| 1. |
- Majchrzak, Paulina, et al.
(författare)
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Spectroscopic view of ultrafast charge carrier dynamics in single- and bilayer transition metal dichalcogenide semiconductors
- 2021
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Ingår i: Journal of Electron Spectroscopy and Related Phenomena. - : Elsevier BV. - 0368-2048 .- 1873-2526. ; 250
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Tidskriftsartikel (refereegranskat)abstract
- The quasiparticle spectra of atomically thin semiconducting transition metal dichalcogenides (TMDCs) and their response to an ultrafast optical excitation critically depend on interactions with the underlying substrate. Here, we present a comparative time- and angle-resolved photoemission spectroscopy (TR-ARPES) study of the transient electronic structure and ultrafast carrier dynamics in the single- and bilayer TMDCs MoS2 and WS2 on three different substrates: Au(111), Ag(111) and graphene/SiC. The photoexcited quasiparticle bandgaps are observed to vary over the range of 1.9-2.5 eV between our systems. The transient conduction band signals decay on a sub-50 fs timescale on the metals, signifying an efficient removal of photoinduced carriers into the bulk metallic states. On graphene, we instead observe a fast timescale on the order of 170 fs, followed by a slow dynamics for the conduction band decay in MoS2. These timescales are explained by Auger recombination involving MoS2 and in-gap defect states. In bilayer TMDCs on metals we observe a complex redistribution of excited holes along the valence band that is substantially affected by interactions with the continuum of bulk metallic states.
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| 2. |
- Rostami, Habib, et al.
(författare)
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Layer and orbital interference effects in photoemission from transition metal dichalcogenides
- 2019
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 100:23
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Tidskriftsartikel (refereegranskat)abstract
- In this work, we provide an effective model to evaluate the one-electron dipole matrix elements governing optical excitations and the photoemission process of single-layer (SL) and bilayer (BL) transition metal dichalcogenides. By utilizing a k . p Hamiltonian, we calculate the photoemission intensity as observed in angle-resolved photoemission from the valence bands around the (K) over bar valley of MoS2. In SL MoS2, we find a significant masking of intensity outside the first Brillouin zone, which originates from an in-plane interference effect between photoelectrons emitted from the Mo d orbitals. In BL MoS2, an additional interlayer interference effect leads to a distinctive modulation of intensity with photon energy. Finally, we use the semiconductor Bloch equations to model the optical excitation in a time- and angle-resolved pump-probe photoemission experiment. We find that the momentum dependence of an optically excited population in the conduction band leads to an observable dichroism in both SL and BL MoS2.
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| 3. |
- Shokeen, Vishal, et al.
(författare)
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Real-time observation of non-equilibrium phonon-electron energy and angular momentum flow in laser-heated nickel
- 2024
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Ingår i: Science Advances. - : American Association For Cancer Research (AACR). - 2375-2548. ; 10:5
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Tidskriftsartikel (refereegranskat)abstract
- Identifying the microscopic nature of non-equilibrium energy transfer mechanisms among electronic, spin, and lattice degrees of freedom is central to understanding ultrafast phenomena such as manipulating magnetism on the femtosecond timescale. Here, we use time- and angle-resolved photoemission spectroscopy to go beyond the often-used ensemble-averaged view of non-equilibrium dynamics in terms of quasiparticle temperature evolutions. We show for ferromagnetic Ni that the non-equilibrium electron and spin dynamics display pronounced variations with electron momentum, whereas the magnetic exchange interaction remains isotropic. This highlights the influence of lattice-mediated scattering processes and opens a pathway toward unraveling the still elusive microscopic mechanism of spin-lattice angular momentum transfer.
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| 4. |
- Volckaert, Klara, et al.
(författare)
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Momentum-resolved linear dichroism in bilayer MoS2
- 2019
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 100:24
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Tidskriftsartikel (refereegranskat)abstract
- In solid state photoemission experiments it is possible to extract information about the symmetry and orbital character of the electronic wave functions via the photoemission selection rules that shape the measured intensity. This approach can be expanded in a pump-probe experiment where the intensity contains additional information about interband excitations induced by an ultrafast laser pulse with tunable polarization. Here, we find an unexpected strong linear dichroism effect (up to 42.4%) in the conduction band of bilayer MoS2, when measuring energy- and momentum-resolved snapshots of excited electrons by time- and angle-resolved photoemission spectroscopy. We model the polarization-dependent photoemission intensity in the transiently populated conduction band using the semiconductor Bloch equations. Our theoretical analysis reveals a strongly anisotropic momentum dependence of the optical excitations due to intralayer single-particle hopping, which explains the observed linear dichroism.
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