Contatto di riferimento: Laura Basiricò
Partecipanti: Stefan K. Estreicher: Physics Department, Texas Tech University
In 1929, Rudolf Peierls first discussed the interactions between heat flow and defects. He thought of defects as local perturbations to Peter Debye’s perfect-crystal ‘lattice waves’ and therefore described these interactions in terms of scattering. This was - and still is - universally accepted and never questioned. Today, phonon scattering is assumed to occur whenever thermal phonons encounter impurities, interfaces, surfaces, grain boundaries, amorphous regions, and other types of defects. But what is the nature of this scattering at the atomic level? And where are the dynamic properties of the defects?
First-principles theory shows that defects in semiconductors introduce vibrational modes that are distinct from the host crystal’s modes because they are spatially localized around the defect. We call them Spatially-Localized Modes (SLMs). Low-frequency SLMs can be excited thermally, and the localized vibrational excitations survive for long periods of time, a phenomenon called phonon trapping. It is temperature dependent. Defects reduce the flow of heat because they trap phonons in SLMs, not because they scatter thermal phonons. The trapped phonons decay into combinations of lower-frequency bulk phonons. Controlling this decay should be possible at heterojunctions by carefully selecting the two materials involved.
My talk will deal with the key ingredients of these processes, including the critical issue of temperature control in (strictly microcanonical) non-equilibrium MD simulations. Then, I will define ‘localization’, SLMs, and discuss vibrational lifetimes. Controlling the decay of phonons trapped at interfaces and controlling heat flow with d layers are active topics of research in my group. Our most recent results (and some speculations) will be discussed. My group needs help from interested experimentalists to guide a project that is so far purely theoretical.
Stefan K. Estreicher received his PhD in theoretical physics from the University of Zürich in 1982, and then spent four years as a postdoc at Rice University in Houston. He joined the Physics faculty at Texas Tech University in 1986 and is now a Paul Whitfield Horn Professor. His research deals with the theory of defects in semiconductors and nanostructures, and their impact on the electrical and thermal properties of the material. He is a Fellow of the American Physical Society and of the Institute of Physics (UK). He won a Humboldt research award in 2001. One of his hobbies is the history of wine, and he has published a book and several papers in this area.