Experimental evidence for coupled-mode phonon gaps in superlattice structures

Abstract

Frequency gaps for acoustic phonons with a propagation direction oblique to the superlattice layers are investigated by phonon transmission spectroscopy in amorphous superlattices. In addition to the gaps at the center and at the boundaries of the superlattice mini-Brillouin-zone, the phonon transmission spectra show gaps associated with stop bands for coupled phonon modes with mixed longitudinal and transverse character. The position of the gaps is well explained by a description of the phonons in the elastic continuum limit.

Folded Acoustic Phonons in Amorphous SeTe/CdSe Superlattices

“…( 1)) exhibits gaps AV corresponding to the regions where the wave equations have solutions only with an imaginary part of the phonon wave vector. These gaps could be measured if the Raman experiments are carried out in forward-scattering geometry (qz = 0) [4,12,13]. If, however, in back-scattering geometry SL of small total thicknesses and weakly absorbing light are used, one could simultaneously measure both spectra because of the reflection from the SL/substrate interface (fig.…”

Transmission of coherent phonons through a metallic multilayer

“…The crystal is a cuboid measuring 10ϫ10ϫ5 mm 3 with the c axis parallel to the larger sides. A periodic superlattice consisting of 12 alternate Au and Ag layers was deposited by physical vapor deposition onto half of a ͑001͒ face.…”

“…Superlattices, periodic or quasiperiodic layered arrays, derive their interest primarily from the occurrence of frequency zones that are forbidden for the transmission of waves that are commensurate with the underlying periodicity. With regard to acoustic waves, in the course of the last two decades a number of experimental studies have been devoted to the observation of stop bands in strictly periodic superlattices [1][2][3][4] as well as quasiperiodic superlattices based on the Fibonacci sequence. 5,6 The experimental work has concentrated on superlattices made up of bilayers of crystalline or amorphous semiconductors, notably GaAs/Al x Ga 1Ϫx As.…”

“…5,6 The experimental work has concentrated on superlattices made up of bilayers of crystalline or amorphous semiconductors, notably GaAs/Al x Ga 1Ϫx As. Use was made of phonon transmission spectroscopy based on superconducting tunnel junctions, [1][2][3] which despite the limited resolution detected dips in the phonon transmission in specific frequency regions, and further frequency-selective phonon imaging 4,6 and Raman scattering. 5,7 Theoretical studies, which are not hindered by the limited frequency and angular resolution of common phonon sources and detectors, have addressed a great variety of systems, varying from periodic superlattices to randomly stacked multilayers.…”

Progress towards terahertz acoustic phonon generation in doping superlattices

“…The density of a:Si0 2 is 2.3 g/cm 3 , yielding an acoustic impedance of 1.34 x 10 6 , in the above units. For a:Si, the longitudinal phonon velocity can be found from the TA phonon velocity, given by Koblinger, when one makes use of the simplifying assumption 43 that the TA and LA phonon velocities scale by a common factor r = v TA /v LA = 0.62. For a:Si in the units above, the acoustic impedance for longitudinal phonons becomes 1.64 x 10 6 .…”