Session S12: Adatom and Proximity Driven Electronic Interactions in Graphene

Strained-induced Landau levels in momentum space

It is well known that certain strain patterns in graphene couple to the low-energy Dirac fermions as pseudo-magnetic fields -- i.e. magnetic fields that preserve time-reversal symmetry. This can lead to the formation of quantized Landau levels in the absence of magnetic fields, as first predicted by [Guinea et al., Nat. Phys. 6, 30 (2010)] and subsequently identified in local STM measurements [Levy et al., Science 329, 544 (2010)]. Here we extend this body of work by considering momentum-resolved signatures of large-scale, strain-induced pseudo-magnetic fields in graphene. Our theoretical modeling supports the first observation of Landau levels in ARPES. The corresponding pseudo-magnetic field of 41 T is attributed to strain generated by shallow triangular nanoprisms on our SiC substrates, over which graphene grows without grain boundaries, as revealed by AFM and STM measurements. Our work demonstrates the feasibility of generating strain-induced quantum phases in 2D Dirac materials on a wafer-scale size.   

Emergence of Kondo Resonance in Graphene Intercalated with Cerium

The interaction of a magnetic impurity with surrounding electrons has been a core problem in modern physics to understand fundamentals of many-body effects and its relation to magnetism. In particular, antiferromagnetic screening of the local magnetic moment by conduction electrons leads to the formation of a new resonant-type many-body ground state, so-called Kondo resonance. Here we report the realization of the Kondo resonance in a prototypical two-dimensional system, graphene, induced by the presence of cerium with the localized spin of a 4felectron. The combination of two complementary techniques, angle-resolved photoemission spectroscopy and dynamic mean-field theory, reveals the development of new spectral weight near Fermi energy at lower temperature that is hybridized with the graphene π-band. The observed T-dependence provides not only a direct evidence of the formation of the many-body ground state in graphene, but also novel insight how Kondo physics emerges in the sea of two-dimensional Dirac electrons.
  

Gate Modulated Interaction Between Quantum Dots in a Graphene/hBN Heterostructure

Understanding interactions between atoms is fundamental in condensed matter systems. In heterostructures of graphene and hexagonal boron nitride (hBN) we are able to create taylormade quantum dots in a background of Dirac fermions. Recent studies have explored the nature of single quantum dots and how their behavior is influenced by the application of a perpendicular magnetic field. However, no efforts have been made towards understanding the interaction between a pair of Dirac fermion quantum dots. We will discuss and present spatially-resolved scanning tunneling spectroscopy studies performed on a pair of quantum dots. We will also show how their behaviour is influenced by changes in their shape, size, and separation attained through the modulation of an electrostatic backgate.   

Proximity-induced Ising spin orbit coupling in bilayer graphene/WSe2 van der Waals heterostructures

Interlayer proximity effects at van der Waals interfaces provide a tool for engineering electronic phases that do not occur in isolated two-dimensional materials. A longstanding goal is proximity induced spin orbit coupling (SOC) in graphene. While previous experimental efforts have reported signatures of SOC in graphene-transition metal dichalcogenide (TMD) heterostructures, these studies have primarily relied on indirect charge and spin transport measurements preventing definitive resolution of the strength, nature, or origin of the induced SOC. Here we report on the direct observation of proximity induced SOC in bilayer graphene in contact with tungsten selenide (WSe2). Using spin- and layer-resolved broken symmetry Landau levels as a probe of layer specific proximity-induced SOC, we observe rearrangement of these levels consistent with an Ising (valley-Zeeman) type SOC with a strength of ≈2 meV. Our observations pave the way toward new engineered topological phases in van der Waals heterostructures.   

Strain engineering of a pseudo gauge field superlattice in a suspended graphene nanoribbon studied by scanning probe microscopy

Engineering exotic electronic states in novel materials, which are unrealizable in traditional systems, has been the focus of intense research over the past decade. Unlike present electronics, which rely on controlling the flow of charges primarily by electric fields, strain, for example, can control electrons by generating pseudo gauge fields in 2D materials. In this talk, we report strain engineering of pseudo electric and magnetic superlattices in suspended graphene nanoribbons by extreme (>10%) strain and study the local electronic density of states by scanning tunneling microscopy. These measurements reveal Landau levels in the presence of highly non-uniform pseudo-magnetic fields. DFT and tight binding calculations imply the existence of counterpropagating snake states along the edges of the nanoribbons reminiscent of topological materials.   

Charge density wave induced proximity spin-orbit coupling effects in graphene on 1T-TaS2

Graphene on transition-metal dichalcogenides (TMDCs) exhibits proximity spin-orbit effects opening new venues for optospintronics [1], and provides route for exploring robust helical edge states [2, 3]. 1T-TaS₂ is a layered TMDC showing metal-insulator transition and the sequence of different charge density wave (CDW) transformations [4]. We present our first-principles results for the electronic band structures of graphene on 1T-TaS₂ in normal state and for periodic lattice distorted David star pattern providing commensurate CDW phase. We discuss the orbital and spin-orbital proximity effects with phenomenological symmetry-based Hamiltonian that we use to fit the first-principles data. The extracted spin-orbit coupling parameters are of the order of meV. A fascinating finding is that induced proximity effects in graphene on 1T-TaS₂ are significantly influenced by the presence of the commensurate CDW in 1T-TaS₂.

[1] M. Gmitra, J. Fabian, Phys. Rev. B 92, 155403 (2015).
[2] M. Gmitra, D. Kochan, P. H\"{o}gl, J. Fabian, Phys. Rev. B 93, 155104 (2016).
[3] T. Frank et al., Phys. Rev. Lett. 120, 156402 (2018).
[4] I. Lutsyk et al., accepted for publication in Phys. Rev. B.   

Synthetic topological phases in bilayer graphene via van der Waals proximity induced spin orbit coupling

Spin orbit coupling (SOC) forms the basis of time reversal invariant topological insulators. While graphene provided the first theoretical model for a topological insulator, the small intrinsic SOC in carbon make such phases unobservable. Here we engineer a synthetic topological insulator-like phase in bilayer graphene via proximity induced SOC from WSe2 in a van der Waals heterostructure. At zero magnetic field, we observe a new incompressible phase at charge neutrality due to proximity induced Ising-type SOC, having equal and opposite magnitude on each layer. Transport measurements show enhanced conductance within this new phase, which is rapidly suppressed by an in-plane magnetic field, consistent with modeling that suggests the new phase hosts spin filtered edge states and has a nearly quantized spin Hall conductance. Interestingly, spin is approximately conserved in this system despite large Rashba spin orbit coupling, whose effects are strongly suppressed in the spin-filtered edge states. Remarkably, electric field can be used to tune the system between the quantum spin Hall phase and a trivial insulator, permitting reconfigurable topological edge state circuits.   

Topological phases in intercalated epitaxial graphene

Intercalation is one of the effective methods to functionally manipulate the electronic structure of epitaxial graphene. Here, using first-principles density functional theory calculations, we design 2D topological insulators in epitaxial graphene via intercalation. We find that the electronic band structure near the Fermi energy shows Dirac-cone type or quadratic band dispersion depending on the type of intercalants, and topologically nontrivial band gap opens due to the considerable strength of the spin-orbit coupling. Our results indicate that the intercalation is a promising way to realize topological phases in graphene, and could be important in future nano device applications of graphene.   

Effect of High gate-voltage application on the molecular adsorption on Graphene

The electronic properties of graphene are well tuned by guest chemical species due to its 2D nature. We have clarified that the charge transfer between molecules and graphene proceeds by a electrochemical process based on the fact that the kinetics of the charge transfer decreases with down and up shift of the Fermi energy of graphene for electrons and holes, respectively. In this study, the electrochemical potential is widely varied by graphene FET in order to induce various electro chemical reactions for the charge transfer phenomena. In the case of the hole doping by oxygen adsorption on graphene, the lower gate-voltage, the slower hole doping reactions as we reported [1] and finally no charge doping occurs at enough higher gate-voltage around -80 V. Interestingly, further lowering of the gate-voltage results in the electron doping by the oxygen adsorption on graphene. Even in this condition, the doping kinetics are accelerated by the presence of water molecule. This suggests that different electrochemical reactions are responsible for the charge transfer by oxygen adsorption on graphene between the higher gate-voltage region and lower voltage region.
[1] Y. Sato, et al Nano Lett., 11 (2011) 3468   

Graphene-Bi (111) interface: atomic structure and electronic properties

Interfacial interactions are widely used to engineer desired electronic properties. In this context, the interface between graphene and nanostructured bismuth is especially interesting since both the materials
exhibit unique properties associated respectively with the Dirac cones and quantum spin-Hall states along the edges and hinges. In this work, using first-principles calculations, we study the interfacial interaction between graphene and Bi (111) surfaces. Since the actual structure of graphene on Bi (111) can be incommensurate, we investigated a large number of crystal approximants to the incommensurate interface and found the most stable one. Despite hybridization between graphene and substrate orbitals, the graphene-derived Dirac cones are preserved, although shifted with respect to the Fermi level due to the n-doping of graphene. We also find that a small gap opens up at the original Dirac points in these cones due to interface–induced spin–orbit coupling within graphene. At the same time, the bands near the gamma point are mostly Bi-derived and resemble the corresponding surface states in pure Bi. Using the obtained results, we discuss possible applications of graphene/Bi interfaces in future electronic devices.   

Comparative Mapping of the Local Potential of Graphene/hBN Systems Using Scanned Probe Techniques

Mapping the local electrostatic potential of 2D materials is an important step in understanding how defects and strain affect local electrostatic structure. Potential mapping can also reveal the strength of el-el interactions in a material, and – when performed under transport conditions – can directly probe the nature of electronic flow. Specifically, probing the local potential gradient of a current-driven graphene sheet could provide insight into how electronic transport shifts between ballistic and diffusive regimes. We present preliminary results comparing three scanned probe methods for mapping electrostatic potential of a graphene sheet: Kelvin probe force microscopy (KPFM), scanning tunneling potentiometry (STP), and scanning tunneling spectroscopy (STS). We discuss the comparative advantages of each technique, describing how each method performs when measuring a graphene/hBN system with current applied in situ.   

Concomitant enhancement of electron-phonon coupling and electron-electron interaction in graphene decorated with ytterbium

The presence of foreign atoms induces exotic many-body physics in a condensed matter system. Especially, a two-dimensional system is very sensitive to such an external perturbation to lead metal-insulator transition, Kondo effect, strongly enhanced electron-phonon coupling, etc. Here we report that the presence of Yb results in the enhancement of not only electron-phonon coupling, but also electron-electron interaction in graphene. When the band structure of graphene with Yb exhibits the characteristics of electron-phonon coupling that is not clearly observed in as-grown graphene, the slope of the energy-momentum dispersion was also found to decrease by the presence of Yb using angle-resolved photoemission spectroscopy. Within the Fermi liquid theory, the decreased slope indicates enhanced electron-electron interaction, suggesting possibility interplay between electron-phonon coupling and electron-electron interaction in graphene decorated with foreign atoms.   

Nanoscale distribution of strains and defects in graphene transferred onto polymethylmethacrylate

Graphene has attracted a wide interest over the last decade and deeper understanding of formation of defects and strains controlling its properties is one of the major goals. One of the commonly used methods for transferring graphene on a substrate of interest utilizes polymethylmethacrylate (PMMA) as a temporary support. While several studies demonstrated that the length scale of strain can be as small as a few nm for graphene on SiO₂ substrates [1], the strain coherence length and distribution of defects on a nanoscale for graphene laying on PMMA remains poorly understood. Here we present tip-enhanced Raman Spectroscopy (TERS) study of single layer graphene on top of PMMA. Our results provide a clear experimental evidence that the locations of defects correlate with topological features of PMMA, which on the other hand replicates topological structure of copper foil. We found that the strain is anisotropically distributed with a length scale varies from tens hundred nm.
1. Teague, M. L. et al. Nano Lett. 2009, 9, 2542   

Deposition of single-molecule magnets on graphene quantum dots.

Gapless graphene is a single-atom-thick material that yields ultra-broadband photodetectors for applications that require high sensitivity. These applications include spectroscopy of single molecule magnets (SMMs), metal-ion complexes that exhibit quantum behavior at low temperatures and have great potential as components for quantum computing and molecular spintronics. By grafting small amounts of SMMs directly to the surface of a graphene quantum dot, we can utilize the high sensitivity of a graphene hot-electron bolometer to perform spectroscopy on the SMMs. We have investigated various methods of grafting transition-metal-based SMMs to the graphene and characterizing the deposited molecules.