Angle-resolved photoemission spectroscopy (ARPES) has recently emerged as a direct probe of excitonic correlations in two-dimensional semiconductors, resolving their dispersion and dynamics in energy–momentum space, including dark exciton states inaccessible to optical techniques. However, the ARPES fingerprint of charged excitons (trions), which plays a key role in all doped and gated two-dimensional (2D) material systems, has remained unknown so far. We present a first theoretical analysis of trion signatures in monolayer transition-metal dichalcogenides, highlighting how the additional charge carrier modifies the spectral position and shape relative to neutral excitons in ARPES spectra. Interestingly, we further predict that mass-imbalanced trions yield a characteristic double-peak structure, clearly separated in energy and line shape from neutral excitons. The predicted temperature dependence of these features offers guidance for experimental investigations aimed at identifying trionic states, thereby establishing a framework for ARPES studies of many-body Coulomb complexes in doped 2D semiconductors.

Extended efforts have been devoted to the study of strongly-interacting excitons and their dynamics, towards macroscopic quantum states of matter such as Bose-Einstein condensates of excitons and polaritons. Momentum-direct layer-hybridized excitons in transition metal dichalcogenides have attracted considerable attention due to their high oscillator strength and dipolar nature. However, the tunability of their interactions and dynamics remains unexplored. Here, we achieve an unprecedented control over the nonlinear properties of dipolar layer-hybridized excitons in an electrically gated van der Waals homobilayer monitored by transient optical spectroscopy. By applying a vertical electric field, we reveal strong Coulomb interactions of dipolar hybrid excitons, leading to opposite density-dependent energy shifts of the two main hybrid species based on their dipolar orientation, together with a strongly enhanced optical saturation of their absorption. Furthermore, by electrically tuning the interlayer tunneling between the hybridized carriers, we significantly extend the formation time of hybrid excitons, while simultaneously increasing their decay times. Our findings have implications for the search on quantum blockade and condensation of excitons and dipolaritons in two-dimensional materials.

Moiré materials have emerged as a powerful platform for exploring exotic quantum phases. While recent experiments have unveiled fractional Chern insulators exhibiting the fractional quantum anomalous Hall effect based on electrons or holes, the exploration of analogous many-body states with bosonic constituents remains largely uncharted. In this work, we predict the emergence of bosonic fractional Chern insulators arising from long-lived excitons in a moiré superlattice formed by twisted bilayer WSe₂ stacked on monolayer MoSe₂. Performing exact diagonalization on the exciton flat Chern band present in this structure, we provide compelling evidence for the existence of Abelian and non-Abelian phases at band filling and 1, respectively, through multiple robust signatures, including ground-state degeneracy, spectral flow, many-body Chern number, and particle-cut entanglement spectrum. The obtained energy gap of ∼10 meV for the Abelian states suggests a remarkably high stability of this phase, which persists for a relatively wide range of twist angles and vertical electric fields. Our findings establish the presence of robust bosonic fractional Chern insulators in highly tunable and experimentally accessible moiré heterostructures and unveil a promising pathway for realizing non-Abelian anyons.

Moiré materials provide a remarkably tunable platform for topological and strongly correlated quantum phases of matter. Very recently, the first Abelian fractional Chern insulators (FCIs) at zero magnetic field have been experimentally demonstrated, and it has been theoretically predicted that non-Abelian states with Majorana fermion excitations may be realized in the nearly dispersionless minibands of these systems. Here, we provide telltale evidence based on many-body exact diagonalization for the even more exotic possibility of moiré-based non-Abelian FCIs exhibiting Fibonacci parafermion excitations. In particular, we obtain low-energy quantum numbers, spectral flow, many-body Chern numbers, and entanglement spectra consistent with the Read–Rezayi parafermion phase in an exemplary moiré system with tunable quantum geometry. Our results hint towards the robustness of moiré-based parafermions and encourage the pursuit in moiré systems of these non-Abelian quasiparticles that are superior candidates for topological quantum computing.

The realization of fractional Chern insulators in moiré materials has sparked the search for further novel phases of matter in this platform. In particular, recent works have demonstrated the possibility of realizing quantum anomalous Hall crystals (QAHCs), which combine the zero-field quantum Hall effect with spontaneously broken discrete translation symmetry. Here, we employ exact diagonalization to demonstrate the existence of stable QAHCs arising from -filled moiré bands with Chern number C = 2. Our calculations show that these topological crystals, which are characterized by a quantized Hall conductivity of 1 (in units of e²/h) and a tripled unit cell, are robust in an ideal model of twisted bilayer-trilayer graphene—providing a novel explanation for experimental observations in this heterostructure. Furthermore, we predict that the QAHC remains robust in a realistic model of twisted double bilayer graphene and, in addition, we provide a range of optimal tuning parameters, namely twist angle and electric field, for experimentally realizing this phase. Overall, our work demonstrates the stability of QAHCs at odd-denominator filling of C = 2 bands, provides specific guidelines for future experiments, and establishes chiral multilayer graphene as a theoretical platform for studying topological phases beyond the Landau-level paradigm.

Two-dimensional van der Waals semiconductors feature a variety of stable Coulomb-bound electron-hole complexes, which determine the optical response of the materials and serve as primary carriers of energy and spin-valley encoded information. Importantly, transitions between different excitonic states are found in the terahertz spectral range, motivating the use of strong THz radiation for their manipulation on ultrafast timescales. In this work, we apply this technique to efficiently transfer populations within the manifold of excitonic complexes in monolayer WSe₂, combining pulsed optical injection with a perturbation induced by a THz free-electron laser source. Monitoring time-resolved photoluminescence, we show conversion between different Coulomb-bound species across biexcitonic and excitonic regimes. Depending on the lattice temperature, these processes involve both short-lived bright and long-lived dark states. Combining experimental findings with theory support, we outline possible dissociation and formation pathways of charged excitons and biexcitons induced by the THz radiation. Finally, we demonstrate access to the formation dynamics of charged biexcitons under controlled conditions of thermalized populations of their constituents, avoiding complications of excess energies that otherwise occur after nonresonant optical excitation.

In monolayer MoSe2 strong photoluminescence from excitons and trions, resembling hydrogen-atom and hydrogen-ion like quasiparticles, can be observed after photoexcitation. We show that THz excitation with a photon energy in resonance or above the trion binding energy results in photo-dissociation of trions into excitons and free electrons. Utilizing this process, the photoluminescence wavelength can be switched on ps timescales in a controllable manner. The experiments pave the way for studies of increasingly complex Coulomb-bound many-body systems and offer perspectives for device applications combining THz and visible photonics.

Doped van der Waals heterostructures host layer-hybridized trions, i.e. charged excitons with layer-delocalized constituents holding promise for highly controllable optoelectronics. Combining a microscopic theory with photoluminescence (PL) experiments, we demonstrate the electrical tunability of the trion energy landscape in naturally stacked WSe₂ bilayers. We show that an out-of-plane electric field modifies the energetic ordering of the lowest lying trion states, which consist of layer-hybridized Λ-point electrons and layer-localized K-point holes. At small fields, intralayer-like trions yield distinct PL signatures in opposite doping regimes characterized by weak Stark shifts in both cases. Above a doping-asymmetric critical field, interlayer-like species are energetically favored and produce PL peaks with a pronounced Stark red-shift and a counter-intuitively large intensity arising from efficient phonon-assisted recombination. Our work presents an important step forward in the microscopic understanding of layer-hybridized trions in van der Waals heterostructures and paves the way towards optoelectronic applications based on electrically controllable atomically-thin semiconductors.

Transition metal dichalcogenides (TMDs) constitute an intriguing platform for studying charge-ordered states including conventional and generalized Wigner crystals as well as Mott insulating states. In this work, we combine a phonon mode expansion of the electronic crystal vibrations with the Lindemann criterion to investigate the quantum and thermal stability of these strongly correlated phases in the exemplary materials of MoSe2 monolayers and twisted MoSe2-WSe2 heterostructures. We find that the moiré potential in heterobilayers acts as a harmonic trap, flattening the energy dispersion of phonon excitations and resulting in an order of magnitude larger melting temperatures compared to monolayer Wigner crystals. Furthermore, we explore the tunability of the correlated states with respect to dielectric environment and bilayer stacking. In particular, we show that the reduced screening in freestanding TMDs results in a tenfold increase in the melting temperature compared to hBN-encapsulated TMDs. Moreover, the deeper moiré potential in R-type stacked heterostructures makes generalized Wigner crystals more stable than in H-type stacking. Overall, our study provides important microscopic insights on the stability and tunability of charge-ordered states in TMD-based structures.

External control of optical excitations is key for manipulating light–matter coupling and is highly desirable for photonic technologies. Excitons in monolayer semiconductors emerged as a unique nanoscale platform in this context, offering strong light–matter coupling, spin–valley locking and exceptional tunability. Crucially, they allow electrical switching of their optical response due to efficient interactions of excitonic emitters with free charge carriers, forming new quasiparticles known as trions and Fermi polarons. However, there are major limitations to how fast the light emission of these states can be tuned, restricting the majority of applications to an essentially static regime. Here we demonstrate switching of excitonic light emitters in monolayer semiconductors on ultrafast picosecond time scales by applying short pulses in the terahertz spectral range following optical injection. The process is based on a rapid conversion of trions to excitons by absorption of terahertz photons inducing photodetachment. Monitoring time-resolved emission dynamics in optical-pump/terahertz-push experiments, we achieve the required resonance conditions as well as demonstrate tunability of the process with delay time and terahertz pulse power. Our results introduce a versatile experimental tool for fundamental research of light-emitting excitations of composite Bose–Fermi mixtures and open up pathways towards technological developments of new types of nanophotonic device based on atomically thin materials.