Steady Floquet–Andreev states in graphene Josephson junctions

Sein Park, Wonjun Lee, Seong Jang, Yong-Bin Choi, Jinho Park, Woochan Jung, Kenji Watanabe, Takashi Taniguchi, Gil Young Cho () and Gil-Ho Lee ()
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Sein Park: Pohang University of Science and Technology
Wonjun Lee: Pohang University of Science and Technology
Seong Jang: Pohang University of Science and Technology
Yong-Bin Choi: Pohang University of Science and Technology
Jinho Park: Pohang University of Science and Technology
Woochan Jung: Pohang University of Science and Technology
Kenji Watanabe: National Institute for Materials Science
Takashi Taniguchi: National Institute for Materials Science
Gil Young Cho: Pohang University of Science and Technology
Gil-Ho Lee: Pohang University of Science and Technology

Nature, 2022, vol. 603, issue 7901, 421-426

Abstract: Abstract Engineering quantum states through light–matter interaction has created a paradigm in condensed-matter physics. A representative example is the Floquet–Bloch state, which is generated by time-periodically driving the Bloch wavefunctions in crystals. Previous attempts to realize such states in condensed-matter systems have been limited by the transient nature of the Floquet states produced by optical pulses1–3, which masks the universal properties of non-equilibrium physics. Here we report the generation of steady Floquet–Andreev states in graphene Josephson junctions by continuous microwave application and direct measurement of their spectra by superconducting tunnelling spectroscopy. We present quantitative analysis of the spectral characteristics of the Floquet–Andreev states while varying the phase difference of the superconductors, the temperature, the microwave frequency and the power. The oscillations of the Floquet–Andreev-state spectrum with phase difference agreed with our theoretical calculations. Moreover, we confirmed the steady nature of the Floquet–Andreev states by establishing a sum rule of tunnelling conductance4, and analysed the spectral density of Floquet states depending on Floquet interaction strength. This study provides a basis for understanding and engineering non-equilibrium quantum states in nanodevices.

Date: 2022
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DOI: 10.1038/s41586-021-04364-8

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