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0–π phase-controllable thermal Josephson junction

Abstract

Two superconductors coupled by a weak link support an equilibrium Josephson electrical current that depends on the phase difference ϕ between the superconducting condensates1. Yet, when a temperature gradient is imposed across the junction, the Josephson effect manifests itself through a coherent component of the heat current that flows opposite to the thermal gradient for |ϕ| < π/2 (refs 24). The direction of both the Josephson charge and heat currents can be inverted by adding a π shift to ϕ. In the static electrical case, this effect has been obtained in a few systems, for example via a ferromagnetic coupling5,6 or a non-equilibrium distribution in the weak link7. These structures opened new possibilities for superconducting quantum logic6,8 and ultralow-power superconducting computers9. Here, we report the first experimental realization of a thermal Josephson junction whose phase bias can be controlled from 0 to π. This is obtained thanks to a superconducting quantum interferometer that allows full control of the direction of the coherent energy transfer through the junction10. This possibility, in conjunction with the completely superconducting nature of our system, provides temperature modulations with an unprecedented amplitude of 100 mK and transfer coefficients exceeding 1 K per flux quantum at 25 mK. Then, this quantum structure represents a fundamental step towards the realization of caloritronic logic components such as thermal transistors, switches and memory devices11,10. These elements, combined with heat interferometers3,4,12 and diodes13,14, would complete the thermal conversion of the most important phase-coherent electronic devices and benefit cryogenic microcircuits requiring energy management, such as quantum computing architectures and radiation sensors.

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Figure 1: Structure of the 0–π phase-tunable thermal Josephson junction.
Figure 2: Electrical behaviour of the Josephson interferometer.
Figure 3: Thermal behaviour of the 0–π phase-controllable Josephson junction at 25 mK.
Figure 4: Performance of the 0–π phase-tunable Josephson junction at different bath temperatures.

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Acknowledgements

The authors acknowledge financial support from the MIUR-FIRB2013–Project Coca (grant no. RBFR1379UX), the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement no. 615187 – COMANCHE, and the European Union (FP7/2007-2013)/REA grant agreement no. 630925 – COHEAT.

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Contributions

A.F. fabricated the samples. A.F. and G.T. performed the measurements. A.F. and G.T. analysed the data and carried out the simulations with inputs from P.V., P.S. and F.G. A.F. and F.G. conceived the experiment. All authors discussed the results and their implications equally at all stages, and wrote the manuscript.

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Correspondence to Francesco Giazotto.

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The authors declare no competing financial interests.

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Fornieri, A., Timossi, G., Virtanen, P. et al. 0–π phase-controllable thermal Josephson junction. Nature Nanotech 12, 425–429 (2017). https://doi.org/10.1038/nnano.2017.25

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