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Christian Schuller - One of the best experts on this subject based on the ideXlab platform.
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momentum space indirect interlayer Excitons in transition metal dichalcogenide van der waals heterostructures
Nature Physics, 2018Co-Authors: Jens Kunstmann, Fabian Mooshammer, Philipp Nagler, Andrey Chaves, Frederick Stein, Nicola Paradiso, Gerd Plechinger, Christoph Strunk, Christian SchullerAbstract:Monolayers of transition-metal dichalcogenides feature exceptional optical properties that are dominated by tightly bound electron–hole pairs, called Excitons. Creating van der Waals heterostructures by deterministically stacking individual monolayers can tune various properties via the choice of materials1 and the relative orientation of the layers2,3. In these structures, a new type of exciton emerges where the electron and hole are spatially separated into different layers. These interlayer Excitons4–6 allow exploration of many-body quantum phenomena7,8 and are ideally suited for valleytronic applications9. A basic model of a fully spatially separated electron and hole stemming from the K valleys of the monolayer Brillouin zones is usually applied to describe such Excitons. Here, we combine photoluminescence spectroscopy and first-principles calculations to expand the concept of interlayer Excitons. We identify a partially charge-separated electron–hole pair in MoS2/WSe2 heterostructures where the hole resides at the Γ point and the electron is located in a K valley. We control the emission energy of this new type of momentum-space indirect, yet strongly bound exciton by variation of the relative orientation of the layers. These findings represent a crucial step towards the understanding and control of excitonic effects in van der Waals heterostructures and devices. A new type of exciton is observed in transition-metal dichalcogenide heterobilayers that is indirect in both real space and momentum space. It consists of a paired electron in MoS2 at the K point and hole spread across MoS2 and WSe2 at the Γ point.
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trion fine structure and coupled spin valley dynamics in monolayer tungsten disulfide
Nature Communications, 2016Co-Authors: Gerd Plechinger, Philipp Nagler, Christian Schuller, Ashish Arora, Robert Schmidt, Alexey Chernikov, Andres Granados Del Aguila, P C M Christianen, Rudolf Bratschitsch, Tobias KornAbstract:Monolayer transition-metal dichalcogenides have recently emerged as possible candidates for valleytronic applications, as the spin and valley pseudospin are directly coupled and stabilized by a large spin splitting. The optical properties of these two-dimensional crystals are dominated by tightly bound electron-hole pairs (Excitons) and more complex quasiparticles such as charged Excitons (trions). Here we investigate monolayer WS2 samples via photoluminescence and time-resolved Kerr rotation. In photoluminescence and in energy-dependent Kerr rotation measurements, we are able to resolve two different trion states, which we interpret as intravalley and intervalley trions. Using time-resolved Kerr rotation, we observe a rapid initial valley polarization decay for the A exciton and the trion states. Subsequently, we observe a crossover towards exciton-exciton interaction-related dynamics, consistent with the formation and decay of optically dark A Excitons. By contrast, resonant excitation of the B exciton transition leads to a very slow decay of the Kerr signal.
Jie Shan - One of the best experts on this subject based on the ideXlab platform.
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evidence of high temperature exciton condensation in two dimensional atomic double layers
Nature, 2019Co-Authors: Zefang Wang, Daniel Rhodes, Kenji Watanabe, Takashi Taniguchi, James Hone, Jie Shan, Kin Fai MakAbstract:A Bose–Einstein condensate is the ground state of a dilute gas of bosons, such as atoms cooled to temperatures close to absolute zero1. With much smaller mass, Excitons (bound electron–hole pairs) are expected to condense at considerably higher temperatures2–7. Two-dimensional van der Waals semiconductors with very strong exciton binding are ideal systems for the study of high-temperature exciton condensation. Here we study electrically generated interlayer Excitons in MoSe2–WSe2 atomic double layers with a density of up to 1012 Excitons per square centimetre. The interlayer tunnelling current depends only on the exciton density, which is indicative of correlated electron–hole pair tunnelling8. Strong electroluminescence arises when a hole tunnels from WSe2 to recombine with an electron in MoSe2. We observe a critical threshold dependence of the electroluminescence intensity on exciton density, accompanied by super-Poissonian photon statistics near the threshold, and a large electroluminescence enhancement with a narrow peak at equal electron and hole densities. The phenomenon persists above 100 kelvin, which is consistent with the predicted critical condensation temperature9–12. Our study provides evidence for interlayer exciton condensation in two-dimensional atomic double layers and opens up opportunities for exploring condensate-based optoelectronics and exciton-mediated high-temperature superconductivity13. Condensation of interlayer Excitons at temperatures above 100 kelvin is demonstrated in a van der Waals heterostructure consisting of two-dimensional atomic double layers of transition metal chalcogenides.
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Opportunities and challenges of interlayer exciton control and manipulation
Nature Nanotechnology, 2018Co-Authors: Kin Fai Mak, Jie ShanAbstract:Advances in van der Waals heterostructures allow the control of interlayer Excitons by electrical and other means, promising exciting opportunities for high-temperature exciton condensation and valley–spin optoelectronics. This Commentary discusses practical prospects of using electrical control of interlayer Excitons in van der Waals heterostructures for high-temperature exciton condensation and valley–spin optoelectronics.
Wang Yao - One of the best experts on this subject based on the ideXlab platform.
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Brightened spin-triplet interlayer Excitons and optical selection rules in van der Waals heterobilayers
arXiv: Mesoscale and Nanoscale Physics, 2018Co-Authors: Guibin Liu, Wang YaoAbstract:We investigate the optical properties of spin-triplet interlayer Excitons in heterobilayer transition metal dichalcogenides in comparison with the spin-singlet ones. Surprisingly, the optical transition dipole of the spin-triplet exciton is found to be in the same order of magnitude to that of the spin-singlet exciton, in sharp contrast to the monolayer Excitons where the spin-triplet species is considered as dark compared to the singlet. Unlike the monolayer Excitons whose spin-conserved (spin-flip) transition dipole can only couple to light of in-plane (out-of-plane) polarisation, such restriction is removed for the interlayer Excitons due to the breaking of the out-of-plane mirror symmetry. We find that as the interlayer atomic registry changes, the optical transition dipole of interlayer exciton crosses between in-plane ones of opposite circular polarizations and the out-of-plane one for both the spin-triplet and spin-singlet species. As a result, Excitons of both species have non-negligible coupling into photon modes of both in-plane and out-of-plane propagations, another sharp difference from the monolayers where the exciton couples predominantly into the out-of-plane propagation channel. At given atomic registry, the spin-triplet and spin-singlet Excitons have distinct valley polarisation selection rules, allowing the selective optical addressing of both the valley configuration and the spin-singlet/triplet configuration of interlayer Excitons.
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Brightened spin-triplet interlayer Excitons in vdW heterobilayers and their contrasted optical selection rules from spin-singlet Excitons
2018Co-Authors: Guibin Liu, Wang YaoAbstract:We investigate the optical properties of spin-triplet interlayer Excitons in heterobilayer transition metal dichalcogenides in comparison with the spin-singlet ones. Surprisingly, the optical transition dipole of the spin-triplet exciton is found to be in the same order of magnitude to that of the spin-singlet exciton, in sharp contrast to the monolayer Excitons where the spin triplet species is considered as dark compared to the singlet. Unlike the monolayer Excitons whose spin-conserved (spin-flip) transition dipole can only couple to light of in-plane (out-of-plane) polarization, such restriction is removed for the interlayer Excitons due to the breaking of the out-of-plane mirror symmetry. We find that as the interlayer atomic registry changes, the optical transition dipole of interlayer exciton crosses between in-plane ones of opposite circular polarization and the out-of-plane one for both the spin-triplet and spin-singlet species. As a result, Excitons of both species have non-negligible coupling into photon modes of both in-plane and out-of-plane propagations, another sharp difference from the monolayers where the exciton couples predominantly into the out-of-plane propagation channel. At given atomic registry, the spin-triplet and spin-singlet Excitons have distinct valley polarization selection rules, allowing the selective optical addressing of both the valley configuration and the spin singlet/triplet configuration of interlayer Excitons.
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dirac cones and dirac saddle points of bright Excitons in monolayer transition metal dichalcogenides
Nature Communications, 2014Co-Authors: Guibin Liu, Pu Gong, Wang YaoAbstract:In monolayer transition metal dichalcogenides, tightly bound Excitons have been discovered with a valley pseudospin optically addressable through polarization selection rules. Here, we show that this valley pseudospin is strongly coupled to the exciton centre-of-mass motion through electron-hole exchange. This coupling realizes a massless Dirac cone with chirality index I = 2 for Excitons inside the light cone, that is, bright Excitons. Under moderate strain, the I = 2 Dirac cone splits into two degenerate I = 1 Dirac cones, and saddle points with a linear Dirac spectrum emerge. After binding an extra electron, the charged exciton becomes a massive Dirac particle associated with a large valley Hall effect protected from intervalley scattering. Our results point to unique opportunities to study Dirac physics, with exciton's optical addressability at specifiable momentum, energy and pseudospin. The strain-tunable valley-orbit coupling also implies new structures of exciton condensates, new functionalities of excitonic circuits and mechanical control of valley pseudospin.
Kin Fai Mak - One of the best experts on this subject based on the ideXlab platform.
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evidence of high temperature exciton condensation in two dimensional atomic double layers
Nature, 2019Co-Authors: Zefang Wang, Daniel Rhodes, Kenji Watanabe, Takashi Taniguchi, James Hone, Jie Shan, Kin Fai MakAbstract:A Bose–Einstein condensate is the ground state of a dilute gas of bosons, such as atoms cooled to temperatures close to absolute zero1. With much smaller mass, Excitons (bound electron–hole pairs) are expected to condense at considerably higher temperatures2–7. Two-dimensional van der Waals semiconductors with very strong exciton binding are ideal systems for the study of high-temperature exciton condensation. Here we study electrically generated interlayer Excitons in MoSe2–WSe2 atomic double layers with a density of up to 1012 Excitons per square centimetre. The interlayer tunnelling current depends only on the exciton density, which is indicative of correlated electron–hole pair tunnelling8. Strong electroluminescence arises when a hole tunnels from WSe2 to recombine with an electron in MoSe2. We observe a critical threshold dependence of the electroluminescence intensity on exciton density, accompanied by super-Poissonian photon statistics near the threshold, and a large electroluminescence enhancement with a narrow peak at equal electron and hole densities. The phenomenon persists above 100 kelvin, which is consistent with the predicted critical condensation temperature9–12. Our study provides evidence for interlayer exciton condensation in two-dimensional atomic double layers and opens up opportunities for exploring condensate-based optoelectronics and exciton-mediated high-temperature superconductivity13. Condensation of interlayer Excitons at temperatures above 100 kelvin is demonstrated in a van der Waals heterostructure consisting of two-dimensional atomic double layers of transition metal chalcogenides.
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Opportunities and challenges of interlayer exciton control and manipulation
Nature Nanotechnology, 2018Co-Authors: Kin Fai Mak, Jie ShanAbstract:Advances in van der Waals heterostructures allow the control of interlayer Excitons by electrical and other means, promising exciting opportunities for high-temperature exciton condensation and valley–spin optoelectronics. This Commentary discusses practical prospects of using electrical control of interlayer Excitons in van der Waals heterostructures for high-temperature exciton condensation and valley–spin optoelectronics.
Gerd Plechinger - One of the best experts on this subject based on the ideXlab platform.
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momentum space indirect interlayer Excitons in transition metal dichalcogenide van der waals heterostructures
Nature Physics, 2018Co-Authors: Jens Kunstmann, Fabian Mooshammer, Philipp Nagler, Andrey Chaves, Frederick Stein, Nicola Paradiso, Gerd Plechinger, Christoph Strunk, Christian SchullerAbstract:Monolayers of transition-metal dichalcogenides feature exceptional optical properties that are dominated by tightly bound electron–hole pairs, called Excitons. Creating van der Waals heterostructures by deterministically stacking individual monolayers can tune various properties via the choice of materials1 and the relative orientation of the layers2,3. In these structures, a new type of exciton emerges where the electron and hole are spatially separated into different layers. These interlayer Excitons4–6 allow exploration of many-body quantum phenomena7,8 and are ideally suited for valleytronic applications9. A basic model of a fully spatially separated electron and hole stemming from the K valleys of the monolayer Brillouin zones is usually applied to describe such Excitons. Here, we combine photoluminescence spectroscopy and first-principles calculations to expand the concept of interlayer Excitons. We identify a partially charge-separated electron–hole pair in MoS2/WSe2 heterostructures where the hole resides at the Γ point and the electron is located in a K valley. We control the emission energy of this new type of momentum-space indirect, yet strongly bound exciton by variation of the relative orientation of the layers. These findings represent a crucial step towards the understanding and control of excitonic effects in van der Waals heterostructures and devices. A new type of exciton is observed in transition-metal dichalcogenide heterobilayers that is indirect in both real space and momentum space. It consists of a paired electron in MoS2 at the K point and hole spread across MoS2 and WSe2 at the Γ point.
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trion fine structure and coupled spin valley dynamics in monolayer tungsten disulfide
Nature Communications, 2016Co-Authors: Gerd Plechinger, Philipp Nagler, Christian Schuller, Ashish Arora, Robert Schmidt, Alexey Chernikov, Andres Granados Del Aguila, P C M Christianen, Rudolf Bratschitsch, Tobias KornAbstract:Monolayer transition-metal dichalcogenides have recently emerged as possible candidates for valleytronic applications, as the spin and valley pseudospin are directly coupled and stabilized by a large spin splitting. The optical properties of these two-dimensional crystals are dominated by tightly bound electron-hole pairs (Excitons) and more complex quasiparticles such as charged Excitons (trions). Here we investigate monolayer WS2 samples via photoluminescence and time-resolved Kerr rotation. In photoluminescence and in energy-dependent Kerr rotation measurements, we are able to resolve two different trion states, which we interpret as intravalley and intervalley trions. Using time-resolved Kerr rotation, we observe a rapid initial valley polarization decay for the A exciton and the trion states. Subsequently, we observe a crossover towards exciton-exciton interaction-related dynamics, consistent with the formation and decay of optically dark A Excitons. By contrast, resonant excitation of the B exciton transition leads to a very slow decay of the Kerr signal.