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The impressive physics and applications of intra- and interlayer excitons in a transition metal dichalcogenide twisted-bilayer make these systems compelling platforms for exploring the manipulation of their optoelectronic properties through electrical fields. This work studies the electrical control of excitons in twisted MoSe$_2$ homobilayer devices at room temperature. Gate-dependent micro-photoluminescence spectroscopy reveals an energy tunability of several meVs originating from the emission of excitonic complexes. Furthermore, it investigates the twist-angle dependence of valley properties by fabricating devices with distinct stacking angles. Strengthened by density functional theory calculations, the results suggest that, depending on the rotation angle, the conduction band minima and hybridized states at the Q-point promote the formation of inter-valley trions involving the Q-and K-points in the conduction band and the K-point in the valence band. Besides exploring the charge-carrier manipulation in twisted homobilayers, our findings open new avenues for engineering multifunctional optoelectronic devices based on ultrathin semiconducting systems.

Spin and charge are the two most important degrees of freedom of electrons. Their interplay lies at the heart of numerous strongly correlated phenomena including Hubbard model physics and high temperature superconductivity. Such interplay for bosons, on the other hand, is largely unexplored in condensed matter systems. Here we demonstrate a unique realization of the spin-1/2 Bose-Hubbard model through excitons in a semiconducting moir\'e superlattice. We find evidence of a transient in-plane ferromagnetic (FM-$xy$) order of exciton spin - here valley pseudospin - around exciton filling $\nu_{ex}$ = 1, which transitions into a FM-$z$ order both with increasing exciton filling and a small magnetic field of 10 mT. The phase diagram is different from the fermion case and is qualitatively captured by a simple phenomenological model, highlighting the unique consequence of Bose-Einstein statistics. Our study paves the way for engineering exotic phases of matter from spinor bosons, as well as for unconventional devices in optics and quantum information science., Comment: Supplementary Materials attached

One-dimensional (1D) interacting electrons are often described as a Luttinger liquid1-4 having properties that are intrinsically different from Fermi liquids in higher dimensions5,6. 1D electrons in materials systems exhibit exotic quantum phenomena that can be tuned by both intra- and inter-1D-chain electronic interactions, but their experimental characterization can be challenging. Here we demonstrate that layer-stacking domain walls (DWs) in van der Waals heterostructures form a broadly tunable Luttinger liquid system including both isolated and coupled arrays. We have imaged the evolution of DW Luttinger liquids under different interaction regimes tuned by electron density using a novel scanning tunneling microscopy (STM) technique. Single DWs at low carrier density are highly susceptible to Wigner crystallization consistent with a spin-incoherent Luttinger liquid, while at intermediate densities dimerized Wigner crystals form due to an enhanced magneto-elastic coupling. Periodic arrays of DWs exhibit an interplay between intra- and inter-chain interactions that gives rise to new quantum phases. At low electron densities inter-chain interactions are dominant and induce a 2D electron crystal composed of phased-locked 1D Wigner crystal in a staggered configuration. Increased electron density causes intra-chain fluctuation potentials to dominate, leading to an electronic smectic liquid crystal phase where electrons are ordered with algebraical correlation decay along the chain direction but disordered between chains. Our work shows that layer-stacking DWs in 2D heterostructures offers new opportunities to explore Luttinger liquid physics.

Two-dimensional (2D) Janus Transition Metal Dichalcogenides (TMDs) have attracted much interest due to their exciting quantum properties arising from their unique two-faced structure, broken-mirror symmetry, and consequent colossal polarisation field within the monolayer. While efforts have been made to achieve high-quality Janus monolayers, the existing methods rely on highly energetic processes that introduce unwanted grain-boundary and point defects with still unexplored effects on the material's structural and excitonic properties Through High-resolution scanning transmission electron microscopy (HRSTEM), density functional theory (DFT), and optical spectroscopy measurements; this work introduces the most encountered and energetically stable point defects. It establishes their impact on the material's optical properties. HRSTEM studies show that the most energetically stable point defects are single (Vs and Vse) and double chalcogen vacancy (Vs-Vse), interstitial defects (Mi), and metal impurities (MW) and establish their structural characteristics. DFT further establishes their formation energies and related localized bands within the forbidden band. Cryogenic excitonic studies on h-BN-encapsulated Janus monolayers offer a clear correlation between these structural defects and observed emission features, which closely align with the results of the theory. The overall results introduce the defect genome of Janus TMDs as an essential guideline for assessing their structural quality and device properties.

Coulomb interactions in atomically thin materials are uniquely sensitive to variations in the dielectric screening of the environment, which can be used to control quasiparticles and exotic quantum many-body phases. A static approximation of the dielectric response, where increased dielectric screening is predicted to cause an energy redshift of the exciton resonance, has been until now sufficient. Here, we use charge-tunable exciton resonances to study screening effects in transition metal dichalcogenide monolayers embedded in materials with dielectric constants ranging from 4 to more than 1000. In contrast to expectations, we observe a blueshift of the exciton resonance exceeding 30 meV for larger dielectric constant environments. By employing a dynamical screening model, we find that while the exciton binding energy remains mostly controlled by the static dielectric response, the exciton self-energy is dominated by the high-frequency response. Dielectrics with markedly different static and high-frequency screening enable the selective addressing of distinct many-body effects in layered materials and their heterostructures, expanding the tunability range and offering new routes to detect and control correlated quantum many-body states and to design optoelectronic and quantum devices., Comment: 10 pages, 4 figures. SI: 11 pages, 6 figures. We welcome your feedback!

Bosonic condensation and lasing of exciton-polaritons in microcavities is a fascinating solid-state phenomenon. It provides a versatile platform to study out-of-equilibrium many-body physics and has recently appeared at the forefront of quantum technologies. Here, we study the photon statistics via the second-order temporal correlation function of polariton lasing emerging from an optical microcavity integrated with an atomically thin MoSe2 crystal. Furthermore, we investigate the macroscopic polariton phase transition for varying excitation powers and temperatures. The lower-polariton exhibits photon bunching below the threshold, implying a dominant thermal distribution of the emission, while above the threshold, the second-order correlation transits towards unity, which evidences the formation of a coherent state. Our findings are in agreement with a microscopic numerical model, which explicitly includes scattering with phonons on the quantum level., Comment: This manuscript was published in Phys. Rev. Lett., see https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.131.206901

The behavior of two-dimensional electron gas (2DEG) in extreme coupling limits are reasonably well-understood, but our understanding of intermediate region remains limited. Strongly interacting electrons crystalize into a solid phase known as the Wigner crystal at very low densities, and these evolve to a Fermi liquid at high densities. At intermediate densities, however, where the Wigner crystal melts into a strongly correlated electron fluid that is poorly understood partly due to a lack of microscopic probes for delicate quantum phases. Here we report the first imaging of a disordered Wigner solid and its quantum densification and quantum melting behavior in a bilayer MoSe2 using a non-invasive scanning tunneling microscopy (STM) technique. We observe a Wigner solid with nanocrystalline domains pinned by local disorder at low hole densities. With slightly increasing electrostatic gate voltages, the holes are added quantum mechanically during the densification of the disordered Wigner solid. As the hole density is increased above a threshold (p ~ 5.7 * 10e12 (cm-2)), the Wigner solid is observed to melt locally and create a mixed phase where solid and liquid regions coexist. With increasing density, the liquid regions gradually expand and form an apparent percolation network. Local solid domains appear to be pinned and stabilized by local disorder over a range of densities. Our observations are consistent with a microemulsion picture of Wigner solid quantum melting where solid and liquid domains emerge spontaneously and solid domains are pinned by local disorder.

Dark excitons in transition metal dichalcogenides (TMD) have been so far neglected in the context of polariton physics due to their lack of oscillator strength. However, in tungsten-based TMDs, dark excitons are known to be the energetically lowest states and could thus provide important scattering partners for polaritons. In this joint theory-experiment work, we investigate the impact of the full exciton energy landscape on polariton absorption and reflectance. By changing the cavity detuning, we vary the polariton energy relative to the unaffected dark excitons in such a way that we open or close specific phonon-driven scattering channels. We demonstrate both in theory and experiment that this controlled switching of scattering channels manifests in characteristic sharp changes in optical spectra of polaritons. These spectral features can be exploited to extract the position of dark excitons. Our work suggests new possibilities for exploiting polaritons for fingerprinting nanomaterials via their unique exciton landscape.

Semiconductor moir\'e superlattices provide a versatile platform to engineer new quantum solids composed of artificial atoms on moir\'e sites. Previous studies have mostly focused on the simplest correlated quantum solid - the Fermi-Hubbard model - where intra-atom interactions are simplified to a single onsite repulsion energy U. These studies have revealed novel quantum phases ranging from Mott insulators to quantum anomalous Hall insulators at a filling of one electron per moir\'e unit cell. New types of quantum solids should arise at even higher filling factors where the multi-electron configuration of moir\'e artificial atoms provides new degrees of freedom. Here we report the experimental observation of Wigner molecular crystals emerging from multi-electron artificial atoms in twisted bilayer WS2 moir\'e superlattices. Moir\'e artificial atoms, unlike natural atoms, can host qualitatively different electron states due to the interplay between quantized energy levels and Coulomb interactions. Using scanning tunneling microscopy (STM), we demonstrate that Wigner molecules appear in multi-electron artificial atoms when Coulomb interactions dominate. Three-electron Wigner molecules, for example, are seen to exhibit a characteristic trimer pattern. The array of Wigner molecules observed in a moir\'e superlattice comprises a new crystalline phase of electrons: the Wigner molecular crystal. We show that these Wigner molecular crystals are highly tunable through mechanical strain, moir\'e period, and carrier charge type. Our study presents new opportunities for exploring quantum phenomena in moir\'e quantum solids composed of multi-electron artificial atoms.

The combination of repulsive and attractive Coulomb interactions in a quantum electron(e)-hole(h) fluid can give rise to novel correlated phases of multiparticle charge complexes such as excitons, trions and biexcitons. Here we report the first experimental realization of an electrically controlled interlayer trion fluid in two-dimensional van der Waals heterostructures. We demonstrate that in the strong coupling regime of electron-hole bilayers, electrons and holes in separate layers can spontaneously form three-particle trion bound states that resemble positronium ions in high energy physics. The interlayer trions can assume 1e-2h and 2e-1h configurations, where electrons and holes are confined in different transition metal dichalcogenide layers. We show that the two correlated holes in 1e-2h trions form a spin-singlet state with a spin gap of ~1meV. By electrostatic gating, the equilibrium state of our system can be continuously tuned into an exciton fluid, a trion fluid, an exciton-trion mixture, a trion-charge mixture or an electron-hole plasma. Upon optical excitation, the system can host novel high-order multiparticle charge complexes including interlayer four-particle complex (tetrons) and five-particle complex (pentons). Our work demonstrates a unique platform to study novel correlated phases of tunable Bose-Fermi mixtures and opens up new opportunities to realize artificial ions/molecules in electronic devices.

Controlling excitons at the nanoscale in semiconductor materials represents a formidable challenge in the fields of quantum photonics and optoelectronics. Achieving this control holds great potential for unlocking strong exciton-exciton interaction regimes, enabling exciton-based logic operations, exploring exotic quantum phases of matter, facilitating deterministic positioning and tuning of quantum emitters, and designing advanced optoelectronic devices. Monolayers of transition metal dichalcogenides (TMDs) offer inherent two-dimensional confinement and possess significant binding energies, making them particularly promising candidates for achieving electric-field-based confinement of excitons without dissociation. While previous exciton engineering strategies have predominantly focused on local strain gradients, the recent emergence of electrically confined states in TMDs has paved the way for novel approaches. Exploiting the valley degree of freedom associated with these confined states further broadens the prospects for exciton engineering. Here, we show electric control of light polarization emitted from one-dimensional (1D) quantum confined states in MoSe2. By employing non-uniform in-plane electric fields, we demonstrate the in-situ tuning of the trapping potential and reveal how gate-tunable valley-hybridization gives rise to linearly polarized emission from these localized states. Remarkably, the polarization of the localized states can be entirely engineered through either the spatial geometry of the 1D confinement potential or the application of an out-of-plane magnetic field.

Van der Waals heterostructures based on TMDC semiconducting materials have emerged as promising materials due to their spin-valley properties efficiently contrived by the stacking-twist angle. The twist angle drastically alters the interlayer excitonic response by determining the spatial modulation, confining moir\'e potential, and atomic reconstruction in those systems. Nonetheless, the impact of the interlayer twist angle on the band alignment of the monolayers composing the heterostructure has received scant attention in the current research. Here, we systematically investigate the twist-angle dependence of intra- and inter-layer excitons in twisted WSe2/MoSe2 heterobilayers. By performing photoluminescence excitation spectroscopy, we identify the twist-angle dependence of interlayer emission response, where an energy redshift of about 100 meV was observed for increasing twist angles. The applied microscopic theory predicts, on the contrary, a blueshift, which suggests that additional features, such as atomic reconstruction, may also surpass the moir\'e potential confinement. Those findings also prompt the effects of dielectric screening by addressing the redshift response to the stacking layer order. Furthermore, our findings support the evidence of a band offset dependence on the twist angle for the adjacent monolayers composing the heterobilayer system. Our fundamental study of exciton resonances deepens the current understanding of the physics of twisted TMDC heterostructures and paves the way for future experiments and theoretical works.

Strongly coupled two-dimensional electron-hole bilayers can give rise to novel quantum Bosonic states: electrons and holes in electrically isolated layers can pair into interlayer excitons, which can form a Bose-Einstein condensate below a critical temperature at zero magnetic field. This state is predicted to feature perfect Coulomb drag, where a current in one layer must be accompanied by an equal but opposite current in the other, and counterflow superconductivity, where the excitons form a superfluid with zero viscosity. Electron-hole bilayers in the strong coupling limit with an excitonic insulator ground state have been recently achieved in semiconducting transition metal dichalcogenide heterostructures, but direct electrical transport measurements remain challenging. Here we use a novel optical spectroscopy to probe the electrical transport of correlated electron-hole fluids in MoSe2/hBN/WSe2 heterostructures. We observe perfect Coulomb drag in the excitonic insulator phase up to a temperature as high as ~15K. Strongly correlated electron and hole transport is also observed at unbalanced electron and hole densities, although the Coulomb drag is not perfect anymore. Meanwhile, the counterflow resistance of interlayer excitons remains finite. These results indicate the formation of an exciton gas in the excitonic insulator which does not condensate into a superfluid at low temperature. Our work also demonstrates that dynamic optical spectroscopy provides a powerful tool for probing novel exciton transport behavior and possible exciton superfluidity in correlated quantum electron-hole fluids.

Moir\'e superlattices of semiconducting transition metal dichalcogenides (TMDCs) enable unprecedented spatial control of electron wavefunctions in an artificial lattice with periodicities more than ten times larger than that of atomic crystals, leading to emerging quantum states with fascinating electronic and optical properties. The breaking of translational symmetry further introduces a new degree of freedom inside each moir\'e unit cell: high symmetry points of energy minima called moir\'e sites, behaving as spatially separated quantum dots. The superposition of a quasiparticle wavefunction between different moir\'e sites will enable a new platform for quantum information processing but is hindered by the suppressed electron tunneling between moir\'e sites. Here we demonstrate the superposition between two moir\'e sites by constructing an angle-aligned trilayer WSe2/monolayer WS2 moir\'e heterojunction. The two moir\'e sites with energy minimum allow the formation of two different interlayer excitons, with the hole residing in either moir\'e site of the first WSe2 layer interfacing the WS2 layer and the electron in the third WSe2 layer. An external electric field can drive the hybridization of either of the interlayer excitons with the intralayer excitons in the third WSe2 layer, realizing the continuous tuning of interlayer exciton hopping between two moir\'e sites. Therefore, a superposition of the two interlayer excitons localized at different moir\'e sites can be realized, which can be resolved in the electric-field-dependent optical reflectance spectra, distinctly different from that of the natural trilayer WSe2 in which the moir\'e modulation is absent. Our study illustrates a strategy of harnessing the new moir\'e site degree of freedom for quantum information science, a new direction of twistronics.

Strongly enhanced electron-electron interaction in semiconducting moir\'e superlattices formed by transition metal dichalcogenides (TMDCs) heterobilayers has led to a plethora of intriguing fermionic correlated states. Meanwhile, interlayer excitons in a type-II aligned TMDC heterobilayer moir\'e superlattice, with electrons and holes separated in different layers, inherit this enhanced interaction and strongly interact with each other, promising for realizing tunable correlated bosonic quasiparticles with valley degree of freedom. We employ photoluminescence spectroscopy to investigate the strong repulsion between interlayer excitons and correlated electrons in a WS2/WSe2 moir\'e superlattice and combine with theoretical calculations to reveal the spatial extent of interlayer excitons and the band hierarchy of correlated states. We further find that an excitonic Mott insulator state emerges when one interlayer exciton occupies one moir\'e cell, evidenced by emerging photoluminescence peaks under increased optical excitation power. Double occupancy of excitons in one unit cell requires overcoming the energy cost of exciton-exciton repulsion of about 30-40 meV, depending on the stacking configuration of the WS2/WSe2 heterobilayer. Further, the valley polarization of the excitonic Mott insulator state is enhanced by nearly one order of magnitude. Our study demonstrates the WS2/WSe2 moir\'e superlattice as a promising platform for engineering and exploring new correlated states of fermion, bosons, and a mixture of both.

Moir\'e superlattices of transitional metal dichalcogenides exhibit strong electron-electron interaction that has led to experimental observations of Mott insulators and generalized Wigner crystals. In this letter, we report direct measurements of the thermodynamic gaps of these correlated insulating states in a dual-gate WS2/WSe2 moir\'e bilayer. We employ the microwave impedance microscopy to probe the electronic features in both the graphene top gate and the moir\'e bilayer, from which we extract the doping dependence of the chemical potential of the moir\'e bilayer and the energy gaps for various correlated insulating states utilizing the Landau quantization of graphene. These gaps are relatively insensitive to the application of an external electric field to the WS2/WSe2 moir\'e bilayer.

Transition metal dichalcogenide (TMDC) moir\'e superlattices, owing to the moir\'e flatbands and strong correlation, can host periodic electron crystals and fascinating correlated physics. The TMDC heterojunctions in the type-II alignment also enable long-lived interlayer excitons that are promising for correlated bosonic states, while the interaction is dictated by the asymmetry of the heterojunction. Here we demonstrate a new excitonic state, quadrupolar exciton, in a symmetric WSe2-WS2-WSe2 trilayer moir\'e superlattice. The quadrupolar excitons exhibit a quadratic dependence on the electric field, distinctively different from the linear Stark shift of the dipolar excitons in heterobilayers. This quadrupolar exciton stems from the hybridization of WSe2 valence moir\'e flatbands. The same mechanism also gives rise to an interlayer Mott insulator state, in which the two WSe2 layers share one hole laterally confined in one moir\'e unit cell. In contrast, the hole occupation probability in each layer can be continuously tuned via an out-of-plane electric field, reaching 100% in the top or bottom WSe2 under a large electric field, accompanying the transition from quadrupolar excitons to dipolar excitons. Our work demonstrates a trilayer moir\'e system as a new exciting playground for realizing novel correlated states and engineering quantum phase transitions.

Emitter dephasing is one of the key issues in the performance of solid-state single photon sources. Among the various sources of dephasing, acoustic phonons play a central role in adding decoherence to the single photon emission. Here, we demonstrate, that it is possible to tune and engineer the coherence of photons emitted from a single WSe$_2$ monolayer quantum dot via selectively coupling it to a spectral cavity resonance. We utilize an open cavity to demonstrate spectral enhancement, leveling, and suppression of the highly asymmetric phonon sideband, finding excellent agreement with a microscopic description of the exciton-phonon dephasing in a truly two-dimensional system. Moreover, the impact of cavity tuning on the dephasing is directly assessed via optical interferometry, which points out the capability to utilize light-matter coupling to steer and design dephasing and coherence of quantum emitters in atomically thin crystals.

Coupled two-dimensional electron-hole bilayers provide a unique platform to study strongly correlated Bose-Fermi mixtures in condensed matter. Electrons and holes in spatially separated layers can bind to form interlayer excitons, composite Bosons expected to support high-temperature exciton superfluids. The interlayer excitons can also interact strongly with excess charge carriers when electron and hole densities are unequal. Here, we use optical spectroscopy to quantitatively probe the local thermodynamic properties of strongly correlated electron-hole fluids in MoSe2/hBN/WSe2 heterostructures. We observe a discontinuity in the electron and hole chemical potentials at matched electron and hole densities, a definitive signature of an excitonic insulator ground state. The excitonic insulator is stable up to a Mott density of ~$0.8\times {10}^{12} \mathrm{cm}^{-2}$ and has a thermal ionization temperature of ~70 K. The density dependence of the electron, hole, and exciton chemical potentials reveals strong correlation effects across the phase diagram. Compared with a non-interacting uniform charge distribution, the correlation effects lead to significant attractive exciton-exciton and exciton-charge interactions in the electron-hole fluid. Our work highlights the unique quantum behavior that can emerge in strongly correlated electron-hole systems.

Moir\'e superlattices provide a highly tunable and versatile platform to explore novel quantum phases and exotic excited states ranging from correlated insulators1-17 to moir\'e excitons7-10,18. Scanning tunneling microscopy has played a key role in probing microscopic behaviors of the moir\'e correlated ground states at the atomic scale1,11-15,19. Atomic-resolution imaging of quantum excited state in moir\'e heterostructures, however, has been an outstanding experimental challenge. Here we develop a novel photocurrent tunneling microscopy by combining laser excitation and scanning tunneling spectroscopy (laser-STM) to directly visualize the electron and hole distribution within the photoexcited moir\'e exciton in a twisted bilayer WS2 (t-WS2). We observe that the tunneling photocurrent alternates between positive and negative polarities at different locations within a single moir\'e unit cell. This alternating photocurrent originates from the exotic in-plane charge-transfer (ICT) moir\'e exciton in the t-WS2 that emerges from the competition between the electron-hole Coulomb interaction and the moir\'e potential landscape. Our photocurrent maps are in excellent agreement with our GW-BSE calculations for excitonic states in t-WS2. The photocurrent tunneling microscopy creates new opportunities for exploring photoexcited non-equilibrium moir\'e phenomena at the atomic scale.

Two-dimensional Janus transition metal dichalcogenides (TMDs) have attracted attention due to their emergent properties arising from broken mirror symmetry and self-driven polarisation fields. While it has been proposed that their vdW superlattices hold the key to achieving superior properties in piezoelectricity and photovoltiacs, available synthesis has ultimately limited their realisation. Here, we report the first packed vdW nanoscrolls made from Janus TMDs through a simple one-drop solution technique. Our results, including ab-initio simulations, show that the Bohr radius difference between the top sulphur and the bottom selenium atoms within Janus M_Se^S (M=Mo, W) results in a permanent compressive surface strain that acts as a nanoscroll formation catalyst after small liquid interaction. Unlike classical 2D layers, the surface strain in Janus TMDs can be engineered from compressive to tensile by placing larger Bohr radius atoms on top (M_S^Se) to yield inverted C scrolls. Detailed microscopy studies offer the first insights into their morphology and readily formed Moir\'e lattices. In contrast, spectroscopy and FETs studies establish their excitonic and device properties and highlight significant differences compared to 2D flat Janus TMDs. These results introduce the first polar Janus TMD nanoscrolls and introduce inherent strain-driven scrolling dynamics as a catalyst to create superlattices.

Moir\'e heterobilayers host interlayer excitons in a natural, periodic array of trapping potentials. Recent work has elucidated the structure of the trapped interlayer excitons and the nature of photoluminescence (PL) from trapped and itinerant charged complexes such as interlayer trions in these structures. In this paper, our results serve to add to the understanding of the nature of PL emission and explain its characteristic blueshift with increasing carrier density, along with demonstrating a significant difference between the interlayer exciton-trion conversion efficiency as compared to both localized and itinerant intra-layer species in conventional monolayers. Our results show the absence of optical generation of trions in these materials, which we suggest arises from the highly localized, near sub-nm confinement of trapped species in these Moir\'e potentials., Comment: 3 figures, Supplementary information available on request

Janus transition metal dichalcogenides are an emerging class of atomically thin materials with engineered broken mirror symmetry that gives rise to long-lived dipolar excitons, Rashba splitting, and topologically protected solitons. They hold great promise as a versatile nonlinear optical platform due to their broadband harmonic generation tunability, ease of integration on photonic structures, and nonlinearities beyond the basal crystal plane. Here, we study second and third harmonic generation in MoSSe and WSSe Janus monolayers. We use polarization-resolved spectroscopy to map the full second-order susceptibility tensor of MoSSe, including its out-of-plane components. In addition, we measure the effective third-order susceptibility, and the second-order nonlinear dispersion close to exciton resonances for both MoSSe and WSSe at room and cryogenic temperatures. Our work sets a bedrock for understanding the nonlinear optical properties of Janus transition metal dichalcogenides and probing their use in the next-generation on-chip multifaceted photonic devices., Comment: 10 pages, 3 figures. SI: 8 pages, 5 figures

We revisit and extend the standard bosonic interpretation of interlayer excitons in the moir\'e potential of twisted heterostructures of transition-metal dichalcogenides. In our experiments, we probe a high quality MoSe$_2$/WSe$_2$ van der Waals bilayer heterostructure via density-dependent photoluminescence spectroscopy and reveal strongly developed, unconventional spectral shifts of the emergent moir\'e exciton resonances. The observation of saturating blueshifts of successive exciton resonances allow us to explain their physics in terms of a model utilizing fermionic saturable absorbers. This approach is strongly inspired by established quantum-dot models, which underlines the close analogy of interlayer excitons trapped in pockets of the moir\'e potential, and quantum emitters with discrete eigenstates.

Solid-state single photon sources are central building blocks in quantum communication networks and on-chip quantum information processing. Atomically thin crystals were established as possible candidates to emit non-classical states of light, however, the performance of monolayer-based single photon sources has so far been lacking behind state-of-the-art devices based on volume crystals. Here, we implement a single photon source based on an atomically thin sheet of WSe2 coupled to a spectrally tunable optical cavity. It is characterized by a high single photon purity with a $g^{(2)}(0)$ value as low as $4.7 \pm 0.7 \%$ and a record-high first lens brightness of linearly polarized photons as large as $65 \pm 4 \%$. Interestingly, the high performance of our devices allows us to observe genuine quantum interference phenomena in a Hong-Ou-Mandel experiment. Our results demonstrate that open cavities and two-dimensional materials constitute an excellent platform for ultra-bright quantum light sources: the unique properties of such two-dimensional materials and the versatility of open cavities open an inspiring avenue for novel quantum optoelectronic devices., Comment: 12 pages, 7 figures

Janus transition-metal dichalcogenide monolayers are fully artificial materials, where one plane of chalcogen atoms is replaced by chalcogen atoms of a different type. Theory predicts an in-built out-of-plane electric field, giving rise to long-lived, dipolar excitons, while preserving direct-bandgap optical transitions in a uniform potential landscape. Previous Janus studies had broad photoluminescence (>15 meV) spectra obfuscating their excitonic origin. Here, we identify the neutral, and negatively charged inter- and intravalley exciton transitions in Janus WSeS monolayer with $\sim 6$ meV optical linewidth. We combine a recently developed synthesis technique, with the integration of Janus monolayers into vertical heterostructures, allowing doping control. Further, magneto-optic measurements indicate that monolayer WSeS has a direct bandgap at the K points. This work provides the foundation for applications such as nanoscale sensing, which relies on resolving excitonic energy shifts, and photo-voltaic energy harvesting, which requires efficient creation of long-lived excitons and integration into vertical heterostructures.

Ultrafast charge transfer processes provide a facile way to create interlayer excitons in directly contacted transition metal dichalcogenide (TMD) layers. More sophisticated heterostructures composed of TMD/hBN/TMD enable new ways to control interlayer exciton properties and achieve novel exciton phenomena, such as exciton insulators and condensates, where longer lifetimes are desired. In this work, we experimentally study the charge transfer dynamics in a heterostructure composed of a 1 nm thick hBN spacer between MoSe$_{2}$ and WSe$_{2}$ monolayers. We observe the hole transfer from MoSe$_{2}$ to WSe$_{2}$ through the hBN barrier with a time constant of 500 ps, which is over 3 orders of magnitude slower than that between TMD layers without a spacer. Furthermore, we observe strong competition between the interlayer charge transfer and intralayer exciton-exciton annihilation processes at high excitation densities. Our work opens possibilities to understand charge transfer pathways in TMD/hBN/TMD heterostructures for the efficient generation and control of interlayer excitons.

Transition metal dichalcogenide-based moire superlattices exhibit very strong electron-electron correlations, thus giving rise to strongly correlated quantum phenomena such as generalized Wigner crystal states. Theoretical studies predict that unusual quasiparticle excitations across the correlated gap between upper and lower Hubbard bands can arise due to long-range Coulomb interactions in generalized Wigner crystal states. Here we describe a new scanning single-electron charging (SSEC) spectroscopy technique with nanometer spatial resolution and single-electron charge resolution that enables us to directly image electron and hole wavefunctions and to determine the thermodynamic gap of generalized Wigner crystal states in twisted WS2 moire heterostructures. High-resolution SSEC spectroscopy was achieved by combining scanning tunneling microscopy (STM) with a monolayer graphene sensing layer, thus enabling the generation of individual electron and hole quasiparticles in generalized Wigner crystals. We show that electron and hole quasiparticles have complementary wavefunction distributions and that thermodynamic gaps of order 50meV exist for the 1/3 and 2/3 generalized Wigner crystal states.

The formation of a charge density wave state is characterized by an order parameter. The way it is established provides unique information on both the role that correlation plays in driving the charge density wave formation and the mechanism behind its formation. Here we use time and angle resolved photoelectron spectroscopy to optically perturb the charge-density phase in 1T-TiSe$_2$ and follow the recovery of its order parameter as a function of energy, momentum and excitation density. Our results reveal that two distinct orders contribute to the gap formation, a CDW order and pseudogap-like order, manifested by an overall robustness to optical excitation. A detailed analysis of the magnitude of the the gap as a function of excitation density and delay time reveals the excitonic long-range nature of the CDW gap and the short-range Jahn-Teller character of the pseudogap order. In contrast to the gap, the intensity of the folded Se$_{4p}$* band can only give access to the excitonic order. These results provide new information into the the long standing debate on the origin of the gap in TiSe$_2$ and place it in the same context of other quantum materials where a pseudogap phase appears to be a precursor of long-range order., Comment: 6 pages, 4 figures

High mobility is a crucial requirement for a large variety of electronic device applications. The state-of-the-art for high quality graphene devices is based on heterostructures made with graphene encapsulated in $>80\,$nm-thick flakes of hexagonal boron nitride (hBN). Unfortunately, scaling up multilayer hBN while precisely controlling the number of layers remains an elusive challenge, resulting in a rough material unable to enhance the mobility of graphene. This leads to the pursuit of alternative, scalable materials, which can be simultaneously used as substrate and encapsulant for graphene. Tungsten disulfide (WS$_2$) is a transition metal dichalcogenide, which was successfully grown in large ($\sim$mm-size) multi-layers by chemical vapour deposition. However, the resistance \textit{vs} gate voltage characteristics when gating graphene through WS$_2$ exhibit largely hysteretic shifts of the charge neutrality point (CNP) in the order of $\Delta n\sim$2.6$\cdot$10$^{11}$ cm$^{-2}$, hindering the use of WS$_2$ as a reliable encapsulant. The hysteresis originates due to the charge traps from sulfur vacancies present in WS$_2$. In this work, we report for the first time the use of WS$_2$ as a substrate and the overcoming of hysteresis issues by chemically treating WS$_2$ with a super-acid, which passivates these vacancies and strips the surface from contaminants. The hysteresis is significantly reduced below the noise level by at least a factor five (to $\Delta n<$5$\cdot$10$^{10}$ cm$^{-2}$) and, simultaneously, the room-temperature mobility of WS$_2$-encapsulated graphene is as high as $\sim$6.2$\cdot$10$^{4}$ cm$^{-2}$V$^{-1}$s$^{-1}$ at a carrier density $n$ $\sim$1$\cdot$ 10$^{12}$ cm$^{-2}$. Our results promote WS$_2$ to a valid alternative to hBN as encapsulant for high-performance graphene devices.

A panoply of unconventional electronic states has been observed in moir\'e superlattices. Engineering similar bosonic phases remains, however, largely unexplored. We report the observation of a bosonic correlated insulator in WSe2/WS2 moir\'e superlattices composed of excitons, i.e., tightly bound electron-hole pairs. We develop a pump probe spectroscopy method that we use to observe an exciton incompressible state at exciton filling {\nu}_ex = 1 and charge neutrality, indicating a correlated insulator of excitons. With varying charge density, the bosonic correlated insulator continuously transitions into an electron correlated insulator at charge filling {\nu}_e = 1, suggesting a mixed correlated insulating state between the two limits. Our studies establish semiconducting moir\'e superlattices as an intriguing platform for engineering bosonic phases., Comment: Science (2023)

We present a comprehensive theory of the magnetic phases in monolayer nickel-based Janus dihalides (NiIBr, NiICl, NiBrCl) through a combination of first-principles calculations and atomistic simulations. Phonon band structure calculations and finite temperature molecular dynamics simulations show that Ni-dihalide Janus monolayers are stable. The parameters of the interacting spin Hamiltonian extracted from ab initio calculations show that nickel-dihalide Janus monolayers exhibit varying degrees of magnetic frustration together with a large Dzyaloshinskii-Moriya interaction due to their inherent inversion symmetry breaking. The atomistic simulations reveal a delicate interplay between these two competing magnetic interactions in giving rise to skyrmion formation.

Understanding the origin of valence band maxima (VBM) splitting in transition metal dichalcogenides (TMDs) is important because it governs the unique spin and valley physics in monolayer and multilayer TMDs. In this work, we present our systematic study of VBM splitting ($\Delta$) in atomically thin MoS$_2$ and WS$_2$ by employing photocurrent spectroscopy as we change the temperature and the layer numbers. We found that VBM splitting in monolayer MoS$_2$ and WS$_2$ depends strongly on temperature, which contradicts the theory that spin-orbit coupling solely determines the VBM splitting in monolayer TMDs. We also found that the rate of change of VBM splitting with respect to temperature ($m=\frac{\partial\Delta}{\partial T}$) is the highest for monolayer (-0.14 meV/K for MoS$_2$) and the rate decreases as the layer number increases ($m ~ 0$ meV/K for 5 layers MoS$_2$). We performed density functional theory (DFT) and the GW with Bethe-Salpeter Equation (GW-BSE) calculations to determine the electronic band structure and optical absorption for a bilayer MoS$_2$ with different interlayer separations. Our simulations agree with the experimental observations and demonstrate that the temperature dependence of VBM splitting in atomically thin monolayer and multilayer TMDs originates from the changes in the interlayer coupling strength between the neighboring layers. By studying two different types of TMDs and many different layer thicknesses, we also demonstrate that VBM splitting also depends on the layer numbers and type of transition metals. Our study will help understand the role spin-orbit coupling and interlayer interaction play in determining the VBM splitting in quantum materials and develop next-generation devices based on spin-orbit interactions., Comment: 15 pages, 5 figures (accepted to JPCC)

Charge density waves (CDW) are states of broken symmetry with a periodic modulation of charge and lattice typically leading to the opening of a gap in the band structure. In the model CDW system 1T-TiSe$_2$ such a gap opens up between its Se$_{4p}$ valence and Ti$_{3d}$ conduction band, accompanied by a change of dispersion. These changes are crucial in understanding the CDW phase, as they provide a measure of the Se$_{4p}$-Ti$_{3d}$ hybridization strength and characteristic mechanistic features. Using time- and angle-resolved photoelectron spectroscopy (trARPES), the unoccupied band structure is populated with near-infrared (NIR) pump pulses which allows to to directly visualize the parabolically-shaped Ti$_{3d}$ conduction band. Furthermore, we observe a transient change of effective mass in the Se$_{4p}$ valence band following photoexcitation. This occurs alongside an overall reduction due to weakening of the CDW phase and is accompanied by an oscillating component with the frequency of the characteristic A$_{1g}$ phonon. These observations, enabled by trAPRES, highlight the importance of the lattice contributions in establishing the CDW order in 1T-TiSe$_2$., Comment: 4 figures

Engineering the properties of quantum materials via strong light-matter coupling is a compelling research direction with a multiplicity of modern applications. Those range from modifying charge transport in organic molecules, steering particle correlation and interactions, and even controlling chemical reactions. Here, we study the modification of the material properties via strong coupling and demonstrate an effective inversion of the excitonic band-ordering in a monolayer of WSe2 with spin-forbidden, optically dark ground state. In our experiments, we harness the strong light-matter coupling between cavity photon and the high energy, spin-allowed bright exciton, and thus creating two bright polaritonic modes in the optical bandgap with the lower polariton mode pushed below the WSe2 dark state. We demonstrate that in this regime the commonly observed luminescence quenching stemming from the fast relaxation to the dark ground state is prevented, which results in the brightening of this intrinsically dark material. We probe this effective brightening by temperature-dependent photoluminescence, and we find an excellent agreement with a theoretical model accounting for the inversion of the band ordering and phonon-assisted polariton relaxation.

Two-dimensional (2D) materials possess great potential for flexible devices, ascribing to their outstanding electrical, optical, and mechanical properties. However, their mechanical deformation property and fracture mechanism, which are inescapable in many applications like flexible optoelectronics, are still unclear or not thoroughly investigated due methodology limitations. In light of this, such mechanical properties and mechanisms are explored on example of gallium telluride (GaTe), a promising optoelectronic candidate with an ultrahigh photo-responsibility and a high plasticity within 2D family. Considering the driving force insufficient in atomic force microscopy (AFM)-based nanoindentation method, here the mechanical properties of both substrate-supported and suspended GaTe multilayers were systematically investigated through full-scale Berkovich-tip nanoindentation, micro-Raman spectroscopy, AFM, and scanning electron microscopy. An unusual concurrence of multiple pop-in and load-drop events in loading curve was observed. By further correlating to molecular dynamics calculations, this concurrence was unveiled originating from the interlayer sliding mediated layers-by-layers fracture mechanism within GaTe multilayers. The van der Waals force between GaTe multilayers and substrates was revealed much stronger than that between GaTe interlayers, resulting in the easy sliding and fracture of multilayers within GaTe. This work provides new insights into the deformation and fracture mechanisms of GaTe and other similar 2D multilayers in flexible applications., Comment: 21 pages, 5 figures

Moir\'e patterns of transition metal dichalcogenide (TMD) heterobilayers have proven to be an ideal platform to host unusual correlated electronic phases, emerging magnetism, and correlated exciton physics. While the existence of novel moir\'e excitonic states is established through optical measurements, the microscopic nature of these states is still poorly understood, often relying on empirically fit models. Here, combining large-scale first-principles GW-BSE calculations and micro-reflection spectroscopy, we identify the nature of the exciton resonances in WSe$_2$/WS$_2$ moir\'e superlattices, discovering a surprisingly rich set of moir\'e excitons that cannot be even qualitatively captured by prevailing continuum models. Our calculations reveal moir\'e excitons with distinct characters, including modulated Wannier excitons and previously unindentified intralayer charge-transfer excitons. Signatures of these distinct excitonic characters are confirmed experimentally via the unique carrier-density and magnetic-field dependences of different moir\'e exciton resonances. Our study highlights the highly non-trivial exciton states that can emerge in TMD moir\'e superlattices, and suggests novel ways of tuning many-body physics in moir\'e systems by engineering excited-states with specific spatial characters.

Moir\'e superlattices in van der Waals heterostructures have emerged as a powerful tool for engineering novel quantum phenomena. Here we report the observation of a correlated interlayer exciton insulator in a double-layer heterostructure composed of a WSe2 monolayer and a WS2/WSe2 moir\'e bilayer that are separated by an ultrathin hexagonal boron nitride (hBN). The moir\'e WS2/WSe2 bilayer features a Mott insulator state at hole density p/p0 = 1, where p0 corresponds to one hole per moir\'e lattice site. When electrons are added to the Mott insulator in the WS2/WSe2 moir\'e bilayer and an equal number of holes are injected into the WSe2 monolayer, a new interlayer exciton insulator emerges with the holes in the WSe2 monolayer and the electrons in the doped Mott insulator bound together through interlayer Coulomb interactions. The excitonic insulator is stable up to a critical hole density of ~ 0.5p0 in the WSe2 monolayer, beyond which the system becomes metallic. Our study highlights the opportunities for realizing novel quantum phases in double-layer moir\'e systems due to the interplay between the moir\'e flat band and strong interlayer electron interactions.

The Wigner crystal state, first predicted by Eugene Wigner in 1934, has fascinated condensed matter physicists for nearly 90 years2-14. Studies of two-dimensional (2D) electron gases first revealed signatures of the Wigner crystal in electrical transport measurements at high magnetic fields2-4. More recently optical spectroscopy has provided evidence of generalized Wigner crystal states in transition metal dichalcogenide (TMDC) moir\'e superlattices. Direct observation of the 2D Wigner crystal lattice in real space, however, has remained an outstanding challenge. Scanning tunneling microscopy (STM) in principle has sufficient spatial resolution to image a Wigner crystal, but conventional STM measurements can potentially alter fragile Wigner crystal states in the process of measurement. Here we demonstrate real-space imaging of 2D Wigner crystals in WSe2/WS2 moir\'e heterostructures using a novel non-invasive STM spectroscopy technique. We employ a graphene sensing layer in close proximity to the WSe2/WS2 moir\'e superlattice for Wigner crystal imaging, where local STM tunneling current into the graphene sensing layer is modulated by the underlying electron lattice of the Wigner crystal in the WSe2/WS2 heterostructure. Our measurement directly visualizes different lattice configurations associated with Wigner crystal states at fractional electron fillings of n = 1/3, 1/2, and 2/3, where n is the electron number per site. The n=1/3 and n=2/3 Wigner crystals are observed to exhibit a triangle and a honeycomb lattice, respectively, in order to minimize nearest-neighbor occupations. The n = 1/2 state, on the other hand, spontaneously breaks the original C3 symmetry and forms a stripe structure in real space. Our study lays a solid foundation toward the fundamental understanding of rich Wigner crystal states in WSe2/WS2 moir\'e heterostructures.

The emergence of spatial and temporal coherence of light emitted from solid-state systems is a fundamental phenomenon, rooting in a plethora of microscopic processes. It is intrinsically aligned with the control of light-matter coupling, and canonical for laser oscillation. However, it also emerges in the superradiance of multiple, phase-locked emitters, and more recently, coherence and long-range order have been investigated in bosonic condensates of thermalized light, as well as in exciton-polaritons driven to a ground state via stimulated scattering. Here, we experimentally show that the interaction between photons in a Fabry-Perot microcavity and excitons in an atomically thin WSe$_2$ layer is sufficient such that the system enters the hybridized regime of strong light-matter coupling at ambient conditions. Via Michelson interferometry, we capture clear evidence of increased spatial and temporal coherence of the emitted light from the spatially confined system ground-state. The coherence build-up is accompanied by a threshold-like behaviour of the emitted light intensity, which is a fingerprint of a polariton laser effect. Valley-physics is manifested in the presence of an external magnetic field, which allows us to manipulate K and K' polaritons via the Valley-Zeeman-effect. Our findings are of high application relevance, as they confirm the possibility to use atomically thin crystals as simple and versatile components of coherent light-sources, and in valleytronic applications at room temperature., Comment: 13 pages, 4 figures

Engineering non-linear hybrid light-matter states in tailored optical lattices is a central research strategy for the simulation of complex Hamiltonians. Excitons in atomically thin crystals are an ideal active medium for such purposes, since they couple strongly with light and bear the potential to harness giant non-linearities and interactions while presenting a simple sample-processing and room temperature operability. We demonstrate lattice polaritons, based on an open, high-quality optical cavity, with an imprinted photonic lattice strongly coupled to excitons in a WS$_{2}$ monolayer. We experimentally observe the emergence of the canonical band-structure of particles in a one-dimensional lattice at room temperature, and demonstrate frequency reconfigurability over a spectral window exceeding 12 meV, as well as the systematic variation of the nearest neighbour coupling, reflected by a tuneability in the bandwidth of the p-band polaritons by 7 meV. The technology presented in this work is a critical demonstration towards reconfigurable photonic emulators operated with non-linear photonic fluids, offering a simple experimental implementation and working at ambient conditions.