Dynamic phases of synthetic bath at negative temperatures.

The article focuses on the dynamic operational modes of a quantum thermal device modeled as a three-level donor–acceptor molecular system coupled to two physical baths at positive temperatures and driven by an external field. By synthesizing a bath with continuously tunable inverse temperature—including negative values—through the coupling of the two baths, the device can function as a heat engine, refrigerator, thermal accelerator, or thermal ratchet, with energy flow directions dependent on the synthetic bath temperature and the driving field frequency. The study reveals that the counter-rotating component of the driving field, particularly at negative frequencies, enables heat flow from the cold to the hot bath while performing output work, a behavior distinct from classical thermodynamics. Additionally, periodic switching of the external drive induces transient underdamped oscillations in heat and work fluxes, offering insights into optimizing work extraction beyond steady-state conditions. This work provides a theoretical framework for multifunctional quantum thermal devices with potential applications in molecular-scale energy manipulation.