Latest News

• Maja Feierabend: Doctoral thesis successfully defended. Congratulations!
• Three different PhD awards for Samuel Brem Congratulations!
• Visualization of dark excitons (published in Nano Lett.) In an excellent collaboration with the groups of Ulrich Höfer and Rupert Huber, we have combined time-resolved ARPES measurements with microscopic theory to directly visualize the ultrafast formation dynamics of dark excitons in WS2 monolayers. Our work was just published in Nano Letters.
• New insights into non-classical exciton transport (published in PRL) In an excellent collaboration with Alexey Chernikov (University of Dresden, Germany) und Mikhail Glazov (Ioffe Institute, Russia), we have gained new microscopic insights into non-classical exciton transport at low temperatures in WSe2 monolayers. Our work was published in Physical Review Letters (with Editors’s Suggestion).
• Rapid exciton dynamics in MoS2 bilayers (published in PRL) In a joint experiment-theory study with the groups of Elaine Li (University of Texas), Ulrike Woggon and Andreas Knorr (TU Berlin), we have revealed a much faster exciton dynamics in MoS2 bilayers compared to the monolayer case. This is traced back to an efficient intervalley exciton-phonon scattering involving momentum-dark exciton states. Our work was published in Physical Review Letters.
• Valley-exchange coupling probed by angle-resolved photoluminescence (published in Nanoscale Horizons) We have explored the role of the valley-exchange interaction on the optical properties of a TMD monolayer. In particular we propose that, by changing the optical polarisation, temperature and ex-ternal magnetic field, angle-resolved photoluminescence acts as an unambiguous probe of the valley-exchange interaction. This work was done in collaboration with the research groups of Witlef Wieczorek &Saroj Dash (Chalmers) and Christian Schneider (Oldenburg). Our work was published in Nanoscale Horizons.
• Dark exciton anti-funneling in atomically thin semiconductors (published in Nature Communications) In this joint theory-experiment work, we have combined spatiotemporal photoluminescence measurements with microscopic many-particle theory to track the way of excitons in time, space and energy. We found that excitons surprisingly move away from high-strain regions. This anti-funneling behavior can be traced back to the crucial role of propagating dark excitons which possess an opposite strain-induced energy variation compared to bright excitons. The findings open new possibilities to control the transport in materials dominated by excitons. This work was done in collaboration with the research group of Rudolf Bratschitsch (University of Münster) and it was published in Nature Communications.
• Terahertz Fingerprint of Monolayer Wigner Crystals (published in Nano Letters) Wigner crystals are solid, crystalline phases of electrons, formed at low temperatures in order to minimize their repulsive energy. This formation is one of the most intriguing quantum phase transitions and their experimental realization remains challenging since their theoretical prediction. However, the strong Coulomb interaction in monolayer semiconductors represents a unique opportunity for the realization of Wigner crystals without external magnetic fields. In this work, we predicted that the formation of monolayer Wigner crystals can be detected by their terahertz response spectrum, which exhibits a characteristic sequence of internal optical transitions. Moreover, a characteristic shift of the peak position as a function of charge density for different atomically thin materials was predicted and showed how the results can be generalized to an arbitrary two-dimensional system. The results will guide future experiments toward the detection of Wigner crystallization and help to study the interaction dynamics in pure and generalized Wigner crystals in twisted bilayers.
• Moire Exciton Polaritons in twisted materials (published in Nano Letters) Twisted atomically thin semiconductors are characterized by moire excitons. Their hybridization with photons in the strong coupling regime for heterostructures integrated in an optical cavity has not been well understood yet. In this work, we combined an excitonic density matrix formalism with a Hopfield approach to provide microscopic insights into moire exciton polaritons. In particular, they show that exciton-light coupling, polariton energy, and even the number of polariton branches can be controlled via the twist angle. These new hybrid light-exciton states become delocalized relative to the constituent excitons due to the mixing with light and higher-energy excitons. The system can be interpreted as a natural quantum metamaterial with a periodicity that can be engineered via the twist angle. The work has been published in Nano Letters.
• Formation of moiré interlayer excitons in space and time (published in Nature) Moiré superlattices in atomically thin van der Waals heterostructures hold great promise for extended control of electronic and valleytronic lifetimes, the confinement of excitons in artificial moiré lattices and the formation of exotic quantum phases. Such moiré-induced emergent phenomena are particularly strong for interlayer excitons, where the hole and the electron are localized in different layers of the heterostructure. To exploit the full potential of correlated moiré and exciton physics, a thorough understanding of the ultrafast interlayer exciton formation process and the real-space wavefunction confinement is indispensable. Here we show that femtosecond photoemission momentum microscopy provides quantitative access to these key properties of the moiré interlayer excitons. First, we elucidate that interlayer excitons are dominantly formed through femtosecond exciton–phonon scattering and subsequent charge transfer at the interlayer-hybridized Σ valleys. Second, we show that interlayer excitons exhibit a momentum fingerprint that is a direct hallmark of the superlattice moiré modification. Third, we reconstruct the wavefunction distribution of the electronic part of the exciton and compare the size with the real-space moiré superlattice. Our work provides direct access to interlayer exciton formation dynamics in space and time and reveals opportunities to study correlated moiré and exciton physics for the future realization of exotic quantum phases of matter.
• Interface engineering of charge-transfer excitons in 2D lateral heterostructures (published in Nature Communications) The existence of bound charge transfer (CT) excitons at the interface of monolayer lateral heterojunctions has been debated in literature, but contrary to the case of interlayer excitons in vertical heterostructure their observation still has to be confirmed. Here, we present a microscopic study investigating signatures of bound CT excitons in photoluminescence spectra at the interface of hBN-encapsulated lateral MoSe2-WSe2 heterostructures. Based on a fully microscopic and material-specific theory, we reveal the many-particle processes behind the formation of CT excitons and how they can be tuned via interface- and dielectric engineering. For junction widths smaller than the Coulomb-induced Bohr radius we predict the appearance of a low-energy CT exciton. The theoretical prediction is compared with experimental low-temperature photoluminescence measurements showing emission in the bound CT excitons energy range. We show that for hBN-encapsulated heterostructures, CT excitons exhibit small binding energies of just a few tens meV and at the same time large dipole moments, making them promising materials for optoelectronic applications (benefiting from an efficient exciton dissociation and fast dipole-driven exciton propagation). Our joint theory-experiment study presents a significant step towards a microscopic understanding of optical properties of technologically promising 2D lateral heterostructures.
• Electrical control of hybrid exciton transport in a van der Waals heterostructure (published in Nature Photonics) Interactions between out-of-plane dipoles in bosonic gases enable the long-range propagation of excitons. The lack of direct control over collective dipolar properties has so far limited the degrees of tunability and the microscopic understanding of exciton transport. In this work we modulate the layer hybridization and interplay between many-body interactions of excitons in a van der Waals heterostructure with an applied vertical electric field. By performing spatiotemporally resolved measurements supported by microscopic theory, we uncover the dipole-dependent properties and transport of excitons with different degrees of hybridization. Moreover, we find constant emission quantum yields of the transporting species as a function of excitation power with radiative decay mechanisms dominating over nonradiative ones, a fundamental requirement for efficient excitonic devices. Our findings provide a complete picture of the many-body effects in the transport of dilute exciton gases, and have crucial implications for studying emerging states of matter such as Bose–Einstein condensation and optoelectronic applications based on exciton propagation.
• Day of Physics - Seeing the world through different eyes The physics department invited interested people to an extensive experience program on May 6 and guided them through the world of physics throughout the day. There were various experiments, guided tours through research laboratories and the observatory, an exciting physics show and finally the stage opened for the "Science Slam". A great and successful day with many visitors and accompanied by a film team.
• Bosonic Delocalization of Dipolar Moiré Excitons (published in Nano Letters) In superlattices of twisted semiconductor monolayers, tunable moiré potentials emerge, trapping excitons into periodic arrays. In particular, spatially separated interlayer excitons are subject to a deep potential landscape and they exhibit a permanent dipole providing a unique opportunity to study interacting bosonic lattices. Recent experiments have demonstrated density-dependent transport properties of moiré excitons, which could play a key role for technological applications. However, the intriguing interplay between exciton−exciton interactions and moiré trapping has not been well understood yet. In this work, we develop a microscopic theory of interacting excitons in external potentials allowing us to tackle this highly challenging problem. We find that interactions between moiré excitons lead to a delocalization at intermediate densities, and we show how this transition can be tuned via twist angle and temperature. The delocalization is accompanied by a modification of optical moiré resonances, which gradually merge into a single free exciton peak.
• Trion Photoluminescence and Trion Stability in Atomically Thin Semiconductors (published in Physical Review Letters) The optical response of doped monolayer semiconductors is governed by trions, i.e. photoexcited electron-hole pairs bound to doping charges. While their photoluminescence (PL) signatures have been identified in experiments, a microscopic model consistently capturing bright and dark trion peaks is still lacking. In this work, we derive a generalized trion PL formula on a quantum-mechanical footing, considering direct and phonon-assisted recombination mechanisms. We show the trion energy landscape in WSe2 by solving the trion Schrödinger equation. We reveal that the mass imbalance between equal charges results in less stable trions exhibiting a small binding energy and, interestingly, a large energetic offset from exciton peaks in PL spectra. Furthermore, we compute the temperature-dependent PL spectra for n- and p-doped monolayers and predict yet unobserved signatures originating from trions with an electron at the Λ point. Our work presents an important step toward a microscopic understanding of the internal structure of trions determining their stability and optical fingerprint.
• Samuel Brem: Doctoral thesis successfully defended Congratulations to Samuel Brem, who has successfully defended his PhD thesis "Microscopic Theory of Exciton Dynamics in Two-Dimensional Materials"!
• Raul Perea Causin: Doctoral thesis successfully defended! Congratulations to Raul Perea Causin who has successfully defended his PhD thesis "Microscopic Theory of Charge Complexes in Atomically-Thin Materials".
• Probing correlations in the exciton landscape of a moiré heterostructure (published in Science Advances) Excitons are two-particle correlated bound states that are formed due to Coulomb interaction between single-particle holes and electrons. In the solid-state, cooperative interactions with surrounding quasiparticles can strongly tailor the exciton properties and potentially even create new correlated states of matter. It is thus highly desirable to access such cooperative and correlated exciton behavior on a fundamental level. Here, we find that the ultrafast transfer of an exciton's hole across a type-II band-aligned moiré heterostructure leads to a surprising sub-200-fs upshift of the single-particle energy of the electron being photoemitted from the two-particle exciton state. While energy relaxation usually leads to an energetic downshift of the spectroscopic signature, we show that this unusual upshift is a clear fingerprint of the correlated interactions of the electron and hole parts of the exciton quasiparticle. In this way, time-resolved photoelectron spectroscopy is straightforwardly established as a powerful method to access exciton correlations and cooperative behavior in two-dimensional quantum materials. Our work highlights this new capability and motivates the future study of optically inaccessible correlated excitonic and electronic states in moiré heterostructures.
• Raul Perea Causin obtains the best PhD thesis award We congratulate our PhD student, Raul Perea-Causin, for the Best Thesis Award from the Department of Physics at Chalmers. The newly qualified physics doctor was awarded for his doctoral theses titled “Microscopic Theory of Charge Complexes in Atomically-Thin Materials”.
• Willy Knorr: Doctoral thesis successfully defended Willy Knorr has successfully defended his doctoral thesis entitled "Microscopic Modeling of Exciton Transport in Twisted Van der Waals Heterostructures".
• Giuseppe Meneghini: Doctoral thesis successfully defended Giuseppe Meneghini has successfully defended his doctoral thesis entitled "Hybrid Exciton Thermalization in Atomically-Thin Semiconductors".
• Phonon-Bottleneck Enhanced Exciton Emission in 2D Perovskites (published in Advanced Energy Materials) Layered halide perovskites exhibit remarkable optoelectronic properties and technological promise, driven by strongly bound excitons. The interplay of spin-orbit and exchange coupling creates a rich excitonic landscape, determining their optical signatures and exciton dynamics. Despite the dark excitonic ground state, surprisingly efficient emission from higher-energy bright states has puzzled the scientific community, sparking debates on relaxation mechanisms. Combining low-temperature magneto-optical measurements with sophisticated many-particle theory, we elucidate the origin of the bright exciton emission in perovskites by tracking the thermalization of dark and bright excitons under a magnetic field. We clearly attribute the unexpectedly high emission to a pronounced phonon-bottleneck effect, considerably slowing down the relaxation towards the energetically lowest dark states. We demonstrate that this bottleneck can be tuned by manipulating the bright-dark energy splitting and optical phonon energies, offering valuable insights and strategies for controlling exciton emission in layered perovskite materials that is crucial for optoelectronics applications.
• Roberto Rosati: poster award Roberto Rosati obtained a poster award at the International Conference on Internal Interfaces in Marburg.
• Moiré superlattices in twisted two-dimensional halide perovskites (published in Nature Materials) Moiré superlattices have emerged as a new platform for studying strongly correlated quantum phenomena, but these systems have been largely limited to van der Waals layer two-dimensional materials. Here we introduce moiré superlattices leveraging ultrathin, ligand-free halide perovskites, facilitated by ionic interactions. Square moiré superlattices with varying periodic lengths are clearly visualized through high-resolution transmission electron microscopy. Twist-angle-dependent transient photoluminescence microscopy and electrical characterizations indicate the emergence of localized bright excitons and trapped charge carriers near a twist angle of ~10°. The localized excitons are accompanied by enhanced exciton emission, attributed to an increased oscillator strength by a theoretically predicted flat band. This research showcases the promise of two-dimensional perovskites as unique room-temperature moiré materials.
• Electrically tunable layer-hybridized trions in doped WSe2 bilayers (published in Nature Communications) 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 -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.
• Strain fingerprinting of exciton valley character in 2D semiconductors (published in Nature Communications) Intervalley excitons with electron and hole wavefunctions residing in different valleys determine the long-range transport and dynamics observed in many semiconductors. However, these excitons with vanishing oscillator strength do not directly couple to light and, hence, remain largely unstudied. Here, we develop a simple nanomechanical technique to control the energy hierarchy of valleys via their contrasting response to mechanical strain. We use our technique to discover previously inaccessible intervalley excitons associated with K, Γ, or Q valleys in prototypical 2D semiconductors WSe2 and WS2. We also demonstrate a new brightening mechanism, rendering an otherwise “dark” intervalley exciton visible via strain-controlled hybridization with an intravalley exciton. Moreover, we classify various localized excitons from their distinct strain response and achieve large tuning of their energy. Overall, our valley engineering approach establishes a new way to identify intervalley excitons and control their interactions in a diverse class of 2D systems.
• Joakim Hagel: Doctoral thesis successfully defended Joakim Hagel has successfully defended his doctoral thesis entitled "Quantum theory of moiré excitons in atomically thin semiconductors".
• Ultrafast switching of trions in 2D materials by terahertz photons (published in Nature Photonics) 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.