transition from antiferroelectric to ferroelectric properties was studied at different fields as a function of temperature. The field forced ferroelectric phase transition was elucidated by the presence of double-polarization hysteresis and double-butterfly characteristics from polarization versus applied electric field and capacitance and voltage measurements, respectively. The measured forward and reverse switching fields were 25 kV/cm and 77 kV/cm, respectively. The measured dielectric constant and dissipation factor were 540 and 0.001 at 100 kHz, respectively, at room temperature.

Simple theoretical analysis of the Fowler-Nordheim field emission from microstructures and quantum wires of optoelectronic materials

We present a simplified theoretical formulation of the Fowler-Nordheim field emission (FNFE) under magnetic quantization and also in quantum wires of optoelectronic materials on the basis of a newly formulated electron dispersion law in the presence of strong electric field within the framework of k.p formalism taking InAs, InSb, GaAs, Hg(1-x)Cd(x)Te and In(1-x)Ga(x) As(y)P(1-y) lattice matched to InP as examples. The FNFE exhibits oscillations with inverse quantizing magnetic field and electron concentration due to SdH effect and increases with increasing electric field. For quantum wires the FNFE increases with increasing film thickness due to the existence van-Hove singularity and the magnitude of the quantum jumps are not of same height indicating the signature of the band structure of the material concerned. The appearance of the humps of the respective curves is due to the redistribution of the electrons among the quantized energy levels when the quantum numbers corresponding to the highest occupied level changes from one fixed value to the others. Although the field current varies in various manners with all the variables in all the limiting cases as evident from all the curves, the rates of variations are totally band-structure dependent. Under certain limiting conditions, all the results as derived in this paper get transformed in to well known Fowler-Nordheim formula. (C) 2011 Elsevier Ltd. All rights reserved.

Studies on Field Dependent Domain Structures in Multi-Grained 0.85PbMg(1/3)Nb(2/3)O3-0.15PbTiO(3) Thin Films by Scanning Force Microscopy

0.85PbMg(1/3)Nb(2/3)O(3)-0.15PbTiO(3) (0.85PMN-0.15PT) ferroelectric relaxor thin films have been deposited on La0.5Sr0.5CoO3/(111) Pt/TiO2/SiO2/Si by pulsed laser ablation by varying the oxygen partial pressures from 50 mTorr to 400 mTorr. The X-ray diffraction pattern reveals a pyrochlore free polycrystalline film. The grain morphology of the deposited films was studied using scanning electron microscopy and was found to be affected by oxygen pressure. By employing dynamic contact-electrostatic force microscopy we found that the distribution of polar nanoregions is majorly affected by oxygen pressure. Finally, the electric field induced switching in these films is discussed in terms of domain wall pinning.

Self Repair in Circuits-Automating Open Fault Repair in Integrated Circuits Using Field-Induced Aggregation of Carbon Nanotubes

This paper presents studies on the use of carbon nanotubes dispersed in an insulating fluid to serve as an automaton for healing open-circuit interconnect faults in integrated circuits. The physics behind the repair mechanism is the electric-field-induced diffusion limited aggregation. On the occurrence of an open fault, the repair is automatically triggered due to the presence of an electric field across the gap. We perform studies on the repair time as a function of the electric field and dispersion concentrations with the above application in mind.