Hybrid Exciton Signatures in ARPES Spectra of van der Waals Materials

Van der Waals heterostructures show fascinating physics including trapped moire exciton states, anomalous moire exciton transport, generalized Wigner crystals, etc. Bilayers of transition metal dichalcogenides (TMDs) are characterized by long-lived, spatially separated interlayer excitons. Provided strong interlayer tunneling, hybrid exciton states consisting of interlayer and intralayer excitons can be formed. Here, electrons and holes are in a superposition of both layers. Although crucial for optics, dynamics, and transport, hybrid excitons are usually optically inactive and have therefore not yet been directly observed yet. Based on microscopic and material-specific theory, we show that time- and angle-resolved photoemission spectroscopy (tr-ARPES) is a direct technique to visualize these hybrid excitons. Concretely, we predict a characteristic double-peak ARPES signal arising from the hybridized hole in the MoS2 homobilayer. The relative intensity is proportional to the quantum mixture of the two hybrid valence bands at the Γ point. Due to the strong hybridization, a peak separation of more than 0.5 eV can be resolved in ARPES experiments. Our study provides a concrete recipe for how to directly visualize hybrid excitons and how to distinguish them from the usually observed regular excitonic signatures

Microscopic View on the Ultrafast Photoluminescence from Photoexcited Graphene

We present a joint theory-experiment study on ultrafast photoluminescence from photoexcited graphene. On the basis of a microscopic theory, we reveal two distinct mechanisms behind the occurring photoluminescence: besides the well-known incoherent contribution driven by nonequilibrium carrier occupations, we found a coherent part that spectrally shifts with the excitation energy. In our experiments, we demonstrate for the first time the predicted appearance and spectral shift of the coherent photoluminescence

Microscopic View on the Ultrafast Photoluminescence from Photoexcited Graphene

We present a joint theory-experiment study on ultrafast photoluminescence from photoexcited graphene. On the basis of a microscopic theory, we reveal two distinct mechanisms behind the occurring photoluminescence: besides the well-known incoherent contribution driven by nonequilibrium carrier occupations, we found a coherent part that spectrally shifts with the excitation energy. In our experiments, we demonstrate for the first time the predicted appearance and spectral shift of the coherent photoluminescence

Strain Control of Exciton–Phonon Coupling in Atomically Thin Semiconductors

Semiconducting transition metal dichalcogenide (TMDC) monolayers have exceptional physical properties. They show bright photoluminescence due to their unique band structure and absorb more than 10% of the light at their excitonic resonances despite their atomic thickness. At room temperature, the width of the exciton transitions is governed by the exciton–phonon interaction leading to strongly asymmetric line shapes. TMDC monolayers are also extremely flexible, sustaining mechanical strain of about 10% without breaking. The excitonic properties strongly depend on strain. For example, exciton energies of TMDC monolayers significantly redshift under uniaxial tensile strain. Here, we demonstrate that the width and the asymmetric line shape of excitonic resonances in TMDC monolayers can be controlled with applied strain. We measure photoluminescence and absorption spectra of the A exciton in monolayer MoSe₂, WSe₂, WS₂, and MoS₂ under uniaxial tensile strain. We find that the A exciton substantially narrows and becomes more symmetric for the selenium-based monolayer materials, while no change is observed for atomically thin WS₂. For MoS₂ monolayers, the line width increases. These effects are due to a modified exciton–phonon coupling at increasing strain levels because of changes in the electronic band structure of the respective monolayer materials. This interpretation based on steady-state experiments is corroborated by time-resolved photoluminescence measurements. Our results demonstrate that moderate strain values on the order of only 1% are already sufficient to globally tune the exciton–phonon interaction in TMDC monolayers and hold the promise for controlling the coupling on the nanoscale