Observation of hydrodynamic plasmons and energy waves in graphene

Wenyu Zhao, Shaoxin Wang, Sudi Chen, Zuocheng Zhang, Kenji Watanabe, Takashi Taniguchi, Alex Zettl and Feng Wang ()
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Wenyu Zhao: University of California at Berkeley
Shaoxin Wang: University of California at Berkeley
Sudi Chen: Kavli Energy NanoScience Institute, University of California at Berkeley
Zuocheng Zhang: University of California at Berkeley
Kenji Watanabe: National Institute for Materials Science
Takashi Taniguchi: National Institute for Materials Science
Alex Zettl: University of California at Berkeley
Feng Wang: University of California at Berkeley

Nature, 2023, vol. 614, issue 7949, 688-693

Abstract: Abstract Thermally excited electrons and holes form a quantum-critical Dirac fluid in ultraclean graphene and their electrodynamic responses are described by a universal hydrodynamic theory. The hydrodynamic Dirac fluid can host intriguing collective excitations distinctively different from those in a Fermi liquid1–4. Here we report the observation of the hydrodynamic plasmon and energy wave in ultraclean graphene. We use the on-chip terahertz (THz) spectroscopy technique to measure the THz absorption spectra of a graphene microribbon as well as the propagation of the energy wave in graphene close to charge neutrality. We observe a prominent high-frequency hydrodynamic bipolar-plasmon resonance and a weaker low-frequency energy-wave resonance of the Dirac fluid in ultraclean graphene. The hydrodynamic bipolar plasmon is characterized by the antiphase oscillation of massless electrons and holes in graphene. The hydrodynamic energy wave is an electron-hole sound mode with both charge carriers oscillating in phase and moving together. The spatial–temporal imaging technique shows that the energy wave propagates at a characteristic speed of $${V}_{{\rm{F}}}/\sqrt{{\rm{2}}}$$ V F / 2 near the charge neutrality2–4. Our observations open new opportunities to explore collective hydrodynamic excitations in graphene systems.

Date: 2023
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DOI: 10.1038/s41586-022-05619-8

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