Sent on behalf of Ali Shakouri. Dear Colleagues, Next week Prof. Xavier Alvarez from Univ. Autonoma in Barcelona is visiting us and will be at the Birck center on July 25-29. He is an expert in extended thermodynamics and transport (heat, sound). His recent research has focused on phonon hydrodynamics, quasi ballistic effects and the observation of 2nd sound. He will give a seminar (see below the abstract) on Tuesday July 26th from 1:00pm-2:00pm in BRK 1001. Please let me know if you would be interested to meet with Prof. Alvarez next week. Best regards, Ali Shakouri ************* TITLE: HYDRODYNAMIC PHENOMENA IN THERMAL TRANSPORT Xavier Alvarez Universitat Autònoma de Barcelona ABSTRACT: A large number of experimental observations incompatible with the classical Fourier description of thermal transport at the nanometer and in the picosecond scales has been reported in the last decade. Despite the theoretical efforts done in the topic, a model able to describe the gathered data at all length and time scales is still not available. Two different descriptions have been proposed. Phonon hydrodynamics has been used as a framework to model thermal transport in materials where momentum conservation in phonon-phonon collisions is important. For other situations, a kinetic description based on the propagation of independent phonons, in what is called quasiballistic description, has been developed. The fundamental difference between them is in the number of length or time scales required to describe the observations. While in the hydrodynamic approach, a single scale is enough, in the quasiballistic description, the full set of phonon scales is necessary. For graphene and other 2D materials, the hydrodynamic approach has been the traditional main stream, while the quasiballistic approach has been more used for classical bulk semiconductors. In the last years, some experiments and theoretical descriptions seems to be challenging this traditional splitting. On the one side, some predictions of the hydrodynamic regime for 2D materials like the second sound velocity have put on doubt the standard approach. On the other side, collective phonon behavior like the use of a single time scale to describe thermal decay in a silicon substrate or the observation of second sound in germanium seem to indicate that the hydrodynamic description could be used in these semiconductors. This could be an indication that a more unified framework could be proposed. The talk will cover some of the most recent evidences in the theoretical and experimental research on thermal transport and we will analyze them in the framework of the Kinetic/Collective model (KCM), developed to give a more generalized framework to describe thermal experiments.