Main Content
Publications 2022
164. D. Schmitt, J. P. Bange, W. Bennecke, A. A. Mutairi, G. Meneghini, K. Watanabe, T. Taniguchi, D. Steil, D. R. Luke, R. T. Weitz, S. Steil, G. S. M. Jansen, S. Brem, E. Malic, S. Hofmann, M. Reutzel and S. Mathias, "Formation of moiré interlayer excitons in space and time", Nature 608, 499 (2022)
Moiré superlattices in atomically thin van der Waals heterostructures hold great promise for extended control of electronic and valleytronic lifetimes, the confinement of excitons in artificial moiré lattices and the formation of exotic quantum phases. Such moiré-induced emergent phenomena are particularly strong for interlayer excitons, where the hole and the electron are localized in different layers of the heterostructure. To exploit the full potential of correlated moiré and exciton physics, a thorough understanding of the ultrafast interlayer exciton formation process and the real-space wavefunction confinement is indispensable. Here we show that femtosecond photoemission momentum microscopy provides quantitative access to these key properties of the moiré interlayer excitons. First, we elucidate that interlayer excitons are dominantly formed through femtosecond exciton–phonon scattering and subsequent charge transfer at the interlayer-hybridized Σ valleys. Second, we show that interlayer excitons exhibit a momentum fingerprint that is a direct hallmark of the superlattice moiré modification. Third, we reconstruct the wavefunction distribution of the electronic part of the exciton and compare the size with the real-space moiré superlattice. Our work provides direct access to interlayer exciton formation dynamics in space and time and reveals opportunities to study correlated moiré and exciton physics for the future realization of exotic quantum phases of matter.
Nature 608, 499 (2022)163. S. Brem and E. Malic, “Terahertz fingerprint of monolayer Wigner crystals”, Nano Lett. 3, 1311 (2022)
The strong Coulomb interaction in monolayer semiconductors represents a unique opportunity for the realization of Wigner crystals without external magnetic fields. In this work, we predict that the formation of monolayer Wigner crystals can be detected by their terahertz response spectrum, which exhibits a characteristic sequence of internal optical transitions. We apply the density matrix formalism to derive the internal quantum structure and the optical conductivity of the Wigner crystal and to microscopically analyze the multi-peak shape of the obtained terahertz spectrum. Moreover, we predict a characteristic shift of the peak position as function of charge density for different atomically thin materials and show how our results can be generalized to an arbitrary two-dimensional system.
Nano Lett. 3, 1311 (2022)
162. J. M. Fitzgerald, J. J. P. Thompson and E. Malic, "Twist Angle Tuning of Moiré Exciton Polaritons in van der Waals Heterostructures", Nano Lett. 11, 4468 (2022)
Twisted atomically thin semiconductors are characterized by moiré excitons. Their optical signatures and selection rules are well understood. However, their hybridization with photons in the strong coupling regime for heterostructures integrated in an optical cavity has not been in the focus of research yet. Here, we combine an excitonic density matrix formalism with a Hopfield approach to provide microscopic insights into moiré exciton polaritons. In particular, we show that exciton-light coupling, polariton energy, and even the number of polariton branches can be controlled via the twist angle. We find that these new hybrid light-exciton states become delocalized relative to the constituent excitons due to the mixing with light and higher-energy excitons. The system can be interpreted as a natural quantum metamaterial with a periodicity that can be engineered via the twist angle. Our study presents a significant advance in microscopic understanding and control of moiré exciton polaritons in twisted atomically thin semiconductors.
Nano Lett. 11, 4468 (2022)161. T. Siday, F. Sandner, S. Brem, M. Zizlsperger, R. Perea-Causin, F. Schiegl, S. Nerreter, M. Plankl, P. Merkl, F. Mooshammer, M. A. Huber, E. Malic, and R. Huber, “Ultrafast Nanoscopy of High-Density Exciton Phases in WSe2”, Nano Lett. 6, 2561 (2022)
The density-driven transition of an exciton gas into an electron–hole plasma remains a compelling question in condensed matter physics. In two-dimensional transition metal dichalcogenides, strongly bound excitons can undergo this phase change after transient injection of electron–hole pairs. Unfortunately, unavoidable nanoscale inhomogeneity in these materials has impeded quantitative investigation into this elusive transition. Here, we demonstrate how ultrafast polarization nanoscopy can capture the Mott transition through the density-dependent recombination dynamics of electron–hole pairs within a WSe2 homobilayer. For increasing carrier density, an initial monomolecular recombination of optically dark excitons transitions continuously into a bimolecular recombination of an unbound electron–hole plasma above 7 × 1012 cm–2. We resolve how the Mott transition modulates over nanometer length scales, directly evidencing the strong inhomogeneity in stacked monolayers. Our results demonstrate how ultrafast polarization nanoscopy could unveil the interplay of strong electronic correlations and interlayer coupling within a diverse range of stacked and twisted two-dimensional materials.
Nano Lett. 6, 2561 (2022)160. G. Pasquale, Z. Sun, K. Cernevics, R. Perea-Causin, F. Tagarelli, K. Watanabe, T. Taniguchi, E. Malic, O. V. Yazyev, A. Kis, "Flat-band-induced many-body interactions and exciton complexes in a layered semiconductor", Nano Lett. 22,22, 8883 (2022)
Interactions among a collection of particles generate many-body effects in solids resulting in striking modifications of material properties. The heavy carrier mass that yields strong interactions and gate control of carrier density over a wide range, make two-dimensional semiconductors an exciting playground to explore many-body physics. The family of III-VI metal monochalcogenides emerges as a new platform for this purpose due to its excellent optical properties and the flat valence band dispersion with a Mexican-hat-like inversion. In this work, we present a complete study of charge-tunable excitons in few-layer InSe by photoluminescence spectroscopy. From the optical spectra, we establish that free excitons in InSe are more likely to be captured by ionized donors due to the large exciton Bohr radius, leading to the formation of bound exciton complexes. Surprisingly, a pronounced redshift of the exciton energy accompanied by a decrease of the exciton binding energy upon hole-doping reveals a significant band gap renormalization and dynamical screening induced by the presence of the Fermi reservoir. Our findings establish InSe as a reproducible and potentially manufacturable platform to explore electron correlation phenomena without the need for twist-angle engineering.
Nano Lett. 22,22, 8883 (2022)159. B. Han, S. Stephan, J. J.P. Thompson, M. Esmann, C. Anton-Solanas, H. Shan, N. Kunte, S. Brem, S. Tongay, K. Wantanabe, T. Taniguchi, M. Silies, E. Malic, und C. Schneider, "Angle- and polarization-resolved luminescence in suspended and hexagonal Boron Nitride encapsulated MoSe2 monolayers", Optica 9, 10, 1190 (2022)
We apply combined angle- and polarization resolved spectroscopy to explore the interplay of excitonic physics and phenomena arising from the commonly utilized encapsulation procedure on the optical properties of atomically thin transition metal dichalcogenides. In our study, we probe MoSe2 monolayers which are prepared in both a suspended, as well as an encapsulated manner. We show that the hBN encapsulation significantly enhances the linear polarization of exciton PL emission at large emission angles. This degree of linear polarization of excitons can increase up to 17 % in the hBN encapsulated samples. As we confirm by finite difference time-domain simulations, it can be directly connected to the optical anisotropy of the hBN layers. In comparison, the linear polarization at finite exciton momenta is significantly reduced in suspended MoSe2 monolayer, and only becomes notable at cryogenic conditions. This phenomenon strongly hints, that the effect is rooted in the k-dependent anisotropic exchange coupling inherent in 2D excitons.
Optica 9, 10, 1190 (2022)158. J. J. P. Thompson, S. Brem, H. Fang, C. Anton-Solanas, B. Han, H. Shan, S. P. Dash, W. Wieczorek, C. Schneider and E. Malic, “Valley-Exchange Coupling Probed by Angle-Resolved Photoluminescence”, Nanoscale Horiz. 7, 77 (2022)
The optical properties of monolayer transition metal dichalcogenides are dominated by tightly- bound excitons. They form at distinct valleys in reciprocal space, and can interact via the valley-exchange coupling, modifying considerably their dispersion. Here, we demonstrate that angle- resolved photoluminescence can be used to probe the changes of the excitonic dispersion. The exchange-coupling leads to a unique angle dependence of the emission intensity for both circularly and linearly-polarised light. We show that these emission characteristics can be strongly tuned by an external magnetic field due to the valley-specific Zeeman-shift. We propose that angle-dependent photoluminescence measurements involving both circular and linear polarisation as well as magnetic fields should act as strong verification of the role of valley-exchange coupling on excitonic dispersion and its signatures in optical spectra.
Nanoscale Horiz. 7, 77 (2022)157. G. Meneghini, S. Brem, E. Malic, "Ultrafast phonon-driven charge transfer in van der Waals heterostructures", Natural Sciences 2, e20220014 (2022)
Van der Waals heterostructures built by vertically stacked transition metal dichalcogenides (TMDs) exhibit a rich energy landscape, including interlayer and intervalley excitons. Recent experiments demonstrated an ultrafast charge transfer in TMD heterostructures. However, the nature of the charge transfer process has remained elusive. Based on a microscopic and material-realistic exciton theory, we reveal that phonon-mediated scattering via strongly hybridized intervalley excitons governs the charge transfer process that occurs on a sub-100fs timescale. We track the time-, momentum-, and energy-resolved relaxation dynamics of optically excited excitons and determine the temperature- and stacking-dependent charge transfer time for different TMD bilayers. The provided insights present a major step in microscopic understanding of the technologically important charge transfer process in van der Waals heterostructures.
Natural Sciences 2, e20220014 (2022)156. J. J. P. Thompson, D. Muth, S. Anhäuser, D. Bischof, M. Gerhard, G. Witte, E. Malic, "Singlet exciton optics and phonon-mediated dynamics in oligoacene semiconductor crystals", Natural Sciences 3, e20220040 (2022)
Organic semiconductor crystals stand out as an efficient, cheap and diverse platform for realising optoelectronic applications. The optical response of these crystals is governed by a rich tapestry of exciton physics. So far, little is known on the phonon-driven singlet exciton dynamics in this class of materials. In this joint theory-experiment work, we combine the fabrication of a high-quality oligoacene semiconductor crystal and characterization via photoluminescence measurements with a sophisticated approach to the microscopic modeling in these crystals. This allows us to investigate singlet exciton optics and dynamics. We predict phonon-bottleneck effects in pentacene crystals, where we find dark excitons acting as crucial phonon-mediated relaxation scattering channels. While the efficient singlet fission in pentacene crystals hampers the experimental observation of this bottleneck effect, we reveal both in theory and experiment a distinct polarisation- and temperature-dependence in absorption and photoluminescence spectra of tetracene crystals, including microscopic origin of exciton linewidths, the activation of the higher Davydov states at large temperatures, and polarisation-dependent quenching of specific exciton resonances. Our joint theory-experiment study represents a significant advance in microscopic understanding of singlet exciton optics and dynamics in oligoacene crystals.
Natural Sciences 3, e20220040 (2022)155. A. Usman, M. Adel Aly, H. Masenda, J. J. P. Thompson, S. M. Gunasekera, M. Mucha-Kruczynski, S. Brem, E. Malic und M. Koch, "Enhanced excitonic features in an anisotropic ReS2/WSe2 heterostructure", Nanoscale 14, 10851 (2022)
Two-dimensional (2D) semiconductors have opened new horizons for future optoelectronic applications through efficient light–matter and many-body interactions at quantum level. Anisotropic 2D materials like rhenium disulphide (ReS2) present a new class of materials with polarized excitonic resonances. Here, we demonstrate a WSe2/ReS2 heterostructure which exhibits a significant photoluminescence quenching at room temperature as well as at low temperatures. This indicates an efficient charge transfer due to the electron–hole exchange interaction. The band alignment of two materials suggests that electrons optically injected into WSe2 are transferred to ReS2. Polarization resolved luminescence measurements reveal two additional polarization-sensitive exciton peaks in ReS2 in addition to the two conventional exciton resonances X1 and X2. Furthermore, for ReS2 we observe two charged excitons (trions) with binding energies of 18 meV and 15 meV, respectively. The bi-excitons of WSe2 become polarization sensitive and inherit polarizing properties from the underlying ReS2 layers, which act as patterned substrates for top layer. Overall, our findings provide a better understanding of optical signatures in 2D anisotropic materials.
Nanoscale 14, 10851 (2022)154. B. Ferreira, R. Rosati, J. M. Fitzgerald, E. Malic "Signatures of dark excitons in exciton-polariton optics of transition metal dichalcogenides", 2D Materials 10, 015012 (2022)
Integrating 2D materials into high-quality optical microcavities opens the door to fascinating many-particle phenomena including the formation of exciton-polaritons. These are hybrid quasi-particles inheriting properties of both the constituent photons and excitons. In this work, we investigate the so-far overlooked impact of dark excitons on the momentum-resolved absorption spectra of hBN-encapsulated WSe2 and MoSe2 monolayers in the strong-coupling regime. In particular, thanks to the efficient phonon-mediated scattering of polaritons into energetically lower dark exciton states, the absorption of the lower polariton branch in WSe2 is much higher than in MoSe2. It shows unique step-like increases in the momentum-resolved profile indicating opening of specific scattering channels. We study how different externally accessible quantities, such as temperature or mirror reflectance, change the optical response of polaritons. Our study contributes to an improved microscopic understanding of exciton-polaritons and their interaction with phonons, potentially suggesting experiments that could determine the energy of dark exciton states via momentum-resolved polariton absorption.
2D Materials 10, 015012 (2022)153. J. J. P. Thompson, S. Brem, M. Verjans, R. Schmidt, S. Michaelis de Vasconcellos, R. Bratschitsch and E. Malic, „Anisotropic exciton diffusion in atomically-thin semiconductors”, 2D Materials 9, 025008 (2022)
Energy transport processes are critical for the efficiency of many optoelectronic applications. The energy transport in technologically promising transition metal dichalcogenides is determined by exciton diffusion, which strongly depends on the underlying excitonic and phononic dispersion. Based on a fully microscopic theory we demonstrate that the valley-exchange interaction leads to an enhanced exciton diffusion due to the emergence of a linear excitonic dispersion and the resulting decreased exciton-phonon scattering. Interestingly, we find that the application of an uniaxial strain can drastically boost the diffusion speed and even give rise to a pronounced anisotropic diffusion, which persists up to room temperature. We reveal that this behaviour originates from the highly anisotropic exciton dispersion in presence of strain, displaying parabolic and linear behaviour perpendicular and parallel to the strain direction, respectively. Our work demonstrates the possibility to control the speed and direction of exciton diffusion via strain and dielectric engineering. This opens avenues for more efficient and exotic optoelectronic applications of atomically thin materials.
2D Materials 9, 025008 (2022)152. R. Perea-Causin, D. Erkensten, J. M. Fitzgerald, J. J. P. Thompson, R. Rosati, S. Brem, E. Malic, "Exciton optics, dynamics and transport in atomically thin semiconductors", APL Materials 10, 100701 (2022)
Atomically thin semiconductors such as transition metal dichalcogenide (TMD) monolayers exhibit a very strong Coulomb interaction, giving rise to a rich exciton landscape. This makes these materials highly attractive for efficient and tunable optoelectronic devices. In this article, we review the recent progress in the understanding of exciton optics, dynamics and transport, which crucially govern the operation of TMD-based devices. We highlight the impact of hBN-encapsulation, which reveals a plethora of many-particle states in optical spectra, and we outline the most novel breakthroughs in the field of exciton-polaritonics. Moreover, we underline the direct observation of exciton formation and thermalization in TMD monolayers and heterostructures in recent time-resolved ARPES studies. We also show the impact of exciton density, strain and dielectric environment on exciton diffusion and funneling. Finally, we put forward relevant research directions in the field of atomically thin semiconductors for the near future.
APL Materials 10, 100701 (2022)151. B. Ferreira, R. Rosati, E. Malic “Microscopic modelling of exciton-polariton diffusion coefficients in atomically thin semiconductors”, Phys. Rev. Mat. 6, 034008 (2022)
In the strong light-matter coupling regime realized e.g. by integrating semiconductors into optical microcavities, polaritons as new hybrid light-matter quasi-particles are formed. The corresponding change in the dispersion relation has a large impact on optics, dynamics and transport behaviour of semiconductors. In this work, we investigate the strong-coupling regime in hBN-encapsulated MoSe2 monolayers focusing on exciton-polariton diffusion. Applying a microscopic approach based on the exciton density matrix formalism combined with the Hopfield approach, we predict a drastic increase of the diffusion coefficients by two to three orders of magnitude in the strong coupling regime. We explain this behaviour by the much larger polariton group velocity and suppressed polariton-phonon scattering channels with respect to the case of bare excitons. Our study contributes to a better microscopic understanding of polariton diffusion in atomically thin semiconductors.
Phys. Rev. Materials 6, 034008 (2022)150. D. Erkensten, S. Brem, R. Perea-Causin, E. Malic, "Microscopic origin of anomalous interlayer exciton transport in van der Waals heterostructures", Phys. Rev. Mat. 6, 094006 (2022)
Van der Waals heterostructures constitute a platform for investigating intriguing many-body quantum phenomena. In particular, transition-metal dichalcogenide (TMD) hetero-bilayers host long-lived interlayer excitons which exhibit permanent out-of-plane dipole moments. Here, we develop a microscopic theory for interlayer exciton-exciton interactions including both the dipolar nature of interlayer excitons as well as their fermionic substructure, which gives rise to an attractive fermionic exchange. We find that these interactions contribute to a drift force resulting in highly non-linear exciton propagation at elevated densities in the MoSe2-WSe2 heterostructure. We show that the propagation can be tuned by changing the number of hBN spacers between the TMD layers or by adjusting the dielectric environment. In particular, although counter-intuitive, we reveal that interlayer excitons in free-standing samples propagate slower than excitons in hBN-encapsulated TMDs - due to an enhancement of the net Coulomb-drift with stronger environmental screening. Overall, our work contributes to a better microscopic understanding of the interlayer exciton transport in technologically promising atomically thin semiconductors.
Phys. Rev. Mat. 6, 094006 (2022)149. W. Knorr, S. Brem, G. Meneghini, E. Malic, "Exciton transport in a moiré potential: from hopping to dispersive regime", Phys. Rev. Mat. 6, 124002 (2022)
The propagation of excitons in TMD monolayers has been intensively studied revealing interesting many-particle effects, such as halo formation and non-classical diffusion. Initial studies have investigated how exciton transport changes in twisted TMD bilayers, including Coulomb repulsion and Hubbard-like exciton hopping. In this work, we investigate the twist-angle-dependent transition of the hopping regime to the dispersive regime of effectively free excitons. Based on a microscopic approach for excitons in the presence of a moiré potential, we show that the hopping regime occurs up to an angle of approximately 2° and is well described by the Hubbard model. At large angles, however, the Hubbard model fails due to increasingly delocalized exciton states. Here, the quantum mechanical dispersion of free particles with an effective mass determines the propagation of excitons. Overall, our work provides microscopic insights into the character of exciton propagation in twisted van der Waals heterostructures.
Phys. Rev. Mat. 6, 124002 (2022)148. R. Perea-Causin, S. Brem, E. Malic, "Trion-phonon interaction in atomically thin semiconductors", Phys. Rev. B 106, 115407 (2022)
Optical and transport properties of doped monolayer semiconductors are dominated by trions, which are three-particle compounds formed by two electrons and one hole or vice versa. In this work, we investigate the trion-phonon interaction on a microscopic footing and apply our model to the exemplary case of a molybdenum diselenide (MoSe2) monolayer. We determine the trion series of states and their internal quantum structure by solving the trion Schrödinger equation. Transforming the system into a trion basis and solving equations of motion, including the trion-phonon interaction within the second-order Born-Markov approximation, provides a microscopic access to the trion dynamics. In particular, we investigate trion propagation and compute the diffusion coefficient and mobility. In the low density limit, we find that trions propagate less efficiently than excitons and electrons due to their stronger coupling with phonons and their larger mass. For increasing densities, we predict a drastic enhancement of diffusion caused by the build-up of a large pressure by the degenerate trion gas, which is a direct consequence of the fermionic character of trions. Our work provides microscopic insights into the trion-phonon interaction and its impact on the diffusion behaviour in atomically thin semiconductors.
Phys. Rev. B 106, 115407 (2022)