Effect of pore packing defects in 2-D ordered mesoporous carbons on ionic transport

Ordered mesoporous materials show great importance in energy, environmental, and chemical engineering. The diffusion of guest species in mesoporous networks plays an important role in these applications, especially for energy storage, such as supercapacitors based on ordered mesoporous carbons ( OMCs). The ion diffusion behavior in two different 2-D hexagonal OMCs was investigated by using cyclic voltametry and electrochemical impedance spectroscopy. In addition, transmission electron microscopy, small-angle X-ray diffraction, and nitrogen cryosorption methods were used to study the pore structure variations of these two OMCs. It was found that, for the OMC with defective pore channels ( termed as pore packing defects), the gravimetric capacitance was greatly decayed when the voltage scan rate was increased. The experimental results suggest that, for the ion diffusion in 2-D hexagonal OMCs with similar mesopore size distribution, the pore packing defect is a dominant dynamic factor.

X-ray scattering studies of mixed langmuir monolayers and Langmuir-Blodgett films of a noncentrosymmetric porphyrin with cadmium arachidate

The structures of mixed Langmuir (floating) monolayers and Langmuir-Blodgett (LB) films of a phenanthroline-porphyrin with cadmium arachidate (PhenPor + CdAr) have been investigated by synchrotron X-ray grazing incidence diffraction (GIXD) and specular X-ray reflectivity (SXR). GIXD measurements of the floating monolayers showed only one peak, arising from the CdAr domains in the films, at a scattering angle of 21.5 degrees. This is consistent with a hexagonal structure (alpha = 4.77 Angstrom). The correlation length in these domains is 250 Angstrom. GMD measurements of the LB films, however, show two sets of diffraction features: one arises from CdAr domains with a rectangular in-plane structure (alpha = 7.44 Angstrom and b = 4.90 Angstrom) and a correlation length of 85 Angstrom; the other is from porphyrin domains with an oblique in-plane structure (alpha (p) 15.2 Angstrom, b(p) = 8.86 Angstrom, and gamma (p) = 80 degrees) and a correlation length of 105 Angstrom. These dimensions are consistent with the surface pressure-area isotherm measurements and indicate that the two components are immiscible. The thickness of the bilayer is 57 Angstrom, and there is no correlation between the bilayers. Introduction of a trigger compound does not alter the structure of the films but slightly increases the bilayer thickness. The SXR measurements of the floating monolayers also support the suggested immiscibility of the two components in the films.

Measuring geometric phases of scattering states in nanoscale electronic devices

We show how a quantum property, a geometric phase, associated with scattering states can be exhibited in nanoscale electronic devices. We propose an experiment to use interference to directly measure the effect of this geometric phase. The setup involves a double-path interferometer, adapted from that used to measure the phase evolution of electrons as they traverse a quantum dot (QD). Gate voltages on the QD could be varied cyclically and adiabatically, in a manner similar to that used to observe quantum adiabatic charge pumping. The interference due to the geometric phase results in oscillations in the current collected in the drain when a small bias across the device is applied. We illustrate the effect with examples of geometric phases resulting from both Abelian and non-Abelian gauge potentials.

Magic-angle effects in the interlayer magnetoresistance of quasi-one-dimensional metals due to interchain incoherence

The dependence of the magnetoresistance of quasi-one-dimensional metals on the direction of the magnetic field show dips when the field is tilted at the so-called magic angles determined by the structural dimensions of the materials. There is currently no accepted explanation for these magic-angle effects. We present a possible explanation. Our model is based on the assumption that, the intralayer transport in the second most conducting direction has a small contribution from incoherent electrons. This incoherence is modeled by a small uncertainty in momentum perpendicular to the most conducting (chain) direction. Our model predicts the magic angles seen in interlayer transport measurements for different orientations of the field. We compare our results to predictions by other models and to experiment.

Scattering and tangential momentum accommodation at a 2D adsorbate-solid interface

We calculate tangential momentum coefficients for the exchange of momentum between molecules in transport and the internal surface of a membrane pore, modelled as a simple atomic structure. We introduce a local specular reflection (LSR) hypothesis, which states that impinging molecules undergo mirror-like reflection in a plane tangent to a surface atom at the point of impact. As a consequence, the components of the velocity, parallel to the direction of flow will (in general) change on impact. The overall effect is a loss of tangential momentum, since more is lost in the upstream direction than is gained in the downstream direction. The loss of tangential momentum is greater when the size ratio of fluid to solid atom is small, allowing more steeply inclined impact planes to become accessible to the fluid phase molecules. (c) 2005 Elsevier B.V. All rights reserved.

Anisotropic scattering and anomalous normal-state transport in a high-temperature superconductor

The metallic state of high-temperature copper-oxide superconductors, characterized by unusual and distinct temperature dependences in the transport properties(1-4), is markedly different from that of textbook metals. Despite intense theoretical efforts(5-11), our limited understanding is impaired by our inability to determine experimentally the temperature and momentum dependence of the transport scattering rate. Here, we use a powerful magnetotransport probe to show that the resistivity and the Hall coefficient in highly doped Tl2Ba2CuO6+delta originate from two distinct inelastic scattering channels. One channel is due to conventional electron electron scattering; the other is highly anisotropic, has the same symmetry as the superconducting gap and a magnitude that grows approximately linearly with temperature. The observed form and anisotropy place tight constraints on theories of the metallic state. Moreover, in heavily doped non-superconducting La2-xSrxCuO4, this anisotropic scattering term is absent(12), suggesting an intimate connection between the origin of this scattering and superconductivity itself.

Fundamentals of optical detection of small particles

This paper reviews a number of used and/or proposed ideas for optical detection of small particles including single molecules. Different techniques (direct absorption and scattering, interferometry, use of sub Poissonian statistics, cavity enhancement, and thermal lens detection) are compared in terms of signal-to-noise ratio. It is shown that scattering (resonance and non resonance) fundamentally remains the method of choice for most applications.

Visual Display Height

We examined the influence of backrest inclination and vergence demand on the posture and gaze angle that-workers adopt to view visual targets placed in different vertical locations. In the study 12 participants viewed a small video monitor placed in 7 locations around a 0.65-m radius arc (from 650 below to 300 above horizontal eye height). Trunk posture was manipulated by changing the backrest inclination of an adjustable chair. Vergence demand was manipulated by using ophthalmic lenses and prisms to mimic the visual consequences of varying target distance. Changes in vertical target location caused large changes in atlantooccipital posture and gaze angle. Cervical posture was altered to a lesser extent by changes in vertical target location. Participants compensated for changes in backrest inclination by changing cervical posture, though they did not significantly alter atlanto-occipital posture and gaze angle. The posture adopted to view any target represents a compromise between visual and musculoskeletal demands. These results provide support for the argument that the optimal location of visual targets is at least 15 below horizontal eye level. Actual or potential applications of this work include the layout of computer workstations and the viewing of displays from a seated posture.

Turbulence in Small Sub-Tropical Estuary with Semi-Diurnal Tides

In natural estuaries, contaminant transport is driven by the turbulent momentum mixing. The predictions of scalar dispersion can rarely be predicted accurately because of a lack of fundamental understanding of the turbulence structure in estuaries. Herein detailed turbulence field measurements were conducted at high frequency and continuously for up to 50 hours per investigation in a small subtropical estuary with semi-diurnal tides. Acoustic Doppler velocimetry was deemed the most appropriate measurement technique for such small estuarine systems with shallow water depths (less than 0.5 m at low tides), and a thorough post-processing technique was applied. The estuarine flow is always a fluctuating process. The bulk flow parameters fluctuated with periods comparable to tidal cycles and other large-scale processes. But turbulence properties depended upon the instantaneous local flow properties. They were little affected by the flow history, but their structure and temporal variability were influenced by a variety of mechanisms. This resulted in behaviour which deviated from that for equilibrium turbulent boundary layer induced by velocity shear only. A striking feature of the data sets is the large fluctuations in all turbulence characteristics during the tidal cycle. This feature was rarely documented, but an important difference between the data sets used in this study from earlier reported measurements is that the present data were collected continuously at high frequency during relatively long periods. The findings bring new lights in the fluctuating nature of momentum exchange coefficients and integral time and length scales. These turbulent properties should not be assumed constant.

Integrability of the Russian doll BCS model

We show that integrability of the BCS model extends beyond Richardson's model (where all Cooper pair scatterings have equal coupling) to that of the Russian doll BCS model for which the couplings have a particular phase dependence that breaks time-reversal symmetry. This model is shown to be integrable using the quantum inverse scattering method, and the exact solution is obtained by means of the algebraic Bethe ansatz. The inverse problem of expressing local operators in terms of the global operators of the monodromy matrix is solved. This result is used to find a determinant formulation of a correlation function for fluctuations in the Cooper pair occupation numbers. These results are used to undertake exact numerical analysis for small systems at half-filling.

Detailed Measurements during a Transient Front in a Small Subtropical Estuary

High-resolution measurements of velocity and physio-chemistry were conducted before, during and after the passage of a transient front in a small subtropical system about 2.1 km upstream of the river mouth. Detailed acoustic Doppler velocimetry measurements, conducted continuously at 25 Hz, showed the existence of transverse turbulent shear between 300 s prior to the front passage and 1300 s after. This was associated with an increased level of suspended sediment concentration fluctuations, some transverse shear next to the bed and some surface temperature anomaly.

Comparison of BR and QR Eigenvalue Algorithms for Power System Small Signal Stability Analysis

The BR algorithm is a novel and efficient method to find all eigenvalues of upper Hessenberg matrices and has never been applied to eigenanalysis for power system small signal stability. This paper analyzes differences between the BR and the QR algorithms with performance comparison in terms of CPU time based on stopping criteria and storage requirement. The BR algorithm utilizes accelerating strategies to improve its performance when computing eigenvalues of narrowly banded, nearly tridiagonal upper Hessenberg matrices. These strategies significantly reduce the computation time at a reasonable level of precision. Compared with the QR algorithm, the BR algorithm requires fewer iteration steps and less storage space without depriving of appropriate precision in solving eigenvalue problems of large-scale power systems. Numerical examples demonstrate the efficiency of the BR algorithm in pursuing eigenanalysis tasks of 39-, 68-, 115-, 300-, and 600-bus systems. Experiment results suggest that the BR algorithm is a more efficient algorithm for large-scale power system small signal stability eigenanalysis.

High-Frequency Suspended Sediment Flux Measurements in a Small Estuary

In small estuaries, the predictions of scalar dispersion can rarely be predicted accurately because of a lack of fundamental understanding of the turbulence structure. Herein detailed turbulence measurements and suspended sediment concentrations were conducted simultaneously and continuously at high-frequency for 50 hours per investigation in a small subtropical estuary with semi-diurnal tides. The data analyses provided an unique characterisation of the turbulent mixing processes and suspended sediment fluxes. The turbulence was neither homogeneous nor isotropic, and it was not a Gaussian process. The integral time scales for turbulence and suspended sediment concentration were about equal during flood tides, but differed significantly during ebb tides. The field experiences showed that the turbulence measurements must be conducted at high-frequency to characterise the small eddies and the viscous dissipation process, while a continuous sampling was necessary to characterise the time-variations of the instantaneous velocity field, Reynolds stress tensor and suspended sediment flux during the tidal cycles.

Gaze angle: a possible mechanism of visual stress in virtual reality headsets

It is known that some Virtual Reality (VR) head-mounted displays (HMDs) can cause temporary deficits in binocular vision. On the other hand, the precise mechanism by which visual stress occurs is unclear. This paper is concerned with a potential source of visual stress that has not been previously considered with regard to VR systems: inappropriate vertical gaze angle. As vertical gaze angle is raised or lowered the 'effort' required of the binocular system also changes. The extent to which changes in vertical gaze angle alter the demands placed upon the vergence eye movement system was explored. The results suggested that visual stress may depend, in part, on vertical gaze angle. The proximity of the display screens within an HMD means that a VR headset should be in the correct vertical location for any individual user. This factor may explain some previous empirical results and has important implications for headset design. Fortuitously, a reasonably simple solution exists.

Resonance fluorescence spectrum in a weak squeezed field with an arbitrary bandwidth

We analyze the linewidth narrowing in the fluorescence spectrum of a two-level atom driven by a squeezed vacuum field of a finite bandwidth. It is found that the fluorescence spectrum in a low-intensity squeezed field can exhibit a (omega - omega(0))(-6) frequency dependence in the wings. We show that this fast fall-off behavior is intimately related to the properties of a narrow-bandwidth squeezed field and does not extend into the region of broadband excitation. We apply the Linear response model and find that the narrowing results from a convolution of the atom response with the spectrum of the incident field. On the experimental side, we emphasize that the linewidth narrowing is not sensitive to the solid angle of the squeezed modes coupled to the atom. We also compare the fluorescence spectrum with the quadrature-noise spectrum and find that the fluorescence spectrum for an off-resonance excitation does not reveal the noise spectrum. We show that this difference arises from the competing three-photon scattering processes. [S1050-2947(98)04308-X].