  Stochastic heat engines are devices that generate work from random thermal
motion using a small number of highly fluctuating degrees of freedom. Proposals
for such devices have existed for more than a century and include the Maxwell
demon and the Feynman ratchet. Only recently have they been demonstrated
experimentally, using e.g., thermal cycles implemented in optical traps.
However, the recent demonstrations of stochastic heat engines are
nonautonomous, since they require an external control system that prescribes a
heating and cooling cycle, and consume more energy than they produce. This
Report presents a heat engine consisting of three coupled mechanical resonators
(two ribbons and a cantilever) subject to a stochastic drive. The engine uses
geometric nonlinearities in the resonating ribbons to autonomously convert a
random excitation into a low-entropy, nonpassive oscillation of the cantilever.
The engine presents the anomalous heat transport property of negative thermal
conductivity, consisting in the ability to passively transfer energy from a
cold reservoir to a hot reservoir.
