Session N55: Excitons in Twisted Transition Metal Dichalcogenides

Electrical control of intralayer moiré excitons and trions in a twisted bilayer of MoS2

Moiré superlattices (mSLs) in twisted bilayers (tBLs) induce multiple sub-bands for the quasiparticles. We experimentally demonstrate the modulation of mSL-induced intralayer excitons and trions in a MoS₂-tBL with dual gating using near-0⁰ twisted samples in H-type stacking. We attribute the formation of intralayer trions to the carriers injected at the indirect band minima. While the spatial modulation of excitonic bandgap remains constant with gating, the variation in moiré potential for individual carriers tunes their corresponding localization and thereby modulates the formation of trions. We present the conceptual model and the corresponding experimental evidence for the localized and delocalized phases of both moiré excitons and trions using different levels of gating and optical excitation. We observe that the moiré excitons also exhibit strong valley coherence that changes in a striking W-shape with gating under near-resonant excitation. We further demonstrate the electrical tunability of moiré exciton-resonant Raman scattering due to the modulation of the oscillator strength of the moiré excitons. Such an electrical control of various optical properties of MoS₂-tBL offers numerous possibilities of designing photonic and valleytronic devices with mSLs.   

Magnetic interactions between excitons and strongly correlated electrons in MoSe2/WS2 moiré superlattices

Van der Waals heterobilayers with small twist angles exhibit moiré potentials that not only confine excitons and electrons, but also enhance their mutual interactions. Such strong correlation has been demonstrated for hole-doped WSe₂/WS₂ heterobilayers, as manifested in enhanced antiferromagetic interactions between the correlated holes and excitons. Here, we show such interactions with a ferromagnetic character for electron-doped MoSe₂/WS₂ heterobilayers. We observed a large enhancement of exciton g factors over g₀ (=4.5), with maxima approaching 100. The magnetic interactions peak at a filling factor of 1 electron per moiré site (ν=1) for R-stacked samples, but at ν=1.5 & 2.3 for H-stacking. At these filling factors, the temperature dependence of the g factors follows a Curie-Weiss law, yielding Weiss constants ~ 2 K and suggesting ferromagnetic interactions. Our results imply a phase diagram that differs from that of WSe₂/WS₂, expanding the range of correlated electron physics in moiré superlattices.   

Signatures of moiré trions in WSe2/MoSe2 heterobilayers

We present the optical signatures of trions coupled to moiré potential in WSe₂/MoSe₂ heterobilayers. The moiré trions exhibit highly irregular and sharp emission lines with complex charge-density dependence, in stark contrast to the behavior of conventional trions. We infer distinct contributions to the trion emission from radiative decay channels in which the remaining carrier resides in different moiré minibands. The complex moiré trion features motivate further theoretical and experimental studies on trions in moiré superlattices.   

Diffusion of dipolar excitons in charge-tunable, reconstructed WSe2/MoSe2 heterostructures

Semiconducting van der Waals heterostructures represent an exceptionally interesting platform to study electronic many-body phenomena. The combination of structural complexity from crystallographic alignment with strong Coulomb interactions leads to drastic changes in the electronic structure challenging the description of the optical excitations, predominantly governed by excitons. To develop better fundamental understanding in our study we address the nature of exciton propagation in gate-tunable field-effect devices based on hBN-encapsulated WSe₂/MoSe₂ heterobilayers in the moiré-free limit of atomically reconstructed domains. Employing transient microscopy, we demonstrate efficient and gate-dependent exciton diffusion. Our results challenge the more common expectation of exciton localization at low densities and provide a solid basis to understand increasingly complex excitonic phenomena in these systems.   

Fine structure of charged moiré interlayer excitons in MoSe2-WSe2 heterostructures

Heterobilayers of transition metal dichalcogenide (TMD) such as MoSe₂ and WSe₂ provides a rich landscape for studying interlayer excitons (IXs) composed of bound electron-hole pairs localized in opposite layers. In the past five years, there have been numerous studies on twisted TMD bilayer samples, including reports moire trapping of single IXs and correlated states with integer filling effects. Recently, charged moire excitons have been reported, demonstrating the possibility of realizing optical control of electron spin in 2D heterostructure. Here, we report on the fine structure of moire-trapped IXs in MoSe₂/WSe₂ with controllable doping level. By varying the carrier density and magnetic field, we show that charged moire IXs exhibit an anomalous magnetic field dependence and investigate the origin of this effect through a combination of low temperature photoluminescence and reflection spectroscopies.    

Strongly correlated electronic states and dielectric engineering in heterobilayer, heterotrilayer and bilayer-bilayer MoSe2/WSe2 heterostructures

Moiré heterostructures formed from twisted layers of transition metal dichalcogenides (TMDs) have been shown to host strongly correlated states including Mott insulators and Wigner crystals. However, experimental investigations have been mostly limited to two stacked monolayers. Here we use white light reflection spectroscopy to investigate the intralayer excitons in a unique device of stacked WSe₂/MoSe₂ where there are 1L/1L, 2L/1L, 1L/2L and 2L/2L regions with the same twist angle (~57 degrees).  Changing the carrier concentration we compare the correlated electron and hole states formed in each region. We observe, relative to the 1L/1L heterobilayer, correlations are suppressed in heterostructures using 2L MoSe₂, while they are enhanced using 2L WSe₂. Due to different dielectric environments we also observe an energetic splitting of the intralayer excitons localised in seperate layers of the bilayers. In 2L WSe₂/1L MoSe₂ we probe the effects of the correlated electronic insulating phases on each layer-localised exciton and find larger changes to the peak position and linewidth in the layer adjacent to the heterostructure interface. Our results introduce layer thickness as a new tuning knob for the formation of correlated electronic states in TMD moiré heterostructures.    

Topologically protected moire exciton at a twist-boundary in a van der Waals heterostructure

A twin boundary in one of the layers of a twisted van der Waals heterostructure separates regions with near opposite inter-layer twist angles. In a MoS₂/WSe₂ bilayer, the regions with R_h^h and R_h^X stacking registry that defined the sub-lattices of the moiré honeycomb pattern would be interchanged across such a twist boundary. In that case, we demonstrate that topologically protected chiral moiré exciton states are confined at the twist boundary. These are one-dimensional and uni-directional excitons with opposite velocities for opposite spins, enabling intrinsic, guided, and far reaching spin-polarized exciton currents.   

The strong modulation limit of excitons and trions in moiré materials

The optical properties of weakly-doped two-dimensional materials are dominated by strong exciton and trion absorption and luminescence features. In this talk I will discuss the influence of moiré patterns in semiconductor heterobilayers on exciton and trion states in the limit of strong moiré modulation potentials, commenting on similarities and differences compared to the case of excitons and trions in semiconductor quantum dots. One important difference in the moiré case is the regular spatial arrangement of trapping centers, which gives rise to exciton bands that can be tuned between narrow-band and nearly-free-exciton limits. I will also discuss strategies for using optical properties as quantitative probes of moiré materials, and the prospects for exploiting moiré materials to design unique light emitters.   

Localized dipolar excitons as a probe of correlated electronic states in van der Waals heterotrilayers

Correlated electronic states are discovered in moiré heterostructures, such as twist transition metal dichalcogenides. Here, we fabricate gated WSe₂/MoSe₂/WSe₂ heterotrilayer and use photoluminescence spectroscopy to study correlated electron behavior. We observe non-jittering localized interlayer excitons with linewidths as sharp as 26 μeV, which can sensitively probe electric fields, either from graphite gates or electrostatically doped electrons in the trilayer, by their dipolar energy shift. When the trilayer is doped with electrons, the localized interlayer excitons first form moiré trions with ~7 meV binding energy. Upon further electron doping, exciton energy shows several reproducible red- and blue-shifts correlated between different excitons at different sample positions. An analysis of the energy shifts with respect to fractional fillings suggests that these shifts arise from the switching of long-range charge ordered states, which gives rise to reconfiguration of electrons in the moiré structure. Furthermore, the degree of circular polarization of the emission depends on the filling - becoming unpolarized in certain filling range and finally recovering at higher fillings. These results establish localized dipolar excitons as a sensitive probe for correlated electron physics in moiré materials.   

Attractive and Repulsive Dipolar Interactions in Semiconductor van der Waals Heterotrilayers

Single-atom thick layers of transition metal dichalcogenides (TMD monolayers) have emerged as a promising platform for investigating quantum many-body physics through the study of interactions between their optical excitations. A key feature of TMD monolayers is their direct band-gap, which allows for efficient optical generation of excitons. Heterobilayers made by stacking two monolayers of different TMDs can host optically active interlayer excitons (IXs), where the electron and hole occupy different layers. Previous work has shown that heterobilayer IXs share a mutually parallel permanent electric dipole and therefore interact via dipole-dipole repulsion. In this work we show that, unlike in heterobilayers, heterotrilayer TMD structures show both attractive and repulsive dipole-dipole interactions because they allow for IXs with both parallel and antiparallel dipole arrangements. These findings are a step towards understanding more complicated quantum many-body states that have been predicted for these systems, including exciton lattices and droplets.   

Valley and charge pumping in time-modulated moiré of semiconductors

Long range moiré patterns can emerge by vertically stacking two monolayer semiconductors with a small twist angle. In such moiré lattices, electrons gain a layer pseudospin internal degree of freedom. The spatially periodic moiré potential effectively introduces a pseudo-Zeeman field, which exhibits vortex/antivortex textures. In the adiabatic regime, the layer pseudospin of an electron moving in the moiré adapts to the local direction of the pseudo-Zeeman field constantly. Such dynamics of the layer pseudospin can be understood through an emergent magnetic field acting on the electron. When the moiré is time-modulated, the magnetic field changes dynamically and a sizable emergent electric field can emerge due to Faraday's law of induction. In this work, we study the emergent fields and the resultant transport responses in time-modulated moiré by applying, for example, interlayer bias or THz pulses. We show that valley/spin and charge currents can be pumped between different domains. Our study offers a simple scheme to realize emergent electrodynamics for valley/spintronics applications.   

g tensor anisotropy and chiral light-matter interface in anomalous light cones of interlayer moiré exciton

We investigate interlayer excitons (IXs) in misaligned double layer transition metal dichalcogenides (TMDs) in applied magnetic field and strain induced pseudo-gauge potential. We show these IXs feature an unusual g tensor inside the light cones that are located at the finite kinematic momenta, where Zeeman splitting in the out-of-plane direction can be introduced by both the out-of-plane and in-plane applied magnetic fields. The response to the in-plane magnetic field arises from the magneto-Stark effect since IXs carry a permanent electric dipole. In addition, we show that through the in-plane magnetic field or heterostrain induced pseudo-gauge potential effects on these light cones, chiral light-matter interface could be realized. With the light cone degeneracy lifted by in-plane magnetic field, unidirectional exciton current can be injected using resonant excitation at selected frequencies. In the presence of heterostrain, broad-band circular polarized excitation can also inject unidirectional exciton current. These effects are expected in TMD/BN/TMD van der Waals heterostructures or TMD bilayers of relatively large twisting angle where moiré modulation of the potential landscape is insignificant.   

Modulating optical emission from dipolar excitons in vdW heterostructures by surface acoustic waves

Surface acoustic waves (SAW) can couple to excitons via strain fields allowing for dynamic optoelectronics. Recently, interlayer excitons with permanent out-of-plane dipole moment have been discovered in transition metal dichalcogenide heterostructures. Due to their long lifetimes, interlayer excitons are expected to interact strongly with SAW. The strain field through deformation potential or piezoelectric coupling can create a dynamic localization for excitons. In order to analyze the effects of the SAW on the properties of excitons, we fabricated a device with a MoSe₂/WSe₂ heterobilayer on a LiNbO₃ substrate with interdigitated transducers to propagate SAW. Low temperature photoluminescence spectroscopy was used to characterize the system. We observe that the dipolar interlayer excitons couple to SAW and their optical emission is modulated accordingly.   

Nanoscale Imaging of moiré excitons in WS2/WSe2 heterostructures using low-loss EELS.

After the discovery of superconductivity in magic angle bilayer graphene, there is tremendous interest to explore new electronic states arising in 2D heterostructures due to the moiré superlattice potential.¹ Signatures of moiré excitons in 2D semiconductor heterostructures of transition metal dichalcogenides have typically been  probed using far-field optical techniques.    However, the nature of the moiré excitons is difficult to probe using these techniques.Theoretically, these excitons are predicted to have interesting spatial modulations within the moiré unit-cell. In this study, we use low-loss electron energy loss spectroscopy to probe excitons in rotationally aligned WS₂/WSe₂ heterostructures with nanometer-scale spatial resolution to experimentally determine the spatial distribution of the moiré excitons in the superlattice.