Electronic rotons and Wigner crystallites in a two-dimensional dipole liquid

Soobin Park, Minjae Huh, Chris Jozwiak, Eli Rotenberg, Aaron Bostwick and Keun Su Kim ()
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Soobin Park: Yonsei University
Minjae Huh: Yonsei University
Chris Jozwiak: E. O. Lawrence Berkeley National Laboratory
Eli Rotenberg: E. O. Lawrence Berkeley National Laboratory
Aaron Bostwick: E. O. Lawrence Berkeley National Laboratory
Keun Su Kim: Yonsei University

Nature, 2024, vol. 634, issue 8035, 813-817

Abstract: Abstract A key concept proposed by Landau to explain superfluid liquid helium is the elementary excitation of quantum particles called rotons1–8. The irregular arrangement of atoms in a liquid leads to the aperiodic dispersion of rotons, which played a pivotal role in understanding fractional quantum Hall liquids (magneto-rotons)9,10 and the supersolidity of Bose–Einstein condensates11–13. Even for a two-dimensional electron or dipole liquid, in the absence of a magnetic field, the repulsive interactions have been predicted to form a roton minimum14–19, which can be used to trace the transition to Wigner crystals20–24 and superconductivity25–27, although this has not yet been observed. Here, we report the observation of such electronic rotons in a two-dimensional dipole liquid of alkali-metal ions donating electrons to surface layers of black phosphorus. Our data reveal the striking aperiodic dispersion of rotons, which is characterized by a local minimum of energy at finite momentum. As the density of dipoles decreases so that interactions dominate over the kinetic energy, the roton gap reduces to 0, as in a crystal, signalling Wigner crystallization. Our model shows the importance of short-range order arising from repulsion between dipoles, which can be viewed as the formation of Wigner crystallites (bubbles or stripes) floating in the sea of a Fermi liquid. Our results reveal that the primary origin of electronic rotons (and the pseudogap) is strong correlations.

Date: 2024
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DOI: 10.1038/s41586-024-08045-0

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