skip to main content
US FlagAn official website of the United States government
dot gov icon
Official websites use .gov
A .gov website belongs to an official government organization in the United States.
https lock icon
Secure .gov websites use HTTPS
A lock ( lock ) or https:// means you've safely connected to the .gov website. Share sensitive information only on official, secure websites.

Explore Research Products in the PAR
It may take a few hours for recently added research products to appear in PAR search results.


Title: Electronic correlations in twisted bilayer graphene near the magic angle
Twisted bilayer graphene with a twist angle of around 1.1° features a pair of isolated flat electronic bands and forms a platform for investigating strongly correlated electrons. Here, we use scanning tunnelling microscopy to probe the local properties of highly tunable twisted bilayer graphene devices and show that the flat bands deform when aligned with the Fermi level. When the bands are half-filled, we observe the development of gaps originating from correlated insulating states. Near charge neutrality, we find a previously unidentified correlated regime featuring an enhanced splitting of the flat bands. We describe this within a microscopic model that predicts a strong tendency towards nematic ordering. Our results provide insights into symmetry-breaking correlation effects and highlight the importance of electronic interactions for all filling fractions in twisted bilayer graphene.  more » « less
Award ID(s):
1723367
PAR ID:
10155872
Author(s) / Creator(s):
; ; ; ; ; ; ; ; ; ; ; ; ;
Date Published:
Journal Name:
Nature physics
Volume:
15
ISSN:
1745-2473
Page Range / eLocation ID:
1174-1180
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. ; ; ; ; ; ; ; (, Nature Communications)
    Abstract Moiré superlattices created by the twisted stacking of two-dimensional crystals can host electronic bands with flat energy dispersion in which enhanced interactions promote correlated electron states. The twisted double bilayer graphene (TDBG), where two Bernal bilayer graphene are stacked with a twist angle, is such a moiré system with tunable flat bands. Here, we use gate-tuned scanning tunneling spectroscopy to directly demonstrate the tunability of the band structure of TDBG with an electric field and to show spectroscopic signatures of electronic correlations and topology for its flat band. Our spectroscopic experiments are in agreement with a continuum model of TDBG band structure and reveal signatures of a correlated insulator gap at partial filling of its isolated flat band. The topological properties of this flat band are probed with the application of a magnetic field, which leads to valley polarization and the splitting of Chern bands with a large effective g-factor. 
    more » « less
  2. ; ; ; ; ; ; ; ; ; (, Nature Physics)
    Recent experiments on magic-angle twisted bilayer graphene have shown the formation of flat bands, suggesting that electronic correlation effects are likely to dominate in this material. However, a global transport measurement showing distinct signatures of strong correlations—such as local moments arising from the flat bands—is missing. Here we demonstrate the presence of emergent local moments through their impact on entropy extracted from thermopower measurements. In addition to sign changes in the thermopower at the Dirac point and full filling of the flat bands, we observe sign changes near the quarter-filled bands that do not vary with temperature from 5 K to 60 K. This is in contrast to temperature-dependent crossing points seen in our study on twisted bilayer graphene devices with weaker correlations. Furthermore, we find that applying a magnetic field reduces the thermopower, consistent with spin entropy suppression observed in layered oxides under partial spin polarization. Neither the robust crossing points nor the suppression by a magnetic field can be explained solely from the contributions of band fermions; instead, our data suggest a dominant contribution coming from the entropy of the emergent localized moments of a strongly correlated flat band. 
    more » « less
  3. ; ; ; ; ; ; ; ; ; ; et al (, Science)
    Fermi nesting Correlated states have been shown to emerge in bilayer and trilayer graphene with the two-dimensional layers at just the right angle with respect to each other. Key to the enhanced importance of interactions are the so-called moiré electronic bands that form in such systems. Rickhauset al. explored a related system of two graphene bilayers twisted with respect to each other. The twist angle was set so that the layer coupling was strong enough to form moiré bands but weak enough for the carrier concentration in the top and bottom bilayer to be controlled separately. Doping the top bilayer with electrons and the bottom bilayer with holes, the researchers created a correlated state with nested Fermi surfaces. —JS 
    more » « less
  4. ; ; ; ; ; ; ; ; ; ; et al (, Nature Communications)
    null (Ed.)
    Abstract The discovery of interaction-driven insulating and superconducting phases in moiré van der Waals heterostructures has sparked considerable interest in understanding the novel correlated physics of these systems. While a significant number of studies have focused on twisted bilayer graphene, correlated insulating states and a superconductivity-like transition up to 12 K have been reported in recent transport measurements of twisted double bilayer graphene. Here we present a scanning tunneling microscopy and spectroscopy study of gate-tunable twisted double bilayer graphene devices. We observe splitting of the van Hove singularity peak by ~20 meV at half-filling of the conduction flat band, with a corresponding reduction of the local density of states at the Fermi level. By mapping the tunneling differential conductance we show that this correlated system exhibits energetically split states that are spatially delocalized throughout the different regions in the moiré unit cell, inconsistent with order originating solely from onsite Coulomb repulsion within strongly-localized orbitals. We have performed self-consistent Hartree-Fock calculations that suggest exchange-driven spontaneous symmetry breaking in the degenerate conduction flat band is the origin of the observed correlated state. Our results provide new insight into the nature of electron-electron interactions in twisted double bilayer graphene and related moiré systems. 
    more » « less
  5. ; ; ; ; ; ; ; ; ; ; et al (, Nature Physics)
    Collective excitations contain key information regarding the electronic order of the ground state of strongly correlated systems. Various collective modes in the spin and valley isospin channels of magic-angle graphene moiré bands have been alluded to by a series of recent experiments. However, a direct observation of collective excitations has been impossible due to the lack of a spin probe. Here we observe low-energy collective excitations in twisted bilayer graphene near the magic angle, using a resistively detected electron spin resonance technique. Two independent observations show that the generation and detection of microwave resonance relies on the strong correlations within the flat moiré energy band. First, the onset of the resonance response coincides with the spontaneous flavour polarization at moiré half-filling, but is absent in the isospin unpolarized density range. Second, we perform the same measurement on various systems that do not have flat bands and observe no indication of a resonance response in these samples. Our explanation is that the resonance response near the magic angle originates from Dirac revivals and the resulting isospin order. 
    more » « less
  • Citation Formats
  • MLA
  • APA
  • Chicago
  • BibTeX
  • Save / Share this Record
  • Send to Email