Distribution and thermophysical properties of n-heptacosane molecules confined in carbon nanotube

Molecular dynamics (MD) simulations were performed to provide an insight about the molecule distribution and thermophysical properties of n-heptacosane confined in the (25, 25) single-walled carbon nanotube (CNT). The results show that an orderly distribution of n-heptacosane molecules along the CNT inner wall is clearly observed. Meanwhile, n-heptacosane confined in CNT exhibits an increased self-diffusion coefficient, a decreased melting point and an enhanced thermal conductivity, compared to the bulk. The simulations reveal that MD is an effective and convenient method to understand the variation characteristics of alkane-based phase change materials confined in CNT on molecular and atomic scale.     

Source characterization using recordings made in a reverberant underwater channel

The ability to accurately characterize an underwater sound source is an important prerequisite for many applications including detection, classification, monitoring and mitigation. Unfortunately, anechoic underwater recording environments required to make ideal recordings are generally not available. This paper presents a practical approach to source characterization when working in an imperfect recording environment; the source spectrum is obtained by equalizing the recording with the inverse of the channel’s impulse response (IR). An experiment was conducted in a diving well (depth of 5.18 m) using a logarithmic chirp to obtain the IR. IR length is estimated using methods borrowed from room acoustics and inversion of non-minimum phase IR is accomplished separately in the time and frequency domain to allow for a direct comparison. Results indicate that the energy of controlled sources can be recovered with root-mean-square error of −70 dB (10–70 kHz band). Two equations, one coherent and the other incoherent, are presented to calculate source spectral levels of an unknown source in a reverberant environment. This paper introduces a practical procedure outlining steps to obtain an anechoic estimate of an unknown source using equipment generally available in an acoustic laboratory.     

trans-Stilbene Photochemistry Beyond 500 nm

trans-Stilbene and its derivatives have been widely investigated over the past decades in terms of both photophysics and photochemistry. This study presents an intramolecular energy transfer within a triple chromophore from an optically excited fluorescein onto an extremely low-lying trans-stilbene T₁ state. As in many stilbenes an isomerization along the triplet pathway was sufficiently effective to be monitored by a decrease in the trans- and a concomitant increase in the cis-stilbene absorption bands. Semiempirical calculations and sensitizing experiments were performed to obtain a good estimate of the S₀–T₁ energy difference, which was found to be about 142 kJ/mol. This uncommonly low value enables a long-wavelength photochemistry of trans-stilbene at exciting light wavelengths ranging from 500 nm (fluorescein) up to at least 680 nm (applying an external triplet sensitizer such as methylene blue).     

The trion as an exciton interacting with a carrier

The X⁻ trion is essentially an electron bound to an exciton. However, due to the composite nature of the exciton, there is no way to write an exciton-electron interaction potential. We can overcome this difficulty by using a commutation technique similar to the one we introduced for excitons interacting with excitons, which allows to take exactly into account the close-to-boson character of the excitons. From it, we can obtain the X⁻ trion creation operator in terms of excitons and electrons. We can also derive the X⁻ trion ladder diagram between an exciton and an electron. These are the basic tools for future works on many-body effects involving trions.     

A generalization of Wigner's R-matrix method

A generalization of the Wigner's non-relativistic R-matrix theory of scattering by a central potential field is proposed. The idea is to use an R-matrix expansion basis generated by a Sturm-Liouville problem with an eigenparameter included both in a differential equation and in a boundary condition (in the standard theory an R-matrix basis is obtained by solving an eigenvalue problem with fixed boundary conditions). A partial fraction expansion of an R^(η)-matrix introduced is derived and shown to converge faster than a partial fraction expansion of Wigner's R-matrix used in the standard theory.     

Analytical electron microscopy of a crack tip extracted from a stressed Alloy 800 sample exposed to an acid sulfate environment

Alloy 800 (Fe–21Cr–33Ni) has been found susceptible to cracking in acid sulfate environments, but the mechanism is not well understood. Alloy 800 C-ring samples were exposed to an acid sulfate environment at 315 °C and cracks were found with depths in excess of 300 μm after 60 h. Preparation of a TEM sample containing crack tips is challenging, but the ability to perform high-resolution microscopy at the crack tip would lend insight to the mechanism of acid sulfate stress corrosion cracking (AcSCC). The lift-out technique combined with a focused ion beam sample preparation was used to extract a crack tip along the cross-section of an acid sulfate crack in an Alloy 800 C-ring. TEM elemental analysis was done using EDS and EELS which identified a duplex oxide within the crack; an inner oxide consisting of a thin 3–4 nm Cr-rich oxide and an outer oxide enriched in Fe and Cr. Preliminary conclusions and hypotheses resulted with respect to the mechanism of AcSCC in Alloy 800.     

Anion Sensing with Luminescent Quantum Dots – A Modular Approach Based on the Photoinduced Electron Transfer (PET) Mechanism

A CdSe–ZnS quantum dot (QD) has been surface functionalised with 1-(2-mercapto-ethyl)-3-phenyl-thiourea in the fluorophore–spacer–receptor format typical of Photoinduced Electron Transfer (PET) based organic dye sensors. The resulting QD conjugate was tested for selectivity toward the tetrabutylammonium salts of fluoride, chloride, bromide, hydrogen sulfate and acetate. Addition of fluoride, chloride and acetate ions resulted in an approximate 90% quenching of the original fluorescence intensity, while bromide and hydrogen sulfate had almost no effect. The observed quench was attributed to an increase in the reduction potential of the receptor upon anion binding resulting in an increase in PET from the excited QD to the receptor and a concomitant reduction in fluorescence intensity. The selectivity and sensitivity were comparable to an analagous organic dye based sensor where a similar receptor was bound to an anthracene fluorophore. Thus a modular approach is evident where a receptor used in an organic dye based sensor can be adapted and successfully used with QD’s.     

Subthreshold swing characteristics of nanowire tunneling FETs with variation in gate coverage and channel diameter

In this study, we demonstrate the simulated subthreshold swing (SS) of silicon nanowire tunneling field-effect transistors (NWTFETs) by varying both the channel diameter from 10 nm to 40 nm and the gate coverage ratio from 30% to 100%. Our simulation work reveals that both a decrease in the channel diameter and an increase in the gate coverage ratio contribute to a reduction in the SS. Additionally, our work shows that the magnitude of the on-current depends linearly on the gate coverage ratio and that the drain current increases with a decrease in the channel diameter. Thus, an NWTFET with a channel diameter of 10 nm and a gate coverage ratio of 100% exhibits superior electrical characteristics over other silicon NWTFETs in that the NWTFET shows a point SS of 22.7 mV/dec, an average SS of 56.3 mV/dec, an on/off current ratio of ∼10¹³, and an on-current of ∼10⁻⁵ A/μm.     

Symbols of a cosmic order

The world runs on networks over which signals communicate sequences of symbols, e.g. numerals. Examining both engineered and natural communications networks reveals an unsuspected order that depends on contact with an unpredictable entity. This order has three roots. The first is a proof within quantum theory that no evidence can ever determine its explanation, so that an agent choosing an explanation must do so unpredictably. The second root is the showing that clocks that step computers do not “tell time” but serve as self-adjusting symbol-handling agents that regulate “logically synchronized” motion in response to unpredictable disturbances. Such a clock-agent has a certain independence as well as the capacity to communicate via unpredictable symbols with other clock-agents and to adjust its own tick rate in response to that communication. The third root is the noticing of unpredictable symbol exchange in natural systems, including the transmission of symbols found in molecular biology. We introduce a symbol-handling agent as a role played in some cases by a person, for example a physicist who chooses an explanation of given experimental outcomes, and in other cases by some other biological entity, and in still other cases by an inanimate device, such as a computer-based detector used in physical measurements. While we forbear to try to explain the propensity of agents at all levels from cells to civilizations to form and operate networks of logically synchronized symbol-handling agents, we point to this propensity as an overlooked cosmic order, an order structured by the unpredictability ensuing from the proof. Appreciating the cosmic order leads to a conception of agency that replaces volition by unpredictability and reconceives the notion of objectivity in a way that makes a place for agency in the world as described by physics. Some specific implications for physics are outlined.     

Nickel doped indium tin oxide anode and effect on dark spot development of organic light-emitting devices

This article demonstrated that introducing nickel (Ni) atoms into an indium tin oxide (ITO) anode could considerably decrease ITO surface roughness and eliminate the formation of dark spots of an organic light-emitting device (OLED). A dramatic drop in surface roughness from 6.52 nm of an conventional ITO to 0.46 nm of an 50 nm Ni(50 W)-doped ITO anode was observed, and this led to an improved lifetime performance of an Alq3 based OLED device attributed to reduced dark spots. Reducing thickness of Ni-doped ITO anode was found to worsen surface roughness. Meanwhile, the existence of Ni atoms showed little effect on deteriorating the light-emitting mechanism of OLED devices.     

Fast electro-optical switching and high contrast ratio in epoxy-based polymer dispersed liquid crystals

We report the observation of an electro-optical switching effect from an opaque to a transparent state occurring at a threshold value of the applied field in a polymer dispersed liquid crystal (PDLC). Optical responses of the composite film under the conditions of an externally applied ac electric field (2–5 V_p−p/μ) and a film thickness (50 μm), were determined using an Argon laser (wavelength 514 nm). The experimental results showed promising switching times with a rise time of 190 μs and a decay time of 2 ms and an exceptionally high contrast ratio up to 410. These results demonstrate the validity of employing this new PDLC in electro-optical devices.     

High resolution hydrogen resonance spectra of some substituted ethylenes

The olefinic proton resonance spectra of a number of mono- and 1, 2 di-substituted ethylenes have been analysed as AB, ABC or ABX systems to obtain the coupling constants and chemical shifts. The values of J _trans (range observed 13·7 to 18·0 c/s in 14 compounds) were significantly larger than J _cis in similar molecules. The values of J _cis (range observed 6·5 to 12·3 c/s in 8 compounds) are abnormally large when the olefinic bond is conjugated to an aromatic ring. Coupling between adjacent protons in an olefinic methylene group (J _gem) is much smaller, and sometimes negative. The shielding of the lone vinyl proton in mono-substituted ethylenes is smaller than that of the methylene group and in two cases (the vinyl group directly bonded to an oxygen atom) the difference is so great as to give an approximation to the ABX condition, so that the methylene protons appear to be shielded to an unusually large extent.     

Human echolocation: Acoustic gaze for burst trains and continuous noise

This study explored the ability of blind and sighted listeners to detect reflections, “echoes”, of burst trains or continuous noise. Echo detection was compared by presenting 5 ms bursts, rates from 1 to 64 bursts, with a continuous white noise, all during 500 ms. Sounds were recorded in an ordinary room through an artificial binaural head, the loudspeaker 1 m behind it. The reflecting object was an aluminum disk, diameter 0.5 m, placed at 1 m. The sounds were presented to 12 blind and 26 sighted participants in a laboratory using a 2-Alternative-Forced-Choice methodology. The task was to detect which of two sounds contained an echo. In Experiment 2, 1.5 m distance sounds were presented to the blind only. At 1 m, detection for the blind increased up to 64 bursts/500 ms, but for the sighted up to 32 bursts. At 1.5 m, the peak performance for the blind was at 32 bursts. At the 1 m, but not at the 1.5 m distance, the blind performed best with continuous white noise. The overlap in time of signal and echo at 1 m for 64 bursts was 60%, but at 1.5 m 82%. Avoiding an overlap between emitted bursts and returning echoes seems important for echolocation, indicating that an acoustic gaze, analogous to in echolocating animals, may also exist in humans.     

Effect of spin of F3-center on changes in its charge under the action of radiation

Methods of pulse spectroscopy with time resolution were used to investigate creation and annihilation processes of F₂ ⁺-, F₂-, F ₂ ^– -centers, which are noninertial with respect to the duration of an electron pulse of nanosecond duration acting on the crystal. It is concluded on the basis of these studies that the effect of radiation on an F₂-center is a change in its charge and spin states. It is established that the direction of change of an F₂ -center charged state is determined by the value of its spin: localization occurs of a band hole on an F₂-center in the singlet ground state, and of an electron in the on an F₂-center in the triplet state. An F₂-center triplet state is formed during the electron localization process on an F₂ ⁺-center or subsequent localization of band holes and an electron on an F₂-center as a result of a thermally activated intercombination transition from a higher state than the radiating singlet.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 109–120, October, 1991.In conclusion the author is grateful to V. A. Kravchenko for participation in the experiments, and to V. M. Lisitsyn and V. Yu. Yakovlev for interesting discussions.     

Effective diffusivity of solute in multiphase materials with segregation

The effective diffusion coefficient D_eff for solute in a multiphase microstructure in which solute segregation can occur is related to D_eff for an isomorphic microstructure in which no segregation occurs. This permits analytical expressions (approximations, bounds, etc.) and methods of numerical calculation for D_eff that neglect solute segregation to be applied to systems that exhibit segregation. As an example, exact results are obtained for solute diffusion and segregation to grain boundaries in an idealized polycrystalline microstructure.     

Left-Symmetric Algebras in Hydrodynamics

Left-Symmetric algebras are shown to appear naturally in integrable hydrodynamical systems. First, to a data a Left-Symmetric algebra and an operator of strong deformation on it is attached an infinite commuting hierarchy of integrable systems of hydrodynamical type in 1+1−d. Second, this picture (without deformation) is embedded into an infinite-component integrable hydrodynamic chain.     

Analysis of carrier transport properties in GaN/Al0.3Ga0.7N multiple quantum well nanostructures

Analysis of carrier transport properties in GaN based multiple quantum well nanostructure has been carried out with an applied bias. Effect of an applied bias and aluminium mole composition in the barrier on the scattering rate, capture time and escape rate has been investigated. The scattering rate was found to be decreased with an increase of applied bias voltage and aluminium mole composition. Capture time shows oscillatory nature with variations in mole composition of aluminium under biasing conditions. The escape rate was found to be increasing from 0.01 ps⁻¹ to 0.69 ps⁻¹ with applied bias voltage.     

Temperature corresponding to ultrasonic velocity maximum studies in aqueous solutions

Ultrasonic velocities in aqueous solutions of acetamide, dimethyl urea andβ-alanine at different concentrations in the temperature range of about 60°C to 80°C have been measured using an interferometer technique with an accuracy of ±0.003%. The temperature corresponding to velocity maximum at different concentrations is evaluated with an accuracy of ±0.2°C. The results are discussed in the light of the structure-breaking property of these substances in water.     

Photoreflectance investigation of hydrogenated (InGa)(AsN)/GaAs heterostructures

The optical response of as grown and hydrogenated In_0.32Ga_0.68As_1-yN_y/GaAs single quantum wells (y = 0, 0.027) has been investigated from T = 80 K to room temperature by photoreflectance. Three excitonic spectral features detected in the N free sample shift to lower energy in the N containing sample and back to higher energy upon H irradiation of the N containing sample. In the hydrogenated sample, a progressive change with increasing temperature of the nature of the lowest energy transition from an excitonic to a band-to-band character has been explained in terms of an increasing release of carriers from traps formed by H and N clusters. A reduction in the oscillator strength of the lowest energy transition and an increase in the binding energy of the heavy-hole exciton have been explained in terms of an increase in the electron effective mass upon N introduction into the In_xGa_1-xAs lattice. Received 23 June 2002 Published online 19 November 2002     

An expansion of continuum wave functions in terms of resonant states

The poles and residues of the complete outgoing Green function in the complex momentum plane are used to obtain, in the case of finite range potentials, an eigenfunction expansion of the continuum wave solution. It is found that in the region r<a the wave solution may be expressed as an infinite sum of discrete terms involving the bound, antibound and resonant states of the problem. At the boundary radius r = a a different expansion is obtained. In this case, in order to get an infinite discrete sum, one has to introduce two subtraction terms. Otherwise the expansion is given by a finite sum of discrete terms and an