Unraveling Photoinduced Spin Dynamics in the Topological Insulator<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>Bi</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi>Se</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

Wang, M. C.; Qiao, Shuang; Jiang, Zhigang; Luo, Sheng‐Nian; Qi, J.

doi:10.1103/physrevlett.116.036601

Cited by 67 publications

(57 citation statements)

References 64 publications

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“…Following excitation, the charge carriers undergo multiple scattering processes, finally resulting in an isotropic distribution. The time constant (22 fs) of this process found here is consistent with other measurements of anisotropy relaxation at the TI surface: ultrafast optical Kerr effect (time constant of ∼25 fs)49 and equilibrium transport experiments (substantially smaller than 300 fs, see Supplementary Note 2)5051. Note that due to spin-momentum locking at the TI surface, the decay of the anisotropy of the charge-carrier distribution is strongly connected with the decay of the transient surface spin polarization52.…”

Section: Discussionsupporting

confidence: 89%

Ultrafast photocurrents at the surface of the three-dimensional topological insulator Bi2Se3

Braun¹,

Mußler²,

Hruban³

et al. 2016

Nat Commun

185

176

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Three-dimensional topological insulators are fascinating materials with insulating bulk yet metallic surfaces that host highly mobile charge carriers with locked spin and momentum. Remarkably, surface currents with tunable direction and magnitude can be launched with tailored light beams. To better understand the underlying mechanisms, the current dynamics need to be resolved on the timescale of elementary scattering events (∼10 fs). Here, we excite and measure photocurrents in the model topological insulator Bi2Se3 with a time resolution of 20 fs by sampling the concomitantly emitted broadband terahertz (THz) electromagnetic field from 0.3 to 40 THz. Strikingly, the surface current response is dominated by an ultrafast charge transfer along the Se–Bi bonds. In contrast, photon-helicity-dependent photocurrents are found to be orders of magnitude smaller than expected from generation scenarios based on asymmetric depopulation of the Dirac cone. Our findings are of direct relevance for broadband optoelectronic devices based on topological-insulator surface currents.

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Section: Discussionsupporting

confidence: 89%

Ultrafast photocurrents at the surface of the three-dimensional topological insulator Bi2Se3

Braun¹,

Mußler²,

Hruban³

et al. 2016

Nat Commun

185

176

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“…The spin lifetimes of few‐layer WTe 2 and MoTe 2 are three orders of magnitude longer than that of Bi 2 Se 3 (<3 ps)19 at room temperature. The remarkable long‐lived spin lifetimes in WTe 2 and MoTe 2 thin films are the consequence of unique band structures of WTe 2 and MoTe 2 .…”

mentioning

confidence: 94%

Room‐Temperature Nanoseconds Spin Relaxation in WTe₂ and MoTe₂ Thin Films

Wang

Besbas

et al. 2018

Advanced Science

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The Weyl semimetal WTe2 and MoTe2 show great potential in generating large spin currents since they possess topologically protected spin‐polarized states and can carry a very large current density. In addition, the intrinsic non‐centrosymmetry of WTe2 and MoTe2 endows with a unique property of crystal symmetry‐controlled spin–orbit torques. An important question to be answered for developing spintronic devices is how spins relax in WTe2 and MoTe2. Here, a room‐temperature spin relaxation time of 1.2 ns (0.4 ns) in WTe2 (MoTe2) thin film using the time‐resolved Kerr rotation (TRKR) is reported. Based on ab initio calculation, a mechanism of long‐lived spin polarization resulting from a large spin splitting around the bottom of the conduction band, low electron–hole recombination rate, and suppression of backscattering required by time‐reversal and lattice symmetry operation is identified. In addition, it is found that the spin polarization is firmly pinned along the strong internal out‐of‐plane magnetic field induced by large spin splitting. This work provides an insight into the physical origin of long‐lived spin polarization in Weyl semimetals, which could be useful to manipulate spins for a long time at room temperature.

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“…The answer to these questions might in fact prove crucial to further understand the role of the electron spin in the relaxation pathways of the generated spin currents, and whether or not such currents directly follow the time scale of the Dirac-cone electron-momentum relaxation. These aspects are especially important as, in the presence of strong spin-orbit coupling, the ultrafast processes of electron-momentum relaxation might be strongly constrained by spin selection at the surface [32,33] or other scattering events that involve the electron spin such as the ones arising from the effective coupling between bulk and surface-state electrons [34,35].…”

Section: Introductionmentioning

confidence: 99%

Subpicosecond spin dynamics of excited states in the topological insulator Bi2Te3

et al. 2017

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Using time-, spin-and angle-resolved photoemission, we investigate the ultrafast spin dynamics of hot electrons on the surface of the topological insulator Bi2Te3 following optical excitation by fs-infrared pulses. We observe two surface-resonance states above the Fermi level coexisting with a transient population of Dirac fermions that relax in about ∼2 ps. One state is located below ∼0.4 eV just above the bulk continuum, the other one at ∼0.8 eV inside a projected bulk band gap. At the onset of the excitation, both states exhibit a reversed spin texture with respect to that of the transient Dirac bands, in agreement with our one-step photoemission calculations. Our data reveal that the high-energy state undergoes spin relaxation within ∼0.5 ps, a process that triggers the subsequent spin dynamics of both the Dirac cone and the low-energy state, which behave as two dynamically-locked electron populations. We discuss the origin of this behavior by comparing the relaxation times observed for electrons with opposite spins to the ones obtained from a microscopic Boltzmann model of ultrafast band cooling introduced into the photoemission calculations. Our results demonstrate that the nonequilibrium surface dynamics is governed by electron-electron rather than electron-phonon scattering, with a characteristic time scale unambiguously determined by the complex spin texture of excited states above the Fermi level. Our findings reveal the critical importance of detecting momentum and energy-resolved spin textures with fs resolution to fully understand the sub-ps dynamics of transient electrons on the surface of topological insulators.

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Unraveling Photoinduced Spin Dynamics in the Topological InsulatorBi2Se3

Cited by 67 publications

References 64 publications

Ultrafast photocurrents at the surface of the three-dimensional topological insulator Bi2Se3

Ultrafast photocurrents at the surface of the three-dimensional topological insulator Bi2Se3

Room‐Temperature Nanoseconds Spin Relaxation in WTe₂ and MoTe₂ Thin Films

Subpicosecond spin dynamics of excited states in the topological insulator Bi2Te3

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Unraveling Photoinduced Spin Dynamics in the Topological InsulatorBi2Se3

Cited by 67 publications

References 64 publications

Ultrafast photocurrents at the surface of the three-dimensional topological insulator Bi2Se3

Ultrafast photocurrents at the surface of the three-dimensional topological insulator Bi2Se3

Room‐Temperature Nanoseconds Spin Relaxation in WTe2 and MoTe2 Thin Films

Subpicosecond spin dynamics of excited states in the topological insulator Bi2Te3

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Product

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Room‐Temperature Nanoseconds Spin Relaxation in WTe₂ and MoTe₂ Thin Films