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Broken Symmetries and Kohn’s Theorem in Graphene Cyclotron Resonance

Jordan Pack, B. Jordan Russell, Yashika Kapoor, Jesse Balgley, Jeffrey Ahlers, Takashi Taniguchi, Kenji Watanabe, and Erik A. Henriksen
Phys. Rev. X 10, 041006 – Published 8 October 2020
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Abstract

The cyclotron resonance of monolayer graphene, encapsulated in hexagonal boron nitride and with a graphite backgate, is explored via infrared transmission magnetospectroscopy as a function of the filling factor at fixed magnetic fields. The impact of many-particle interactions in the regime of broken spin and valley symmetries is observed spectroscopically. As the occupancy of the zeroth Landau level is increased from half-filling, a nonmonotonic progression of multiple cyclotron resonance peaks is seen for several interband transitions, revealing the evolution of underlying many-particle-enhanced gaps. Analysis of the peak energies shows significant exchange enhancements of spin gaps both at and below the Fermi energy, a strong filling-factor dependence of the substrate-induced Dirac mass, and also the smallest particle-hole asymmetry reported to date in graphene cyclotron resonance.

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  • Received 23 January 2020
  • Revised 9 July 2020
  • Accepted 18 August 2020

DOI:https://doi.org/10.1103/PhysRevX.10.041006

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Jordan Pack*, B. Jordan Russell*, Yashika Kapoor, Jesse Balgley, and Jeffrey Ahlers

  • Department of Physics, Washington University in St. Louis, 1 Brookings Drive, St. Louis, Missouri 63130, USA

Takashi Taniguchi

  • International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan

Kenji Watanabe

  • Research Center for Functional Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan

Erik A. Henriksen

  • Department of Physics, Washington University in St. Louis, 1 Brookings Drive, St. Louis, Missouri 63130, USA and Institute for Materials Science & Engineering, Washington University in St. Louis, 1 Brookings Drive, St. Louis, Missouri 63130, USA

  • *J. P. and B. J. R. contributed equally to this work.
  • henriksen@wustl.edu

Popular Summary

The energies of electrons constrained to move in two dimensions become quantized when placed in a strong magnetic field. In devices with very low disorder, the motion of the electrons can become correlated because of the mutually repelling Coulomb interactions, leading to remarkable patterns of quantization in the electrical resistance. Moreover, the absorption of light by such a system can reveal details of this dance of correlated electrons. Yet, a quirk of fate has prevented such light-absorption measurements from seeing the collective motions in most materials, almost as if the Coulomb interaction had been turned off. Here, we overcome this hurdle and investigate the correlated movement of electrons in graphene, whose massless electronic system is not subject to this prohibition.

In our work, infrared transmission through very clean graphene reveals a remarkable sequence of absorption peaks as the number of electrons in the system (and hence the energies of collective motions) are changed. These absorption peaks represent transitions between different electronic states, which, in principle, contain multiple sublevels. The change in the number of absorption peaks directly measures the energies of these sublevels. Remarkably, we can follow a smooth evolution of the sublevels from one state to another: As the number of electrons in the system increases, the energies of the gaps between levels grow and saturate.

With even cleaner devices in future investigations, we expect to directly observe absorptions in the fractionally quantized regime, providing a new window on this most remarkable aspect of the 2D quantum motion of electrons.

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Vol. 10, Iss. 4 — October - December 2020

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