Session G51: Graphene: Electronic Structure and Interactions III; Moire, Correlations, & Topology

Electronic Collective Modes in Twisted Bilayer Graphene

Twisted Bilayer Graphene (TBLG) is a structure composed of two graphene sheets stacked with a relative twist angle between them. Cao et al. [Nature 556, 43 (2018)] reported the discovery of a superconducting state in TBLG at a "magic angle" of about 1.05°. There is no consensus yet on the mechanism that causes this state.
We study electronic collective modes in bilayer graphene at various twist angles with a particular emphasis on low-energy plasmons. The calculation is performed for two band structures: the tight-binding model that displays a long-range periodic Moiré pattern at commensurate twist angles, and the continuum model that allows for the calculation of collective modes at and near the magic angle. We discuss the implications of the electronic collective modes spectrum for superconductivity in twisted bilayer graphene.   

Charge Smoothening and Band Flattening due to Hartree corrections in Twisted Bilayer Graphene

Doping twisted bilayer graphene away from charge neutrality leads to an enormous buildup of charge inhomogeneities within each Moiré unit cell. Here we show, using unbiased real-space self-consistent Hartree calculations on a relaxed lattice, that Coulomb interactions smoothen this charge imbalance by changing the occupation of earlier identified ‘ring’ orbitals in the AB/BA region and ‘center’ orbitals at the AA region. For hole doping, this implies an increase of the energy of the states at the Γ point, leading to a further flattening of the flat bands and a pinning of the Van Hove singularity at the Fermi level. The charge smoothening will affect the subtle competition between different possible correlated phases.   

Electrostatic Tuning of the Coulomb Interaction in Single Layer Graphene

The Landau-Fermi Liquid Theory maps an interacting liquid of electrons to a non-interacting gas of quasiparticles. This picture breaks down in Graphene, because the bare Coulomb interaction is preserved near the charge neutrality point. In this talk, I will discuss recent in-operando angle-resolved photoemission studies on single layer graphene where we directly visualize modifications of its electronic band structure upon tuning the Fermi Energy.   

Streamlining twisted bilayer graphene measurements

Twisted bilayer graphene (TBG) has emerged as a promising platform for studying strongly correlated physics. When the twist angle is near the magic angle of approximately one degree, transport measurements have revealed a host of interesting phenomena including correlated insulating states, superconductivity, and ferromagnetism. However, fabricating and effectively measuring TBG samples remains challenging. The twist angle varies widely from sample to sample, and even from contact to contact within the same sample. In this work, I present a few tools we have used in order to streamline the process of making and measuring TBG samples, along with some interesting results encountered along the way.   

Fabrication and Scanning Tunneling Spectroscopy of Magic Angle Twisted Bilayer Graphene

Twisted bilayer graphene has recently gained significant attention due to the discovery of superconductivity and correlated insulating behavior near the magic angle of 1.1 degrees. Other twisted 2D van der Waals heterostructures are expected to host similar phenomena. Thus, methods for consistent and precise fabrication of twisted 2D materials are essential to enable their further study. We discuss the dry transfer technique that enables high accuracy rotational alignment that we have used to fabricate magic angle twisted bilayer graphene devices for scanning probe microscopy measurements. Using low temperature scanning tunneling microscopy and spectroscopy, we have verified the localization properties of the flat bands and directly probed the correlated insulating states at commensurate filings. These results can lead to a better understanding of the local electronic properties of magic angle twisted bilayer graphene.   

Theory of Current-induced Magnetization Switching in Twisted Bilayer Graphene

Recently, signatures of quantum anomalous Hall states with spontaneous ferromagnetism were observed in twisted bilayer graphenes (TBGs) near 3/4 filling [Science 365, 605-608 (2019), arXiv: 1907. 00261]. Importantly, it was demonstrated that an extremely small current can switch the direction of the magnetization. This opens the prospect of realizing low energy dissipation magnetic memories. However, the mechanism of the current-driven magnetization switching is poorly understood as the charge currents in graphene layers are generally believed to be non-magnetic. In this work, we demonstrate that, in TBGs, the twist-induced reduction of lattice symmetry allows a charge current to generate net orbital magnetization at a general filling factor through magnetoelectric effects. Substrate-induced strain and sublattice symmetry breaking further reduce the symmetry such that an out-of-plane orbital magnetization can be generated. Due to the large non-trivial Berry phase of the bands, the orbital magnetization of a Bloch state can be as large as tens of Bohr magnetons and therefore a small current would be sufficient to generate a large orbital magnetization. We further demonstrate how the charge current can switch the magnetization of TBGs as observed in experiments.   

Berry Curvature Dipole in Strained Graphene: a Fermi Surface Warping Effect

It has been recently established that optoelectronic and non-linear transport experiments can give direct access to the dipole moment of the Berry curvature in non-magnetic and non-centrosymmetric materials. Thus far, non-vanishing Berry curvature dipoles have been shown to exist in materials with substantial spin-orbit coupling where low-energy Dirac quasiparticles form tilted cones. Here, we prove that this topological effect does emerge in two-dimensional Dirac materials even in the complete absence of spin-orbit coupling. In these systems, it is the warping of the Fermi surface that triggers sizeable Berry dipoles. We show indeed that uniaxially strained monolayer and bilayer graphene, with substrate-induced and gate-induced band gaps respectively, are characterized by Berry curvature dipoles comparable in strength to those observed in monolayer and bilayer transition metal dichalcogenides.

References:

R. Battilomo, N. Scopigno and C. Ortix, arXiv:1910.09872 [cond-mat.mes-hall] (to appear in Physical Review Letters)
  

Electron-phonon interactions in twisted bilayer graphene

Interlayer electron-electron (el-el) and electron-phonon (el-ph) interactions emerge from the coupling of atomic layers in 2D heterostructures and are essential for describing their physical properties. The additional possibility of controlling the twisting angle between layers opens new possibilities for tunable devices. Raman spectroscopy is a fundamental tool to investigate el-ph interactions and the possibility of using multiple-excitation spectroscopy allows the distinction between intralayer and interlayer el-ph interactions. We will discuss a resonance Raman study of twisted bilayer graphene (TBG) samples with different twisting angles and measured using different laser lines. Results show that the extra Raman peaks that appear in the spectra come from different physical mechanisms: the intralayer process, where the el-ph scattering occurs in a single graphene layer and the other layer only imposes a periodic potential that scatters the excited electron, and the interlayer el-ph process, where the scattering occurs between states in the Dirac cones of adjacent graphene layers.
  

Atomistic Caculation of Electron-Phonon Coupling in Twisted Graphene Layers

We investigate electron-phonon coupling in twisted graphene layers based on atomistic calculations of electronic structure, phonon dispersion, and electron-phonon matrix elements. First, we calculate twisted double bilayer graphene (TDBG). Then, we analyze the characteristics of important phonon modes that have strong contributions to the total coupling strength. Also, we compare the electron-phonon coupling in TDBG and magic-angle twisted bilayer graphene in the context of the recent experimental observations of superconductivity in these systems. Our work provides microscopic understanding of the electron-phonon coupling in twisted graphene layers, which is a basis for explaining superconducting mechanism of these systems.   

Observation of Substrate Induced Multiple Dirac Cones in Graphene/ SiC Heterostructure

The coupling between epitaxial graphene layers and various substrates give rise to many interesting phenomena such as the energy gap at the Dirac node and double Dirac cones. Electronic structure of graphene is modulated by the substrate effects. In this work, we measured the electronic structure of bilayer graphene/SiC heterostructure. Apart from the duplicated Dirac cones arising from the interaction with the interfacial reconstruction, 24 additional Dirac cones are clearly observed by angle-resolved photoemission spectroscopy (ARPES) within the 1^stBrillouin zone of graphene. These 24 Dirac cones are directly generated from the coupling between graphene layers and the supporting SiC lattice, a result consistent with our tight-binding simulations. This work paves the way for understanding the direct electronic coupling between graphene and SiC substrates.
  

Localized doping through lithium intercalation in twisted bilayer graphene

Lithium atoms intercalating between layers of graphene show an energetic preference for regions of AA stacking. Density functional theory calculations employing commensurate supercells with twist angles of 7.3° and 2.5° are used to study the energetics, atomic relaxation, and electronic structure of Li intercalation in twisted graphene bilayers, with results extrapolated to the magic angle.   

Scanning Tunneling Microscopy Studies of Electronic Correlations in Magic Angle Twisted Bilayer Graphene

Discovery of superconductivity and correlated insulating states in the magic angle(~1.1deg) twisted bilayer graphene sparked the search for other broken symmetry states in this system. Previously we used scanning tunneling microscopy (STM), to investigate interaction effects at charge neutrality, spectroscopic gaps originating from insulating states at the half and quarter filling, and signatures of spatial ordering [1]. In this talk, we will present our latest STM results and discuss spatial spectroscopic maps revealing new insights into this system.
[1] Y. Choi et al. Nature Physics (2019)   

Hofstadter butterfly and quantum Hall effect in twisted double bilayer graphene

In this talk, I will discuss the energy spectrum and quantum Hall effect in twisted double bilayer graphene (TDBG, a pair of Bernal-stacked bilayer graphenes stacked with a rotational stacking fault). The electronic band structures of TDBG with AB-AB configuration and AB-BA configuration look similar in the absence of magnetic field, but their topological nature are different [1]. Therefore, the twins of AB-AB and AB-BA TDBG exhibit very different spectrum in magnetic field, by directly reflecting the difference in Chern numbers of the two pair structures [2]. For example, the first gap in the electron side of AB-AB TDBG remains opened in increasing magnetic fields, while that of AB-BA TDBG vanishes at some points due to the non-zero valley Chern number in AB-BA TDBG.
[1] M. Koshino, Phys. Rev. B 99, 235406 (2019).
[2] J. A. Crosse*, M. Koshino, P. Moon^†, in preparation.