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Brillouin zone
About: Brillouin zone is a research topic. Over the lifetime, 13849 publications have been published within this topic receiving 383077 citations.
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TL;DR: In this article, the authors derived the correlation functions of mass, momentum, and energy densities for a fluid subject to a large temperature gradient, and the deviations of the intensity of each Brillouin line from its equilibrium value as a function of the temperature gradient were obtained for the optimal scattering geometry.
Abstract: The equations derived in the first paper of this series for the correlation functions of mass, momentum, and energy densities are solved for a fluid subject to a large temperature gradient. The shape and intensity of the Rayleigh line show deviations from equilibrium that are proportional to the square of the temperature gradient. The deviations of the intensity of each Brillouin line from its equilibrium value as a function of the temperature gradient is obtained for the optimal scattering geometry. The intensity of one of these two Brillouin lines shows a maximum or minimum as a function of the temperature gradient, depending on the sign of the temperature derivatives of the coefficient of sound attenuation and thermal conductivity and on the orientation of the momentum transfer between fluid and light with respect to the temperature gradient. Further, the difference in intensity of the two Brillouin lines is found to be about three times smaller than predicted by the linear theory, consistent with the experiments of Beysens et al. Since all these results are due to mode-coupling effects, an experimental verification would constitute the first observation of mode-coupling effects away from criticality. The connection between (a) the mode-coupling effects responsible for the changes in the intensities of the Rayleigh and Brillouin lines, (b) the long-time tail contributions to the transport coefficients, and (c) the nonexistence of a virial expansion of the transport coefficients is discussed.
119 citations
TL;DR: In this article, the effects of chain confinement and the free surface on the mechanical properties of thin polystyrene (PS) films were investigated by measuring the velocity of film-guided acoustic phonons in films with thickness varying from less than to greater than the average end-to-end distance of the unperturbed molecules.
Abstract: We have used Brillouin light scattering (BLS) to measure the room-temperature, high-frequency mechanical properties of thin freely standing polystyrene (PS) films. We have investigated the effects of chain confinement and the free surface on the mechanical properties by measuring the velocity of film-guided acoustic phonons in films with thicknesses ranging from less than to greater than the average end-to-end distance of the unperturbed molecules ${R}_{\mathrm{EE}}.$ We find that the measured, room-temperature sound velocities are the same, to within $\ifmmode\pm\else\textpm\fi{}1%,$ for all films that have glass transition temperatures that differ by as much as 65 K. Our results, which differ markedly from those of recent picosecond acoustic measurements, are discussed in terms of models proposed to explain anomalous glass transition behavior in thin polymer films. A careful analysis of the BLS data reveals that, at room temperature, the mechanical stiffness, mass density, and thermal expansion of thin, freely standing PS films in the glassy state are consistent with bulk values for all film thicknesses.
119 citations
TL;DR: In this article, the electronic structure of an ordinary two-dimensional electron gas (2DEG) under an appropriate external periodic potential of hexagonal symmetry was analyzed and it was shown that massless Dirac fermions are generated near the corners of the supercell Brillouin zone.
Abstract: Analysis of the electronic structure of an ordinary two-dimensional electron gas (2DEG) under an appropriate external periodic potential of hexagonal symmetry reveals that massless Dirac fermions are generated near the corners of the supercell Brillouin zone. The required potential parameters are found to be achievable under or close to laboratory conditions. Moreover, the group velocity is tunable by changing either the effective mass of the 2DEG or the lattice parameter of the external potential, and it is insensitive to the potential amplitude. The finding should provide a new class of systems other than graphene for investigating and exploiting massless Dirac fermions using 2DEGs in semiconductors.
119 citations
TL;DR: In this paper, the authors reviewed recent advances in the understanding of anomalous thermal expansion behavior of open frame-work compounds and found that the phonons responsible for anomalous behavior are different in all these compounds.
119 citations
Patent•
18 Dec 2000
TL;DR: In this paper, the relative frequency separation between the lasers is adjusted to align the frequency of the backscattered Brillouin signal with a desired sideband in the carrier signal to effect a Brillousin gain on the sideband.
Abstract: Opto-electronic devices and techniques using Brillouin scattering to select a sideband in a modulated optical carrier signal for amplification. Two lasers respectively provide a carrier signal beam and a Brillouin pump beam which are fed into an Brillouin optical medium in opposite directions. The relative frequency separation between the lasers is adjusted to align the frequency of the backscattered Brillouin signal with a desired sideband in the carrier signal to effect a Brillouin gain on the sideband. This effect can be used to implement photonic RF signal mixing and conversion with gain, conversion from phase modulation to amplitude modulation, photonic RF frequency multiplication, optical and RF pulse generation and manipulation, and frequency-locking of lasers.
119 citations