Crossover from incoherent to coherent phonon scattering in epitaxial oxide superlattices

Abstract

Elementary particles such as electrons or photons are frequent subjects of wave-nature-driven investigations, unlike collective excitations such as phonons. The demonstration of wave-particle crossover, in terms of macroscopic properties, is crucial to the understanding and application of the wave behaviour of matter. We present an unambiguous demonstration of the theoretically predicted crossover from diffuse (particle-like) to specular (wave-like) phonon scattering in epitaxial oxide superlattices, manifested by a minimum in lattice thermal conductivity as a function of interface density. We do so by synthesizing superlattices of electrically insulating perovskite oxides and systematically varying the interface density, with unit-cell precision, using two different epitaxial-growth techniques. These observations open up opportunities for studies on the wave nature of phonons, particularly phonon interference effects, using oxide superlattices as model systems, with extensive applications in thermoelectrics and thermal management.

Characteristic length of phonon transport within periodic nanoporous thin films and two-dimensional materials

“…In comparable superlattices with well-defined periodic interfaces, phonon coherence is important only for sub-10 nm periods at 300 K, whereas diffusive phonon scattering by interfaces is dominant for larger periods. 29,30 Consistently, more recent analysis shows that diffusive pore-edge phonon scattering is sufficient to explain the data of nanoporous films with feature sizes larger than 100 nm at 300 K. 31 As the second mechanism, pore-edge amorphization and oxidation, as found in real Si thin films, 8 may largely affect k L for fine nanoporous structures. 1 Such amorphous and surface oxidation layers usually have a few nm thickness.…”

Thermal conductance of superlattice junctions

“…7 It is known that the thermal conductivity of a bulk superlattice is sensitive to its period (i.e., the minimum in superlattice thermal conductivity) and can be partitioned into a regime where vibrational wave interference is significant and one where interference is not. 17,19 Hence, another hypothesis is that the lead effect is partially attributable to vibrational wave interference in the superlattice junction. Increasing the period of the superlattice junction attenuates the lead effect by moving the system out of the regime where interference is significant.…”

“…Previous works on superlattices 17,19 and superlattice junctions 16,34 suggest that thermal transport in superlattice junctions depends on both period length and junction length, and is modified by disorder. In this work, we vary both period length and junction length, and consider the effects of lead composition, finite temperature, and disorder.…”

Phonon interference in crystalline and amorphous confined nanoscopic films

“…It was found in numerous experiments and numerical simulations that the specular reflection and transmission of phonon waves at interfaces of nanostructured components may result in phonon interference effects which can be used for the modification of phonon dispersion and for controlling nanoscale heat transport. [1][2][3][4][5][6][7][8][9] To achieve strong phonon interference effects, the thickness of confined thin films should be smaller than the phonon mean free path (MFP) such that the phonons can travel ballistically between two interfaces of the thin film. Phonon MFPs of typical crystalline semiconductors such as Si, GaN, and graphite are on the order of tens of nanometers to micrometers.…”