Thursday, May 23, 2013

1305.5084 (Livia Conti et al.)

Effects of breaking vibrational energy equipartition on measurements of
temperature in macroscopic oscillators subject to heat flux

Livia Conti, Paolo De Gregorio, Gagik Karapetyan, Claudia Lazzaro, Matteo Pegoraro, Michele Bonaldi, Lamberto Rondoni
The principle of energy equipartition is of central importance in classical statistical mechanics, it is based on the equilibrium hypothesis and it is extensively used. The energy content of resonant modes of a crystalline solid can be directly measured, and coincides with temperature except for proportionality factors, due to equipartition. However, most systems found in nature are not in thermodynamic equilibrium and thus the principle cannot be granted. We show by direct measure- ment that the low-frequency modes can defy energy equipartition, when a heat flux draws the solid in a nonequilibrium steady state however close to equilibrium. We found, both experimentally and numerically, that the energy separately associated with normal modes of low frequency depends strongly on the heat flux, so that it decouples sensibly from temperature. A few percent increase in the relative temperature difference around room temperature across the object suffices to excite two modes of a macroscopic oscillator as if they were at equilibrium, respectively, at an equivalent temperature of about 400 K and 900 K. We interpret the result in terms of new flux-mediated correlations between modes in the nonequilibrium state, which are absent at equilibrium.
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