Main Content
Publications 2020
133. P. Merkl, F. Mooshammer, S. Brem, A. Girnghuber, K. Lin, L. Weigl, C. Yong, R. Gillen, J. Maultzsch, J. Lupton, E. Malic and R. Huber, "Twist-tailoring Coulomb correlations in van der Waals homobilayers", Nature Communications 11, 2167 (2020)
The recent discovery of artificial phase transitions induced by stacking monolayer materials at magic twist angles represents a paradigm shift for solid state physics. Twist-induced changes of the single-particle band structure have been studied extensively, yet a precise understanding of the underlying Coulomb correlations has remained challenging. Here we reveal in experiment and theory, how the twist angle alone affects the Coulomb-induced internal structure and mutual interactions of excitons. In homobilayers of WSe2, we trace the internal 1s–2p resonance of excitons with phase-locked mid-infrared pulses as a function of the twist angle. Remarkably, the exciton binding energy is renormalized by up to a factor of two, their lifetime exhibits an enhancement by more than an order of magnitude, and the exciton-exciton interaction is widely tunable. Our work opens the possibility of tailoring quasiparticles in search of unexplored phases of matter in a broad range of van der Waals heterostructures.
132. S. Brem, C. Linderälv, P. Erhart and E. Malic, "Tunable phases of moire excitons in van der Waals heterostructures", Nano Letters 20, 8534 (2020)
Stacking monolayers of transition metal dichalcogenides into aheterostructure with afinite twist-angle gives rise to artificial moirésuperlattices with a tunable periodicity. As a consequence, excitons experiencea periodic potential, which can be exploited to tailor optoelectronic propertiesof these materials. Whereas recent experimental studies have confirmed twist-angle-dependent optical spectra, the microscopic origin of moiréexcitonresonances has not been fully clarified yet. Here, we combinefirst-principlescalculations with the excitonic density matrix formalism to study transitionsbetween different moiréexciton phases and their impact on optical propertiesof the twisted MoSe2/WSe2heterostructure. At angles smaller than 2°,wefindflat, moiré-trapped states for inter- and intralayer excitons. This moiréexcitonphase changes into completely delocalized states at 3°. We predict a linear andquadratic twist-angle dependence of excitonic resonances for the moiré-trappedand delocalized exciton phases, respectively.
131. S. Brem, A. Ekman, D. Christiansen, F. Katsch, M. Selig, C. Robert, X. Marie, B. Urbaszek, A. Knorr, and E. Malic,"Phonon-assisted photoluminescence from dark excitons in monolayers of transition metal dichalcogenides", Nano Letters 20, 2849 (2020)
The photoluminescence (PL) spectrum of transition-metal dichalcogenides (TMDs) shows a multitude of emission peaks below the bright exciton line, and not all of them have been explained yet. Here, we study the emission traces of phonon-assisted recombinations of indirect excitons. To this end, we develop a microscopic theory describing simultaneous exciton, phonon, and photon interaction and including consistent many-particle dephasing. We explain the drastically different PL below the bright exciton in tungsten- and molybdenum-based materials as the result of different configurations of bright and momentum-dark states. In good agreement with experiments, our calculations predict that WSe2 exhibits clearly visible low-temperature PL signals stemming from the phonon-assisted recombination of momentum-dark K–K′ excitons.
130. S. Thurakkal, D. Feldstein, R. Causin, E. Malic and X. Zhang, "The Art of Constructing Black Phosphorus Nanosheets based Heterostructures: from 2D to 3D", Adv. Mater. 2020, 205254 (2020)
Assembling different kinds of 2D nanosheets into heterostructures presents a promising way of designing novel artificial materials with new and improved functionalities by combining the unique properties of each component. In the past few years, black phosphorus nanosheets (BPNSs) have been recognized as a highly feasible 2D material with outstanding electronic properties, a tunable bandgap, and strong in‐plane anisotropy, highlighting their suitability as a material for constructing heterostructures. In this study, recent progress in the construction of BPNS‐based heterostructures ranging from 2D hybrid structures to 3D networks is discussed, emphasizing the different types of interactions (covalent or noncovalent) between individual layers. The preparation methods, optical and electronic properties, and various applications of these heterostructures—including electronic and optoelectronic devices, energy storage devices, photocatalysis and electrocatalysis, and biological applications—are discussed. Finally, critical challenges and prospective research aspects in BPNS‐based heterostructures are also highlighted.
129. S. Brem, K. Lin, R. Gillen, J. Bauer, J. Maultzsch, J. Lupton and E. Malic, "Hybridized intervalley moiré excitons and flat bands in twisted WSe2 bilayers", Nanoscale 12, 11088 (2020)
The large surface-to-volume ratio in atomically thin 2D materials allows to efficiently tune their properties through modifications of their environment. Artificial stacking of two monolayers into a bilayer leads to an overlap of layer-localized wave functions giving rise to a twist angle-dependent hybridization of excitonic states. In this joint theory-experiment study, we demonstrate the impact of interlayer hybridization on bright and momentum-dark excitons in twisted WSe2 bilayers. In particular, we show that the strong hybridization of electrons at the Λ point leads to a drastic redshift of the momentum-dark K–Λ exciton, accompanied by the emergence of flat moiré exciton bands at small twist angles. We directly compare theoretically predicted and experimentally measured optical spectra allowing us to identify photoluminescence signals stemming from phonon-assisted recombination of layer-hybridized dark excitons. Moreover, we predict the emergence of additional spectral features resulting from the moiré potential of the twisted bilayer lattice.
128. R. Rosati, R. Perea-Causín, S. Brem, and E. Malic, "Negative excitonic diffusion in transition metal dichalcogenide", Nanoscale 12, 356 (2020)
While exciton relaxation in monolayers of transition metal dichalcogenides (TMDs) has been intensively studied, spatial exciton diffusion has received only a little attention – in spite of being a key process for optoelectronics and having already shown interesting unconventional behaviours (e.g. spatial halos). Here, we study the spatiotemporal dynamics in TMD monolayers and track optically excited excitons in time, momentum, and space. In particular, we investigate the temperature-dependent exciton diffusion including the remarkable exciton landscape constituted by bright and dark states. Based on a fully quantum mechanical approach, we show at low temperatures an unexpected negative effective diffusion characterized by a shrinking of the spatial exciton distributions. This phenomenon can be traced back to the existence of dark exciton states in TMD monolayers and is a result of an interplay between spatial exciton diffusion and intervalley exciton–phonon scattering.
127. R. Rosati, S. Brem, R. Perea-Causin, K. Wagner, E. Wietek, J. Zipfel, M. Selig, T. Taniguchi, K. Watanabe, A. Knorr, A. Chernikov, E. Malic, "Temporal evolution of low-temperature phonon sidebands in WSe2 monolayers", ACS Photonics 7, 2756 (2020)
Low-temperature photoluminescence (PL) of hBN-encapsulated monolayer tungsten diselenide (WSe2) shows a multitude of sharp emission peaks below the bright exciton. Some of them have been recently identified as phonon sidebands of momentum-dark states. However, the exciton dynamics behind the emergence of these sidebands has not been revealed yet. In this joint theory–experiment study, we theoretically predict and experimentally observe time-resolved PL, providing microscopic insights into the thermalization of hot excitons formed after optical excitation. In very good agreement between theory and experiment, we demonstrate a spectral red-shift of phonon sidebands on a time scale of tens of picoseconds, reflecting the phonon-driven thermalization of hot excitons in momentum-dark states. Furthermore, we predict the emergence of a transient phonon sideband that vanishes in the stationary PL. The obtained microscopic insights are applicable to a broad class of 2D materials with multiple exciton valleys.
126. D. Feldstein, R. Perea-Causin, S. Wang, M. Dyksik, K. Watanabe, T. Taniguchi, P. Plochocka, E. Malic, "Microscopic picture of electron-phonon interaction in two-dimensional halide perovskites", accepted at J. Phys. Chem. Lett. (2020), J. Phys. Chem. Lett. 11, 9975 (2020)
Perovskites have attracted much attention due to their remarkable optical properties. While it is well established that excitons dominate their optical response, the impact of higher excitonic states and formation of phonon sidebands in optical spectra still need to be better understood. Here, we perform a theoretical study of excitonic properties of monolayered hybrid organic perovskites—supported by temperature-dependent photoluminescence measurements. Solving the Wannier equation, we obtain microscopic access to the Rydberg-like series of excitonic states including their wave functions and binding energies. Exploiting the generalized Elliot formula, we calculate the photoluminescence spectra demonstrating a pronounced contribution of a phonon sideband for temperatures up to 50 K, in agreement with experimental measurements. Finally, we predict temperature-dependent line widths of the three energetically lowest excitonic transitions and identify the underlying phonon-driven scattering processes.
125. M. Selig, F. Katsch, S. Brem, G. Mkrtchian, E. Malic, A. Knorr, "Suppresion of Intervalley Exchange Coupling in the Presence of Momentum-Dark States in Transition Metal Dichalcogenides", Phys. Rev. Res. 2, 023322 (2020)
Monolayers of transition metal dichalcogenides (TMDCs) are promising materials for valleytronic appli-cations, since they possess two individually addressable excitonic transitions at the nonequivalentKandK′points with different spins, selectively excitable with light of opposite circular polarization. Here, it is of crucialimportance to understand the elementary processes determining the lifetime of optically injected valley excitons.In this study, we perform microscopic calculations based on a Heisenberg equation of motion formalism toinvestigate the efficiency of the intervalley coupling in the presence (W-based TMDCs) and absence (Mo-basedTMDCs) of energetically low-lying momentum-dark exciton states after pulsed excitation. While we predicta spin polarization lifetime on the order of some hundreds of femtoseconds in the absence of low-lyingmomentum-dark states, we demonstrate a strong elongation of the spin-polarization lifetime in the presenceof such states due to a suppression of the intervalley exchange coupling.
124. J. Thompson, S. Brem, H. Fang, J. Frey, S. Dash, W. Wieczorek and E. Malic, "Criteria for single-photon emission in two-dimensional atomic crystals", Phys. Rev. Mat. 4, 084006 (2020)
The deterministic production of single photons from two-dimensional materials promises to usher in a new generation of photonic quantum devices. In this work, we outline criteria by which single-photon emission can be realized in two-dimensional materials: spatial isolation, spectral filtering, and low excitation of quantum emitters. We explore how these criteria can be fulfilled in atomically thin transition metal dichalcogenides, where excitonic physics dictates the observed photoemission. In particular, we model the effect of defects and localized strain, in accordance with the most common experimental realizations, on the photon statistics of emitted light. Moreover, we demonstrate that an optical cavity has a negative impact on the photon statistics, suppressing the single-photon character of the emission by diminishing the effect of spectral filtering on the emitted light. Our work provides a theoretical framework revealing criteria necessary to facilitate single-photon emission in two-dimensional materials and thus can guide future experimental studies in this field.
123. J. Zipfel, M. Kulig,R. Perea-Causín, S. Brem, J. Ziegler, R. Rosati, T. Taniguchi, K. Watanabe, M. Glazov, E. Malic and A. Chernikov, "Exciton diffusion in monolayer semiconductors with suppressed disorder", Phys. Rev. B 101, 115430 (2020, Editor's Suggestion)
Tightly bound excitons in monolayer semiconductors represent a versatile platform to study two-dimensional propagation of neutral quasiparticles. Their intrinsic properties, however, can be severely obscured by spatial energy fluctuations due to a high sensitivity to the immediate environment. Here, we take advantage of the encapsulation of individual layers in hexagonal boron nitride to strongly suppress environmental disorder. Diffusion of excitons is then directly monitored using time and spatially resolved emission microscopy at ambient conditions. We consistently find very efficient propagation with linear diffusion coefficients up to 10cm2/s, corresponding to room-temperature effective mobilities as high as 400cm2/Vs as well as a correlation between rapid diffusion and short population lifetime. At elevated densities we detect distinct signatures of many-particle interactions and consequences of strongly suppressed Auger-type exciton-exciton annihilation. A combination of analytical and numerical theoretical approaches is employed to provide pathways toward comprehensive understanding of the observed linear and nonlinear propagation phenomena. We emphasize the role of dark exciton states and present a mechanism for diffusion facilitated by free-electron hole plasma from entropy-ionized excitons.
122. R. Perea-Causin, S. Brem, E. Malic, "Microscopic Modeling of Pump-Probe Spectroscopy and Population Inversion Transition Metal Dichalcogenides", Phys. Status Solidi B 257, 2000223 (2020)
Optical properties of transition metal dichalcogenide (TMD) monolayers are dominated by excitonic effects. These are significantly altered at high carrier densities, leading to energy renormalization, absorption bleaching, and even optical gain. Such effects are experimentally accessible in ultra‐fast pump–probe measurements. Herein, the semiconductor Bloch equations are combined with the generalized Wannier equation to investigate the effect that excited carriers have on the excitonic properties of TMD monolayers. In particular, the dynamics of carrier occupation, energy renormalization, and absorption bleaching as well as population inversion and optical gain are investigated.