Dr Alexei Maznev, MIT

Non-diffusive thermal transport and THz ultrasonics at room temperature: in search for the phonon mean free path
When May 28, 2015
from 02:00 PM to 03:00 PM
Where LR8, IEB Building, Engineering Science
Contact Name
Contact Phone 01865-273925
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In non-metallic solids, heat is carried primarily by acoustic phonons. At cryogenic temperatures, the phonon mean free path (MFP) can be large and the connection between heat transport and acoustics is well established as exemplified by the studies of “phonon imaging”. At room temperature (RT) phonon frequencies significantly contributing to heat transport (typically above 1 THz) have been out of reach of acoustics research. The two fields are now converging thanks to two recent developments. On one hand, studies of the fundamentals of thermal conductivity showed that low-frequency phonons with long MFP play a much larger role in heat transport at RT than previously thought. On the other hand, acoustic frequencies ~1 THz and above are becoming accessible in laser-based picosecond ultrasonics experiments. In this talk, I will give an overview of our recent research on both thermal transport and high-frequency acoustics aimed at bridging the gap between the two fields. We will discuss thermal transport measurements of thin Si membranes and bulk GaAs with the laser-induced thermal grating technique where we detected a significant deviation from the diffusive transport at distances exceeding 1 mm, in stark contrast with the 40 nm average phonon MFP in Si at RT typically cited in textbooks. Furthermore, I will describe experiments with laser-generated coherent phonons at frequencies 0.3-1.5 THz in GaAs and GaN-based structures as well as Si membranes aimed at direct measurements of the phonon lifetime. We will also see how methods used in these studies can be employed by materials scientists for non-contract characterization of thermal and elastic properties of materials.