Effect of non-energetic coherence in the bath on the performance of quantum thermal machines

In this paper, we propose an autonomous thermal machine described by a structured collision model (CM), in which the working substance is a qubit that repeatedly interacts with two local reservoirs at distinct temperatures. Each reservoir is simulated by a collection of numerous identical and uncorrelated ancillas, with each ancilla consisting of multiple correlated qubits. We find that manipulation of non-energetic coherence within the reservoirs can induce transitions among distinct operating regimes of the thermal machine: engine, refrigerator, accelerator, and hybrid refrigerator. The thermal machine exhibits phase-sensitive modulation, wherein the non-energetic coherence in the reservoirs can either enhance or reduce the performance of the machine depending on its phase. Remarkably, non-energetic coherence with some specific phases in cold (hot) reservoirs can even enable the efficiency (coefficient of performance, COP) at maximum power of the machine operating as an engine (refrigerator) to surpass (approach) the Carnot bound achievable under equilibrium reservoirs. We further demonstrate that the effects of the non-energetic coherence in the reservoir on the performance and operating regimes of the machine stem from its modulation of the apparent temperatures of the non-thermal reservoirs.