Phonons scattering experiments

Some experiments outlined the frequency dependence of phonon scattering on surfaces [74]. Thus, Swartz made the hypothesis that a similar phenomenon could take place at the interface between solids and proposed the diffuse mismatch model [72]. The latter model represents the theoretic limit in which all phonons are heavily scattered at the interface, whereas the basic assumption in the acoustic mismatch model is that no scattering phenomenon takes place at the interface of the two materials. In the reality, phonons may be scattered at the interface with a clear reduction of the contact resistance value as calculated by the acoustic model. 

The elementary excitations mentioned so far are not related in any special way to the solid state and will therefore not be treated in this article. We will discuss here the following low-lying quantized excitations or quasi-particles which have been investigated by Raman spectroscopic methods phonons, polaritons, plasmons and coupled plasmon-phonon states, plasmaritons, mag-nons, and Landau levels. Finally, phase transitions were also studied by light scattering experiments however, they cannot be dealt with in this article. 

Abstract Electronic correlation effects in La2.ISrINi04 lead to spontaneous phase separation into microscopic spin/charge stripes with commensurate or incommensurate order. Raman scattering experiments on such single crystalline materials show a rich phenomenology of phonon and magnon anomalies due to the new, self-organized periodicities. These effects are observable as function of temperature. 

Most stimulated scattering experiments are carried out by overlapping spatially and temporally two laser outputs of appropriately tuned frequencies and wave vectors (tui, kj and (coj, k2) to excite coherent phonons (or other Raman-active modes) of the difference frequency and wave vector (Q = oji — coj, Qo = Itj — k2) [37]. The excitation process can be described by the stimulated scattering equation of motion [9] for a damped, nondispersive vibrational mode. 

In general, the exact functional form of the scattering law is complex. For most experiments on hydrogeneous polymers at low temperatures, the scattering probability has been taken to be proportional to the one-phonon scattering cross-section 74, 34) for crystals of cubic symmetry, namely ... 

C. K. Loong et ai, High-Energy Oxygen Phonon Modes and Superconductivity in Bai j,Kj,Bi03 An Inelastic-Neutron-Scattering Experiment and Molecular-Dynamics Simulation, Phys. Rev. Lett. 62, 2628-2631 (1989). 

The fundamental role of the dynamic electron-phonon coupling in the Jahn-Teller crystals was clearly demonstrated by the Raman light scattering experiments [18],... 

Now we can show the explicit relation with experiment. What is usually measured in spectroscopic or scattering experiments is the spectral density function /(to), which is the Fourier transform of some correlation function. For example, the absorption intensity in infrared spectroscopy is given by the Fourier transform of the time-dependent dipole-dipole correlation function <[/x(r), ju,(0)]>. If one expands the observables, i.e., the dipole operator in the case of infrared spectroscopy, as a Taylor series in the molecular displacement coordinates, the absorption or scattering intensity corresponding to the phonon branch r at wave vector q can be written as (Kobashi, 1978)... 

If these holes emit phonons, they will move to the edge of the HOMO band edge, and if they absorb phonons, they will move away from the HOMO band edge. Each hole will experience a different sequence of phonon absorption and emission events that have probabihties given by the carrier-phonon scattering rate, as a function of carrier... 

The main message from this class of experiments is that the details of the surface do affect the carrier relaxation. In the presence of surface defects associated with conventional surface preparation, the carrier relaxation in the surface region is exceptionally fast relative to bulk processes (10-100 fs dynamics). As can be seen by comparing the dynamics shown in Fig. 2.9, the effect of the surface is to increase the rate of relaxation and thermalisation. The asymmetry, more anharmonic character to the surface modes and increased mixing of states at defect sites all conspire to speed up the relaxation processes. With proper attention to surface structure, it is possible to intervene in the relaxation process and achieve carrier and phonon scattering rates that approach bulk processes. In this limit, 200 fs to picosecond dynamics define the operative time scales. 

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