Raman Detection of One-Phonon—Two-Phonon Interactions in CuCl

Abstract

A large ^-direction dependence of the broad band close to the TO peak is observed and explained qualitatively and quantitatively. The TO peak is found to be repelled out of a two-phonon band by a strong third-order anharmonic interaction.In a crystal having the zinc-blende structure, the first-order Raman scattering allows detection of either the TO(g~0) phonon or the LO(#~0) phonon, according to the selected direction q of the transferred momentum. We have measured at 40 K the scattered intensities of a CuCl single crystal for various q directions (Fig. 1). Because all the spectra were recorded with the same spectral slit width (2 cm" 1 ), the heights (eventually above the continuum) of the sharp lines at 171 and 208 cm" 1 could be taken to fix a common intensity scale for the drawings of Fig. 1 by making use of the fact that the chosen geometries impose I B = 0.5(JA+IC)-The unusual features in CuCl are the broad and strong band of the TO spectrum 1 " 3 (curve A) in the 140-170 cm" 1 range, reaching a very sharp line at 171 cm" 1 with a maximum at 151 cm" 1 , and its replacement by a weak and featureless continuum in the LO spectrum (curve C). This observation of ^-dependent, first-orderlike spectra obviously differs from the recent results of Shand et al. 3 and excludes the assignment of the broad band to a pure second-order Raman scattering.We propose to explain these features by the existence of strong anharmonic interactions between one of the <7~0 optical phonons and a twophonon continuum limited on the high-frequency side by a P 3 type of singularity 4 due to a Bril-where i labels the TO or LO phonon with frequency w,, R* y is the usual first-order Raman tensor element, the anharmonic third-order interaction being given by the usual formula 5 :(F 3 0 2 [A(a>) + iT(a>)]^ ^ TJ vfiirqJu -qiJ 2 ) G (u,qih, -Qi^ QsJJs* -(fa^M^', 4^3> -Qs^) • ( 2 ) q 3 ,J 3f i 4

Raman Scattering in CuCl under Pressure

“…The anharmonic interactions result in a shift and broadening of the TO mode and a transfer of oscillator strength to the two-phonon Raman scattering [2,6]. The strength of the anharmonic coupling is determined by Fourier coefficients of the third derivatives of the lattice potential with respect to atomic displacements.…”

“…Instead of narrow Raman lines corresponding to the characteristic transverse (TO) and longitudinal (LO) optical modes of zincblende crystals, CuCl shows, in the region of the TO mode frequency, a broad structure consisting of three maxima with TO-like polarization characteristics. There exist two different explanations for the anomalous TO structure, the Fermi-resonance model (FRM) [2,6] and the off-center model (OCM) [7,8]. The FRM assumes a strong anharmonic coupling of the bare TO phonon with acoustic twophonon states.…”

“…It exhibits several anomalous physical properties, see e.g. [1], among them a very unusual first-order Raman spectrum [2,3,4,5]. Instead of narrow Raman lines corresponding to the characteristic transverse (TO) and longitudinal (LO) optical modes of zincblende crystals, CuCl shows, in the region of the TO mode frequency, a broad structure consisting of three maxima with TO-like polarization characteristics.…”

The curious case of cuprous chloride: Giant thermal resistance and anharmonic quasiparticle spectra driven by dispersion nesting

“…In fact, according to Phillip's criterion for ionicity 13 , CuCl is just outside the rocksalt stability range. Many properties of CuCl suggest strong anharmonicity: negative thermal expansion 14 , negative Grüneisen parameters 14,15 , unusual and broad Raman peaks [16][17][18] , asymmetric phonon line shapes 19 and a rich phase diagram 20,21 . Slack and Andersson 12 measured κ(P) of CuCl and found very low κ (< 1W/m-K at room temperature (RT)) that decreased with increasing P. This pressure response is unusual, though found in a few other systems 12,[22][23][24] , and was attributed to large negative Grüneisen parameters of transverse acoustic (TA) modes.…”

Pressure‐Raman study of resonant TO(Γ)–two‐phonon decay processes in ZnS: Comparison of three isotope compositions

“…However, this quasiparticle approach must be modified whenever accidental degeneracies occur between quasiharmonic phonons and singularities in the two-phonon density of states (DOS). Prior work has found that the line-shapes of the TO(Γ) Raman peaks in GaP, [2,3] CuCl, [4,5] and ZnS [6,7] are profoundly altered by resonant mixing between the TO(Γ) mode and Brillouin Zone boundary (ZB) TA + LA combinations. In fact, this mixing is sufficiently large in CuCl and ZnS at room temperature (RT) and 1 atm.…”