Surface barrier effects in non-resonant microwave absorption by superconducting thin films of YBa2Cu3O7??

We present an unusual temperature dependence of hysteresis in the Lion resonant microwave absorption (NRMA) signals from superconducting thin films of YBa2Cu3O7-delta. We observe that the hysteresis increases with increase in temperature till T-c which we interpret as evidence for the presence of Bean-Livingston surface barriers (BLSB) in the single crystalline films.

Kinetics of self-assembled InN quantum dots grown on Si (111) by plasma-assisted MBE

One of the scientific challenges of growing InN quantum dots (QDs), using Molecular beam epitaxy (MBE), is to understand the fundamental processes that control the morphology and distribution of QDs. A systematic manipulation of the morphology, optical emission, and structural properties of InN/Si (111) QDs is demonstrated by changing the growth kinetics parameters such as flux rate and growth time. Due to the large lattice mismatch, between InN and Si (similar to 8%), the dots formed from the Strannski-Krastanow (S-K) growth mode are dislocated. Despite the variations in strain (residual) and the shape, both the dot size and pair separation distribution show the scaling behavior. We observed that the distribution of dot sizes, for samples grown under varying conditions, follow the scaling function.

Kinetics and mechanism for dye degradation with ionic Pd-substituted ceria

The degradation of the dye, Orange G, was carried out in the presence of H2O2 and Pd-substituted/impregnated CeO2. The effects of pH, initial dye concentration, initial H2O2 concentration, temperature, catalyst loading, and Pd content in the catalyst on the degradation of the dye were investigated. Eight to twelve percent degradation of the dye was obtained in 1 h when the reaction was carried out in the presence of CeO2 or H2O2 or Pd-substituted/impregnated CeO2 while 17% and 97% degradation was obtained when H2O2 was used with Pd-impregnated CeO2 and Pd-substituted CeO2, respectively. This difference clearly indicated that the ionic substitution of Pd played a key role in the degradation of the dye. A mechanism for the reaction was proposed based upon the catalyst structure and the electron transfer processes that take place in the metal ion substituted system in a reducible oxide. The reaction was found to follow first order kinetics and the influence of all the parameters on the degradation kinetics was compared using the rate constants. (c) 2011 Elsevier B.V. All rights reserved.

''Phase reversal'' of non-resonant microwave absorption in superconducting YBaCuO/CuO powder mixtures

Non-resonant microwave absorption is studied as a function of temperature and composition in superconducting YBa2Cu3O7/CuO ceramic composite samples. In pure YBa2Cu3O7 only normal field dependence of the absorption is observed, where as in composites an anomalous non-monotonic field dependence is seen. The results are explained using an extended resistively shunted junction model and invoking the occurrence of junctions with phase difference psi(0) such that pi/2 < psi(0) < 3 pi/2. Copyright (C) 1996 Elsevier Science Ltd

Synthesis, structure and IR absorption studies of LnBaCuCoO(5) (Ln equal earth) oxides

A series of oxides LnBaCuCoO(5) (Ln = Pr, Nd, Sm, Dy, Gd, Ho and Er) have been synthesized by ceramic method. The oxides crystallize in a tetragonal structure, isostructural to YBaCuCoO5. All the oxides in the series are semiconducting. IR spectra of these oxides show distinct absorption bands at 630 cm(-1), 550 cm(-1) and 330 cm(-1) which are assigned to E, A(2) and A(1) modes respectively. Doping of holes in these oxides, by calcium substitution in Er1-xCaxBaCuCoO5-x (up to x similar or equal to 0.3) was done but, these oxides did not show metallic behaviour.

Synthesis Based on Cyclohexadienes. 5. A New Approach to the Synthesis of a-Ring Aromatic Steroids - a Formal Total Synthesis of (+/-)-Estrone

A new strategy for the construction of A-ring aromatic steroids which resulted in the formal total synthesis of estrone is described. Thus reaction of the adduct (9), obtained from 1-methoxy-4-methylcyclohexa-1,4-diene and acrolein, with 3-(m-methoxyphenyl)propylmagnesium bromide followed by oxidation afforded the bicyclo[2.2.2]octene derivative (14). Acid-catalysed rearrangement of (14) followed by an intramolecular Michael addition resulted in the cis tetraenone (18) which was dehydrogenated with palladium chloride to the pentaenone (22). Li/NH3 reduction of (22) gave 3-methoxy-D-homoestra-1,3,5(10)-trien-16-one (31) which has been converted into the methyl ether (37) of marrianolic acid, and its methyl ester (38).

Thermodynamic modelling of absorption-resorption heating cycles with some new working pairs

This paper presents the results of a thermodynamic cycle analysis of single stage resorption heat pump (RHP) and resorption heat transformer (RHT) cycles with the new working pairs R22-NMP and R22-DMA. The coefficients of performance (COP) are correlated with the low grade source temperature, temperature at which useful heat is obtained and ambient temperature. The COPs are in the range 1.20–1.60 for the RHP mode and 0.25–0.45 for the RHT mode. Absorber temperatures (useful temperatures) as high as 50°C in the RHP mode and 87°C in the RHT mode have been obtained. It is observed that absorption-resorption systems are inflexible in their range of operating temperature and necessitate a higher pump work as compared with simple single-stage absorption heating systems. However, single stage RHTs show higher temperature boosts than simple absorption heat transformers.

The stretching of single poly-ubiquitin molecules: Static versus dynamic disorder in the non-exponential kinetics of chain unfolding

Static disorder has recently been implicated in the non-exponential kinetics of the unfolding of single molecules of poly-ubiquitin under a constant force Kuo, Garcia-Manyes, Li, Barel, Lu, Berne, Urbakh, Klafter, and Fernandez, Proc. Natl. Acad. Sci. U. S. A. 107, 11336 (2010)]. In the present paper, it is suggested that dynamic disorder may provide a plausible, alternative description of the experimental observations. This suggestion is made on the basis of a model in which the barrier to chain unfolding is assumed to be modulated by a control parameter r that evolves in a parabolic potential under the action of fractional Gaussian noise according to a generalized Langevin equation. The treatment of dynamic disorder within this model is pursued using Zwanzig's indirect approach to noise averaging Acc. Chem. Res. 23, 148 (1990)]. In conjunction with a self-consistent closure scheme developed by Wilemski and Fixman J. Chem. Phys. 58, 4009 (1973); ibid. 60, 866 (1974)], this approach eventually leads to an expression for the chain unfolding probability that can be made to fit the corresponding experimental data very closely. (C) 2011 American Institute of Physics.

Reactivation kinetics of acceptors in hydrogenated InP during unbiased annealing

The reactivation kinetics of passivated Mg acceptors in hydrogenated InP during unbiased annealing of a Schottky diode is reported. The reactivation is found to slow down gradually with annealing time and this phenomenon is attributed to substantial retrapping of H at the acceptor sites. It is found from the concentration profiles and the kinetics data that the reactivation is most likely limited by H2 molecule formation processes for longer annealing times; for shorter annealing times, contributions from in-diffusion of H also become significant. The diffusion of H during the initial period follows an Arrhenius relation with an activation energy for the effective diffusion coefficient of 1.13±0.10 eV. In the H2 formation regime, the reactivation is thermally activated with an activation energy for the annealing parameter of 1.71±0.10 eV. The H2 formation-limited regime of reactivation occurs sooner as the annealing temperature is increased.

Nonresonant microwave absorption studies of surface passivation of superconducting YBa2Cu3O7−δ thin films

The surfaces of laser ablated thin films of YBa2Cu3O7?? have been passivated with about 100 Å thick textured layer of Ca0.95Sr0.025Ba0.025Zr0.98Ta0.01Ti0.01O3. It is shown that this low loss dielectric material preserves the quality of the surface and also prolongs the aging process. The films (both passivated and as?deposited) have been studied for degradation on exposure to atmosphere and also on dipping directly in water. The technique of nonresonant microwave absorption is used to study the effects and extent of degradation in these films. © 1995 American Institute of Physics.

Wavepacket dynamical studies on trans-azobenzene: absorption spectrum and resonance Raman excitation profiles of the n??* transition

Time-dependent wavepacket propagation techniques have been used to calculate the absorption spectrum and the resonance Raman excitation profiles of the n-pi* transition in azobenzene. A comparison of both the calculated absorption spectrum and excitation profiles with experiment has been made. From an analysis of the data, it is concluded that the Raman intensities are mainly due to resonance from the n-pi* transition and not from the pre-resonance of the pi-pi* transition, as reported earlier. We find that the isomerization pathway is through the inversion mechanism rather than by rotation. This is the first direct spectroscopic evidence for the isomerization pathway in trans-azobenzene.

Photoinduced transparency of effective three-photon absorption coefficient for femtosecond laser pulses in Ge16As29Se55 thin films

We report a dramatic change in effective three-photon absorption coefficient of amorphous Ge16As29Se55 thin films, when its optical band gap decreases by 10 meV with 532 nm light illumination. This large change provides valuable information on the higher excited states, which are otherwise inaccessible via normal optical absorption. The results also indicate that photodarkening in chalcogenide glasses can serve as an effective tool to tune the multiphoton absorption in a rather simple way. (C) 2011 American Institute of Physics.

Kinetics of copolyurethane network formation

Cure kinetics for the formation of copolyurethane networks of various compositions based on hydroxy-terminated polybutadiene(HTPB), poly(12-hydroxy stearic acid-co-TMP) ester polyol(PEP), and different isocyanates has been studied through viscosity build up during the cure reaction. The viscosity (N)-time (t) plots conform to the equation N = ae(bt), where a and b are empirical constants, dependent on the composition and the nature of the polyols and the isocyanates. The rate constants (b) for viscosity build up, evaluated from the slopes of dN/dt versus N plots at different temperatures, were found to vary significantly from 0.0073 to 0.25 min(-1); and the activation energies for gelation were found to be in the range 20 to 40 kJ mol(-1). The results have been interpreted in terms of the dependence of the rate constants on structural characteristics of the prepolymers. (C) 1997 John Wiley & Sons, Inc.

Non-exponentiality in electron transfer kinetics: Static versus dynamic disorder models

Non-exponential electron transfer kinetics in complex systems are often analyzed in terms of a quenched, static disorder model. In this work we present an alternative analysis in terms of a simple dynamic disorder model where the solvent is characterized by highly non-exponential dynamics. We consider both low and high barrier reactions. For the former, the main result is a simple analytical expression for the survival probability of the reactant. In this case, electron transfer, in the long time, is controlled by the solvent polarization relaxation-in agreement with the analyses of Rips and Jortner and of Nadler and Marcus. The short time dynamics is also non-exponential, but for different reasons. The high barrier reactions, on the other hand, show an interesting dynamic dependence on the electronic coupling element, V-el.