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Unraveling Fluctuations, Dissipation and Nonlinearities in Nanotube and Graphene Mechanical Resonators
Speaker Prof. Joel Moser, Soochow University
Date 9 April 2019 (Tuesday)
Time 10:30 - 12:00
Venue Room 4472 (Lifts 25-26), HKUST

The study of vibrations is a cornerstone of condensed matter physics. For more than a century, vibrations in solids and in molecular systems have been explored by means of measurements over macroscopic ensembles containing a large number of degrees of freedom. In recent years, progress in nanofabrication has made it possible to study new vibrational systems in the form of nanomechanical resonators such as nanotubes, nanostrings and nanomembranes. While vibrations of these systems still result from the collective motion of a large number of atoms, they can be engineered and controlled in such a way that well-defined, individual vibrational modes can be studied. I will present the research my colleagues and I conducted in Barcelona and at Michigan State University about vibrations in resonators made of carbon nanotubes and graphene sheets. I will focus on two aspects of their dynamics. Firstly, as a consequence of their low mass density and small size, these resonators experience strong thermal and non-thermal fluctuations. While thermal fluctuations are associated with dissipation and give rise to a finite linewidth of the vibrational spectrum, non-thermal fluctuations are not related to dissipation and yet also lead to spectral broadening. Secondly, by virtue of their high aspect ratio and small cross-section, these resonators respond to driving forces in a strongly nonlinear fashion. Such mechanical nonlinearities lead to the exchange of energy among vibrational modes. Unravelling fluctuations, dissipation and nonlinearities in the mechanical response of these resonators is challenging. To address this problem, we performed (i) spectral measurements of thermal vibrations in nanotube resonators, and (ii) time-resolved energy relaxation measurements in few-layer, ultra-clean graphene resonators, both at cryogenic temperature. In both nanotube and graphene-based resonators, we found that resonant frequency fluctuations of non-thermal origin account for a large part of the response spectrum. We also found an interesting interplay between nonlinearities and dissipation in graphene resonators, with individual dissipation channels closing down as stored mechanical energy would freely decay.