Phonons background

If r is significantly shorter than the time over which the atoms in the lattice vibrate (phonon vibrations), then the carrier appears to move on an essentially rigid background and is termed free . This is the form of conduction found in most simple metals. 

The highest phonon frequency measured so far has been reported for NaF(lOO). By using primary beam energies of 90meV, Brusdeylins et al. have detected optical surface modes with frequencies of 40meV. The intensities, however, were rather low and the multiphonon background dominated the energy loss spectra. 

The first observation of the stimulated Raman effect was reported by Woodbury and Ng 215) j e effect was then thoroughly studied by several authors 216-218) and its theoretical background developed 219.220) (see also the review articles by Zubov et a/.22D). The stimulated Raman effect can be described as a parametric process where the coupling between a light wave at the Stokes frequency (Os and an optical phonon (vibrational wave) at cOy is produced by a pump field at col = (Oj + ojy. 

This is the background for the Lyddane-Sachs-Teller relation to be treated below. For transverse optical vibrations the origin of an is field is less obvious, but it is also present and its reaction on the eigenfrequency of the TO phonon later gives rise to the polaritons. 

Figure 5. Comparison of phonon DoS neutron measurements [34] (symbols) with PC spectra for NbB2 and TaB2 [33] after subtraction of the rising background (solid curves). The dotted curve in the left panel shows the PC spectrum of ZrB2 [33] for comparison.

Figure 9 Images from propagating phonon polaritons produced by a single beam focused to a round spot. Background data have been subtracted as in Fig. 8. The splotch in the center that obscures some data near time t = 0 is due to intense scattered pump light, which could not be completely filtered from the imaging setup. Image dimensions 1020 pm tall x 860 pm wide. (From Ref. 16.)...

The formation of the nickel excitons results in the lattice distortion near them and the induced lattice vibrations. Conditions of their occurrence are defined by the charged impurity because the removed hydrogen-like type carrier practically does not influence the deformation of the lattice near the charged impurity. Thus, our analysis of the vibrational background of the zero phonon line of the EA spectrum of the nickel exciton is based on results of a simulation of the lattice dynamics of the ZnO crystal with NE or Ni ions. 

The background scattering (e.g. from the sample container and cryostat) can be measured separately but the calculation of multi-phonon contributions from equation (11) might not be straightforward. Alternatively a number of self-consistent iteration techniques have also been used, for details see ref. [21]. In most of our work we have compared the observed (S(Q,o)) =>) g(co) obtained from equation (20) with that calculated from... 

---