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Publications 2024

  • 192. X. Sun, E. Malic, Y. Lu "Dipolar many-body complexes and their interactions in stacked 2D heterobilayers", Nature Review Physics (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 (2024)

  • 191. 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", accepted by 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.

    accepted by Advanced Energy Materials (2024)

  • 190. 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)

    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)

  • 189. 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)

  • 188. 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, 036903

  • 187. W. Knorr, S. Brem, G. Meneghini, E. Malic “Polaron-induced changes in moiré exciton propagation in twisted van der Waals heterostructures” accepted by Nanoscale

    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.

    accepted by Nanoscale

  • 186. 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", accepted by ACS Photonics

    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.

    accepted by ACS Photonics

  • 185. C. Chandrakant Palekar, J. Hagel, B. Rosa, S. Brem, C.- W. Shih, I. Limame, M. von Helversen, S. Tongay, E. Malic, S. Reitzenstein, "Twist Angle Dependence of Exciton Resonances in WSe2/MoSe2 Moiré Heterostructures", accepted by 2D Materials

    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.

    accepted by 2D Materials

  • 184. 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)

  • 183. 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)

  • 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