ARPES Signatures of Few-Layer Twistronic Graphenes

Abstract

Diverse emergent correlated electron phenomena have been observed in twisted-graphene layers. Many electronic structure predictions have been reported exploring this new field, but with few momentum-resolved electronic structure measurements to test them. We use angle-resolved photoemission spectroscopy to study the twist-dependent (1° < θ < 8°) band structure of twisted-bilayer, monolayer-on-bilayer, and double-bilayer graphene (tDBG). Direct comparison is made between experiment and theory, using a hybrid k·p model for interlayer coupling. Quantitative agreement is found across twist angles, stacking geometries, and back-gate voltages, validating the models and revealing field-induced gaps in twisted graphenes. However, for tDBG at θ = 1.5 ± 0.2°, close to the magic angle θ = 1.3°, a flat band is found near the Fermi level with measured bandwidth E w = 31 ± 5 meV. An analysis of the gap between the flat band and the next valence band shows deviations between experiment (Δh = 46 ± 5 meV) and theory (Δh = 5 meV), indicative of lattice relaxation in this regime.

Correlation-Induced Symmetry-Broken States in Large-Angle Twisted Bilayer Graphene on MoS2

“…Second, the width W , defined as the full width at the half-maximum (fwhm) of the VHS LDOS peak, is measured to be 111.8 ± 3.1 meV for VHS 1 and 122.0 ± 2.8 meV for VHS 2 . These two observations are consistent with that previously reported in TBGs with θ ∼ 3 ° which are treated to be in a weakly correlated regime and described by single particle model. ,,, With the measured t θ ∼ 212 meV, we calculate the band structure and LDOS of 3.45 ° TBG using the single-particle model in Figures (d), (e) (see Methods). The calculated energy location and fwhm of the two VHSs and two remote bands reproduce well our experimental results in Figure (c).…”

Revealing flat bands and hybridization gaps in a twisted bilayer graphene device with microARPES

“…To further substantiate the previous analysis, we proceed to search for the optimum photon energy where the hybridization effects are most visible. Such a dependence on photon energy is expected due to interference effects involving photoelectrons from the moiré sites, in addition to the two sublattice sites in each graphene layer [19,32]. By performing a scan of hν from 40 to 76 eV we are able to determine substantial redistributions of intensity between the manifold of bands in tBLG near the magic angle, as shown in figure 2.…”

“…Note that our data does not enable us to preclude the presence of faint in-gap states. Furthermore, the electric field leads to different charge carrier concentrations in the top and bottom graphene layers (see [28,32,33] and section 4), which results in an increasingly asymmetric overlap of the top and bottom Dirac cones with voltage and thereby substantially affect the hybridization energies and possible flat band dispersion [1]. These details are difficult to resolve due to the broadening of our spectra.…”

“…Secondly, in situ control of charge carrier concentration is most elegantly achieved by integrating the twisted 2D material in a functional device architecture that permits electrostatic gating. Such in operando micro-and nanoARPES experiments have recently been demonstrated, but they remain highly challenging as the additional electrostatic field may impose detrimental energy-and momentumbroadening [25][26][27][28][29][30][31][32][33]. Moreover, the induced photocurrent from the probing synchrotron light appears to strongly affect the device components, including the hexagonal boron nitride (hBN) dielectric that is customarily used, causing gate leakage currents and degradation of device performance after prolonged exposure [34].…”

“…Moreover, the induced photocurrent from the probing synchrotron light appears to strongly affect the device components, including the hexagonal boron nitride (hBN) dielectric that is customarily used, causing gate leakage currents and degradation of device performance after prolonged exposure [34]. So far, back gated ARPES measurements have been reported for single-layer graphene [26,27,30], Bernal-stacked bilayer graphene [25], tBLG with a twist angle of 12.2 • [28], tBLG with a twist angle around 3 • [33] and twisted monolayeron-bilayer graphene with a twist angle of 3.4 • [32], but there are no such gated ARPES measurements reported for magic angle tBLG.…”