Phonons Brillouin scattering experiments

In the Brillouin scattering experiment a photon is inelastically scattered from a phonon giving rise to two peaks in the scattered light... 

Fig. 2 Normalized dispersion relation for pSi superlattiees showing composite data from large set of superlattice samples obtained by Brillouin scattering experiments. The solid dashed) curves are theoretical longitudinal (transverse) modes obtained from the Rytov model. The horizontal hne identifies a locahzed surface mode lying within die phononic bandgap of the bulk longitudinal mode, trapped at the surface of the phononic crystal. Full details of die samples and acoustic branch identification in Parsons and Andrews (2012) (Reprinted with permission from Journal of applied physics by American Institute of Physics, Copyright 2012, American Institute of Physics)...

The phonon-fracton crossover frequency, C0coi wave vector, q o, are accessible by Brillouin scattering experiments. Spectra can be measured for various sample densities p, and for each p for various momentum exchanges q. Each spectrum is determined by three fitting parameters, namely co oi, qco he absolute intensity. Using the former two, and assuming that mutual self-similarity also extends to dynamical... 

In such experiments information about the phonons is deduced by measurements on the spin system. With the advent of monochromatic laser sources, the technique of Brillouin scattering can be used to track phonons of definite momentum and polarization, together with the number of such phonons. Al tshuler et al. (1971, 1972) used a helium-neon laser in experiments on cerium magnesium nitrate at... 

The first evidence for hypersonic phononic crystal effects was observed in porous silicon superlattices using Brillouin light-scattering experiments (Parsons and Andrews 2009). Comparison between experiment and theory using the Rytov model indicated that the bulk longitudinal phonon branch had been folded. One of the samples exhibited a spectrum consistent with a bandgap of 3.3 GHz for a first gap located at 13.4 GHz. 

In order to probe the entire Brillouin zone of k-space, inelastic neutron scattering experiments must be carried out, where the excitation of lattice phonons draws whatever momentum is needed from the high momentum of the neutron quanta. In addition, the k-vector of the incoming radiation can be properly matched with the phonon k-vector in modulus and direction, so that k-dependent intensities can be obtained, the various branches can be probed separately, and the entire dispersion curves can be experimentally determined. Such experiments have been carried out only for some particularly simple crystals. For a review see e.g. Gonze, X. Rignanese, G.-M. Caracas, R. First-principle studies of the lattice dynamics of crystals and related properties, Z. Krist. 2(X)5,220,458 72. 

The basic experiment consists of measuring the spectrum of the scattered light. It consists of a strong elastic peak at one frequency with additional components whose frequency has been shifted by the inelastic scattering processes. The frequencies of these much weaker phonon peaks are measured relative to the elastic peak. From observation of the shifted Brillouin peak with respect to the central elastic peak, the longitudinal Brillouin splitting, A >i is given by... 

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