Session R51: Graphene: bilayers, imaging transport and electronic properties, adatoms

Spin-momentum Locking in an Array of Defect Lines in Gated Bilayer Graphene

Graphene can have defect lines with pentagons and octagons (8-55) that induce localized states [1]. This domain wall in gated bilayer graphene produces a change between AB to BA stacking and presents topological states in the gap [2-4]. In this work using density functional theory calculations, we investigate an array of these defect lines in bilayer graphene. We found that the band structure shows a magnetic phase in which the spin is locked to the momentum, as in topological insulators. We also follow the topological states that appear even without a gate because of the array of defect lines. We lastly study the differences in spin bands and identified topological states when engineering by doping and/or electric field. All these results are summing to the new interesting data of the correlated behavior of electrons with the stacking in two-dimensional materials [5].

[1] Pelc, M., Chico, L., Ayuela, A., & Jaskólski, W. Phys Rev B, 87(16), 165427 (2013).
[2] Pelc, M., Jaskólski, W., Ayuela, A., & Chico, L. Phys Rev B, 92(8), 085433 (2015).
[3] Jaskólski, W., Pelc, M., Chico, L., & Ayuela, A. Nanoscale, 8(11), 6079-6084 (2016).
[4] Jaskólski, W., Pelc, M., Bryant, G. W., Chico, L., & Ayuela, A. 2D Materials, 5(2), 025006 (2018).
[5] Cao, Y. et al Nature, 556(7699), 43(2018).   

Effect of correlation on nearly flat bands in twisted bilayer graphene

Recently, correlated insulating phases were observed experimentally in twisted bilayer graphene (TBG) at different doping concentrations. To study possible mechanisms for insulating phases, we investigate electronic and magnetic properties in TBG by performing dynamical mean-field theory calculations based on a minimal tight-binding Hamiltonian. We study variation in the spectral function of the nearly flat bands for a range of local interaction parameters, electron filling, and temperature. We compare our results with experimental reports and discuss the possibility of Mott insulating phase in this system.   

Visualization of the flat electronic band in twisted bilayer graphene near the magic angle twist

Bilayer graphene was theorized to host a moiré miniband with flat dispersion if the layers are stacked at specific twist angles known as the “magic angles”. Recently, such twisted bilayer gra-phene (tBLG) with the first magic angle twist was reported to exhibit correlated insulating state and superconductivity, where the presence of the flat miniband in the system is thought to be es-sential for the emergence of these ordered phases in the transport measurements. Tunneling spectroscopy and electronic compressibility measurements in tBLG have revealed a van Hove sin-gularity that is consistent with the presence of the flat miniband. However, a direct observation of the flat dispersion in the momentum-space of such moiré miniband in tBLG is still elusive. Here, we report the visualization of the flat moiré miniband by using angle-resolved photoemission spectros-copy with nanoscale resolution (nanoARPES). The high spatial resolution in nanoARPES enabled the measurement of the local electronic structure of the tBLG. We clearly demonstrate the exist-ence of the flat moiré band near the charge neutrality for tBLG close to the magic angle at room temperature.   

Low Temperature Insulating State in hBN-Encapsulated Multilayer Graphene

Bilayer graphene is susceptible to strong electron-electron interactions at charge neutrality, leading to the possibility for spontaneous symmetry breaking and the associated opening of a gap at low temperatures, even in the absence of any external fields [1]. Recent experiments confirm this possibility for Bernal stacked graphene layers of thickness up to 8 layers [2-4], but this physics is only observed via transport in suspended devices of the highest quality. Here, we present evidence for the opening of a gap at low temperatures in multilayer graphene systems which are encapsulated in hBN. The dielectric environment of hBN not only alleviates the requirement for suspension, but also poses constraints on the nature of the interactions. Our results provide new insights into the insulating state of multilayer graphene systems, and help to facilitate their fabrication for potential applications.

[1] Y. Barlas, R. Côté, K. Nomura, A. H. MacDonald, Phys. Rev. Lett. 101, 097601 (2008).
[2] R. T. Weitz, M. T. Allen, B. E. Feldman, J. Martin, A. Yacoby, Science 330, 812–816 (2010).
[3] Y. Nam, D.-K. Ki, M. Koshino, E. McCann, A. F. Morpurgo, 2D Mater. 3, 045014 (2016).
[4] Youngwoo Nam, Dong-Keun Ki, David Soler-Delgado, Alberto F. Morpurgo, Science 362, 324-328 (2018).   

Signature of Viscous Electron Flow in Graphene Using a Scanning Probe Microscope

Graphene is a single layer of carbon atoms held together in a hexagonal crystalline structure. Graphene has shown great promise in electronics and photonics because of its two-dimensional properties. These properties reduce the scattering of electrons that are seen in metals, and thus graphene has the ability to be much more conductive than commonly used conductors such as copper. In graphene, at certain range of temperature and electron density, electrons begin interacting with each other in such a way that they start acting together as a viscous fluid. The purpose of this paper is to model the viscous flow of electrons in graphene and analyze the fluid behavior. The model of an incompressible fluid using the Navier-Stokes equations will be created to examine this behavior. The goal is to construct the best geometry and determine suitable boundary conditions for the walls and circular obstacle. The walls act as the edge of the graphene strip, and the circular obstacle acts as the tip perturbation of a scanning probe microscope. The objective is to obtain a suitable geometry showing signature of viscous electron flow.   

Engineering of the electronic properties of an epitaxial graphene on an SiC(0001) substrate by manganese adatoms

The application of graphene in electronic devices requires an energy band gap with its remarkable properties. By using angle-resolved photoemission spectroscopy, we show that it is possible to decouple the graphene from substrate by manganese (Mn) intercalation concomitant with energy band gap opening. Upon introducing Mn adatoms, we observe a charge neutrality of the graphene band with energy band gap at the Dirac energy up to ~ 0.4 eV. In addition, the Mn-intercalated graphene shows fully screened energy-momentum dispersion as Mn 3d states play a role of a metallic background on graphene. These observations provides a viable route towards the engineering of the electronic properties of graphene.
  

Lévy flights and Hydrodynamic Superdiffusion on the Dirac Cone of Graphene

It is shown that hydrodynamic collision processes in graphene at the neutrality point can be described in terms of a Fokker-Planck equation with a fractional derivative. This is a consequence of the fact that the phase space dynamics of electrons is governed by Lévy flights: rare large-angle scattering events interrupting the small-angle scattering. Lévy flights give rise to superdiffusive dynamics of collective excitations. Implications for transport and relaxation processes will be discussed.   

Scanning tunneling microscopy and electronic transport of gold-decorated graphene devices

Past studies on graphene show that scattering from disorder can give rise to weak Anderson localization, but a disorder-driven quantum metal-insulator transition has not been observed. Using a low temperature scanning tunneling microscope (STM) on graphene field effect transistors, we present simultaneous STM and electronic transport measurements of graphene devices decorated with gold adatoms. These data show quasiparticle scattering off of gold adatoms on graphene on SiO2/Si. We study the microscopic scattering mechanisms, including the effects of charge puddles versus point-like disorder, and correlate these mechanisms with macroscopic electronic transport. These experiments are the first steps toward understanding the phase space near disorder-tuned metal-insulator quantum phase transitions in 2D materials.   

Atomic Engineering of Monolayer Graphene: Inducing Kekulé Bond Order by Adatom Deposition

The Kekulé distortion (KD) periodically modifies the carbon-carbon bonds in graphene, resulting in a √3×√3 R30° superstructure and symmetry breaking between three previously equivalent hexagonal plaquettes. Previous scanning tunnelling microscopy (STM) experiments have shown that such a distortion induced by vacancies in the graphene substrate can produce charge density modulations corresponding to the superstructure [1]. Here, we induce a KD phase in monolayer graphene by low-flux deposition of a small number of lithium adatoms. Using angle-resolved photoemission spectroscopy (ARPES), we observe backfolding of the Dirac cones to Γ, as well as a gap opening (2Δ = 0.22 ± 0.02 eV) at the Dirac point. Low-energy electron diffraction (LEED) measurements also show the appearance of peaks corresponding to the new periodicity. Finally, we discuss the real-space behaviour of the adatoms by comparing STM data with predictions by a Monte Carlo toy model.

References

[1] C. Gutiérrez, Nature Physics 12, 950–958 (2016).   

Scanning Tunneling Spectroscopy of Coupled Graphene Quantum Dots

In backgated graphene devices with hexagonal boron nitride (hBN) substrates, impurities in the hBN can be locally charged by applying a voltage pulse to the tip of a scanning tunneling microscope. These charges create a writeable, tunable potential landscape for the graphene charge carriers, which we use to create isolated or coupled quantum dots in graphene. Under a strong perpendicular magnetic field, the free carriers condense into Landau levels (LLs). In a single dot, the LLs form a concentric pattern of alternating compressible and incompressible strips at the Fermi energy. The incompressible strips act as tunnel barriers creating a confined, concentric quantum dot system. When two or more such dots are placed next to each other, their charges interact. Addition of carriers to the dots manifests itself as peaks in the tunneling dI/dV, which can be tracked through the space of tip position, sample bias, and backgate voltage. as. Here, we explore the interaction between the dots revealed in the avoided crossings of their two sets of peaks to examine an intricate interplay of Coloumb interaction and tunnel coupling in this novel confined electron system.   

Frictional drag between graphene and LaAlO3/SrTiO3 heterostructures

Vertical stacking of heterostructures that combine layered materials offer new ways of combining interesting properties of dissimilar electronic materials. Over the past few years we have been integrating graphene with complex-oxide heterostructures, specifically, the LaAlO₃/SrTiO₃ system. Furthermore, conducting nanostructures can be written under graphene, producing interesting interactions between the two systems. Here we report Coulomb drag measurements between single-layer graphene and a conductive LaAlO₃/SrTiO₃ interface. Fabry-Perot oscillations are observed in both graphene and drag signal in graphene, which indicate the local doping in graphene by conductive atomic force microscope(c-AFM) lithography. While the drag resistance is greatly enhanced in SrTiO₃ in the superconducting region, the drag in graphene remain unchanged. We also observed key differences between longitudinal drag and hall drag in both graphene and SrTiO₃ .   

Visualizing broken symmetry states in the zeroth Landau level of the graphene quantum Hall system

In graphene, the combination of electron spin and two equivalent sublattice atoms results in a four-fold degeneracy of Landau levels. A number of competing quantum Hall isospin states inside the four-fold degenerate zeroth landau level of graphene have been suggested based on macroscopic transport measurements. Here we observe the broken symmetry states of the zeroth Landau level in graphene with Kelvin Probe Force Microscopy (KPFM) by using a millikelvin scanning probe microscope coupled with qPlus sensor. The field dependence of the energy difference between isospin states is suggestive of transitions between isospin configurations at certain magnetic field values. In addition, we show how the quantum Hall edge channels of zeroth Landau level spatially develop and disperse at the edge of Hall bar.   

Imaging ballistic and hydrodynamic electron flow in the Corbino geometry

Hydrodynamic flow is predicted to exhibit unique voltage patterns which are distinct from both the Ohmic and ballistic regimes. A flow geometry to best highlight these patterns is the Corbino disc, in which the current is driven from a circular central contact into an outer ring. In this geometry there are no walls, and so the electrons can only transfer momentum to either impurities or to other electrons. In the hydrodynamic regime in which the flow is dominated by such electron-electron scattering, momentum is conserved, leading to an expulsion of the electric field across the bulk of the disc and hence a constant electrostatic potential. In this situation, the device resistance may even be lower than in the fully ballistic regime, where the potential drops inversely with the radius. We present our initial results on imaging electron flow in high-mobility graphene Corbino discs using a nanotube single electron transistor. We image the voltage drop of the flowing electrons from the inner to outer contact, with the aim of distinguishing these unique spatial signatures of ballistic and hydrodyanmic flow.   

Correlation-Driven Kekule’ Dimerization and Semimetal-Insulator Transition in Strongly Isotropically Strained Graphene

Freestanding graphene will spontaneously distort and become insulating under a large isotropic tensile strain. We calculate the ground state enthalpy (not just energy) of strongly strained graphene by an off-lattice quantum Monte Carlo correlated approach of great accuracy and variational flexibility, removing the limitations of earlier density-functional or rigid lattice approaches. Beginning with undistorted semimetallic graphene at low strain, we find [1] that multideterminant Heitler-London correlations stabilize between 8.5% and 15% tensile strain an insulating Kekule' dimerized state. Closer to a crystallized resonating-valence bond than to a Peierls state, Kekule’ dimerization of graphene prevails over the competing antiferromagnetic insulating state favored by density-functional calculations which we conduct in parallel. The insulator gap grows from zero at onset to over 1 eV before mechanical failure near 15% strain, and is topological in nature, implying under certain conditions 1D metallic interface states lying in the bulk energy gap. [1] S. Sorella, et al., PRL 121, 066402 (2018).   

Enhancement of Ultrafast Photoluminescence from Deformed Graphene

We studied the correlation between the ultrafast photoluminescence and the morphology of graphene, and observed enhancement of the ultrafast photoluminescence from the deformed graphene. In comparison to the planar graphene, the enhancement factor of ultrafast photoluminescence could be up to several times at the highly curved region. We found that the intensity of photoluminescence from the uniaxially rippled graphene depends on the polarization of excitation light. Furthermore, Raman spectroscopy was used to measure the strain distribution. Pump-probe measurements were conducted to reveal the carrier dynamics. From the experimental results, two mechanisms were confirmed to mainly account for the enhancement of ultrafast photoluminescence from the deformed graphene. One is the deformation-induced strain increases the absorption of graphene. The other is the prolonged carrier relaxation time in the curved graphene.