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

TL;DR

This study investigates how heat flux in non-equilibrium conditions causes low-frequency vibrational modes in solids to deviate from energy equipartition, affecting temperature measurements.

Contribution

It demonstrates experimentally and numerically that heat flux induces mode-specific energy deviations, revealing new flux-mediated correlations in non-equilibrium states.

Findings

Low-frequency modes can be excited as if at higher temperatures.

Energy associated with modes strongly depends on heat flux.

Mode energies decouple from temperature in non-equilibrium conditions.

Abstract

When the energy content of a resonant mode of a crystalline solid in thermodynamic equilibrium is directly measured, assuming that quantum effects can be neglected it coincides with temperature except for a proportionality factor. This is due to the principle of energy equipartition and the equilibrium hypothesis. However, most natural systems found in nature are not in thermodynamic equilibrium and thus the principle cannot be granted. We measured the extent to which the low-frequency modes of vibration of a solid can defy energy equipartition, in presence of a steady state heat flux, even close to equilibrium. We found, experimentally and numerically, that the energy separately associated with low frequency normal modes strongly depends on the heat flux, and decouples sensibly from temperature. A 4% in the relative temperature difference across the object around room temperature suffices to excite two modes of a macroscopic oscillator, as if they were at equilibrium, respectively, at temperatures about 20% and a factor 3.5 higher. We interpret the result in terms of new flux-mediated correlations between modes in the nonequilibrium state, which are absent at equilibrium.