Session N10: Twisted Bilayer Heterostructures

Flat band in twisted graphene I: Evolution of flat band near the Magic Angle in twisted bilayer graphene

Magic-angle twisted bilayer graphene (MATBG) exhibits intriguing correlated phenomena, whose central physics is related to the flat band near the Fermi energy. Such flat band not only depends on the moiré period, but is also expected to be sensitively affected by lattice relaxations, which has remained elusive so far due to the lack of a systematic evolution of energy- and momentum-resolved electronic structure with the twist angle. To figure out the important role of lattice relaxion in MATBG, we combine NanoARPES, AFM measurements and theoretical calculations to reveal the evolution of band structure near magic angle. Our results not only provide direct spectroscopic information for understanding the flat band physics, but also highlight the critical role of lattice relaxations in the electronic structure of twisted bilayer graphene.   

Flat band in twisted graphene II: Field tunable flat band in twisted multilayer graphene

Twisted graphene structures provide ideal platforms for exploring the flat band engineering and correlated phenomena, such as the unconventional superconductivity and Mott insulating states discovered in twisted bilayer graphene. By modifying the number of twisting graphene layers, one can tune both the moiré superlattice potential and the symmetry, which enables to tailor the intriguing physics. In this talk, I will introduce our new progress on the NanoARPES measurement of twisted monolayer-bilayer graphene with operando gating capability. High-quality data with clear isolated flat band near the Fermi energy is revealed. Moreover, by applying bottom gate voltage, we not only tuned the filling of the flat band, but also manipulate the flat band through the gating field. Our results suggest twisted monolayer-bilayer graphene as a fascinating platform for exploring the field tunable phenomena.   

Nature of Superconductivity in Twisted Bilayer Graphene

When two sheets of van der Waals material are twisted with respect to each other, it creates intricate superlattice structures known as moire patterns. These moire materials have become a hot topic of research in the strongly correlated community due to their rich quantum phases such as superconductivity, correlated insulating states, topological bands, etc. Twisted bilayer graphene is one of the moire family which shows robust superconductivity dome only for carrier densities 2 < |n/n_s| <3. We use this fact to constrain the nature of the superconducting phase and possible mechanisms that can drive the superconductivity in this material.   

Kagome and Honeycomb flat bands in moire graphene systems

We propose a class of graphene-based moire systems hosting flat bands on kagome and honeycomb moire superlattices. These systems are formed by inducing approximate Kekule lattice periods in graphene moire systems, for instance, graphene layer stacked on a 2D substrate with lattice constant approximately square root three times that of graphene. When the moiré potentials are induced by a 2D irreducible corepresentation in the substrate, the model shows a rich phase diagram of low energy bands including eigenvalue fragile phases as well as kagome and honeycomb flat bands. A particularly interesting system is twisted bilayer Kekule graphene (graphene with a Kekule order), in which a pair of Kagome flat bands emerge around charge neutrality. The addition of spin-orbit coupling (from substrate, etc) can further lift symmetry protected degeneracies and create spin Chern bands. These systems provide promising new platforms for studying novel strongly correlated phases of matter.   

Oral: Electron-K-Phonon Interaction in Twisted Bilayer Graphene

We develop an analytic theory to describe the interaction between electrons and K-phonons and study its influence on superconductivity in the bare bands of twisted bilayer graphene (TBG). We find that, due to symmetry and the two-center approximation, only one optical K phonon (∼ 160meV) of graphene is responsible for inter-valley electron-phonon interaction. This phonon has recently been found in angular-resolved photo-emission spectroscopy to be responsible for replicas of the TBG flat bands. We analyzed the properties of replica bands induced by electron-K-phonon in TBG and compared it with the experimental observations. By projecting the interaction to the TBG flat bands, we perform the full symmetry analysis of phonon-mediated attractive interaction and pairing channels in the Chern basis and show that several channels are guaranteed to have gapless order parameters. The gap equation from the phonon-mediated attractive interaction is investigated to support our symmetry analysis.   

Electronic structure of lattice-relaxed hBN-encapsulated twisted bilayer graphene

Twisted bilayer graphene (tBG) on hexagonal Boron Nitride (hBN) can host the quantum anomalous Hall effect for nearly aligned moire patterns of tBG and graphene/hBN interfaces, while reproducible experimental evidence is yet to be fully established. In this study, we review the electronic structure of energetically favored magic angle-tBG and hBN heterointerface structures and demonstrate that certain commensurate double moire patterns leads to a gap opening between the valence and conduction flat bands, thereby enhancing the possibility of observing well-resolved topological bands. When the system is deposited on a bulk hBN substrate, both commensurate moire systems exhibit such gap opening. We can further tune the bands at charge neutrality through an additional encapsulation hBN layer or by applying a perpendicular electric field. Our results show that careful alignment and sliding between the moire patterns of tBG and hBN substrate can make a notable impact on the resulting device's electronic properties.   

Heavy Fermion Quantum Simulator Realized in a Graphene Moiré Superlattice

The unexpected discovery of superconductivity in magic angle twisted bilayer graphene immediately generated a wave of intense theoretical and experimental research attracted by its rich phase diagram, which seemingly resembles ones of copper-oxide high-temperature superconductors. Originated in the low-energy narrow electronic bands, a family of magic angle graphene compounds hosts a collection of exotic phases including but not limited to superconductivity, correlated insulators, topological and magnetic orders. Compared to other strongly-correlated systems, 2D multilayers offer a unique opportunity to tune the charge carrier density in situ and adjust system properties in other ways (for example, by alternating the distance to the gate or varying the dielectric environment), thus offering a potentially faster progress in understanding the underlying microscopic mechanisms governing its strong correlations. While the seemingly disagreeing electronic transport and scanning tunneling microscopy experiments brought up a controversy about the locality of the Wannier orbitals in these materials, a definitive experimental evidence merging two patterns together has been much coveted. Here I discuss on the first local thermoelectric measurements in the flat electronic bands of the twisted symmetric trilayer graphene (TSTG). We use a cryogenic near-field optical microscope with an oscillating atomic force microscopy (AFM) tip irradiated by infrared photons to create a nanoscopic hot spot in the planar samples. We observe a breakdown of the non-interacting Mott formalism at low temperatures (~10 K) signaling an importance of the electronic interactions in PV generation. Explained by the interacting topological heavy-fermion model, our data suggest a spatial variation of the interaction strength dependent on the local twist angle. These experimental findings provide the first evidence of heavy fermion behavior in the topological flat bands of moiré graphene and epitomize an avenue to apply local thermoelectric measurements to other strongly correlated materials in the disorder-free limit.   

Efficient momentum space approach to superconductivity in quasiperiodic systems

Superconductivity in quasiperiodic multilayer systems has attracted significant recent interests, including that in twisted bilayer graphene (TBG). While numerical studies on superconductivity in general multilayer heterostructures have been mainly based on the real space picture so far, its momentum space picture has less numerical cost and is desirable. Here, we develop an efficient momentum space approach to superconductivity in quasiperiodic systems, by including quasiperiodicity through the folding of Brillouin zones (BZs). We apply the BZ folding method to the SC state within bilayer systems and discuss how we can understand it through the BZ folding picture.   

Extended critical phase in quasiperiodic quantum Hall systems

We consider the effects of quasiperiodic spatial modulation on the quantum Hall plateau transition, by analyzing the Chalker-Coddington network model for the integer quantum Hall transition with quasiperiodically modulated link phases. In the conventional case (uncorrelated random phases), there is a critical point separating topologically distinct integer quantum Hall insulators. Surprisingly, the quasiperiodic version of the model supports an extended critical phase for some angles of modulation. We characterize this critical phase and the transitions between critical and insulating phases. For quasiperiodic potentials with two incommensurate wavelengths, the transitions we find are in a different universality class from the random transition. Upon adding more wavelengths they undergo a crossover to the uncorrelated random case. We expect our results to be relevant to the quantum Hall phases of twisted bilayer graphene or other Moiré systems with large unit cells.   

New twisted van der Waals fabrication method based on strongly adhesive polymer

Observations of emergent quantum phases in twisted bilayer graphene prompted a flurry of activities in van-der-Waals (vdW) materials beyond graphene. Most current twisted experiments use a so-called tear-and-stack method using a polymer called PPC. However, despite the clear advantage of the current PPC tear-and-stack method, there are also technical limitations, mainly a limited number of vdW materials that can be studied using this PPC-based method. This technical bottleneck has been preventing further development of the exciting field beyond a few available vdW samples. To overcome this challenge and facilitate future expansion, we developed a new tear-and-stack method using a strongly adhesive polycaprolactone (PCL). With similar angular accuracy, our technique ensures an intrinsically clean interface and low working temperatures. More importantly, it can be applied to many other vdW materials that have remained inaccessible with the PPC-based method. We present our results on twist homostructures made with a wide choice of vdW materials – from two well-studied vdW materials (graphene and MoS₂) to the first-ever demonstrations of other vdW materials (NbSe₂, NiPS₃, and Fe₃GeTe₂). Therefore, our new technique will help expand moiré physics beyond few selected vdW materials and open up more exciting developments.   

Charge density waves and moiré reconstruction in twisted NbSe2/NbSe2 bilayers​

Twisted moiré bilayers are two-dimensional materials formed by stacking and rotating two monolayer materials. The lattice mismatch generated by such rotation generates a large moiré cell that can contain thousands of atoms and which can give rise to novel electronic properties. For instance, twisted moiré bilayers of semiconducting transition metal dichalcogenides have been found to host flat electronic bands and correlated electronic phases.

Here we focus on twisted bilayers of metallic NbSe2. On the one hand, the different local stacking arrangements in the moiré cell cause atomic relaxations (both in-plane and out-of-plane). On the other hand, monolayer NbSe2 exhibits spontaneous symmetry breaking into a charge density wave state with a 3x3 unit cell lattice reconstruction. In this project we use first-principles density functional theory calculations to study and understand the coexistence of these two types of atomic relaxation in twisted NbSe2 bilayers.   

Strain-Modulated Emergence of Rectangular Moiré Superlattice in Transition Metal Dichalcogenide van der Waals Heterostructures

The existence of moiré superlattice in van der Waals heterostructures brings highly tunable electronic structure , which enables the moiré heterostructures to simulate quantum phenomena in condensed-matter systems. Currently, the size and symmetry of moiré superlattice are controlled by monolayer material choice and twist angle between layers. However, they are inferior to continuous tuning after heterostructure fabrication. Here we show that strain application is suitable for continuous size and symmetry control of the superlattice. We fabricated twisted bilayer WSe₂ on flexible substrate. By bending the substrate, uniaxial strain was intentionally applied to the sample. Based on the theoretical model, we were able to design the expected moiré superlattice under combined twist angle and strain condition. We then successfully observed the continuous distortion of moiré superlattice from triangular to rectangular. Corresponding band structure calculation indicatess strong anisotropy and tunable flat-band property in strained sample. The established method and observation pave the way for utilizing strain a new tuning parameter to modulate the correlated behavior in moiré superlattices.   

Landau Level Description of Topological Bands in Twisted Homobilayer TMDs

Topological flat bands and nearly ideal quantum geometry have been identified in twisted homobilayer transition metal dichalcogenide (TMD) moiré superlattices, and are thought to be crucial for understanding the recently observed fractional Chern insulating state in twisted MoTe2. Recent theoretical work proposes an adiabatic approximation which maps the bilayer valence bands to a system of charged particles in a nonuniform magnetic field and a periodic potential. Here, we study the non-interacting band structure using a basis constructed from Landau levels (LLs) formed by the spatially averaged part of the effective magnetic field. We find that Landau-level mixing can have a strong effect on the band properties, whereas higher harmonics of the periodic field and potential have a weaker effect. Upon including several LLs, the nonuniformity of Berry curvature, the nearly ideal quantum geometry at the magic angle and the value of the magic angle are qualitatively reproduced by the adiabatic approximation, which works better for WSe2 than MoTe2.   

Continuous Mott transition in correlated moiré TMDs with disorder

Twisted TMDs are by now well-recognized as strong candidate hosts for exotic quantum states, such as spin liquids. Accordingly, it has been proposed that the continuous transition observed between a correlated insulating state and a metal in twisted MoTe2/WSe2 and twisted bilayer WSe2 may be a physical realization of the theorized continuous Mott transition separating a spin liquid with spinon Fermi surface and a Fermi liquid. Nevertheless, experimental observations are not entirely consistent with expectations; in particular, the resistance change across the transition is larger than the universal value calculated for a clean system. As in all moiré systems, twist angle inhomogeneities are unavoidable, and we therefore study the continuous Mott transition in the presence of disorder using a quantum Boltzmann equation. We discuss applications to composite Fermi liquid theories of fractional quantum anomalous Hall and fractional Chern insulating states.   

Oral: Imaging ferroelectric domains in twisted hexagonal boron nitride layers

Hexagonal boron nitride (hBN) has been mostly utilized as passive encapsulation or space layers in van der Waals heterostructures Recent studies of twisted hBN layers suggest that ferroelectric (FE) domains may form due to charge redistribution at the slid or twisted interfaces that break structural inversion symmetry. Here, we present a Kelvin probe force microscopy (KPFM) study on the electric control of FE domains. In addition to KPFM images, one can also monitor the domain flipping via exciton resonances in adjacent transition metal dichalcogenide monolayers.