Observation of Electrically Tunable van Hove Singularities in Twisted Bilayer Graphene from NanoARPES

Result of the Month

Author: Prof. Dr. Søren Ulstrup Institute: ''Department of Physics and Astronomy, Aarhus University, Aarhus C 8000, Denmark'' Advanced Materials
URL: https://doi.org/10.1002/adma.202001656
Date: 12/2020 Instruments: DA30-L

The possibility of triggering correlated phenomena by placing a singularity of the density of states near the Fermi energy remains an intriguing avenue toward engineering the properties of quantum materials. Twisted bilayer graphene is a key material in this regard because the superlattice produced by the rotated graphene layers introduces a van Hove singularity and flat bands near the Fermi energy that cause the emergence of numerous correlated phases, including superconductivity. Direct demonstration of electrostatic control of the superlattice bands over a wide energy range has, so far, been critically missing. This work examines the effect of electrical doping on the electronic band structure of twisted bilayer graphene using a back‐gated device architecture for angle‐resolved photoemission measurements with a nano‐focused light spot. A twist angle of 12.2° is selected such that the superlattice Brillouin zone is sufficiently large to enable identification of van Hove singularities and flat band segments in momentum space. The doping dependence of these features is extracted over an energy range of 0.4 eV, expanding the combinations of twist angle and doping where they can be placed at the Fermi energy and thereby induce new correlated electronic phases in twisted bilayer graphene.

Image Description: Electrostatic tuning of twisted bilayer graphene Dirac cones viewed by nanoARPES. a) Sketch of our nanoARPES experiment with top (red) and bottom (blue) graphene layers contacted by source (S) and drain (D) electrodes and stacked on hBN and on a graphite back-gate (G). A beam of photons is focused to a 690 nm spot using zone plate optics. b,c) Functional region of device presented via optical microscopy (b) and spatially dependent ARPES intensity integrated over E and k (c). The dotted lines demarcate the twBLG flake. d–f) ARPES spectra of the twBLG Dirac cones measured at the given gate voltages. The dashed blue (red) lines represent linearly extrapolated peak positions determined from MDC fits of the bottom (top) layer Dirac cone around KB (KT ) as shown in Figure S3, Supporting Information. g) Gate voltage dependence of n obtained from kF of each Dirac cone. The curves are fits to a linear dependence on gate voltage. The inset presents the resistance of the device measured in situ before exposure to photons. h) Dirac point energies ED determined from the linear extrapolation shown in (d–f) with fits to two separate Vg -dependent functions (curves). The different slopes of the fitted curves in (g,h) result from the smaller amount of charge induced in the top layer (see capacitor model of our device in the inset). i) Demonstration of n-dependence (black curve) of ED for bottom (top) graphene, as expected for non-interacting Dirac cones (see sketch of cones at different doping levels in the inset). The blue (red) markers represent the bottom (top) layer and vertical dashed lines mark the charge neutrality point in (g–i).

Authors

Alfred J. H. Jones, Ryan Muzzio, Paulina Majchrzak, Sahar Pakdel, Davide Curcio, Klara Volckaert, Deepnarayan Biswas, Jacob Gobbo, Simranjeet Singh, Jeremy T. Robinson, Kenji Watanabe, Takashi Taniguchi, Timur K. Kim, Cephise Cacho, Nicola Lanata, Jill A. Miwa, Philip Hofmann, Jyoti Katoch, Søren Ulstrup

Institutes

1) A. J. H. Jones, P. Majchrzak, S. Pakdel, Dr. D. Curcio, K. Volckaert, Dr. D. Biswas, Dr. N. Lanata, Dr. J. A. Miwa, Prof. P. Hofmann, Dr. S. Ulstrup

Department of Physics and Astronomy, Aarhus University, Aarhus C 8000, Denmark

E-mail: ulstrup@phys.au.dk

2) R. Muzzio, J. Gobbo, Dr. S. Singh, Dr. J. Katoch

Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15213, USA

E-mail: jkatoch@andrew.cmu.edu

3) Dr. J. T. Robinson

US Naval Research Laboratory, Washington, D.C. 20375, USA

4) Dr. K. Watanabe, Dr. T. Taniguchi

National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan

5) Dr. T. K. Kim, Dr. C. Cacho

Diamond Light Source, Division of Science, Didcot OX11 0DE, UK

Name and email of corresponding author

Dr. S. Ulstrup
ulstrup@phys.au.dk

Observation of Electrically Tunable van Hove Singularities in Twisted Bilayer Graphene from NanoARPES

Result of the Month

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