Main Content
Publications 2019
121. P. Merkl, F. Mooshammer, P. Steinleitner, A. Girnhguber, K. Lin, P. Nagler, J. Holler, C. Schueller, J. Lupton, T. Korn, S. Ovesen, S. Brem, E. Malic and R. Huber, "Ultrafast transition between exciton phases in van der Waals heterostructures", Nature Materials 18, 691 (2019)
Heterostructures of atomically thin van der Waals bonded monolayers have opened a unique platform to engineer Coulomb correlations, shaping excitonic, Mott insulating or superconducting phases. In transition metal dichalco-genide heterostructures, electrons and holes residing in different monolayers can bind into spatially indirect exci-tons with a strong potential for optoelectronics, val-leytronics, Bose condensation, superfluidity and moiré-induced nanodot lattices. Yet these ideas require a microscopic understanding of the formation, dissociation and thermalization dynamics of correlations including ultra-fast phase transitions. Here we introduce a direct ultrafast access to Coulomb correlations between monolayers, where phase-locked mid-infrared pulses allow us to measure the binding energy of interlayer excitons in WSe2/WS2 hetero-bilayers by revealing a novel 1s–2p resonance, explained by a fully quantum mechanical model. Furthermore, we trace, with subcycle time resolution, the transformation of an exci-ton gas photogenerated in the WSe2 layer directly into inter-layer excitons. Depending on the stacking angle, intra- and interlayer species coexist on picosecond scales and the 1s–2presonance becomes renormalized. Our work provides a direct measurement of the binding energy of interlayer excitons and opens the possibility to trace and control correlations in novel artificial materials.
Nature Materials 18, 691 (2019)120. A. Raja, L. Waldecker, J. Zipfel, Y. Cho, S. Brem, J. Ziegler, M. Kulig, T. Taniguchi, K. Watanabe, E. Malic, T. Berkelbach, T. Heinz and A. Chernikov, "Dielectric disorder in two-dimensional materials", Nature Nanotechnology 14, 832 (2019)
Understanding and controlling disorder is key to nanotechnology and materials science. Traditionally, disorder is attributed to local fluctuations of inherent material properties such as chemical and structural composition, doping or strain. Here, we present a fundamentally new source of disorder in nanoscale systems that is based entirely on the local changes of the Coulomb interaction due to fluctuations of the external dielectric environment. Using two-dimensional semiconductors as prototypes, we experimentally monitor dielectric disorder by probing the statistics and correlations of the exciton resonances, and theoretically analyse the influence of external screening and phonon scattering. Even moderate fluctuations of the dielectric environment are shown to induce large variations of the bandgap and exciton binding energies up to the 100 meV range, often making it a dominant source of inhomogeneities. As a consequence, dielectric disorder has strong implications for both the optical and transport properties of nanoscale materials and their heterostructures.
Nature Nanotechnology 14, 832 (2019)119. R. Perea-Causin, S. Brem, R. Rosati, R.Jago, M. Kulig, J. Ziegler, J. Zipfel, A. Chernikov and E. Malic, "Exciton propagation and halo formation in two-dimensional materials", Nano Lett. 19, 7317 (2019)
The interplay of optics, dynamics, and transport is crucial for the design of novel optoelectronic devices, such as photodetectors and solar cells. In this context, transition-metal dichalcogenides (TMDs) have received much attention. Here, strongly bound excitons dominate optical excitation, carrier dynamics, and diffusion processes. While the first two have been intensively studied, there is a lack of fundamental understanding of nonequilibrium phenomena associated with exciton transport that is of central importance (e.g., for high-efficiency light harvesting). In this work, we provide microscopic insights into the interplay of exciton propagation and many-particle interactions in TMDs. On the basis of a fully quantum mechanical approach and in excellent agreement with photoluminescence measurements, we show that Auger recombination and emission of hot phonons act as a heating mechanism giving rise to strong spatial gradients in excitonic temperature. The resulting thermal drift leads to an unconventional exciton diffusion characterized by spatial exciton halos.
Nano Lett. 19, 7317 (2019)118. S. Ovesen, S. Brem, C. Linderälv, M. Kuisma, P. Erhart, M. Selig and E. Malic, "Interlayer exciton dynamics in van der Waals heterostructures", Communication Physics 2, 23 (2019)
Atomically thin transition metal dichalcogenides can be stacked to van der Waals heterostructures enabling the design of new materials with tailored properties. The strong Coulomb interaction gives rise to interlayer excitons, where electrons and holes are spatially separated in different layers. In this work, we reveal the time- and momentum-dependent elementary processes behind the formation, thermalization and photoemission of interlayer excitons for the exemplary MoSe2–WSe2 heterostructure. We identify tunneling of holes from MoSe2 to WSe2 on a ps timescale as the crucial process for interlayer exciton formation. We also predict a drastic reduction of the formation time as a function of the interlayer energy offset suggesting that interlayer excitons can be externally tuned. Finally, we explain the experimental observation of a dominant photoluminescence from interlayer excitons despite the vanishingly small oscillator strength as a consequence of huge interlayer exciton occupations at low temperatures.
Communication Physics 2, 23 (2019)117. S. Brem, J. Zipfel, M. Selig, A. Raja, L. Waldecker, J. Ziegler, T. Taniguchi, K. Watanabe, A. Chernikov and E. Malic, "Intrinsic Lifetime of Higher Excitonic States in Tungsten Diselenide Monolayers", Nanoscale 11, 12318 (2019)
The reduced dielectric screening in atomically thin transition metal dichalcogenides allows to study the hydrogen-like series of higher exciton states in optical spectra even at room temperature. The width of excitonic peaks provides information about the radiative decay and phonon-assisted scattering channels limiting the lifetime of these quasi-particles. While linewidth studies so far have been limited to the exciton ground state, encapsulation with hBN has recently enabled quantitative measurements of the broadening of excited exciton resonances. Here, we present a joint experiment-theory study combining microscopic calculations with spectroscopic measurements on the intrinsic linewidth and lifetime of higher exciton states in hBN-encapsulated WSe2 monolayers. Surprisingly, despite the increased number of scattering channels, we find both in theory and experiment that the linewidth of higher excitonic states is similar or even smaller compared to the ground state. Our microscopic calculations ascribe this behavior to a reduced exciton–phonon scattering efficiency for higher excitons due to spatially extended orbital functions.
Nanoscale 11, 12318 (2019)116. R. Jago, R. Perea-Causin, S. Brem and E. Malic, "Spatio-temporal dynamics in graphene", Nanoscale 11, 10017 (2019)
Temporally and spectrally resolved dynamics of optically excited carriers in graphene has been intensively studied theoretically and experimentally, whereas carrier diffusion in space has attracted much less attention. Understanding the spatio-temporal carrier dynamics is of key importance for optoelectronic applications, where carrier transport phenomena play an important role. In this work, we provide a microscopic access to the time-, momentum-, and space-resolved dynamics of carriers in graphene. We determine the diffusion coefficient to be D ≈ 360 cm2 s−1 and reveal the impact of carrier–phonon and carrier–carrier scattering on the diffusion process. In particular, we show that phonon-induced scattering across the Dirac cone gives rise to back-diffusion counteracting the spatial broadening of the carrier distribution.
Nanoscale 11, 10017 (2019)115. B. Semnani, R. Jago, S. Safavi-Naeini, A. Majedi, E. Malic and P. Tassin, "Anomalous optical saturation of low-energy Dirac states in graphene and its implication for nonlinear optics", 2D Mater. 6, 031003 (2019)
We reveal that optical saturation of the low-energy states takes place in graphene for arbitrarily weak electromagnetic fields. This effect originates from the diverging field-induced interband coupling at the Dirac point. Using semiconductor Bloch equations to model the electronic dynamics of graphene, we argue that the charge carriers undergo ultrafast Rabi oscillations leading to the anomalous saturation effect. The theory is complemented by a many-body study of the carrier relaxations dynamics in graphene. It will be demonstrated that the carrier relaxation dynamics is slow around the Dirac point, which in turn leads to a more pronounced saturation. The implications of this effect for the nonlinear optics of graphene are then discussed. Our analysis shows that the conventional perturbative treatment of the nonlinear optics, i.e. expanding the polarization field in a Taylor series of the electric field, is problematic for graphene, in particular at small Fermi levels and large field amplitudes.
2D Mater. 6, 031003 (2019)114. Z. Khatibi, M. Feierabend, M. Selig, S. Brem, C. Linderälv,1 P. Erhart and E. Malic, "Impact of strain on the excitonic linewidth in transition metal dichalcogenides", 2D Mater. 6, 015015 (2019)
Monolayer transition metal dichalcogenides (TMDs) are known to be highly sensitive to externally applied tensile or compressive strain. In particular, strain can be exploited as a tool to control the optical response of TMDs. However, the role of excitonic effects under strain has not been fully understood yet. Utilizing the strain-induced modification of electron and phonon dispersion obtained by first principle calculations, we present in this work microscopic insights into the strain-dependent optical response of various TMD materials. We show that the different changes in the excitonic linewidth of diverse TMD monolayers are due to the strain-induced modification of the relative spectral position of bright and dark excitonic states. Our theoretical results explain well the observed partially opposite changes in the excitonic linewidth of different TMDs at room temperature. Furthermore, we predict the linewidth behavior of excitonic resonances in strained TMDs for tensile and compressive strain at low temperatures.
2D Mater. 6, 015015 (2019)113. M. Dwedari, S. Brem, M. Feierabend, E. Malic, "Disorder-induced broadening of excitonic resonances in transition metal dichalcogenides", Phys. Rev. Mat. 3, 074004 (2019)
The presence of impurities and disorder has an important impact on the optical response of monolayertransition metal dichalcogenides (TMDs). Here, we investigate elastic exciton-impurity scattering and itsinfluence on the linewidth of excitonic resonances in different TMD materials. We derive an analytic expressionfor the linewidth broadening within the density matrix formalism. We find that the exciton linewidth increasesfor states up to the 3sexciton due to the scattering with impurities. For higher states, the impurity contributiondecreases, reflecting the reduced scattering cross section. Furthermore, we reveal that the scattering efficiencyis the largest for transitions betweensandpexciton states. Finally, different TMDs show generally a similarbehavior. The quantitatively smaller broadening in tungsten-based TMDs can be ascribed to their smallereffective masses resulting in a less efficient scattering.
112. M. Feierabend, S. Brem and E. Malic, "Optical fingerprint of bright and dark localized excitonic states in atomically thin 2D materials", Phys. Chem. Chem. Phys 21, 26077 (2019)
Point defects, local strain or impurities can crucially impact the optical response of atomically thin two-dimensional materials as they offer trapping potentials for excitons. These trapped excitons appear in photoluminescence spectra as new resonances below the bright exciton that can even be exploited for single photon emission. While large progress has been made in deterministically introducing defects, only little is known about their impact on the optical fingerprint of 2D materials. Here, based on a microscopic approach we reveal direct signatures of localized bright excitonic states as well as indirect phonon-assisted side bands of localized momentum-dark excitons. The visibility of localized excitons strongly depends on temperature and disorder potential width. This results in different regimes, where either the bright or dark localized states are dominant in optical spectra. We trace back this behavior to an interplay between disorder-induced exciton capture and intervalley exciton–phonon scattering processes.
Phys. Chem. Chem. Phys 21, 26077 (2019)111. D. Christiansen, M. Selig, E. Malic, R. Ernstorfer, A. Knorr, "Theory of exciton dynamics in time-resolved ARPES: intra- and intervalley scattering in two-dimensional semiconductors", Phys. Rev. B 100, 205401 (2019).
Time- and angle-resolved photoemission spectroscopy (trARPES) is a powerful spectroscopic method to measure the ultrafast electron dynamics directly in momentum space. However, band gap materials with exceptionally strong Coulomb interaction such as monolayer transition-metal dichalcogenides exhibit tightly bound excitons, which dominate their optical properties. This raises the question of whether excitons, in particular their formation and relaxation dynamics, can be detected in photoemissions. Here, we develop a fully microscopic theory of the temporal dynamics of excitonic time- and angle-resolved photoemission with a particular focus on the phonon-mediated thermalization of optically excited excitons to momentum-forbidden dark exciton states. We find that trARPES is able to probe the ultrafast exciton formation and relaxation throughout the Brillouin zone.
Phys. Rev. B 100, 205401 (2019)110. M. Selig, F. Katsch, R. Schmidt, S. Michaelis de Vasconcellos, R. Bratschitsch, E. Malic and A. Knorr,"Ultrafast dynamics in monolayer TMDs: dark excitons, phonons, and intervalley Coulomb exchange", Phys. Rev. Research 1, 022007(R) (2019)
Understanding the ultrafast coupling and relaxation mechanisms between valleys in transition metal dichalcogenide semiconductors is of crucial interest for future valleytronic devices. Recent ultrafast pump-probe experiments showed an unintuitive significant bleaching at the excitonic B transition after optical excitation of the energetically lower excitonic A transition. Here, we present a possible microscopic explanation for this surprising effect. It is based on the joint action of exchange coupling and phonon-mediated thermalization into dark exciton states and does not involve a population of the B exciton. Our work demonstrates how intra- and intervalley coupling on a femtosecond timescale governs the optical valley response of 2D semiconductors.
Phys. Rev. Research 1, 022007(R) (2019)109. M. Feierabend, Z. Khatibi, G. Berghäuser and E. Malic"Dark-exciton based strain sensing in transition metal dichalcogenides", Phys. Rev. B 99, 195454 (2019)
The trend towards ever smaller high-performance devices in modern technology requires novel materials withnew functionalities. The recent emergence of atomically thin two-dimensional (2D) materials has opened uppossibilities for the design of ultra-thin and flexible nanoelectronic devices. As truly 2D materials, they exhibitan optimal surface-to-volume ratio, which results in an extremely high sensitivity to external changes. Thismakes these materials optimal candidates for sensing applications. Here, we exploit the remarkably diverseexciton landscape in monolayer transition metal dichalcogenides to propose a novel dark-exciton-based conceptfor ultra sensitive strain sensors. We demonstrate that the dark-bright-exciton separation can be controlled bystrain, which has a crucial impact on the activation of dark excitonic states. This results in a pronounced intensitychange of dark excitons in photoluminescence spectra, when only 0.05 % strain is applied. The predictedextremely high optical gauge factors of up to 8000 are promising for the design of optical strain sensors.
Phys. Rev. B 99, 195454 (2019)108. M. Selig, E. Malic, K. Jun Ahn, N. Koch and A. Knorr, "Theory of optically induced Förster coupling in van der Waals coupled heterostructures", Phys. Rev. B 99, 035420 (2019)
We investigate the impact of optically induced Förster coupling in van der Waals heterostructures consisting of graphene and a monolayer transition-metal dichalcogenide (TMD). In particular, we predict the corresponding dephasing rates and a fast energy transfer between the TMD layer and graphene being in the picosecond range. Exemplary we find a transition rate of thermalized excitons of about 4 ps−1 in a MoSe2-graphene stack at room temperature. This time scale is in good agreement with the recently measured exciton lifetime in this heterostructure.
Phys. Rev. B 99, 035420 (2019)107. Roland Jago, Ermin Malic, and Florian Wendler, "Microscopic origin of the bolometric effect in graphene", Phys. Rev. B 99, 035419 (2019)
While the thermoelectric and photoconduction effects are crucial in pristine and low-doped graphene, the bolometric effect is known to dominate the photoresponse in biased graphene. Here, we present a detailed microscopic investigation of the photoresponse due to the bolometric effect in graphene. Based on the semiconductor Bloch equations, we investigate the time- and momentum-resolved carrier dynamics in graphene in the presence of a constant electric field under optical excitation. The magnitude of the bolometric effect is determined by the optically induced increase of temperature times the conductivity change. Investigating both factors independently, we reveal that the importance of the bolometric effect in the high-doping regime can be mostly ascribed to the latter showing a parabolic dependence on the doping.
Phys. Rev. B 99, 035419 (2019)