Scattering of light by entropy fluctuations in dielectric crystals

Wehner, R.K.; Klein, R.

doi:10.1016/0031-8914(72)90122-x

Cited by 79 publications

(28 citation statements)

References 48 publications

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“…13 so that explicit formulae are obtained for one-phonon and multi-phonon scattering processes in·crystals in various approximations. (WEHNER and KLEIN, 1972). An alternative approach proceeds in an analogous way as for infrared absorption in Sect.…”

Section: D) Polarizability Theorymentioning

confidence: 93%

“…In this so-called collision-dominated or hydrodynamic regime, it is no longer possible to specify the final state of the crystal in terms of a few phonons, and perturbation theory fails completely; this does not mean that the phonons themselves are not well defined. Physical effects in this regime are the dielectric losses of perfect crystals at microwave frequencies and the quasi-elastic Rayleigh scattering by entropy fluctuations (WEHNER and KLEIN, 1972). In the static limit, the difference between the isothermal and adiabatic susceptibility is an effect due to the collisions between the thermal phonons.…”

Section: G Dynamical Theory Of Interacting Phonon Systemsmentioning

confidence: 99%

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Vibrational Infrared and Raman Spectra of Non-Metals

Bilz¹,

Strauch²,

Wehner³

1984

Light and Matter Id / Licht Und Materie Id

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The vibrational properties of crystals determine the photon infrared absorption, inelastic neutron scattering and, to a large extent, inelastic photon scattering by phonons, i.e., Raman scattering. The interpretation of infrared and Raman spectra requires, therefore, an understanding of the basic features of lattice dynamics. The quantum theory of solids describes the crystal properties in terms of elementary excitations and their mutual interactions. Dynamic properties are represented by phonons (lattice vibrations) and their interactions mainly with other phonons (anharmonicity), electrons (electron-phonon coupling), and photons (interaction with radiation). This characterizes the scope of the article. Its emphasis is on the interrelation between theory and experiment, i.e., on the microscopic or model interpretation of experimental spectra.Work on infrared absorption and Raman scattering of non-metallic solids was summarized twenty years ago in two review articles in this Encyclopedia by LECOMTE (1958) and by MIZUSHIMA (1958). Since then, many important new experimental data have become available, thanks to very refined techniques for measuring spectra of pure and imperfect single crystals. Furthermore, the theory of the interactions of phonons and photons has been much improved, stimulated by the remarkable development of the theory of lattice vibrations and the mathematical techniques of many-body physics.In this article we describe the dispersion and absorption of infrared radiation and Raman scattering in non-metallic crystals, both perfect and containing point defects. At present, quantitative calculations are usually restricted to diatomic crystals, especially those with simple structures such as the alkali halides or germanium and its homologues, since our knowledge of lattice vibrations is still rather poor for polyatomic and low-symmetry crystals. The theory of lattice vibrations, as reviewed by COCHRAN and COWLEY (1967) in Vol. XXV /2a of this Encyclopedia, constitutes the background and the natural starting point for our investigations. There are other recent reviews by LUDWIG (1967), COCHRAN (1971), MARADUDIN et al. (1971), SINHA (1973), and several articles in: HORTON and MARADUDIN (eds., 1974). A summary of the developments during the last few years is given in Chap. B.An important part of the theory of lattice vibrations is the construction of models. A good example is the so-called shell model for phonons (see Sect. 4) which describes the adiabatic linear electron-ion interaction in terms of localized charges and coupling constants. This provides a natural explanation of some long-range ion-ion forces in insulators in terms of induced dipole forces. Anharmonic extensions of this shell model and its modifications seem very desirable, and first attempts in this direction are discussed.Models can often give a qualitatively and sometimes quantitatively correct description of certain processes in terms of a few parameters. They provide, therefore, an orientation for microscopic approaches. Furthermo...

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Section: D) Polarizability Theorymentioning

confidence: 93%

Section: G Dynamical Theory Of Interacting Phonon Systemsmentioning

confidence: 99%

Vibrational Infrared and Raman Spectra of Non-Metals

Bilz¹,

Strauch²,

Wehner³

1984

Light and Matter Id / Licht Und Materie Id

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“…Wehner and Klein [28] predicted that in various crystals, including SrTiO 3 , the integrated intensities of phonon density fluctuation processes (i.e. 2S or TPDS) could be 10 to 100 times the value given by the Landau-Placzek ratio.…”

Section: T Smentioning

confidence: 99%

“…Second sound is the collective mode of phonon density fluctuation processes [28]. When the window condition is not fulfilled (ω 2S > Γ N ), non-thermalized phonon can still interact via two-phonon difference scattering (TPDS) processes.…”

Section: T Smentioning

confidence: 99%

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The extra brillouin doublets and central peak of KTaO₃: Second soundvs.two-phonon difference scattering

et al. 2000

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Low-T Brillouin spectra of the incipient ferroelectric KTaO 3 exhibit a broad central peak (CP), and additional Brillouin doublets (BD), that can both be related to phonon-density fluctuations. On the basis of new high-resolution neutron data obtained of low-lying phonon branches, we analysed the phonon-kinetics mechanisms that are possibly the origin of these unusual features. Firstly, transverse acoustic (TA) phonons whose normal damping is faster than the BD frequency can produce hydrodynamic second sound. Secondly, two-phonon difference scattering from low damping thermal transverse phonons contribute to the spectra with either a sharp or a broader doublet, depending on the phonon group velocity and anisotropy of dispersion surfaces. The position of the observed sharp doublet is consistent with both mechanisms, but a comparison of the computed and experimental anisotropies favours the second process.

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Renormalization of the Optical Response of Semiconductors by Electron-Phonon Interaction

Cardona

2001

phys. stat. sol. (a)

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In the past five years enormous progress has been made in the ab initio calculations of the optical response of electrons in semiconductors. The calculations include the Coulomb interaction between the excited electron and the hole left behind, as well as local field effects. However, they are performed under the assumption that the atoms occupy fixed equilibrium positions and do not include effects of the interaction of the lattice vibrations with the electronic states (electron-phonon interaction). This interaction shifts and broadens the energies at which structure in the optical spectra is observed, the corresponding shifts being of the order of the accuracy claimed for the ab initio calculations. These shifts and broadenings can be calculated with various degrees of reliability using a number of semiempirical and ab initio techniques, but no full calculations of the optical spectra including electron-phonon interaction are available to date. This article discusses experimental and theoretical aspects of the renormalization of optical response functions by electron-phonon interaction, including both temperature and isotopic mass effects. Some of the theoretical techniques used can also be applied to analyze the renormalization of other response functions, such as the phonon spectral functions, the lattice parameters, and the elastic constants.

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Scattering of light by entropy fluctuations in dielectric crystals

Cited by 79 publications

References 48 publications

Vibrational Infrared and Raman Spectra of Non-Metals

Vibrational Infrared and Raman Spectra of Non-Metals

The extra brillouin doublets and central peak of KTaO₃: Second soundvs.two-phonon difference scattering

Renormalization of the Optical Response of Semiconductors by Electron-Phonon Interaction

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Scattering of light by entropy fluctuations in dielectric crystals

Cited by 79 publications

References 48 publications

Vibrational Infrared and Raman Spectra of Non-Metals

Vibrational Infrared and Raman Spectra of Non-Metals

The extra brillouin doublets and central peak of KTaO3: Second soundvs.two-phonon difference scattering

Renormalization of the Optical Response of Semiconductors by Electron-Phonon Interaction

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The extra brillouin doublets and central peak of KTaO₃: Second soundvs.two-phonon difference scattering