We study two different models of optomechanical systems where a temperature
gradient between two radiation baths is exploited for inducing self-sustained
coherent oscillations of a mechanical resonator. Viewed from a thermodynamic
perspective, such systems represent quantum instances of self-contained thermal
machines converting heat into a periodic mechanical motion and thus they can be
interpreted as nano-scale analogues of macroscopic piston engines. Our models
are potentially suitable for testing fundamental aspects of quantum
thermodynamics in the laboratory and for applications in energy efficient
nanotechnology.Comment: 10 pages, 6 figure
