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Van der Waals (vdW) materials have opened up many avenues for discovery through layer assembly, as epitomized by interlayer dipolar excitons that exhibit electrically tunable luminescence, lasing and exciton condensation. Extending interlayer excitons to more vdW layers, however, raises fundamental questions concerning coherence within excitons and coupling between moiré superlattices at multiple interfaces. Here, by assembling angle-aligned WSe2/WS2/WSe2 heterotrilayers, we demonstrate the emergence of quadrupolar excitons. We confirm the exciton’s quadrupolar nature by the decrease in its energy of 12 meV from coherent hole tunnelling between the two outer layers, its tunable static dipole moment under an external electric field and the reduced exciton–exciton interactions. At high exciton density, we also see signatures of a phase of oppositely aligned dipolar excitons, consistent with a staggered dipolar phase predicted to be driven by attractive dipolar interactions. Our demonstration paves the way for discovering emergent exciton orderings for three vdW layers and beyond.The authors report the emergence of quadrupolar excitons in angle-aligned WSe2/WS2/WSe2 heterotrilayers characterized by a delocalized hole residing in both outer WSe2 layers, electric-field tunability and reduced exciton–exciton interactions.

Atomically thin semiconductor heterostructures provide a two-dimensional (2D) device platform for creating high densities of cold, controllable excitons. Interlayer excitons (IEs), bound electrons and holes localized to separate 2D quantum well layers, have permanent out-of-plane dipole moments and long lifetimes, allowing their spatial distribution to be tuned on demand. Here, we employ electrostatic gates to trap IEs and control their density. By electrically modulating the IE Stark shift, electron-hole pair concentrations above 2 × 10 12  cm −2 can be achieved. At this high IE density, we observe an exponentially increasing linewidth broadening indicative of an IE ionization transition, independent of the trap depth. This runaway threshold remains constant at low temperatures, but increases above 20 K, consistent with the quantum dissociation of a degenerate IE gas. Our demonstration of the IE ionization in a tunable electrostatic trap represents an important step towards the realization of dipolar exciton condensates in solid-state optoelectronic devices. Here, the authors use electrostatic gates to trap interlayer excitons (IE) in MoSe 2 /WSe 2 heterobilayers. They observe an exponential broadening of the IE emission linewidth that signals the IE ionization threshold.

A van der Waals heterostructure built from atomically thin semiconducting transition metal dichalcogenides (TMDs) enables the formation of excitons from electrons and holes in distinct layers, producing interlayer excitons with large binding energy and a long lifetime. By employing heterostructures of monolayer TMDs, we realize optical and electrical generation of long-lived neutral and charged interlayer excitons. We demonstrate that neutral interlayer excitons can propagate across the entire sample and that their propagation can be controlled by excitation power and gate electrodes. We also use devices with ohmic contacts to facilitate the drift motion of charged interlayer excitons. The electrical generation and control of excitons provide a route for achieving quantum manipulation of bosonic composite particles with complete electrical tunability.

Near-field coupling to surface plasmon polaritons enables the observation of spin-forbidden dark excitonic states in monolayer WSe 2 . Transition metal dichalcogenide (TMD) monolayers with a direct bandgap feature tightly bound excitons, strong spin–orbit coupling and spin–valley degrees of freedom 1 , 2 , 3 , 4 . Depending on the spin configuration of the electron–hole pairs, intra-valley excitons of TMD monolayers can be either optically bright or dark 5 , 6 , 7 , 8 . Dark excitons involve nominally spin-forbidden optical transitions with a zero in-plane transition dipole moment 9 , making their detection with conventional far-field optical techniques challenging. Here, we introduce a method for probing the optical properties of two-dimensional materials via near-field coupling to surface plasmon polaritons (SPPs). This coupling selectively enhances optical transitions with dipole moments normal to the two-dimensional plane, enabling direct detection of dark excitons in TMD monolayers. When a WSe 2 monolayer is placed on top of a single-crystal silver film 10 , its emission into near-field-coupled SPPs displays new spectral features whose energies and dipole orientations are consistent with dark neutral and charged excitons. The SPP-based near-field spectroscopy significantly improves experimental capabilities for probing and manipulating exciton dynamics of atomically thin materials, thus opening up new avenues for realizing active metasurfaces and robust optoelectronic systems, with potential applications in information processing and communication 11 .

For years, scientists have imagined the possibilities of 2D materials, pondering how they might behave differently from their thicker counterparts, with predictions pointing towards a paradigm shift in physics when transitioning from the bulk to the atomically thin. Such speculations proposed that the reduced dimensionality would lead to a host of unprecedented physical properties, in particular not observed before half-integer quantum Hall effect and many more. Yet, it was not until the seminal discovery of graphene in 2004—a single layer of carbon atoms — that the exploration of two-dimensional Van der Waals (vdW) materials truly gained momentum. The defining characteristic of these materials lies in their structure, where each layer is interconnected by weak out-of-plane vdW forces, facilitating the exfoliation to smooth single layers, without dangling bonds. These layers exhibit remarkable electronic, thermal, and optoelectronic properties previously unseen. Initial investigations were primarily aimed at deciphering these groundbreaking properties.As the field matured, the intrigue surrounding 2D monolayers evolved, focusing on the possibility of engineering novel materials and exploring new physics by layering different monolayers, each with distinct properties. This method has been particularly successful in the exploration of transition metal dichalcogenides (TMDs), where early research into the properties of monolayer TMDs and their heterostructures has led to significant advancements in fields such as valleytronics, exciton condensates, and the development of novel optoelectronic devices.More recently, the ability to adjust the twist angle between layers introduced the concept of moiré superlattices, presenting even more fascinating physics, including Mott insulating states and Wigner crystal phases. Despite these advancements, the intricate process of fabricating such heterostructures poses considerable challenges, leaving the exploration of more complex moiré heterostructures as a relatively untapped area with the potential to reveal unprecedented new physics. In this thesis, I explore the optical characterization of novel complex TMD heterostructure devices influenced by moiré superlattices.I first present the first conclusive demonstration of so-called quadrupolar excitons in symmetrically stacked vdW heterotrilayers, predicted by theory to exhibit new excitonic states and novel quantum phases. By using sophisticated sample fabrication techniques - including precise allignment of TMD layers, in-situ second harmonic generation and edge contacts—we assemble angle-aligned trilayer structures composed of WSe2/WS2/WSe2 monolayers. By probing these devices by methods such as electrostatic gating, reflectance contrast spectroscopy, photoluminescence spectroscopy and lifetime measurements, we demonstrate the existence of quadrupolar excitons in such heterostructures. We also investigate the interplay between the two moir´e lattices at the interfaces of WSe2/WS and WS2/WSe2and their influence on optical properties of the devices.Next, I explore the dynamics of resident electron’s spin in highly aligned MoSe2/WS2moir´e superlattices. By using pump-probe technique, we measure reflectance magnetic circular dichroism signal from the heterostructure, from which we get valley polarization and spin relaxation times of resident electrons. We find that the later is three orders of magnitude longer than in monolayers and about an order of magnitude longer than in unaligned structures, which we attribute to suppressed momentum scattering for confined electrons in moir´e superlattices. In addition, we find that the efficiency of angular momentum transfer from excitons to electron spins is exeptionaly high.

Moiré materials provide fertile ground for the correlated and topological quantum phenomena. Among them, the quantum anomalous Hall (QAH) effect, in which the Hall resistance is quantized even under zero magnetic field, is a direct manifestation of the intrinsic topological properties of a material and an appealing attribute for low-power electronics applications. The QAH effect has been observed in both graphene and transition metal dichalcogenide (TMD) moiré materials. It is thought to arise from the interaction-driven valley polarization of the narrow moiré bands. Here, we show that the newly discovered QAH state in AB-stacked MoTe2/WSe2 moiré bilayers is not valley polarized but valley coherent. The layer- and helicity-resolved optical spectroscopy measurement reveals that the QAH ground state possesses spontaneous spin (valley) polarization aligned (antialigned) in two TMD layers. In addition, saturation of the out-of-plane spin polarization in both layers occurs only under high magnetic fields, supporting a canted spin texture. Our results call for a new mechanism for the QAH effect and highlight the potential of TMD moiré materials with strong electronic correlations and spin-orbit interactions for exotic topological states.

Moiré materials provide fertile ground for the correlated and topological quantum phenomena. Among them, the quantum anomalous Hall (QAH) effect, in which the Hall resistance is quantized even under zero magnetic field, is a direct manifestation of the intrinsic topological properties of a material and an appealing attribute for low-power electronics applications. The QAH effect has been observed in both graphene and transition metal dichalcogenide (TMD) moiré materials. It is thought to arise from the interaction-driven valley polarization of the narrow moiré bands. Here, we show that the newly discovered QAH state in AB-stacked MoTe 2 / W S e 2 moiré bilayers is not valley polarized but valley coherent. The layer- and helicity-resolved optical spectroscopy measurement reveals that the QAH ground state possesses spontaneous spin (valley) polarization aligned (antialigned) in two TMD layers. In addition, saturation of the out-of-plane spin polarization in both layers occurs only under high magnetic fields, supporting a canted spin texture. Our results call for a new mechanism for the QAH effect and highlight the potential of TMD moiré materials with strong electronic correlations and spin-orbit interactions for exotic topological states.

Moiré materials provide fertile ground for the correlated and topological quantum phenomena. Among them, the quantum anomalous Hall (QAH) effect, in which the Hall resistance is quantized even under zero magnetic field, is a direct manifestation of the intrinsic topological properties of a material and an appealing attribute for low-power electronics applications. The QAH effect has been observed in both graphene and transition metal dichalcogenide (TMD) moiré materials. It is thought to arise from the interaction-driven valley polarization of the narrow moiré bands. Here, we show that the newly discovered QAH state in AB-stacked MoTe_{2}/WSe_{2} moiré bilayers is not valley polarized but valley coherent. The layer- and helicity-resolved optical spectroscopy measurement reveals that the QAH ground state possesses spontaneous spin (valley) polarization aligned (antialigned) in two TMD layers. In addition, saturation of the out-of-plane spin polarization in both layers occurs only under high magnetic fields, supporting a canted spin texture. Our results call for a new mechanism for the QAH effect and highlight the potential of TMD moiré materials with strong electronic correlations and spin-orbit interactions for exotic topological states.

Language models can be prompted to reason through problems in a manner that significantly improves performance. However, \\textit{why} such prompting improves performance is unclear. Recent work showed that using logically \\textit{invalid} Chain-of-Thought (CoT) prompting improves performance almost as much as logically \\textit{valid} CoT prompting, and that editing CoT prompts to replace problem-specific information with abstract information or out-of-distribution information typically doesn't harm performance. Critics have responded that these findings are based on too few and too easily solved tasks to draw meaningful conclusions. To resolve this dispute, we test whether logically invalid CoT prompts offer the same level of performance gains as logically valid prompts on the hardest tasks in the BIG-Bench benchmark, termed BIG-Bench Hard (BBH). We find that the logically \\textit{invalid} reasoning prompts do indeed achieve similar performance gains on BBH tasks as logically valid reasoning prompts. We also discover that some CoT prompts used by previous works contain logical errors. This suggests that covariates beyond logically valid reasoning are responsible for performance improvements.