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Soutenance de thèse

Jeudi 14 décembre à 14h00,
Salle Nevill Mott, D420

Orateur : Adib TAVAKOLI (TPS)
"Phonon transport in the quantum regime"

Abstract

This PhD entitles Phonon transport in the quantum regime is based on the analysis of the thermal properties of confined systems at very low temperature. The objectives of this subject are to put the systems (membrane or nanowire) in two extreme conditions (low temperature and low dimensions) and understand the fundamental thermal properties coming from these criteria. By lowering the temperature, the phonon characteristic lengths like the mean free path (MFP) or the phonon dominant wavelength increase.

When these characteristic lengths exceed lateral dimensions of the system, the boundary scattering will govern the overall thermal properties. In 1D channel between two reservoirs, when the phonon MFP is bigger than the length of the system (Λ_ph>l), and the phonon wavelength is bigger than the section (λ_ph>d), the phonon transport is ballistic. In such case, the phonons are transmitted from one reservoir to the other one when they are kept at different temperatures. The thermal conductance is then expressed as : K=N_α q T where N_α is the number of populated vibrational modes, q=((π²k_B^2)T)⁄3h) is the universal value of the quantum of thermal conductance, and T is the transmission coefficient. We show that the transmission coefficient is the dominant factor that will set the thermal conductance value. It depends on the dimension and the shape of the reservoirs, and the nature of the material in use.

The measurements of specific heat of 2D suspended phonon cavities with different thicknesses show the transition of specific heat from volumetric to surface effect. Below 1 K, when the phonon dominant wavelength becomes bigger than the thickness of the membranes, the specific heat is only governed by the surfaces. In another experimental achievement the effect of internal stress on specific heat of 2D phonon cavities was investigated. It was found that the internal stress has an inverse relation with the formation of the localized excitations that exist in amorphous solids (a-solids) at very low temperature.

By decreasing the internal stress, the formation of these excitations are enhanced, so the heat capacity increases.

According to our experiments, we believe that these When the phonon transport is governed by boundaries, the high contribution of TLSs on the surfaces will dominate the thermal properties.

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