Josephson nanocircuits as a suitable platform for implementing quantum thermoelectric machines

Thermoelectric effects in metals are generally small due to the nearly perfect particle–hole symmetry around their Fermi surface. Additionally, thermo-phase effects and linear thermoelectricity in superconducting systems have only been observed when particle–hole symmetry is explicitly broken, as thermoelectric effects were thought to be impossible in pristine superconductors. In this talk, I will present experimental results showing that superconducting tunnel junctions can display a significant bipolar thermoelectric effect when subjected to a substantial thermal gradient, which arises from the spontaneous breaking of particle-hole symmetry [1].Our junctions display Seebeck coefficients of up to ±300 μV K⁻¹, which is comparable to those found in quantum dots and roughly 100,000 times larger than the expected values for normal metals at subkelvin temperatures [2].
Additionally, by incorporating our junctions into a Josephson interferometer, we have developed a bipolar thermoelectric Josephson engine that generates phase-tunable electric power of up to approximately 140 nW mm–2 [2,3]. Notably, our device also serves as a prototype for a persistent thermoelectric memory cell, which can be written or erased through current injection. We anticipate that our results will lead to advancements in the field of superconducting quantum technologies [4,5].

We acknowledge funding from the EU’s Horizon 2020 Research and Innovation Framework Programme under Grant No. 101057977 - SPECTRUM.

References
[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).