Session W63: STM on twisted 2D heterostructures

Spectroscopic features of the parent phase of magic-angle twisted bilayer graphene

Recent experiments on magic-angle twisted bilayer graphene (MATBG) have shown various intriguing low-temperature electronic phases induced by filling of the moiré flat bands [1][2]. These phases, including correlated insulator and superconducting phases, are derived from a high-temperature “parent phase,” which is currently not well understood. Previously, there have been scanning tunneling microscopy and spectroscopy (STM/STS) measurements of this parent phase that have shown signatures of strong electronic correlations [3]. In this talk, we report on new, higher resolution STM/STS studies of this phase. Our experimental results reveal new distinct filling-dependent spectroscopic features that reflect key information on the nature of many-body correlations in MATBG. We have found that these features can be compared to a model of strongly interacting electrons in the flat bands allowing us to determine the microscopic origin of these features as well as extract an estimate of the strength of electronic correlation in MATBG.

[1] Y. Cao et al. Nature 556, 43-50 (2018)
[2] M. Yankowitz et al. Science 363, 1059-1064 (2019)
[3] Y. Xie et al. Nature 572, 101-105 (2019)   

High-Resolution Spectroscopic Studies of Magic-Angle Twisted Bilayer Graphene at Millikelvin Temperatures

Magic-angle twisted bilayer graphene (MATBG) has a complex gate-accessible electronic phase diagram characterized by a variety of exotic insulating, superconducting, and topological phases. Using a homebuilt dilution-fridge scanning tunneling microscope (STM), we probe the spectroscopic properties of MATBG to gain insight into the underlying mechanisms of its phases. At partial filling of the flat bands, consistent with previous studies, our measurements at high temperatures (< 12K) show significant deviations from the single-particle density of states (DOS), indicating the presence of strong electronic interactions [1]. We extend these STM measurements to millikelvin temperatures, where we explore how the “parent phase” develops into correlated insulating and superconducting phases. Our measurements have uncovered a series of gate-dependent energy gaps in the DOS at the Fermi level corresponding to electron fillings at which electrical transport measurements reveal these phases. These spectra, along with their magnetic field and spatial dependencies, provide key information about the microscopic origin of each energy gap and their connection to the various electronic phases in MATBG.

[1] Xie et al. Nature 572, 101 (2019).   

High-resolution Landau level spectroscopy of magic-angle twisted bilayer graphene

Spectroscopy with scanning tunneling microscopy (STM) in a high magnetic field can be used to probe the quantization of the density of states into Landau levels (LL), which provides important information on electronic properties such as the relevance of broken symmetries and the presence or absence of Dirac cones. We apply this technique to study gated magic-angle twisted bilayer graphene (MATBG) devices, for which recent magnetotransport studies have reported transitions in the LL fan diagrams as a function of the filling of electrons in MATBG’s flat bands [1-3]. These reported changes correlate with the appearance of various correlated insulating, magnetic, and superconducting phases. Our STM measurements are carried out in a dilution-fridge system allowing us to resolve the energy of the LL with high resolution and to probe MATBG devices as a function of filling of its flat band. We will describe these measurements, their connection to the magnetotransport studies, and their interpretations in terms of microscopic properties of MATBG as a function of electron density.

[1] Y. Cao et al. Nature 556, 43 (2018)
[2] M. Yankowitz et al. Science 363, 1059 (2019)
[3] X. Lu et al. arXiv:1903.06513   

Atomic-scale structure and electronic properties of twisted double bilayer graphene

The fundamental properties of 2D materials are dramatically modified when they are brought next to each other to form a vertical heterostructure. The electronic characteristics of such van der Waals materials can be further controlled by the twist angle degree of freedom, inducing electronic flat bands that lead to emergent phases such as correlated insulating (CI) and superconducting (SC) states in twisted bilayer graphene. Such phenomenology is expected in higher order heterostructures where the vertical stacking order plays a major role. Recent works showed that double bilayer graphene (tDBG) at a twist angle of ~1.3° hosts displacement field tunable CI and SC states, as well as ferromagnetic order. We show real-space imaging of tDBG moiré superstructure by means of Scanning Tunneling Microscopy/Spectroscopy (STM/STS). STS reveals the presence of van Hove singularities whose spatial distribution within the moiré unit cell is determined by the inequivalent stacking sites. Tuning carrier density and displacement field reveals long-range broken symmetries that emerge when the Fermi level is brought in the vicinity of such flat bands. Our results shed light into the underlying mechanisms behind electron-electron correlations in tDBG and the emergent ferromagnetic order.   

Identifying the effect of hBN alignment on twisted bilayer graphene using scanning tunneling microscopy

When two graphene layers are stacked with a small twist angle, a moire superlattice arises that modifies the electronic band structure. Specifically, at the magic angle (~1.1deg), the bands become flat. In these flat bands, various correlated electronic states emerge. In some samples, correlated insulating behaviors are observed at integer fillings, accompanied by superconducting states upon doping. In other samples, the quantum anomalous Hall effect replaces the correlated insulator at ¾ filling while superconductivity is absent. Despite the lack of direct experimental evidence, it has been suggested that the latter phase is produced by the alignment between graphene and the hBN substrate. Using a scanning tunneling microscope (STM), we extract the hBN alignment information from graphene-hBN moire lattice superimposed on the graphene-graphene moire lattice. Comparing spectra from different samples, we explore the effect of hBN alignment on the electronic properties of twisted bilayer graphene.   

Imaging moiré superlattices in twisted bilayers of 2D materials

Two-dimensional materials stacked into van der Waals (vdW) heterostructures form moiré superlattices as a result of a relative rotational misalignment between layers as well as a lattice mismatch between two dissimilar materials. The resulting physics of the vdW heterostructures can be dominated by the large scale periodicity of the moiré superlattice which is generally substantially larger than the atomic scale lattice. In this work we outline a method to visualize the real-space moiré superlattices in a variety of twisted vdW heterostructures utilizing piezoresponse force microscopy which locally measures an electromechanical sample surface deformation. Its origin is suggested to arise due to the flexoelectric effect wherein electric polarization is induced through strain gradients present within moiré superlattices.   

Flat bands in small angle twisted bilayer WSe2

The discovery of superconductivity in magic angle twisted bilayer graphene has ignited great interest in exploring flat bands in two-dimensional van der Waals heterostructures. We demonstrate for the first time the existence of flat bands in the electronic structure of a 3° twisted bilayer WSe2 sample using low temperature scanning tunneling microscopy and spectroscopy. Using variable height spectroscopy, we observe sharp peaks in the density of states near the valence band edge related to the flat bands. By decay constant measurements, we assign various peaks in the density of states to different locations in the Brillouin zone. By direct spatial mapping of the wavefunctions at the flat band energy, we have found that the flat band is localized in the form of a hexagonal network, which is in excellent agreement with first-principle density functional theory calculations.   

Effect of Spin-orbit Coupling in Twisted Bilayer and Double Bilayer Transition Metal Dichalcogenides

Since the discovery of flat bands at magic angles in twisted bilayer graphene, the formation of flat bands has been predicted for several other twisted bilayers, especially twisted transition metal dichalcogenides (TMDs). The effect of spin-orbit coupling is quite prominent in TMDs such as WSe₂. Using a multiscale approach, based on classical force-fields for structural relaxation and density functional theory calculations for electronic structure, we investigate the influence of spin-orbit coupling on the flat bands in twisted TMD systems. Owing to spin-valley coupling, spin-orbit interaction affects the flat bands of the twisted bilayer TMDs with twist angle close to 0^o differently than those near 60^o. The spin-orbit splitting of bands near valence band edge decreases with smaller twist angle. We study wavefunction localization near valence and conduction band edges in these bilayers and extend our studies to twisted double bilayer TMDs as well.   

Tunable Moiré Superlattice of Artificially Twisted Monolayers

Twisting between two stacked monolayers modulates periodic potentials and forms the Moiré electronic superlattices, which offers an additional degree of freedom to alter material property. Considerable unique observations, including unconventional superconductivity and quantized interlayer excitons are correlated to the electronic superlattices but further study requires reliable routes to study the Moiré in real space. Scanning tunneling microscopy (STM) is ideal to precisely probe the Moiré superlattice and correlate coupled parameters among local electronic structures, strains, defects, and band alignment at atomic scale. Here, a clean route is developed to construct twisted lattices using synthesized monolayers for fundamental studies. Diverse electronic superlattices are experimentally confirmed with STM at room temperature.   

Unsupervised learning of nematic order from scanning tunneling spectroscopy on twisted bilayer graphene

Moiré materials such as magic angle twisted bilayer graphene (TBG) provide an exciting platform for the study of novel states of matter, but their large unit cells present significant difficulties for atomic resolution probes such as scanning tunneling microscopy (STM). We therefore develop an automated method to extract salient physical quantities from STM data on moiré materials, and apply it to measurements on TBG that visually suggest the breaking of rotational symmetry (i.e. nematic order). We apply the machine learning technique of Gaussian mixture modeling to this data to classify the STM images at different bias voltages into several categories in an unbiased fashion. This classification yields evidence for a nematic order that manifests itself in a manner strongly dependent on bias voltage. In addition to providing evidence for spatial symmetry breaking in TBG, our techniques can be applied in the future to overcome the intrinsic challenges of exploiting STM in the burgeoning field of moiré materials.   

Moiré superlattice of a charge transfer Kondo monolayer and its interplay with superconductivity

Moiré pattern formation has emerged as a powerful tool to create controllable two-dimensional electronic superlattices. Moiré superlattices can exhibit remarkable properties such as unconventional superconductivity and Mott insulators in twisted graphene bilayers, and moiré exciton bands in transition metal dichalcogenide heterobilayers. Here, we report on the observation of a moiré superlattice formed between an organic monolayer and a s-wave superconductor. Although the organic molecule does not contain magnetic atoms, the interlayer charge transfer from the superconductor to the p-orbitals creates a local moment leading to the co-existence of Kondo screening and Cooper pair formation, and to an interplay between superconductivity and the Kondo effect. More specifically, the moiré pattern leads to a strong modulation of the “local” Kondo temperature by a factor of three. In addition, the interplay of Kondo screening and Cooper pair formation leads to a modulation of the pairing gap by about 30%. We show that the periodicity of these superlattices is tunable by the twist angle, making it possible to tailor the properties of this heterostructure.   

Scanning tunneling microscopy and spectroscopy of graphene on in-plane anisotropic rhenium disulfide

Van der Waals heterostructures made of graphene placed on top of other 2D materials have been shown to affect the band structure of graphene and lead to proximity-induced novel phenomena. In this talk, we focus on a system that has graphene placed on top of the in-plane anisotropic semiconductor ReS₂. Using scanning tunneling microscopy and spectroscopy, we characterized the changes at the atomic scale in the structure and electronic properties of Dirac electrons placed in the longitudinal periodic potential of ReS₂.   

Transition Metal Adatoms on Graphyne: Electronic Structure and Thermoelectric Properties

Thermoelectricity is the advantageous property of some materials to convert waste thermal energy into electricity. The research interest of achieving high thermoelectric efficiency increased in the recent years, specially promoted by the demand for eco-friendly energy harvesting technology. It is known that 2D systems provide much higher thermoelectric conversion than bulk materials, due to enhancement of Seebeck coefficient and reduction of phonon thermal conductivity. Therefore, graphene nanostructures and other 2D sp-sp2 carbon allotropes such as graphynes and graphdiynes are promising candidates to thermoelectric applications and spin caloritronics, an emerging technology combining spintronics and thermoelectronics.
Here we investigated structural and electronic properties of Mn and Fe adatoms adsorbed on Graphyne (Gy) using density functional theory. We verified that the band structure of Gy sheets, strongly depends on the adatom distance to the plane. To explore the effects of quantum confinement, we also discuss the functionalization of Gy nanoribbons. Finally, we focus on the electronic transport and thermoelectric quantities with the perspective of spin caloritronics nanodevices.   

Electronic structure of carbon nanotubes on graphene substrates

Allotropes of carbon, including one-dimensional carbon nanotubes and two-dimensional graphene sheets, continue to draw attention as promising platforms for probing the physics of electrons in lower dimensions. Recent research has shown that the electronic properties of graphene multilayers are exquisitely sensitive to the relative orientation between sheets, and in the bilayer case exhibit strong electronic correlations when close to a magic twist angle. Here, we investigate the electronic properties of a carbon nanotube deposited on a graphene sheet by deriving a low-energy theory that accounts both for rotations and rigid displacements of the nanotube with respect to the underlying graphene layer. We show that this heterostructure is described by a translationally invariant, a periodic or a quasi-periodic Hamiltonian, depending on the orientation and the chirality of the nanotube. Furthermore, we find that, even for a vanishing twist angle, rigid displacements of a nanotube with respect to a graphene substrate can alter its electronic structure qualitatively. Our results identify a promising new direction for strong correlation physics in low dimensions.   

Q-Valley Quasiparticle Interference in Few-Layer Transition Metal Dichalcogenides

Quantum transport experiments have provided compelling evidence for the threefold flavors of the Q-valley electrons in few-layer transition metal dichalcogenides (TMD). We study the quasiparticle interference (QPI) due to Q-valley electrons scattering off localized impurities, which modifies the local density of states. In the quantum Hall regime, all the TMD odd-layers thicker than bilayer exhibit Landau level triplets. When a Landau level triplet is one-third filled or empty, the flavor SU(3) symmetry is broken in the ferroelectric nematic ground states tunable by an in-plane electric field and an out-of-plane magnetic field. We show the QPI patterns of such flavor states, which can be probed by scanning tunneling spectroscopy.