Distance dependence of fluorescence resonance energy transfer

Deviations from the usual R (-6) dependence of the rate of fluorescence resonance energy transfer (FRET) on the distance between the donor and the acceptor have been a common scenario in the recent times. In this paper, we present a critical analysis of the distance dependence of FRET, and try to illustrate the non R (-6) type behaviour of the rate for the case of transfer from a localized electronic excitation on the donor, a dye molecule to three different energy acceptors with delocalized electronic excitations namely, graphene,two-dimensional semiconducting sheet and the case of such a semiconducting sheet rolled to obtain a nanotube. We use simple analytic models to understand the distance dependence in each case.

The composition dependence of electrical switching behavior of Ge7Se93-xSbx glasses

Bulk Ge7Se93-xSbx (21 <= x <= 32) glasses are prepared by melt quenching method and electrical switching studies have been undertaken on these samples to elucidate the type of switching and the composition and thickness dependence of switching voltages. On the basis of the compressibility and atomic radii, it has been previously observed that Se-based glasses exhibit memory switching behavior. However, the present results indicate that Ge7Se93-xSbx glasses exhibit threshold type electrical switching with high switching voltages. Further, these samples are found to show fluctuations in the current-voltage (I-V) characteristics. The observed threshold behavior of Ge7Se93-xSbx glasses has been understood on the basis of larger atomic radii and lesser compressibilities of Sb and Ge. Further. the high switching voltages and fluctuations in the I-V characteristics of Ge-Se-Sb samples can be attributed to the high resistance of the samples and the difference in thermal conductivities of different structural units constituting the local structure of these glasses. The switching voltages of Ge7Se93-xSbx glasses have been found to decrease with the increase in the Sb concentration. The observed composition dependence of switching voltages has been understood on the basis of higher metallicity of the Sb additive and also in the light of the Chemically Ordered Network (CON) model. Further, the thickness dependence of switching voltages has been studied to reassert the mechanism of switching.

Aggregation of apolar peptides in organic solvents. Concentration dependence of 1H-nmr parameters for peptide NH groups in 310 helical decapeptide fragment of suzukacillin

Peptide NH chemical shifts and their temperature dependences have been monitored as a function of concentration for the decapeptide, Boc-Aib-Pro-Val-Aib-Val-Ala-Aib-Ala-Aib-Aib-OMe in CDCl3 (0.001-0.06M) and (CD3)2SO (0.001-0.03M). The chemical shifts and temperature coefficients for all nine NH groups show no significant concentration dependence in (CD3)2SO. Seven NH groups yield low values of temperature coefficients over the entire range, while one yields an intermediate value. In CDCl3, the Aib(1) NH group shows a large concentration dependence of both chemical shift and temperature coefficient, in contrast to the other eight NH groups. The data suggest that in (CD3)2SO, the peptide adopts a 310 helical conformation and is monomeric over the entire concentration range. In CDCl3, the 310 helical peptide associates at a concentration of 0.01M, with the Aib(1) NH involved in an intermolecular hydrogen bond. Association does not disrupt the intramolecular hydrogen-bonding pattern in the decapeptide.

Adsorption of Nitrogen on Activated Carbon-Refit of Experimental Data and Derivation of Properties Required for Design of Equipment

Volumetric method based adsorption measurements of nitrogen on two specimens of activated carbon (Fluka and Sarabhai) reported by us are refitted to two popular isotherms, namely, Dubunin−Astakhov (D−A) and Toth, in light of improved fitting methods derived recently. Those isotherms have been used to derive other data of relevance in design of engineering equipment such as the concentration dependence of heat of adsorption and Henry’s law coefficients. The present fits provide a better representation of experimental measurements than before because the temperature dependence of adsorbed phase volume and structural heterogeneity of micropore distribution have been accounted for in the D−A equation. A new correlation to the Toth equation is a further contribution. The heat of adsorption in the limiting uptake condition is correlated with the Henry’s law coefficients at the near zero uptake condition.

Diffusion of flexible, charged, nanoscopic molecules in solution: Size and pH dependence for PAMAM dendrimer

In order to understand self-diffusion (D) of a charged, flexible, and porous nanoscopic molecule in water, we carry out very long, fully atomistic molecular dynamics simulation of PAMAM dendrimer up to eight generations in explicit salt water under varying pH. We find that while the radius of gyration (R-g) varies as N-1/3, the self-diffusion constant (D) scales, surprisingly, as N-alpha, with alpha=0.39 at high pH and 0.5 at neutral pH, indicating a dramatic breakdown of Stokes-Einstein relation for diffusion of charged nanoscopic molecules. The variation in D as a function of radius of gyration demonstrates the importance of treating water and ions explicitly in the diffusion process of a flexible nanoscopic molecule. In agreement with recent experiments, the self-diffusion constant increases with pH, revealing the importance of dielectric friction in the diffusion process. The shape of a dendrimer is found to fluctuate on a nanosecond time scale. We argue that this flexibility (and also the porosity) of the dendrimer may play an important role in determining the mean square displacement of the dendrimer and the breakdown of the Stokes-Einstein relation between diffusion constant and the radius.

Energy identities in water wave theory for free-surface boundary condition with higher-order derivatives

A modified form of Green's integral theorem is employed to derive the energy identity in any water wave diffraction problem in a single-layer fluid for free-surface boundary condition with higher-order derivatives. For a two-layer fluid with free-surface boundary condition involving higher-order derivatives, two forms of energy identities involving transmission and reflection coefficients for any wave diffraction problem are also derived here by the same method. Based on this modified Green's theorem, hydrodynamic relations such as the energy-conservation principle and modified Haskind–Hanaoka relation are derived for radiation and diffraction problems in a single as well as two-layer fluid.

Electrical switching behavior of bulk Si15Te85-xSbx chalcogenide glasses - A study of compositional dependence

Studies on the electrical switching behavior of melt quenched bulk Si15Te85-xSbx glasses have been undertaken in the composition range (1 <= x <= 10), in order to understand the effect of Sb addition on the electrical switching behavior of Si15Te85-x base glass. It has been observed that all the Si15Te85-xSbx glasses studied exhibit a smooth memory type switching. Further, the switching voltages are found to decrease almost linearly with Sb content, which indicates that the metallicity of the dopant plays a dominant role in this system compared to network connectivity/rigidity. The thickness dependence of switching voltage (V-th) indicates a clear thermal origin for the switching mechanism. The temperature variation of switching voltages reveals that the Si15Te85-xSbx glasses studied have a moderate thermal stability. (C) 2009 Elsevier B.V. All rights reserved.

Real-coded genetic algorithm for machining condition optimization

In this paper, we consider the machining condition optimization models presented in earlier studies. Finding the optimal combination of machining conditions within the constraints is a difficult task. Hence, in earlier studies standard optimization methods are used. The non-linear nature of the objective function, and the constraints that need to be satisfied makes it difficult to use the standard optimization methods for the solution. In this paper, we present a real coded genetic algorithm (RCGA), to find the optimal combination of machining conditions. We present various issues related to real coded genetic algorithm such as solution representation, crossover operators, and repair algorithm in detail. We also present the results obtained for these models using real coded genetic algorithm and discuss the advantages of using real coded genetic algorithm for these problems. From the results obtained, we conclude that real coded genetic algorithm is reliable and accurate for solving the machining condition optimization models.

Theory of the unusual doping and temperature dependence of photoemission spectra in manganites

A recent, major, puzzle in the core-level photoemission spectra of doped manganites is the observation of a 1–2 eV wide shoulder with intensity varying with temperature T as the square of the magnetization over a T scale of order 200 K, an order of magnitude less than electronic energies. This is addressed and resolved here, by extending a recently proposed two-fluid polaron–mobile electron model for these systems to include core-hole effects. The position of the shoulder is found to be determined by Coulomb and Jahn-Teller energies, while its spectral weight is determined by the mobile electron energetics which is strongly T and doping dependent, due to annealed disorder scattering from the polarons and the t2g core spins. Our theory accounts quantitatively for the observed T dependence of the difference spectra, and furthermore, explains the observed correspondence between spectral changes due to increasing doping and decreasing T.

Stability of an unsupported vertical circular excavation in clays under undrained condition

By using the axisymmetric finite elements static limit analysis formulation, proposed recently by the authors, the stability numbers (gamma H/c(o)) for an unsupported vertical circular excavation in clays, whose cohesion increases with depth, have been determined under undrained condition; gamma = unit weight, H., height of the excavation and c(o) = cohesion along ground surface. The results are obtained for various values of H/b and m; where b = the radius of the excavation and m = a non-dimensional parameter which accounts for the rate of the increase of cohesion with depth. The values of the stability numbers increase continuously both with increases in H/b and m. The results obtained in this study compare well with those available in literature.(C) 2009 Elsevier Ltd. All rights reserved.

Subquadratic wavenumber dependence of the structural relaxation of supercooled liquid in the crossover regime

As a liquid is progressively supercooled toward its glass transition temperature, an intriguing weakening of the wavenumber (q) dependence of the structural relaxation time tau(q) in the intermediate-to-large q limit is observed both in experiments and simulation studies. Neither continuous Brownian diffusive dynamics nor discontinuous activated events can alone explain the anomalous wavenumber dependence. Here we use our recently developed theory that unifies the mode coupling theory for continuous dynamics, with the random first order transition theory treatment of activated discontinuous motion as a nucleationlike instanton process to understand the wavenumber dependence of density relaxation. The predicted smooth change in mechanism of relaxation from diffusive to activated, in the crossover regime, is wavevector dependent and appears to be responsible for the observed subquadratic,nalmost linear, q dependence of the relaxation time.

A voltage-sensorless control method to balance the input currents of a three-wire boost rectifier under unbalanced input voltages condition

This paper proposes a control method that can balance the input currents of the three-phase three-wire boost rectifier under unbalanced input voltage condition. The control objective is to operate the rectifier in the high-power-factor mode under balanced input voltage condition but to give overriding priority to the current balance function in case of unbalance in the input voltage. The control structure has been divided into two major functional blocks. The inner loop current-mode controller implements resistor emulation to achieve high-power-factor operation on each of the two orthogonal axes of the stationary reference frame. The outer control loop performs magnitude scaling and phase-shifting operations on current of one of the axes to make it balanced with the current on the other axis. The coefficients of scaling and shifting functions are determined by two closed-loop prportional-integral (PI) controllers that impose the conditions of input current balance as PI references. The control algorithm is simple and high performing. It does not require input voltage sensing and transformation of the control variables into a rotating reference frame. The simulation results on a MATLAB-SIMULINK platform validate the proposed control strategy. In implementation Texas Instrument's digital signal processor TMS320F24OF is used as the digital controller. The control algorithm for high-power-factor operation is tested on a prototype boost rectifier under nominal and unbalanced input voltage conditions.

Anomalous temperature dependence of Fermi-edge singularity in modulation-doped AlGaAs/InGaAs/GaAs hetero-structures

The temperature and power dependence of Fermi-edge singularity (FES) in high-density two-dimensional electron gas, specific to pseudomorphic AlxGa1-xAs/InGa1-yAs/GaAs heterostructures is studied by photoluminescence (PL). In all these structures, there are two prominent transitions E-11 and E-21 considered to be the result of electron-hole recombination from first and second electron sub-bands with that of first heavy-hole sub-band. FES is observed approximately 5-10 meV below the E-21 transition. At 4.2 K, FES appears as a lower energy shoulder to the E-21 transition. The PL intensity of all the three transitions E-11, FES and E-21 grows linearly with excitation power. However, we observe anomalous behavior of FES with temperature. While PL intensity of E-11 and E-21 decrease with increasing temperature, FES transition becomes stronger initially and then quenches-off slowly (till 40K). Though it appears as a distinct peak at about 20 K, its maximum is around 7 - 13 K.

Effect of boundary condition description on convergence of solution in a boundary-value problem

It is well known that the numerical accuracy of a series solution to a boundary-value problem by the direct method depends on the technique of approximate satisfaction of the boundary conditions and on the stage of truncation of the series. On the other hand, it does not appear to be generally recognized that, when the boundary conditions can be described in alternative equivalent forms, the convergence of the solution is significantly affected by the actual form in which they are stated. The importance of the last aspect is studied for three different techniques of computing the deflections of simply supported regular polygonal plates under uniform pressure. It is also shown that it is sometimes possible to modify the technique of analysis to make the accuracy independent of the description of the boundary conditions.

ESR study of dibarium copper formate tetrahydrate Part II. Optical absorption and temperature dependence of spin-hamiltonian parameters

The variations in certain spin-Hamiltonian parameters of the Cu++ ion in dibarium copper formate tetrahydrate with temperature have been studied. Optical absorption investigations on single crystals of the salt at room temperature and 90° K. are reported. The results are discussed in terms of a model in which vibronic mixing of certain electron levels of the Cu++ ion play an important role.