| 1. |
- Merkl, Philipp, et al.
(författare)
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Ultrafast Transition from Intra- to Interlayer Exciton Phases in a Van Der Waals Heterostructure
- 2019
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Ingår i: Optics InfoBase Conference Papers. - 2162-2701. - 9781943580576 ; Part F127-CLEO_AT 2019
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Konferensbidrag (refereegranskat)abstract
- The binding energy of interlayer excitons is measured by revealing a novel 1s-2p resonance in the mid infrared. Intralayer excitons transform directly into interlayer species via ultrafast electron tunneling strongly influenced by the stacking angle. (C) 2019 The Author(s)
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| 2. |
- Berghäuser, Gunnar, 1983, et al.
(författare)
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Mapping of the dark exciton landscape in transition metal dichalcogenides
- 2018
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Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 98:2
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Tidskriftsartikel (refereegranskat)abstract
- Transition metal dichalcogenides (TMDs) exhibit a remarkable exciton physics including bright and optically forbidden dark excitonic states. Here, we show how dark excitons can be experimentally revealed by probing the intraexcitonic 1s-2p transition. Distinguishing the optical response shortly after the excitation and after the exciton thermalization allows us to reveal the relative position of bright and dark excitons. We find both in theory and experiment a clear blueshift in the optical response demonstrating the transition of bright exciton populations into lower-lying dark excitonic states.
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| 3. |
- Merkl, Philipp, et al.
(författare)
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Ultrafast transition between exciton phases in van der Waals heterostructures
- 2019
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Ingår i: Nature Materials. - : Springer Science and Business Media LLC. - 1476-4660 .- 1476-1122. ; 18:7, s. 691-696
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- © 2019, The Author(s), under exclusive licence to Springer Nature Limited. Heterostructures of atomically thin van der Waals bonded monolayers have opened a unique platform to engineer Coulomb correlations, shaping excitonic1–3, Mott insulating4 or superconducting phases5,6. In transition metal dichalcogenide heterostructures7, electrons and holes residing in different monolayers can bind into spatially indirect excitons1,3,8–11 with a strong potential for optoelectronics11,12, valleytronics1,3,13, Bose condensation14, superfluidity14,15 and moiré-induced nanodot lattices16. Yet these ideas require a microscopic understanding of the formation, dissociation and thermalization dynamics of correlations including ultrafast 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 exciton gas photogenerated in the WSe2 layer directly into interlayer excitons. Depending on the stacking angle, intra- and interlayer species coexist on picosecond scales and the 1s–2p resonance 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.
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| 4. |
- Steinleitner, Philipp, et al.
(författare)
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Dielectric Engineering of Electronic Correlations in a van der Waals Heterostructure
- 2018
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 18:2, s. 1402-1409
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Tidskriftsartikel (refereegranskat)abstract
- Heterostructures of van der Waals bonded layered materials offer unique means to tailor dielectric screening with atomic-layer precision, opening a fertile field of fundamental research. The optical analyses used so far have relied on interband spectroscopy. Here we demonstrate how a capping layer of hexagonal boron nitride (hBN) renormalizes the internal structure of excitons in a WSe 2 monolayer using intraband transitions. Ultrabroadband terahertz probes sensitively map out the full complex-valued mid-infrared conductivity of the heterostructure after optical injection of 1s A excitons. This approach allows us to trace the energies and line widths of the atom-like 1s-2p transition of optically bright and dark excitons as well as the densities of these quasiparticles. The excitonic resonance red shifts and narrows in the WSe 2 /hBN heterostructure compared to the bare monolayer. Furthermore, the ultrafast temporal evolution of the mid-infrared response function evidences the formation of optically dark excitons from an initial bright population. Our results provide key insight into the effect of nonlocal screening on electron-hole correlations and open new possibilities of dielectric engineering of van der Waals heterostructures.
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| 5. |
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| 6. |
- Merkl, Philipp, et al.
(författare)
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Proximity control of interlayer exciton-phonon hybridization in van der Waals heterostructures
- 2021
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723 .- 2041-1723. ; 12:1
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Tidskriftsartikel (refereegranskat)abstract
- 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. Here, the authors demonstrate proximity-controlled strong-coupling between Coulomb correlations and lattice dynamics in neighbouring van der Waals materials (WSe2 and a gypsum layer), creating electrically neutral hybrid exciton-phonon eigenmodes called excitonic Lyman polarons.
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| 7. |
- Merkl, Philipp, et al.
(författare)
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Tailoring coulomb correlations in twisted WSe 2 bilayers
- 2021
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Ingår i: 2021 Conference on Lasers and Electro-Optics, CLEO 2021 - Proceedings. - 9781557528209
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Konferensbidrag (refereegranskat)abstract
- Phase-locked few-cycle mid-infrared pulses trace how the twist angle alone renormalizes the binding energy of excitons in twisted WSe2 homobilayers by a factor of two and tunes their lifetime by a factor of twenty.
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| 8. |
- Merkl, Philipp, et al.
(författare)
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Twist-tailoring Coulomb correlations in van der Waals homobilayers
- 2020
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723 .- 2041-1723. ; 11:1
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 9. |
- Mooshammer, Fabian, et al.
(författare)
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Excitons in twisted van der Waals bilayers: Internal structure and ultrafast dynamics
- 2020
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Ingår i: 2020 45TH INTERNATIONAL CONFERENCE ON INFRARED, MILLIMETER, AND TERAHERTZ WAVES (IRMMW-THZ). - 2162-2027.
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Konferensbidrag (refereegranskat)abstract
- By probing internal 1s-2p transitions with phaselocked mid-infrared pulses, we reveal the internal structure and binding energy of photo-generated excitons in transition metal dichalcogenide bilayers. Furthermore, we trace how the twist angle precisely controls the ultrafast formation of interlayer excitons and tunes excitonic hybridization in hetero- and homobilayers, respectively.
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| 10. |
- Mooshammer, F., et al.
(författare)
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Twist-Tailoring Hybrid Excitons in Van der Waals Homobilayers
- 2021
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Ingår i: 2021 Conference on Lasers and Electro-Optics Europe and European Quantum Electronics Conference, CLEO/Europe-EQEC 2021.
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Konferensbidrag (refereegranskat)abstract
- In homobilayer systems, the twist angle θ has emerged as a powerful tuning knob for tailoring novel phase transitions in atomically thin layers stacked at magic twist angles [1] - a paradigm shift for condensed-matter physics. For twisted bilayers of transition metal dichalcogenides, topological phases [2] and even potential signatures of a superconducting state [3] have been discussed. Yet, a precise understanding of the underlying Coulomb correlations has remained challenging.
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