Bipolar Thermoelectric Josephson Engine

Thermoelectric effects in metals are typically small due to the nearly-perfect particle-hole (PH) symmetry around their Fermi surface [1, 2]. Despite being initially considered paradoxical [3], thermophase effects [4-8] and linear thermoelectricity [9] in superconducting systems were identified only when PH symmetry is explicitly broken [10-14]. Here, we experimentally demonstrate that a superconducting tunnel junction can develop a very large bipolar thermoelectric effect in the presence of a nonlinear thermal gradient thanks to spontaneous PH symmetry breaking [15]. Our junctions show a maximum thermovoltage of $\pm150\; \mu$ V at $\pm650$ mK, directly proportional to the superconducting gap. Notably, the corresponding Seebeck coefficient of $\pm300\; \mu$V/K is roughly $10^5$ times larger than the one expected for a normal metal at the same temperature [16, 17]. Moreover, by integrating our junctions into a Josephson interferometer, we realize a bipolar thermoelectric Josephson engine (BTJE) [18] with phase-coherent thermopower control [19]. When connected to a generic load, the BTJE generates a phase-tunable electric power up to about 140 mW/m$^2$ at subKelvin temperatures. In addition, our device implements the prototype for a persistent thermoelectric memory cell, written or erased by current injection [20]. We expect that our findings will trigger thermoelectricity in PH symmetric systems, and will lead to a number of groundbreaking applications in superconducting electronics [21], cutting-edge quantum technologies [22-24] and sensing [25].