Main Content
Publications 2021
147. R. Rosati, R. Schmidt, S. Brem, R. Perea-Causín, I. Niehues, J. Kern, J. A. Preuß, R. Schneider, S. Michaelis de Vasconcellos, R. Bratschitsch and E. Malic „Dark exciton anti-funneling in atomically thin semiconductors”, Nature Communications 12, 7221 (2021)
Transport of charge carriers is at the heart of current nanoelectronics. In conventional materials, electronic transport can be controlled by applying electric fields. Atomically thin semiconductors, however, are governed by excitons, which are neutral electron-hole pairs and as such cannot be controlled by electrical fields. Recently, strain engineering has been introduced to manipulate exciton propagation. Strain-induced energy gradients give rise to exciton funneling up to a micrometer range. Here, we combine spatiotemporal photoluminescence measurements with microscopic theory to track the way of excitons in time, space and energy. We find that excitons surprisingly move away from high-strain regions. This anti-funneling behavior can be ascribed to dark excitons which possess an opposite strain-induced energy variation compared to bright excitons. Our findings open new possibilities to control transport in exciton-dominated materials. Overall, our work represents a major advance in understanding exciton transport that is crucial for technological applications of atomically thin materials.
Nature Communications 12, 7221 (2021)
146. P. Merkl, C. Yong, M. Liebich, I. Hofmeister, G. Berghaeuser, E. Malic and R. Huber, "Proximity control of interlayer exciton-phonon hybridization in van der Waals heterostructures", Nature Communications 12, 1719 (2021)
Van der Waals stacking has provided unprecedented flexibility in shaping many-body interactions by controlling electronic quantum confinement and orbital overlap. Theory has predicted that also electron-phonon coupling critically influences the quantum ground state of low-dimensional systems. Here we introduce proximity-controlled strong-coupling between Coulomb correlations and lattice dynamics in neighbouring van der Waals materials, creating new electrically neutral hybrid eigenmodes. Specifically, we explore how the internal orbital 1s-2p transition of Coulomb-bound electron-hole pairs in monolayer tungsten diselenide resonantly hybridizes with lattice vibrations of a polar capping layer of gypsum, giving rise to exciton-phonon mixed eigenmodes, called excitonic Lyman polarons. Tuning orbital exciton resonances across the vibrational resonances, we observe distinct anticrossing and polarons with adjustable exciton and phonon compositions. Such proximity-induced hybridization can be further controlled by quantum designing the spatial wavefunction overlap of excitons and phonons, providing a promising new strategy to engineer novel ground states of two-dimensional systems.
Nature Comm. 12, 1719 (2021)
145. R. Wallauer, R. Perea-Causin, L. Muenster, S. Zajusch, S. Brem, J. Guedde, K. Tanimura, K. Lin, R. Huber, E. Malic and U. Hoefer, "Direct observation of ultrafast dark exciton formation in monolayer WS2", Nano Lett. 21, 5867 (2021)
The exciton landscape in transition metal dichalcogenides is crucial for their optical properties and difficult to measure experimentally, since many exciton states are not accessible to optical interband spectroscopy. Here, we combine a tuneable pump, high-harmonic probe laser source with a 3D momentum imaging technique to map photoemitted electrons from monolayer WS2. This provides momentum-, energy- and time-resolved access to excited states on an ultrafast timescale. The high temporal resolution of the setup allows us to trace the early-stage exciton dynamics and the formation of a momentum-forbidden dark KΣ exciton a few tens of femtoseconds after optical excitation. By tuning the excitation energy we manipulate the temporal evolution of the coherent excitonic polarization and observe its influence on the dark exciton formation. The experimental results are in excellent agreement to a fully microscopic theory, resolving the temporal and spectral dynamics of bright and dark excitons in WS2.
Nano Lett. 21, 5867 (2021)
144. K. Wagner, J. Zipfel, R. Rosati, E. Wietek, J. Ziegler, S. Brem, R. Perea-Causín, T. Taniguchi, K. Watanabe, M. Glazov, E. Malic, and A. Chernikov, "Dark exciton diffusion and non-classical propagation in monolayer WSe2", Phys. Rev. Lett. 127, 076801 (2021), Editors' Suggestion
We experimentally demonstrate time-resolved exciton propagation in a monolayer semiconductor at cryogenic temperatures. Monitoring phonon-assisted recombination of dark states, we find a highly unusual regime of the exciton diffusion. While at 5K the diffusivity is intrinsically limited by acoustic phonon scattering, we observe a pronounced decrease of the diffusion coefficient with increasing temperature, far below the activation threshold of higher-energy phonon modes. This behavior corresponds neither to well-known regimes of semi-classical free-particle transport nor to the thermally activated hopping in systems with strong localization. It is discussed in the framework of both microscopic numerical and semi-phenomenological analytical models illustrating the observed characteristics of non-classical propagation. These results challenge the established description of mobile excitons in monolayer semiconductors and open up avenues to study quantum transport phenomena for excitonic quasiparticles in low-dimensional systems.
Phys. Rev. Letters 127, 076801, Editors Suggestion
143. S. Helmrich, K. Sampson, D. Huang, M. Selig, K. Hao, K. Tran, A. Achstein, C. Young, A. Knorr, E. Malic, U. Woggon, N. Owschimikow, and X. Li, “Phonon-assisted intervalley scattering determines ultrafast exciton dynamics in MoSe2 bilayers”, Phys. Rev. Lett. 127, 157403 (2021)
While valleys (energy extrema) are present in all band structures of solids, their preeminent role in determining exciton resonances and dynamics in atomically thin transition metal dichalcogenides (TMDC) is unique. Using two-dimensional coherent electronic spectroscopy, we find that exciton decoherence occurs on a much faster time scale in MoSe2 bilayers than that in the monolayers. We further identify two population relaxation channels in the bilayer, a coherent and an incoherent one. Our microscopic model reveals that phonon-emission processes facilitate scattering events from the K valley to other lower energy Γ and Λ valleys in the bilayer. Our combined experimental and theoretical studies unequivocally establish different microscopic mechanisms that determine exciton quantum dynamics in TMDC monolayers and bilayers. Understanding exciton quantum dynamics provides critical guidance to manipulation of spin/valley degrees of freedom in TMDC bilayers.
Phys. Rev. Lett. 127, 157403142. R. Krause, S. Aeschlimann, M. Chavez-Cervantes, R. Perea-Causin, S. Brem, E. Malic, S. Forti, F. Fabbri, C. Coletti, I. Gierz, "Microscopic understanding of ultrafast charge transfer in van-der-Waals heterostructures", Phys. Rev. Lett. 127, 276401 (2021)
Van-der-Waals heterostructures show many intriguing phenomena including ultra-fast charge separation following strong excitonic absorption in the visible spectral range.However, despite the enormous potential for future applications in the field of opto-electronics, the underlying microscopic mechanism remains controversial. Here weuse time- and angle-resolved photoemission spectroscopy combined with microscopicmany-particle theory to reveal the relevant microscopic charge transfer channels inepitaxial WS2/graphene heterostructures. We find that the timescale for efficient ul-trafast charge separation in the material is determined by direct tunneling at thosepoints in the Brillouin zone where WS2and graphene bands cross, while the lifetimeof the charge separated transient state is set by defect-assisted tunneling through lo-calized sulphur vacanices. The subtle interplay of intrinsic and defect-related chargetransfer channels revealed in the present work can be exploited for the design of highlyefficient light harvesting and detecting devices.
Phys. Rev. Letters 127, 276401 (2021)
141. S. Dong, M. Puppin, T. Pincelli, S. Beaulieu, D. Christiansen, H. Huebener, C. Nicholson, R. Xian, M. Dendzik, Y. Deng, Y. Windsor, M. Selig, E. Malic, A. Rubio, A. Knorr, M. Wolf, L. Rettig, R. Ernstorfer, "Direct m easurement of key exciton properties: Energy,dynamics, and spatial distribution of the wave function" Natural Sciences 1, e10010 (2021)
Excitons, Coulomb-bound electron-hole pairs, are the fundamental excitations governing optoelectronic properties of semiconductors. While optical signatures of excitons have been studied extensively, experimental access to the excitonic wave function itself has been elusive. Using multidimensional photoemission spectroscopy, we present a momentum-, energy- and time-resolved perspective on excitons in the layered semiconductor WSe2. By tuning the excitation wavelength, we determine the energy-momentum signature of bright exciton formation and its difference from conventional single-particle excited states. The multidimensional data allows to retrieve fundamental exciton properties like the binding energy and the exciton-lattice coupling and to reconstruct the real-space excitonic wave function via Fourier transform. All quantities are in excellent agreement with microscopic calculations. Our approach provides a full characterization of the exciton wave function and is applicable to bright and dark excitons in semiconducting materials, heterostructures and devices.
140. D. Erkensten, S. Brem, K. Wagner, R. Gillen, R. Perea-Causin, J. D. Ziegler, T. Taniguchi, K. Watanabe, J. Maultzsch, A. Chernikov und E. Malic, "Dark exciton-exciton annihilation in monolayer transition-metal dichalcogenides", Phys. Rev. B 104, L241406 (2021), Editors' Suggestion
The exceptionally strong Coulomb interaction in semiconducting transition-metal dichalcogenides (TMDs) gives rise to a rich exciton landscape consisting of bright and dark exciton states. At elevated densities, excitons can interact through exciton-exciton annihilation (EEA), an Auger-like recombination process limiting the efficiency of optoelectronic applications. Although EEA is a well-known and particularly important process in atomically thin semiconductors determining exciton lifetimes and affecting transport at elevated densities, its microscopic origin has remained elusive. In this joint theory-experiment study combining microscopic and material-specific theory with time- and temperature-resolved photoluminescence measurements, we demonstrate the key role of dark intervalley states that are found to dominate the EEA rate in monolayer WSe2. We reveal an intriguing, characteristic temperature dependence of Auger scattering in this class of materials with an excellent agreement between theory and experiment. Our study provides microscopic insights into the efficiency of technologically relevant Auger scattering channels within the remarkable exciton landscape of atomically thin semiconductors.
Phys. Rev. B 104, L241406 (2021), Editors' Suggestion
139. R. Rosati, K. Wagner, S. Brem, R. Perea-Causin, J. D. Ziegler, J. Zipfel, T. Taniguchi, K. Watanabe, A. Chernikov and E. Malic, "Non-equilibrium diffusion of dark excitons in atomically thin semiconductors", Nanoscale 13, 19966 (2021)
Atomically thin semiconductors provide an excellent platform to study intriguing many-particle physics of tightly-bound excitons. In particular, the properties of tungsten-based transition metal dichalcogenides are determined by a complex manifold of bright and dark exciton states. While dark excitons are known to dominate the relaxation dynamics and low-temperature photoluminescence, their impact on the spatial propagation of excitons has remained elusive. In our joint theory-experiment study, we address this intriguing regime of dark state transport by resolving the spatio-temporal exciton dynamics in hBN-encapsulated WSe2 monolayers after resonant excitation. We find clear evidence of an unconventional, time-dependent diffusion during the first tens of picoseconds, exhibiting strong deviation from the steady-state propagation. Dark exciton states are initially populated by phonon emission from the bright states, resulting in creation of hot excitons whose rapid expansion leads to a transient increase of the diffusion coefficient by more than one order of magnitude. These findings are relevant for both fundamental understanding of the spatio-temporal exciton dynamics in atomically thin materials as well as their technological application by enabling rapid diffusion.
Nanoscale 13, 19966 (2021)
138. R. Perea-Causin, S. Brem and E. Malic, "Phonon-assisted Exciton Dissociation in Transition Metal Dichalcogenides", Nanoscale 13, 1884 (2021)
Monolayers of transition metal dichalcogenides (TMDs) have been established in the last years as promising materials for novel optoelectronic devices. However, the performance of such devices is often limited by the dissociation of tightly bound excitons into free electrons and holes. While previous studies have investigated tunneling at large electric fields, we focus in this work on phonon-assisted exciton dissociation that is expected to be the dominant mechanism at small fields. We present a microscopic model based on the density matrix formalism providing access to time- and momentum-resolved exciton dynamics including phonon-assisted dissociation. We track the pathway of excitons from optical excitation via thermalization to dissociation, identifying the main transitions and dissociation channels. Furthermore, we find intrinsic limits for the quantum efficiency and response time of a TMD-based photodetector and investigate their tunability with externally accessible knobs, such as excitation energy, substrate screening, temperature and strain. Our work provides microscopic insights in fundamental mechanisms behind exciton dissociation and can serve as a guide for the optimization of TMD-based optoelectronic devices.
Nanoscale 13, 1884 (2021)
137. R. Rosati, S. Brem, R. Perea-Causín, R. Schmidt, I. Niehues, S. Michaelis de Vasconcellos, R. Bratschitsch and E. Malic, "Strain-dependent exciton diffusion in transition metal dichalcogenides", 2D Mater. 8, 015030 (2021)
Monolayers of transition metal dichalcogenides have a remarkable excitonic landscape with deeplybound bright and dark exciton states. Their properties are strongly affected by lattice distortionsthat can be created in a controlled way via strain. Here, we perform a joint theory-experimentstudy investigating exciton diffusion in strained tungsten disulfide (WS2) monolayers. We reveal anon-trivial and non-monotonic influence of strain. Lattice deformations give rise to differentenergy shifts for bright and dark excitons changing the excitonic landscape, the efficiency ofintervalley scattering channels and the weight of single exciton species to the overall excitondiffusion. We predict a minimal diffusion coefficient in unstrained WS2followed by a steepspeed-up by a factor of 3 for tensile biaxial strain at about 0.6\% strain—in excellent agreementwith our experiments. The obtained microscopic insights on the impact of strain on excitondiffusion are applicable to a broad class of multi-valley 2D materials.
2D Mater. 8, 015030 (2021)
136. M. Feierabend, S. Brem, A. Ekman, E. Malic, "Brightening of spin- and momentum-dark excitons in transition metal dichalcogenides", 2D Mat. 8, 015013 (2021)
Monolayer transition metal dichalcogenides (TMDs) have been in focus of current research,among others due to their remarkable exciton landscape consisting of bright and dark excitonicstates. Although dark excitons are not directly visible in optical spectra, they have a large impact onexciton dynamics and hence their understanding is crucial for potential TMD-based applications.Here, we study brightening mechanisms of dark excitons via interaction with phonons andin-plane magnetic fields. We show clear signatures of momentum- and spin-dark excitons in WS2, WSe2 and MoS2, while the photoluminescence of MoSe2 is only determined by the bright exciton.In particular, we reveal the mechanism behind the brightening of states that are both spin-andmomentum-dark in MoS2. Our results are in good agreement with recent experiments and contribute to a better microscopic understanding of the exciton landscape in TMDs.
2D Mat. 8, 015013 (2021)135. J. Hagel, S. Brem, C. Linderälv, P. Erhart, E. Malic, "Exciton landscape in van der Waals heterostructures" Phys. Rev. Research 3, 043217 (2021)
Van der Waals heterostructures consisting of vertically stacked transition metal dichalcogenides (TMDs) exhibit a rich landscape of bright and dark intra- and interlayer excitons. In spite of a growing literature in this field of research, the type of excitons dominating optical spectra in different van der Waals heterostructures has not yet been well established. The spectral position of exciton states depends strongly on the strength of hybridization and energy renormalization due to the periodic moiré potential. Combining exciton density matrix formalism and density functional theory, we shed light on the exciton landscape in TMD homo- and heterobilayers at different stackings. This allows us to identify on a microscopic footing the energetically lowest lying exciton state for each material and stacking. Furthermore, we disentangle the contribution of hybridization and layer polarization-induced alignment shifts of dark and bright excitons in photoluminescence spectra. By revealing the exciton landscape in van der Waals heterostructures, our work provides the basis for further studies of the optical, dynamical and transport properties of this technologically promising class of nanomaterials.
Phys. Rev. Research 3, 043217134. D. Erkensten, S. Brem, E. Malic, "Exciton-exciton interaction in transition metal dichalcogenide monolayers and van der Waals heterostructures", Phys. Rev. B 103, 045426 (2021)
Due to a strong Coulomb interaction, excitons dominate the excitation kinetics in two-dimensional (2D) materials. While Coulomb scattering between electrons has been well studied, the interaction of excitons is more challenging and remains to be explored. As neutral composite bosons consisting of electrons and holes, excitons show nontrivial scattering dynamics. Here, we study exciton-exciton interaction in transition-metal dichalcogenides and related van der Waals heterostructures on microscopic footing. We demonstrate that the crucial criterion for efficient scattering is a large electron/hole mass asymmetry, giving rise to internal charge inhomogeneities of excitons and emphasizing their cobosonic substructure. Furthermore, both exchange and direct exciton-exciton interactions are boosted by enhanced exciton Bohr radii. We also predict an unexpected temperature dependence that is usually associated with phonon-driven scattering, and we reveal an orders of magnitude stronger interaction of interlayer excitons due to their permanent dipole moment. The developed approach can be generalized to arbitrary material systems and will help to study strongly correlated exciton systems, such as moire super lattices.
Phys. Rev B 103, 045426 (2021)