Session A09: Moiré on Transition Metal Dichalcogenides: Transport and Spectroscopy

Resonant tunneling detection of atomic reconstruction in twisted bilayer WSe2

In twisted bilayer WSe₂ (tBL-WSe₂) at low twist angles, atomic reconstruction is driven based on moiré periodicity. In this study, we perform resonant tunneling measurements and transmission electron microscopy (TEM) observations for tBL-WSe₂ samples with various twist angles θ_BL from 0° to 34°, and reveal the connection between atomic reconstruction and the resulting Γ-point band energy landscape. The resonant tunneling results consistently indicate the presence of two peak currents associated with tunneling into VB-Γ-point states of tBL-WSe₂, regardless of the twist angle. As the twist angle increases from θ_BL = 0°, the energy spacing between these peaks decreases from ~0.7 eV and stabilizes at approximately ~0.6 eV for θ_BL ≥ 6°. In comparison, the TEM observations show atomic reconstruction patterns persisting up to θ_BL ~ 5°. It is worth noting that the energy spacing of the VB-Γ-point is significantly influenced by the interlayer distance of WSe₂. Therefore, we have shown that we can detect atomic reconstruction in tBL-WSe₂ through the changes in interlayer distance, which alter the resonant tunneling behavior into the VB-Γ-point states.   

Electric field tunable correlated insulating states in AB stacked twisted MoTe2

Twisted MoTe₂ homobilayer has emerged as an attractive platform for studying strong-correlation physics. A recent breakthrough is the observation of fractional quantum anomalous hall effect in AA stacked homobilayer. Compared to the AA stacking with a honeycomb moiré superlattice lattice and strong interlayer hybridization, AB stacked twisted homobilayer hosts a triangular lattice with negligible interlayer tunneling and well-defined layer degree of freedom. Here we study the AB stacked homobilayer using combined transport and optical measurements. We observe correlated insulating states at multiple fractional fillings of electrons and holes. In the electron side, a displacement field has no effect on the correlated states. In contrast, the states at fractional hole fillings exhibit transitions between correlated insulating states when electric field induces full layer pseudospin polarization, likely from the change of charge configuration. Our results reveal rich correlated physics in AB stacked MoTe₂ homobilayer.    

Oral: Evidence for excitonic Mott insulator in WS2/WSe2 moiré superlattice

Angle-aligned transition metal dichalcogenide(TMDC) heterojunction exhibits strongly enhanced electron-electron interaction because of the moiré superlattices, which provides an interesting platform for fermionic correlated states studies. This enhancement can also be inherited at the type II heterojunction (WS₂/WSe₂ junction) when the electron and hole are separated in two different layers, which leads to long-lived valley-polarized correlated bosonic quasiparticles. With helicity-resolved PL measurement as a function of excitation power, we are able to study the spatial extent of interlayer excitons, as well as the band hierarchy of correlated states that arise from the strong interaction between interlayer excitons and electrons. The measurements also show evidence that at the filling of one exciton per moiré cell, an excitonic Mott insulator state emerges, with the valley polarization enhanced by nearly one order of magnitude. Our study demonstrates the potential of WS₂/WSe₂ interlayer for the study of correlated states of fermion, bosons, and a mixture of both.   

Electric Field-driven Topological Phase Transitions in Twisted MoTe2

The fractional quantum anomalous Hall effect (FQAHE), a lattice analogue to the fractional quantum Hall effect, displays fractionally quantized Hall conductivity in the absence of an external magnetic field. This remarkable effect has recently been realized in twisted MoTe₂ bilayer in rhombohedral stacking. In this talk, we will report our study of electrically tunable topological phase transitions in this new system. Near -1/2 filling, we observe a transition from the putative zero-field composite fermi-liquid state to a correlated insulating state and then to a metallic state with strong local magnetic interactions. Near -2/3 fillings, we reveal a competition between FQAHE and charge density wave order that breaks the translation symmetry. Our work shows that the large parameter space offered by tuning knobs such as electrostatic doping, twist angle, and electric field provides insight to the FQAHE and proximate phases.   

Layer-hybridized correlated insulating states in a WS2/WSe2 moiré superlattice

In moiré superlattices of transitional metal dichalcogenides (TMD), the enhanced Coulomb interaction gives rise to rich correlated electronic states such as Mott insulators and generalized Wigner crystals. The introduction of layer degree of freedom to the moiré superlattice presents a new method to tune the electronic correlation, exemplified by phenomena like the excitonic insulator in monolayer WS₂/bilayer WSe₂. In this talk, we study angle-aligned monolayerWS₂/trilayer WSe₂ heterostructure in a dual gate geometry and employ microwave impedance microscopy to probe the formation of insulating states. We observe correlated insulating states atfilling factors of, n=-1/3, -2/3, -1 and -2. Notably, these states exhibit insensitivity to the hole distribution among WSe₂ layers, which is controlled by the applied vertical electric field. This result suggests the formation of layer-hybridized correlated states in the heterostructure, with each hole shared between the first and third layer WSe₂ via interlayer hopping of hole with same spin and valley indices. The hybridization across layers is also supported by the observation of hybridization of interlayer and intralayer excitons in the trilayer WSe₂.   

Moiré ferroelectricity engineered plasmonic excitations and nano-photocurrent in graphene/twisted-WSe2 structures

Ferroelectricity, a spontaneous and reversible electric polarization, is found in certain classes of van der Waals (vdW) materials. The discovery of ferroelectricity in twisted vdW layers provides new opportunities to engineer spatially dependent electric and optical properties associated with the configuration of moiré superlattice domains and the network of domain walls. Here, we employ near-field infrared nano-imaging and nano-photocurrent measurements to study ferroelectricity in minimally twisted WSe₂. The ferroelectric domains are visualized through the imaging of the plasmonic response in a graphene monolayer adjacent to the moiré WSe₂ bilayers. Specifically, we find that the ferroelectric polarization in moiré domains is imprinted on the plasmonic response of the graphene. Complementary nano-photocurrent measurements demonstrate that the optoelectronic properties of graphene are also modulated by the proximal ferroelectric domains. Our approach represents an alternative strategy for studying moiré ferroelectricity at native length scales and opens promising prospects for (opto)electronic devices.

Ref.: Zhang, S., Liu, Y., Sun, Z., Chen, X., Li, B., Moore, S. L., ... & Basov, D. N. (2023).  Nature Communications, 14(1), 6200.   

Moiré Zeeman effect in twisted WSe2/WS2 heterobilayer

Moiré superlattices introduce an artificial periodicity to the underlying crystalline structures, leading to the emergence of a wide range of novel electronic properties in two-dimensional materials. In particular, the combination of flat moiré minibands and strong Coulomb interactions has resulted in the observation of periodic electronic crystals in twisted layers of transition metal dichalcogenides. These electronic crystals can also give rise to intricate magnetic phenomena. In this study, we present our findings on the moiré Zeeman effect, which originates from local magnetic moments within well-aligned WSe₂/WS₂ heterobilayers. In addition to the previously reported remarkably large g-factor of the WSe₂ A exciton in this heterostructure, we unveil the existence of another high-energy excitonic resonance with an even larger g-factor (five times that of the WSe₂ A exciton). This resonance occurs near the filling condition of one hole per moiré unit cell (i.e., v = -1). Our investigations, supported by continuous model calculations, reveal that these high-energy states are the result of dark excitons being brightened through Umklapp scattering from the moiré mini-Brillouin zone. The observed colossal g-factors, denoted as the moiré Zeeman effect, in these high-energy states can be attributed to the periodic modulation of the local magnetic field when the electron lattice forms a Mott insulating state. These results underscore the potential of moiré superlattices as solid-state platforms for simulating quantum magnetism.   

Pressure-tuning of minibands in MoS2/WSe2 heterostructures revealed by moiré phonons

Moiré superlattices of two-dimensional heterostructures arose as a new platform to investigate emergent behavior in quantum solids with unprecedented tunability. To glean insights into the physics of these systems, it is paramount to discover new probes of the moiré potential and the moiré minibands, as well as their dependence on external tuning parameters. Hydrostatic pressure is a powerful control parameter, since it allows to continuously and reversibly enhance the moiré potential. In this talk, I will discuss our recent work [1] where we used high pressure to tune the minibands in a rotationally-aligned MoS₂/WSe₂ moiré heterostructure, and showed that their evolution can be probed via moiré phonons. The latter are Raman-inactive phonons from the individual layers that are activated by the moiré potential. Moiré phonons manifest themselves as satellite Raman peaks arising exclusively from the heterostructure region, increasing in intensity and frequency under applied pressure. Further theoretical analysis reveals that their scattering rate is directly connected to the moiré potential strength. By comparing the experimental and calculated pressure-induced enhancement, we obtain numerical estimates for the moiré potential amplitude and its pressure dependence. Our results indicate a considerable flattening of the minibands upon compression up to 5 GPa due to the increase of the moiré potential strengh. The present work establishes moiré phonons as a sensitive probe of the moiré potential and of the electronic structures of moiré systems.

[1] Pimenta Martins, L.G.*, Ruiz-Tijerina*, D. A., et al . "Pressure tuning of minibands in MoS₂/WSe₂ heterostructures revealed by moiré phonons." Nature Nanotechnology (2023): 1-7. DOI: https://doi.org/10.1038/s41565-023-01413-3. * Equal contirbution   

Fractional Quantum Anomalous Hall Effect in Multilayer Graphene

The fractional quantum anomalous Hall effect (FQAHE)¹, the analog of the fractional quantum Hall effect at zero magnetic field, is predicted to exist in topological flat bands under spontaneous time-reversal-symmetry breaking^2-5. The demonstration of FQAHE could lead to non-Abelian anyons which form the basis of topological quantum computation^6-8. So far, FQAHE has been observed only in twisted MoTe2 (t-MoTe2) at moiré filling factor v > 1/2^9-12. Graphene-based moiré superlattices are believed to host FQAHE with the potential advantage of material quality and electron mobility. In this talk, I will report the observation of integer and fractional QAH effects in a rhombohedral pentalayer graphene/hBN moiré superlattice. At zero magnetic field, we observed plateaus of quantized Hall resistance Rxy = h/(ve2) at moiré filling factors v = 1, 2/3, 3/5, 4/7, 4/9, 3/7 and 2/5. In addition, the phase transitions from composite Fermi liquid and FQAH states to other correlated states will also be discussed. The rich family of FQAH states in our graphene-based moiré superlattice provides an ideal platform for exploring charge fractionalization and (non-Abelian) anyonic braiding at zero magnetic field^13-18.   

Field-control of symmetry-broken and quantum disordered phases in frustrated moiré bilayers with population imbalance

We determine the ground states and excitation spectra of the paradigmatic four-flavour Heisenberg model with nearest- and next-nearest-neighbor exchange couplings on the triangular lattice in a field controlling the population imbalance of flavor pairs. Such a system arises in the strongly correlated limit of moiré bilayers of transition metal dichalcogenides in an electric displacement field or in-plane magnetic field, and can be simulated via ultracold alkaline-earth atoms. We argue that the field tunes between effective SU(4) and SU(2) symmetries in the balanced and fully polarised limits and employ a combination of mean-field calculations, flavour-wave theory, and exact diagonalisation to analyse the intermediate, imbalanced regime. We find different symmetry-broken phases with simultaneous spin and excitonic order depending on the field and next-nearest-neighbor coupling. Furthermore, we demonstrate that there is a strongly fluctuating regime without long-range order that connects candidate spin liquids of the SU(2) and SU(4) limit. The strong fluctuations are facilitated by an extensive classical degeneracy of the model, and we argue that they are also responsible for a strong polarisability at 1/3 polarisation that survives from the mean-field level to the exact spectrum.