Spintronic harvesting of thermal fluctuations on paramagnetic molecular centers around a phase transition

Experiments and theory are reexamining how the laws of thermodynamics are expressed in a quantum world. Most quantum thermodynamics research is performed at sub-Kelvin temperatures to prevent thermal fluctuations from smearing the mesoscopic quantum engines discrete energy levels that mediate the asymmetric shuffling of electrons between the electrodes. Meanwhile, several groups report that building an electron-spin based implementation by placing the discrete spin states of paramagnetic centers between ferromagnetic electrodes can not only overcome this drawback, but also induce a net electrical power output despite an apparent thermal equilibrium. We illustrate this apparent thermodynamics conundrum through measurements on several devices of large output power, which endures beyond room temperature. Weve inserted the Co paramagnetic center in Co phthalocyanine molecules between electron spin-selecting Fe/C60 interfaces within vertical molecular nanojunctions. This device class behaves as a spintronically controlled switch of current flow, and of its direction. We observe dc current output over several hours, and output power as high as 450nW(24nW) at 40K(360K). This leapfrogs previous results, as well as other energy harvesting strategies involving a thermal gradient. Our data indicates that the output power is strongly altered when the partly fluctuating paramagnetic centers undergo a magnetic phase transition. This new conceptual ingredient in the spin engine can account for the devices operation beyond the boundaries of classical thermodynamics. Further clarifying the phenomenon and developing this technology could help accelerate the transition to clean energy.