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Publications 2022
Inhalt ausklappen Inhalt einklappen 152. M. Feierabend, V. Lumsargis, J. J. P. Thompson, K. Wang, L. Dou, L. Huang and E. Malic, “Dark interlayer excitons in WS2/tetracene heterostructures”, arXiv: 2111.12400152. M. Feierabend, V. Lumsargis, J. J. P. Thompson, K. Wang, L. Dou, L. Huang and E. Malic, “Dark interlayer excitons in WS2/tetracene heterostructures”, arXiv: 2111.12400
The vertical stacking of two-dimensional materials into heterostructures gives rise to a plethora of intriguing optoelectronic properties and presents an unprecedented potential for technological concepts. While much progress has been made combining different monolayers of transition metal dichalgonenides (TMDs), little is known about TMD-based heterostructures including organic layers of molecules. Here, we present a joint theory-experiment study on a TMD/tetracene heterostructure demonstrating clear signatures of spatially separated interlayer excitons in low temperature photoluminescence spectra. Here, the Coulomb-bound electrons and holes are localized either in the TMD or in the molecule layer, respectively. In particular, we reveal both in theory and experiment that at cryogenic temperatures, signatures of momentum-dark interlayer excitons emerge. Our findings shed light on the microscopic nature of interlayer excitons in TMD/molecule heterostructures and could have important implications for technological applications of these materials.
arXiv:2111.12400Inhalt ausklappen Inhalt einklappen 151. J. M. Fitzgerald, J. J. P. Thompson and E. Malic, “Twist Angle Tuning of Moiré Exciton Polaritons in van der Waals Heterostructures”, accepted by Nano Letters151. J. M. Fitzgerald, J. J. P. Thompson and E. Malic, “Twist Angle Tuning of Moiré Exciton Polaritons in van der Waals Heterostructures”, accepted by Nano Letters
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.
Inhalt ausklappen Inhalt einklappen 150. 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. 22, 6, 2561 (2022)150. 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. 22, 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. 22, 6, 2561 (2022)Inhalt ausklappen Inhalt einklappen 149. B. Ferreira, R. Rosati, E. Malic “Microscopic modelling of exciton-polariton diffusion coefficients in atomically thin semiconductors”, Phys. Rev. Materials 6, 034008149. B. Ferreira, R. Rosati, E. Malic “Microscopic modelling of exciton-polariton diffusion coefficients in atomically thin semiconductors”, Phys. Rev. Materials 6, 034008
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
Inhalt ausklappen Inhalt einklappen 148. S. Brem and E. Malic, “Terahertz fingerprint of monolayer Wigner crystals”, Nano Lett. 22, 3, 1311 (2022)148. S. Brem and E. Malic, “Terahertz fingerprint of monolayer Wigner crystals”, Nano Lett. 22, 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. 22, 3, 1311 (2022)
Inhalt ausklappen Inhalt einklappen 147. J. J. P. Thompson, S. Brem, M. Verjans, R. Schmidt, S. Michaelis de Vasconcellos, R. Bratschitsch and Ermin Malic, „Anisotropic exciton diffusion in atomically-thin semiconductors”, 2D Materials 9, 025008 (2022)147. J. J. P. Thompson, S. Brem, M. Verjans, R. Schmidt, S. Michaelis de Vasconcellos, R. Bratschitsch and Ermin 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 Mat. 9, 025008 (2022)
Inhalt ausklappen Inhalt einklappen 146. 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)146. 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)