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Wave Vector (wave + vector)
Selected AbstractsCompartment size estimation with double wave vector diffusion-weighted imagingMAGNETIC RESONANCE IN MEDICINE, Issue 1 2008Martin A. Koch Abstract Double wave vector diffusion weighting uses gradients along two different directions between excitation and acquisition. It has been shown theoretically that for restricted diffusion the signal amplitude in such an experiment can depend on the angle between the two gradient vectors. The highest amplitude is obtained with antiparallel orientation, and the amplitude difference between parallel and antiparallel gradient orientations depends on the compartment size. The validity of this description is experimentally tested for water between polymer beads, for radish, and for porcine spinal cord, using a clinical MR system with limited gradient strength. The results indicate that the phenomenon is observable; however, the size of the signal difference is considerably diminished when compared with theory. This is attributed to violations of the approximating conditions underlying the theoretical description and to free diffusion contributions. It is concluded that the effect could successfully be used as a basis for developing a new noninvasive method for assessing cell size. Magn Reson Med, 2008. © 2008 Wiley-Liss, Inc. [source] Beam-squint minimization in series-slotted waveguide array antennas using double negative materialsMICROWAVE AND OPTICAL TECHNOLOGY LETTERS, Issue 9 2006Soon-Soo Oh Abstract A 5 × 1 series-slotted waveguide array antenna partly filled with double positive (DPS) and double negative (DNG) materials is investigated. The DNG material has a negative wave vector and its guided-wavelength equals that of the DPS material. Two simple straight slotted waveguides, one filled only with the DPS material and the other with both DPS and DNG materials, are designed and studied. It is found that the latter waveguide has repeated voltage standing wave shapes. This unique characteristic is utilized to minimize the beam squinting effect in the series-fed array antennas. It is shown that in a 5 × 1 array using both DPS and DNG materials, the variation in the direction of its beam and boresight gain are limited to 0.5 ° and 0.8 dB, respectively, within a 10% frequency band. In the corresponding conventional array these parameters were 5 ° beam tilt and 3.3 dB boresight gain loss. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 1789,1792, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21800 [source] Direct observation of Landau damping in a solid state plasmaPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 1 2008M. P. Hasselbeck Abstract We use ultrafast THz spectroscopy to study Landau damping in a coherent plasma system. A series of InSb-based bulk heterostructures provide increasing spatial confinement that restricts the scattering wave vector in the electron-electron interaction. This results in a dramatic reduction of the plasma contribution to the radiation signal. The experimental results are explained by model calculations. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Novel properties of wave propagation in biaxially anisotropic left-handed materialsANNALEN DER PHYSIK, Issue 6 2004J.Q. Shen Abstract Some physically interesting properties and effects (including the quantum effects) of wave propagation in biaxially anisotropic left-handed materials are investigated in this paper: (i) we show that in the biaxially gyrotropic left-handed material, the left-right coupling of circularly polarized light arises due to the negative indices in permittivity and permeability tensors of gyrotropic media; (ii) it is well known that the geometric phases of photons inside a curved fiber in previous experiments often depend on the cone angles of solid angles subtended by a curve traced by the direction of wave vector of light, at the center of photon momentum space. Here, however, for the light propagating inside certain anisotropic left-handed media we will present a different geometric phase that is independent of the cone angles; (iii) the extra phases of electromagnetic wave resulting from the instantaneous helicity inversion at the interfaces between left- and right-handed (LRH) media is also studied in detail by using the Lewis-Riesenfeld invariant theory. Some interesting applications (e.g., controllable position-dependent frequency shift, detection of quantum-vacuum geometric phases and helicity reversals at the LRH interfaces etc.) of above effects and phenomena in left-handed media is briefly discussed. [source] Wave propagation in an inhomogeneous cross-anisotropic mediumINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 7 2010Cheng-Der Wang Abstract Analytical solutions for wave velocities and wave vectors are yielded for a continuously inhomogeneous cross-anisotropic medium, in which Young's moduli (E, E,) and shear modulus (G,) varied exponentially as depth increased. However, for the rest moduli in cross-anisotropic materials, , and ,, remained constant regardless of depth. We assume that cross-anisotropy planes are parallel to the horizontal surface. The generalized Hooke's law, strain,displacement relationships, and equilibrium equations are integrated to constitute governing equations. In these equations, displacement components are fundamental variables and, hence, the solutions of three quasi-wave velocities, VP, VSV, and VSH, and the wave vectors, , and , can be generated for the inhomogeneous cross-anisotropic media. The proposed solutions and those obtained by Daley and Hron, and Levin correlate well with each other when the inhomogeneity parameter, k, is 0. Additionally, parametric study results indicate that the magnitudes and directions of wave velocity are markedly affected by (1) the inhomogeneous parameter, k; (2) the type and degree of geomaterial anisotropy (E/E,, G,/E,, and ,/,,); and (3) the phase angle, ,. Consequently, one must consider the influence of inhomogeneous characteristic when investigating the behaviors of wave propagation in a cross-anisotropic medium. Copyright © 2009 John Wiley & Sons, Ltd. [source] Mechanism of Thermal Transport in Zirconia and Yttria-Stabilized Zirconia by Molecular-Dynamics SimulationJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 12 2001Patrick K. Schelling We present results of molecular-dynamics simulations of the thermal conductivity, ,, of ZrO2 and Y2O3 -stabilized ZrO2 (YSZ). For both pure ZrO2 and YSZ with low concentrations of Y2O3, we find that the high-temperature , is typical of a crystalline solid, with the dominant mechanism being phonon-phonon scattering. With increasing Y2O3 concentration, however, the mechanism changes to one more typical of an amorphous system. In particular, phononlike vibrational modes with well-defined wave vectors appear only at very low frequencies. As in amorphous materials, the vast majority of vibrational modes, while delocalized, do not propagate like ordinary phonon modes but transport energy in a diffusive manner. We also find that the few highest frequency modes are localized and do not contribute to ,. [source] Energy relaxation in CdSe/ZnSe quantum dots under the strong exciton-phonon coupling regimePHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 4 2006A. V. Platonov Abstract Resonant photoluminescence and photoluminescence excitation spectroscopy have been applied to CdSe/ZnSe quantumdots where separation between the quantized levels is close to the optical phonon energy. Astrong relaxation enhancement induced by the optical phonon is observed. The fine structure of the phonon replicas is resolved. It is attributed to the optical phonons with large wave vectors from the symmetrical points of the Brillouin zone. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | |||||||||||||