The Experts below are selected from a list of 189 Experts worldwide ranked by ideXlab platform
Steven T. Bramwell - One of the best experts on this subject based on the ideXlab platform.
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Generalized longitudinal susceptibility for magnetic Monopoles in spin ice
Philosophical Transactions of the Royal Society A, 2012Co-Authors: Steven T. BramwellAbstract:The generalized longitudinal susceptibility χ(q,ω) affords a sensitive measure of the spatial and temporal correlations of magnetic Monopoles in spin ice. Starting with the Monopole model, a mean field expression for χ(q,ω) is derived as well as expressions for the mean square longitudinal field and induction at a Point. Monopole motion is shown to be strongly correlated, and both spatial and temporal correlations are controlled by the dimensionless Monopole density x which defines the ratio of the magnetization relaxation rate and the Monopole hop rate. Thermal effects and spin-lattice relaxation are also considered. The derived equations are applicable in the temperature range where the Wien effect for magnetic Monopoles is negligible. They are discussed in the context of existing theories of spin ice and the following experimental techniques: DC and AC magnetization, neutron scattering, neutron spin echo and longitudinal and transverse field μSR. The Monopole theory is found to unify diverse experimental results, but several discrepancies between theory and experiment are identified. One of these, concerning the neutron scattering line shape, is explained by means of a phenomenological modification to the theory.
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Generalised Longitudinal Susceptibility for Magnetic Monopoles in Spin Ice
Philosophical transactions. Series A Mathematical physical and engineering sciences, 2012Co-Authors: Steven T. BramwellAbstract:The generalised longitudinal susceptibility $\chi({\bf q}, \omega)$ affords a sensitive measure of the spatial and temporal correlations of magnetic Monopoles in spin ice. Starting with the Monopole model, a mean field expression for $\chi({\bf q}, \omega)$ is derived as well as expressions for the mean square longitudinal field and induction at a Point. Monopole motion is shown to be strongly correlated, and both spatial and temporal correlations are controlled by the dimensionless Monopole density $x$ which defines the ratio of the magnetization relaxation rate and the Monopole hop rate. Thermal effects and spin lattice relaxation are also considered. The derived equations are applicable in the temperature range where the Wien effect for magnetic Monopoles is negligible. They are discussed in the context of existing theories of spin ice and the following experimental techniques: dc and ac-magnetization, neutron scattering, neutron spin echo, and longitudinal and transverse field $\mu$SR. The Monopole theory is found to unify diverse experimental results, but several discrepancies between theory and experiment are identified. One of these, concerning the neutron scattering line shape, is explained by means of a phenomenological modification to the theory.
Kai Ming Li - One of the best experts on this subject based on the ideXlab platform.
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The scattering of sound by a long cylinder above an impedance boundary
Journal of the Acoustical Society of America, 2010Co-Authors: Kai Ming LiAbstract:The classical problem of sound scattering by an acoustically hard cylinder due to a Point Monopole and a line airborne source is extended in the present study. The solution to the homogeneous Helmholtz equation is expressed in a cylindrical coordinate system and represented by an expansion of Fourier integrals. Incorporating the image source method and the Bessel function addition theorem, the analytical solution is derived for the prediction of multiple scattering of sound by a hard cylinder placed above a ground surface of finite impedance. The total sound field can be expressed as a sum of four components: the incident field, the reflected wave, and the scattered fields from the cylinder and its image. The total far-field scattered potential was evaluated asymptotically by the method of stationary phase. Experimental measurements by using a Point source were conducted in an anechoic chamber to validate the theoretical formulations. The numerical predictions of using a Point source model give good agree...
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The scattering of sound by a long cylinder above an impedance boundary
The Journal of the Acoustical Society of America, 2010Co-Authors: Wai Keung Lui, Kai Ming LiAbstract:The classical problem of sound scattering by an acoustically hard cylinder due to a Point Monopole and a line airborne source is extended in the present study. The solution to the homogeneous Helmholtz equation is expressed in a cylindrical coordinate system and represented by an expansion of Fourier integrals. Incorporating the image source method and the Bessel function addition theorem, the analytical solution is derived for the prediction of multiple scattering of sound by a hard cylinder placed above a ground surface of finite impedance. The total sound field can be expressed as a sum of four components: the incident field, the reflected wave, and the scattered fields from the cylinder and its image. The total far-field scattered potential was evaluated asymptotically by the method of stationary phase. Experimental measurements by using a Point source were conducted in an anechoic chamber to validate the theoretical formulations. The numerical predictions of using a Point source model give good agreements with all the experimental data but there are obvious discrepancies in the spectral magnitudes between the calculation and experimental results when a line source model is used to simulate the scattering problem due to a Point source excitation.
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A theoretical study for the propagation of rolling noise over a porous road pavement
Journal of the Acoustical Society of America, 2004Co-Authors: Kai Ming LiAbstract:A simplified model based on the study of sound diffracted by a sphere is proposed for investigating the propagation of noise in a hornlike geometry between porous road surfaces and rolling tires. The simplified model is verified by comparing its predictions with the published numerical and experimental results of studies on the horn amplification of sound over a road pavement. In a parametric study, a Point Monopole source is assumed to be localized on the surface of a tire. In the frequency range of interest, a porous road pavement can effectively reduce the level of amplified sound due to the horn effect. It has been shown that an increase in the thickness and porosity of a porous layer, or the use of a double layer of porous road pavement, attenuates the horn amplification of sound. However, a decrease in the flow resistivity of a porous road pavement does little to reduce the horn amplification of sound. It has also been demonstrated that the horn effect over a porous road pavement is less dependent on the angular position of the source on the surface of tires.A simplified model based on the study of sound diffracted by a sphere is proposed for investigating the propagation of noise in a hornlike geometry between porous road surfaces and rolling tires. The simplified model is verified by comparing its predictions with the published numerical and experimental results of studies on the horn amplification of sound over a road pavement. In a parametric study, a Point Monopole source is assumed to be localized on the surface of a tire. In the frequency range of interest, a porous road pavement can effectively reduce the level of amplified sound due to the horn effect. It has been shown that an increase in the thickness and porosity of a porous layer, or the use of a double layer of porous road pavement, attenuates the horn amplification of sound. However, a decrease in the flow resistivity of a porous road pavement does little to reduce the horn amplification of sound. It has also been demonstrated that the horn effect over a porous road pavement is less dependent o...
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A theoretical study for the propagation of rolling noise over a porous road pavement
Journal of the Acoustical Society of America, 2004Co-Authors: Kai Ming LiAbstract:A simplified model based on the study of sound diffracted by a sphere is proposed for investigating the propagation of noise in a hornlike geometry between porous road surfaces and rolling tires. The simplified model is verified by comparing its predictions with the published numerical and experimental results of studies on the horn amplification of sound over a road pavement. In a parametric study, a Point Monopole source is assumed to be localized on the surface of a tire. In the frequency range of interest, a porous road pavement can effectively reduce the level of amplified sound due to the horn effect. It has been shown that an increase in the thickness and porosity of a porous layer, or the use of a double layer of porous road pavement, attenuates the horn amplification of sound. However, a decrease in the flow resistivity of a porous road pavement does little to reduce the horn amplification of sound. It has also been demonstrated that the horn effect over a porous road pavement is less dependent o...
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Sound propagation over concave surfaces
Journal of the Acoustical Society of America, 1999Co-Authors: Qiang Wang, Kai Ming LiAbstract:Diffraction of sound by concave surfaces is investigated theoretically and experimentally. In an earlier study [J. Acoust. Soc. Am. 104, 2683–2691 (1998)], it has been demonstrated that a rigorous analogy exists for the sound field above a convex circular cylinder in an otherwise homogeneous medium. The predicted sound field corresponds to the situation where the sound speed of the medium decreases exponentially with height. Extending the previous work, this paper investigates of the sound field above a concave surface and explores the corresponding analogy. Normal mode solutions have been developed for a downward refracting medium with an exponential sound speed profile. The solutions are used to predict the sound fields diffracted by a cylindrical concave surface. A series of laboratory experiments is conducted using Point Monopole, horizontal dipole, and vertical dipole sources over cylindrical concave surfaces. The experimental measurements are compared with the normal mode predictions. For Monopole a...
David Valenzuela - One of the best experts on this subject based on the ideXlab platform.
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Critical behavior for Point Monopole and dipole electric impurities in uniformly and uniaxially strained graphene
Physical Review B, 2020Co-Authors: Julio César Pérez-pedraza, Erik Díaz-bautista, Alfredo Raya, David ValenzuelaAbstract:We revisit the problem of bound states in graphene under the influence of Point electric Monopole and dipole impurity potentials extended to the case in which the membrane of this material is uniformly and uniaxially strained, which leads to a redefinition of the charge and dipole moment, respectively. By considering an anisotropic Fermi velocity, we analytically solve the resulting Dirac equation for each potential. We observe that the effect of the anisotropy is to promote or inhibit the critical behavior known to occur for each kind of impurity, depending on the direction along which strain is applied: both the atomic collapse, for the Monopole impurity, and the emergence of cascades of infinitely many bound states with a universal Efimov-like scaling, for the dipole impurity, are phenomena that occur under less or more restrictive conditions due to strain.
Hugo Scheer - One of the best experts on this subject based on the ideXlab platform.
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Structure‐Based Calculations of the Optical Spectra of the LH2 Bacteriochlorophyll‐Protein Complex from Rhodopseudomonas acidophila
Photochemistry and Photobiology, 1996Co-Authors: Kenneth Sauer, Richard J. Cogdell, Steve M. Prince, Andrew A. Freer, Neil W. Isaacs, Hugo ScheerAbstract:Abstract— The molecular structure of the light-harvesting complex 2 (LH2) bacteriochlorophyll-protein antenna complex from the purple non-sulfur photosynthetic bacterium Rhodopseudomonas acidophila, strain 10050 provides the positions and orientations of the 27 bacteriochlorophyll (BChl) molecules in the complex. Our structure-based model calculations of the distinctive optical properties (absorption, CD, polarization) of LH2 in the near-infrared region use a Point-Monopole approximation to represent the BChl Qy transition moment. The results of the calculations support the assignment of the ring of 18 closely coupled BChl to B850 (BChl absorbing at 850 nm) and the larger diameter, parallel ring of 9 weakly coupled BChl to B800. All of the significantly allowed transitions in the near infrared are calculated to be perpendicular to the C9 symmetry axis, in agreement with polarization studies of this membrane-associated complex. To match the absorption maxima of the B800 and B850 components using a relative permittivity (dielectric constant) of 2.1, we assign different site energies (12 500 and 12260 cm−1, respectively) for the Qy transitions of the respective BChl in their protein binding sites. Excitonic coupling is particularly strong among the set of B850 chromophores, with pairwise interaction energies nearly 300 cm between nearest neighbors, comparable with the experimental absorption bandwidths at room temperature. These strong interactions, for the full set of 18 B850 chromophores, result in an excitonic manifold that is 1200 cm−1 wide. Some of the upper excitonic states should result in weak absorption and perhaps stronger CD features. These predictions from the calculations await experimental verification.
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structure based calculations of the optical spectra of the lh2 bacteriochlorophyll protein complex from rhodopseudomonas acidophila
Photochemistry and Photobiology, 1996Co-Authors: Kenneth Sauer, Richard J. Cogdell, Steve M. Prince, Andrew A. Freer, Neil W. Isaacs, Hugo ScheerAbstract:Abstract— The molecular structure of the light-harvesting complex 2 (LH2) bacteriochlorophyll-protein antenna complex from the purple non-sulfur photosynthetic bacterium Rhodopseudomonas acidophila, strain 10050 provides the positions and orientations of the 27 bacteriochlorophyll (BChl) molecules in the complex. Our structure-based model calculations of the distinctive optical properties (absorption, CD, polarization) of LH2 in the near-infrared region use a Point-Monopole approximation to represent the BChl Qy transition moment. The results of the calculations support the assignment of the ring of 18 closely coupled BChl to B850 (BChl absorbing at 850 nm) and the larger diameter, parallel ring of 9 weakly coupled BChl to B800. All of the significantly allowed transitions in the near infrared are calculated to be perpendicular to the C9 symmetry axis, in agreement with polarization studies of this membrane-associated complex. To match the absorption maxima of the B800 and B850 components using a relative permittivity (dielectric constant) of 2.1, we assign different site energies (12 500 and 12260 cm−1, respectively) for the Qy transitions of the respective BChl in their protein binding sites. Excitonic coupling is particularly strong among the set of B850 chromophores, with pairwise interaction energies nearly 300 cm between nearest neighbors, comparable with the experimental absorption bandwidths at room temperature. These strong interactions, for the full set of 18 B850 chromophores, result in an excitonic manifold that is 1200 cm−1 wide. Some of the upper excitonic states should result in weak absorption and perhaps stronger CD features. These predictions from the calculations await experimental verification.
Barry Koren - One of the best experts on this subject based on the ideXlab platform.
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An accurate and efficient computational method for time-domain aeroacoustic scattering
Journal of Computational Physics, 2020Co-Authors: Zhao-huan Wang, Sw Sjoerd Rienstra, Barry KorenAbstract:Abstract This paper presents a convective time-domain equivalent-source method for determining the scattered acoustic pressure field in a uniform moving medium. The proposed method is based on the solution of the time-domain convective Ffowcs Williams–Hawkings (FW–H) equation while the strengths of equivalent sources are determined by the required pressure gradient boundary condition on the scattering surface. The scattered acoustic pressure can be calculated once the strengths of equivalent sources have been determined. The current paper adopts the recently published analytical time-domain formulation for the acoustic pressure gradient in a moving medium to evaluate the incident pressure gradient on the scattering surface. This makes the proposed method considerably more efficient and accurate than a direct method. The total acoustic pressure consists of the scattered and the incident components. The latter can be obtained by the time-domain acoustic pressure formulation of the convective FW–H equation. Causes of possible instability in the proposed method are analyzed and an effective stabilizing method is proposed. Three test cases are considered to demonstrate the validity of the proposed method: a Point Monopole source field scattered by: (1) a rigid sphere in a stationary medium, (2) an infinite flat plate in uniform flow parallel to its surface, and (3) a cylinder of infinite length in axial uniform flow. To demonstrate the usefulness of the proposed method in practical engineering applications, the scattering of a Point Monopole source field by a slender wing in uniform flow is considered.