Thermoelectricity with superconducting nanocircuits

Conventional understanding holds that thermoelectric effects in metals are intrinsically weak, being strongly suppressed by the near-perfect particle–hole symmetry at the Fermi surface. Superconductors have traditionally been regarded as even less promising in this respect, effectively thermally inert. In this work, we challenge this paradigm. We demonstrate that superconducting tunnel junctions, when driven out of equilibrium by a thermal gradient, spontaneously break particle–hole symmetry [1]. The resulting response is striking: we observe bipolar thermoelectricity with Seebeck coefficients reaching up to ±300 μV K⁻¹. These values are comparable to those obtained in quantum dots and are approximately five orders of magnitude larger than those typically expected for conventional metals at sub-kelvin temperatures [2].
Furthermore, by integrating these junctions into a Josephson interferometer, we realize a bipolar thermoelectric Josephson engine, namely, a device that converts heat into phase-tunable electrical power, achieving output power densities up to ~140 nW mm⁻² [2, 3], and controllable via an externally applied magnetic flux. Remarkably, the same device also functions as a persistent thermoelectric memory element, whose logical state can be written and erased by current injection, while requiring no power to maintain the stored information. These results establish a versatile thermoelectric platform with direct relevance for superconducting quantum technologies, ranging from quantum refrigeration schemes to highly sensitive single-photon detection architectures [4, 5].

[1] G. Marchegiani, A. Braggio, and F. Giazotto. Nonlinear thermoelectricity with electron-hole symmetric systems. Phys. Rev. Lett. 124, 106801 (2020).
[2] G. Germanese, F. Paolucci, G. Marchegiani, A. Braggio, and F. Giazotto. Bipolar thermoelectric Josephson engine, Nat. Nanotechnol. 17, 1084 (2022).
[3] G. Germanese, F. Paolucci, G. Marchegiani, A. Braggio, and F. Giazotto. Phase-control of bipolar thermoelectricity in Josephson tunnel junctions, Phys. Rev. Applied 19, 014074 (2023).
[4] F. Paolucci, G. Germanese, A. Braggio, and F. Giazotto. A highly-sensitive broadband superconducting thermoelectric single-photon detector, Appl. Phys. Lett. 122, 173503 (2023).
[5] C. Guarcello, R. Citro, F. Giazotto, and A. Braggio. Bipolar thermoelectrical SQUIPT (BTSQUIPT), Appl. Phys. Lett. 123, 152601 (2023).