Beating Carnot efficiency with periodically driven chiral conductors

Classically, the power generated by an ideal thermal machine cannot be larger than the Carnot limit. This profound result is rooted in the second law of thermodynamics. A hot question is whether this bound is still valid for microengines operating far from equilibrium. Here, we demonstrate that a quantum chiral conductor driven by AC voltage can indeed work with efficiencies much larger than the Carnot bound. The system also extracts work from common temperature baths, violating Kelvin-Planck statement. Nonetheless, with the proper definition, entropy production is always positive and the second law is preserved. The crucial ingredients to obtain efficiencies beyond the Carnot limit are: i) irreversible entropy production by the photoassisted excitation processes due to the AC field and ii) absence of power injection thanks to chirality. Our results are relevant in view of recent developments that use small conductors to test the fundamental limits of thermodynamic engines.We acknowledge support from Grants No. MAT2017-82639 and No. PID2020-117347GB-I00 funded by MCIN/AEI/10.13039/501100011033, No. MDM2017-0711 funded by MINECO/AEI/FEDER María de Maeztu Program for Units of Excellence, and No. PDR2020-12 funded by GOIB. S.R. also acknowledges partial support from National Research Foundation of Korea (Grant No. 2021R1A6A3A03040076).Peer reviewe