Main Content
Publications 2024
202. T. Venanzi, M. Cuccu, R. Perea-Causin, X. Sun, S. Brem, D. Erkensten, T. Taniguchi, K. Watanabe, E. Malic, M. Helm, S. Winnerl, A. Chernikov, "Ultrafast switching of trions in 2D materials by terahertz photons", Nature Photonics (2024)
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 for 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 THz spectral range following optical injection. The process is based on a rapid conversion of trions to excitons by absorption of low energy photons. Monitoring time-resolved emission dynamics in optical-pump/THz-push experiments, we realize the required resonance conditions as well as demonstrate tunability of the process with delay time and THz 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 devices based on atomically-thin materials.
Nature Photonics (2024)201. D. Schmitt, J. P. Bange, W. Bennecke, G. Meneghini, A. AlMutairi, M.Merboldt, J. Pöhls, K. Watanabe, T. Taniguchi, S. Steil, D. Steil, R. T. Weitz, S. Hofmann, S. Brem, G. S. Matthijs Jansen, E. Malic, S. Mathias and M. Reutzel "Ultrafast nano-imaging of dark excitons", accepted by Nature Photonics (2024)
The role and impact of spatial heterogeneity in two-dimensional quantum materials represents one of the major research quests regarding the future application of these materials in optoelectronics and quantum information science. In the case of transition-metal dichalcogenide heterostructures, in particular, direct access to heterogeneities in the dark-exciton landscape with nanometer spatial and ultrafast time resolution is highly desired, but remains largely elusive. Here, we introduce ultrafast dark field momentum microscopy to spatio-temporally resolve dark exciton formation dynamics in a twisted WSe2/MoS2 heterostructure with 55 femtosecond time- and 500~nm spatial resolution. This allows us to directly map spatial heterogeneity in the electronic and excitonic structure, and to correlate these with the dark exciton formation and relaxation dynamics. The benefits of simultaneous ultrafast nanoscale dark-field momentum microscopy and spectroscopy is groundbreaking for the present study, and opens the door to new types of experiments with unprecedented spectroscopic and spatiotemporal capabilities.
accepted by Nature Photonics200. S. Zhang, L. Jin, Y. Lu, L. Zhang, J. Yang, Q. Zhao, D. Sun, J. J. P. Thompson, B. Yuan, K. Ma, Akriti, J. Yung Park, Y. Ho Lee, Z. Wei, B. P. Finkenauer, D. D. Blach, S. Kumar, H. Peng, A. Mannodi-Kanakkithodi, Y. Yu, E. Malic, G. Lu, L. Dou, L. Huang, "Moiré superlattices in twisted two-dimensional halide perovskites", Nature Materials 23, 1222 (2024)
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 (2D) materials. Here we introduce moiré superlattices leveraging ultra-thin, 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 forecasted flat band. This work illustrates the potential of extended ionic interaction in realizing moiré physics at room temperature, broadening the horizon for future investigations.
Nature Materials 23, 1222 (2024)199. X. Sun, E. Malic, Y. Lu "Dipolar many-body complexes and their interactions in stacked 2D heterobilayers", Nature Review Physics 6, 439 (2024)
Highly customizable interfaces created by van der Waals stacked 2D materials provide an extremely flexible opportunity for engineering and effectively controlling material properties. The atomic-thin nature and strong scalability of transition metal dichalcogenides (TMDs), the star family of two-dimensional semiconducting materials, allow for the modulation of their inherent optical and electrical characteristics by utilizing various environmental stimuli. In such a material system, the stacking mechanism with spatial separation in the structure enables recent observations of dipolar many-body complexes with the interplay of multi-particles, leading to some exotic and novel excitonic phenomena and enabling the closer study of high-correlated quantum physics. The presence of powerful dipole-dipole interactions among long-lived interlayer excitons can cause the system to enter unique classical and quantum phases with multiparticle correlations, such as dipolar liquids, dipolar crystals and superfluids. The strong binding energy of interlayer excitons in TMD-based hetero-bilayers especially enhances the critical temperature of these exotic phenomena. Here, we provide a concise summary of the recent frontier research progress on dipolar complexes and many-body effects in TMD double layers, encompassing fundamental theory and properties modulation. We reveal the significance and current challenges of this research field and present the potential developing directions of the hetero-bilayers in quantum physics and quantum devices by adding new levels of external control or integration.
Nature Review Physics 6, 439 (2024)198. J. J. P. Thompson, M. Dyksik, P. Peksa, K. Posmyk, A. Joki, R. Perea-Causin, P. Erhart, M. Baranowski, M. Antonietta Loi, P. Plochocka, E. Malic, "Phonon-bottleneck enhanced exciton emission in 2D perovskites", Advanced Energy Materials 14, 20 (2024) on the front cover
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.
Advanced Energy Materials 14, 20 (2024)197. J. P. Bange, D. Schmitt, W. Bennecke, G. Meneghini, A. AlMutairi, K. Watanabe, T. Taniguchi, D. Steil, S. Steil, R. T. Weitz, G. S. M. Jansen, S. Hofmann, S. Brem, E. Malic, M. Reutzel, S. Mathias "Probing correlations in the exciton landscape of a moiré heterostructure", Science Advances 10, 6 (2024) on the front cover
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.
Science Advances 10,6 (2024)196. R. Perea-Causin, S. Brem, F. Buchner, K. Watanabe, T. Taniguchi, J. M. Lupton, K.-Q. Lin, E. Malic, "Electrically tunable layer-hybridized trions in doped WSe2 bilayers", Nature Communications 15, 6713 (2024)
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.
Nature Communications 15, 6713 (2024)195. A. Kumar, D. Yagodkin, R. Rosati, D. J Bock, C. Schattauer, S. Tobisch, J. Hagel, B. Höfer, J. N Kirchhof, P. Hernández López, K. Burfeindt, S. Heeg, C. Gahl, F. Libisch, E. Malic, K. I Bolotin, "Strain fingerprinting of exciton valley character", Nature Communications 15, 7546 (2024)
Momentum-indirect excitons composed of electrons and holes in different valleys define optoelectronic properties of many semiconductors, but are challenging to detect due to their weak coupling to light. The identification of an excitons' valley character is further limited by complexities associated with momentum-selective probes. Here, we study the photoluminescence of indirect excitons in controllably strained prototypical 2D semiconductors (WSe2, WS2) at cryogenic temperatures. We find that these excitons i) exhibit valley-specific energy shifts, enabling their valley fingerprinting, and ii) hybridize with bright excitons, becoming directly accessible to optical spectroscopy methods. This approach allows us to identify multiple previously inaccessible excitons with wavefunctions residing in K, Γ, or Q valleys in the momentum space as well as various types of defect-related excitons. Overall, our approach is well-suited to unravel and tune intervalley excitons in various semiconductors.
Nature Communications 15, 7546 (2024)194. Q. Lin, H. Fang, Y. Liu, Y. Zhang, M. Fischer, J. Li, J. Hagel, S. Brem, E. Malic, N. Stenger, Z. Sun, M. Wubs, S. Xiao, “Moiré-engineered light-matter interactions in MoS2/WSe2 heterobilayers at room temperature”, Nature Communications 15, 8762 (2024)
Moiré superlattices in van der Waals heterostructures represent a highly tunable quantum system, attracting substantial interest in both many-body physics and device applications. However, the influence of the moiré potential on light-matter interactions at room temperature has remained largely unexplored. In our study, we demonstrate that the moiré potential in MoS2/WSe2 heterobilayers facilitates the localization of interlayer exciton (IX) at room temperature. By performing reflection contrast spectroscopy, we demonstrate the importance of atomic reconstruction in modifying intralayer excitons, supported by the atomic force microscopy experiment. When decreasing the twist angle, we observe that the IX lifetime becomes longer and light emission gets enhanced, indicating that non-radiative decay channels such as defects are suppressed by the moiré potential. Moreover, through the integration of moiré superlattices with silicon single-mode cavities, we find that the devices employing moiré-trapped IXs exhibit a significantly lower threshold, one order of magnitude smaller compared to the device utilizing delocalized IXs. These findings not only encourage the exploration of many-body physics in moiré superlattices at elevated temperatures but also pave the way for leveraging these artificial quantum materials in photonic and optoelectronic applications.
Nature Communications 15, 8762 (2024)193. A. de la Torre, D. M. Kennes, E. Malic, S. Kar "Review: Spatial inhomogeneities, moiré potential and moiré excitons", Small 2401474 (2024)
In this short review, we provide an overview of recent progress in deploying advanced characterization techniques to understand the effects of local inhomogeneities in moiré heterostructures over multiple length scales. Particular emphasis is placed on correlating the impact of twist angle misalignment, nano-scale disorder, and atomic relaxation on the moiré potential and its collective excitations, particularly moiré excitons. Finally, we discuss future technological applications leveraging based on moié excitons.
Small 2401474 (2024)192. G. Meneghini, S. Brem, E. Malic "Excitonic thermalization bottleneck in twisted TMD heterostructures", Nano Letters 15, 4505 (2024)
Twisted van der Waals heterostructures show an intriguing interface exciton physics including hybridization effects and emergence of moiré potentials. Recent experiments have revealed that moiré-trapped excitons exhibit a remarkable dynamics, where excited states show lifetimes that are several orders of magnitude longer than those in monolayers. The origin of this behavior is still under debate. Based on a microscopic many-particle approach, we investigate the phonon-driven relaxation cascade of non-equilibrium moiré excitons in the exemplary MoSe2-WSe2 heterostructure. We track the exciton relaxation pathway across different moiré mini-bands and identify the phonon-scattering channels assisting the spatial redistribution of excitons into low-energy pockets of the moiré potential. We unravel a phonon bottleneck in the flat band structure at low twist angles preventing excitons to fully thermalize into the lowest state explaining the measured enhanced emission intensity of excited moiré excitons. Overall, our work provides important insights into exciton relaxation dynamics in flatband exciton materials.
Nano Letters 15, 4505 (2024)191. J. Hagel, S. Brem and E. Malic, “Polarization and charge-separation of moiré excitons in van der Waals heterostructures”, accepted by Nano Letters (2024)
Twisted transition metal dichalcogenide (TMD) bilayers exhibit periodic moiré potentials, which can trap excitons at certain high-symmetry sites. At small twist angles, TMD lattices undergo an atomic reconstruction, altering the moiré potential landscape via the formation of large domains, potentially separating the charges in-plane and leading to the formation of intralayer charge-transfer (CT) excitons. Here, we employ a microscopic, material-specific theory to investigate the intralayer charge-separation in atomically reconstructed MoSe2-WSe2 heterostructures. We identify three distinct and twist-angle-dependent exciton regimes including localized Wannier-like excitons, polarized excitons, and intralayer CT excitons. We calculate the moiré site hopping for these excitons and predict a fundamentally different twist-angle-dependence compared to regular Wannier excitons - presenting an experimentally accessible key signature for the emergence of intralayer CT excitons. Furthermore, we show that the charge separation and its impact on the hopping can be efficiently tuned via dielectric engineering, allowing for a suppression or an enhancement of the charge-separation and its effects on the hopping.
accepted by Nano Letters190. J. M. Fitzgerald, R. Rosati, B. Ferreira, H. Shan, C. Schneider, E. Malic, "Circumventing the polariton bottleneck via dark excitons in 2D semiconductors" Optica, 9, 1346 (2024)
Efficient scattering into the exciton polariton ground state is a key prerequisite for generating Bose-Einstein condensates and low-threshold polariton lasing. However, this can be challenging to achieve at low densities due to the polariton bottleneck effect that impedes phonon-driven scattering into low-momentum polariton states. The rich exciton landscape of transition metal dichalcogenides (TMDs) provides potential intervalley scattering pathways via dark excitons to rapidly populate these polaritons. Here, we present a microscopic study exploring the time- and momentum-resolved relaxation of exciton polaritons supported by a \ce{MoSe2} monolayer integrated within a Fabry-Perot cavity. By exploiting phonon-assisted transitions between momentum-dark excitons and the lower polariton branch, we demonstrate that it is possible to circumvent the bottleneck region and efficiently populate the polariton ground state. Furthermore, this intervalley pathway is predicted to give rise to, yet unobserved, angle-resolved phonon sidebands in low-temperature photoluminescence spectra that are associated with momentum-dark excitons. This represents a distinctive experimental signature for efficient phonon-mediated polariton-dark-exciton interactions.
Optica, 9, 1346 (2024)189. R. Perea-Causin, S. Brem, O. Schmidt, E. Malic "Trion photoluminescence and trion stability in atomically thin semiconductors", Phys. Rev. Lett. 132, 036903 (2024)
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 towards a microscopic understanding of the internal structure of trions determining their stability and optical fingerprint.
Phys. Rev. Lett. 132, 036903188. B. Ferreira, H. Shan, R. Rosati, J. M. Fitzgerald, L. Lackner, B. Han, M. Esmann, P. Hays, G. Liebling, K. Watanabe, T. Taniguchi, F. Eilenberger, S. Tongay, C. Schneider, E. Malic, "Revealing dark exciton signatures in polariton spectra of 2D materials", ACS Photonics 11, 6, 2215 (2024)
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.
ACS Photonics 11, 6, 2215 (2024)187. W. Knorr, S. Brem, G. Meneghini, E. Malic “Polaron-induced changes in moiré exciton propagation in twisted van der Waals heterostructures”, Nanoscale 16, 8996 (2024)
Twisted transition metal dichalcogenides (TMDs) present an intriguing platform for exploring excitons and their transport properties. By introducing a twist angle, a moiré superlattice forms, providing a spatially dependent exciton energy landscape. Based on a microscopic many-particle theory, we investigate in this work polaron-induced changes in exciton transport properties in the MoSe2/WSe2 heterostructure. We demonstrate that polaron formation and the associated enhancement of moiré excitonic mass lead to a significant band flattening. As a result, the hopping rate and the propagation velocity undergo noticeable temperature and twist-angle dependent changes. We predict a reduction of the hopping strength ranging from 80% at a twist angle of 1∘ to 30% at 3∘ at room temperature. The provided microscopic insights into the spatio-temporal exciton dynamics in presence of a moiré potential further deepens our understanding of the intriguing moiré exciton physics.
Nanoscale 16, 8996 (2024)186. C. Chandrakant Palekar, J. Hagel, B. Rosa, S. Brem, C.- W. Shih, I. Limame, M. von Helversen, S. Tongay, E. Malic, S. Reitzenstein, "Anomalous redshift in interlayer exciton emission with increasing twist angle in WSe2/MoSe2 heterostructures", 2D Materials 11, 025034 (2024)
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 moiré 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 interlayer 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 moiré 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.
2D Materials 11, 025034 (2024)185. S. Brem and E. Malic "Optical signatures of moiré trapped biexcitons", 2D Materials 11, 025030 (2024)
Atomically thin heterostructures formed by twisted transition metal dichalcogenides can be used to create periodic moiré patterns. The emerging moiré potential can trap interlayer excitons into arrays of strongly interacting bosons, which form a unique platform to study strongly correlated many-body states. In order to create and manipulate these exotic phases of matter, a microscopic understanding of exciton–exciton interactions and their manifestation in these systems becomes indispensable. Recent density-dependent photoluminescence (PL) measurements have revealed novel spectral features indicating the formation of trapped multi-exciton states providing important information about the interaction strength. In this work, we develop a microscopic theory to model the PL spectrum of trapped multi-exciton complexes focusing on the emission from moiré trapped single- and biexcitons. Based on an excitonic Hamiltonian we determine the properties of trapped biexcitons as function of twist angle and use these insights to predict the luminescence spectrum of moiré excitons for different densities. We demonstrate how side peaks resulting from transitions to excited states and a life time analysis can be utilized as indicators for moiré trapped biexcitons and provide crucial information about the excitonic interaction strength.
2D Materials 11, 025030 (2024)184. J. Jasinski, J. J. P. Thompson, S. Palai, M. Smiertka, M. Dyksik, T. Taniguchi, K. Watanabe, M. Baranowski, D. K. Maude, A. Surrente, E. Malic and P. Plochocka, "Control of the Valley Polarization of Monolayer WSe2 by Dexter-like Coupling", 2D Materials 11, 2 (2024)
Intervalley scattering mechanisms govern the dynamics of excitonic complexes in transition metal dichalcogenide monolayers. Here, we investigate the excitation energy dependence of the valley polarization of excitons in a WSe2 monolayer. We observe that the valley polarization drastically decreases when the excitation is resonant with the B1s resonance. This behaviour can be explained by a Dexter-like coupling in the momentum space between exciton states residing in opposite valleys but with the same spin configuration. This induces a net transfer of the exciton population from the optically driven valley towards the opposite, undriven valley. We observe the long-term fingerprints of this population transfer, as a vanishing valley polarization for the neutral exciton, and a negative valley polarization for biexcitonic complexes, in qualitative agreement with theoretical predictions based on a fully microscopic many-particle approach. This, together with a decrease of the PL energy when the excitation is resonant with the B1s state, points to the prominent role of the Dexter-like coupling in the exciton dynamics of atomically thin semiconductors.
2D Materials 11, 2 (2024)183. D. Erkensten, S. Brem, R. Perea-Causin, and E. Malic "Stability of Wigner crystals and Mott insulators in twisted moiré structures", Phys. Rev. B 110, 155132 (2024)
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 free-standing 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.
Phys. Rev. B 110, 155132 (2024)182. J. Hagel, S. Brem, J. Abelardo Pineiro, E. Malic, "Impact of atomic reconstruction on optical spectra of twisted TMD homobilayers", Phys. Rev. Mat. 8, 034001 (2024)
Twisted bilayers of transition metal dichalcogenides (TMDs) have revealed a rich exciton landscape including hybrid excitons and spatially trapped moiré excitons that dominate the optical response of the material. Recent studies have revealed that in the low-twist-angle regime, the lattice undergoes a significant relaxation in order to minimize local stacking energies. Here, large domains of low energy stacking configurations emerge, deforming the crystal lattices via strain and consequently impacting the electronic band structure. However, so far the direct impact of atomic reconstruction on the exciton energy landscape and the optical properties has not been well understood. Here, we apply a microscopic and material-specific approach and predict a significant change in the potential depth for moiré excitons in a reconstructed lattice, with the most drastic change occurring in TMD homobilayers. We reveal the appearance of multiple flat bands and a significant change in the position of trapping sites compared to the rigid lattice. Most importantly, we predict a multi-peak structure emerging in optical absorption of WSe2 homobilayers - in stark contrast to the single peak that dominates the rigid lattice. This finding can be exploited as an unambiguous signature of atomic reconstruction in optical spectra of moiré excitons in twisted homobilayers.
Phys. Rev. Mat. 8, 034001