Ultrafast Surface Carrier Dynamics in the Topological Insulator Bi<sub>2</sub>Te<sub>3</sub>

Hajlaoui, Mahdi; Papalazarou, E.; Mauchain, Julien; Lantz, Gabriel; Moisan, Nicolas; Boschetto, D.; Jiang, Zhigang; Miotkowski, I.; Chen, Yong P.; Taleb‐Ibrahimi, A.; Perfetti, L.; Marsi, M.

doi:10.1021/nl301035x

Cited by 224 publications

(215 citation statements)

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“…We average the intensity over a narrow energy window (less than 20 meV), and this provides us with a precise information on the energy dependent temporal evolution of the excited electrons. This constitutes an important difference to a previous study on Bi 2 Te 3 , in which large energy windows were selected, enclosing distinct bands [32]. The dynamics is fast for E >> µ (-6-), and apparently slows down when approaching µ (-4-and -3-).…”

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confidence: 71%

Ultrafast photodoping and effective Fermi-Dirac distribution of the Dirac particles in Bi2Se3

et al. 2012

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We exploit time-and angle-resolved photoemission spectroscopy to determine the evolution of the out-ofequilibrium electronic structure of the topological insulator Bi2Se3. The response of the Fermi-Dirac distribution to ultrashort IR laser pulses has been studied by modelling the dynamics of the hot electrons after optical excitation. We disentangle a large increase of the effective temperature (T * ) from a shift of the chemical potential (µ * ), which is consequence of the ultrafast photodoping of the conduction band. The relaxation dynamics of T * and µ * are k-independent and these two quantities uniquely define the evolution of the excited charge population. We observe that the energy dependence of the non-equilibrium charge population is solely determined by the analytical form of the effective Fermi-Dirac distribution.The recent discovery of topological insulators (TIs) is renewing the attention on the effects of spin orbit interactions (SOI) in solids, paving the road for the emergence of new quantum states of matter [1][2][3][4][5][6][7][8][9]. The SOI acquires particular relevance in the case of systems containing high-Z elements, leading to the lifting of the Kramers spin-degeneracy in broken inversion symmetry systems, as described by Rashba [10][11][12], Dresselhaus [13] and 15]. Therefore, understanding the consequence of SOI is of primary importance also for future technological applications in spintronics.TIs are band insulators (semiconductors) where the conduction and the valence band states have opposite parities and their energy ordering is inverted by the SOI [8,16]. The most prominent feature of their electronic structure is the odd number of spin polarized Dirac cones at the surface, connecting the opposite sides of the bulk band gap, resulting in topological protection from backscattering [7]. [5,19]) have been discovered. Among these, Bi 2 Se 3 represents a paradigmatic case, owing to the simplicity of its band structure characterized by a single Dirac cone [2,9].In topological insulators, the spin helicity of the metallic surface state offers the unique possibility to support spincurrent. Hence, spin-polarized charge distributions can be generated by circularly polarized light [20,21]. The topological protection of the linearly dispersing surface state is expected to strongly affect the scattering mechanisms of the Dirac particle, with respect to normal metallic states. In particular, the different couplings to optical and acoustic phonon modes have been recently studied [22] and the optical excitation of long-lived electron population in the surface state might play an important role in forthcoming opto-spintronics devices [22,23].In this paper we report on the study of the out-ofequilibrium electronic properties of Bi 2 Se 3 , investigated by time-and angle-resolved photoemission spectroscopy (tr-ARPES). Although conventional ARPES, with its surface sensitivity, has proven to be effective and rich of information [1,3,6], the combined use of ultrashort laser pulses and angleresolved p...

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confidence: 71%

Ultrafast photodoping and effective Fermi-Dirac distribution of the Dirac particles in Bi2Se3

et al. 2012

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“…At fixed energy, the population decay directly reflects the lifetimes and decay rates. As a result, we observe single scattering events at these delays rather than the dynamics of a hot electron gas, which most previous studies on electron dynamics in TIs have focused on [5][6][7]. The dynamics can be described by a rate-equation model.…”

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confidence: 80%

“…Time-resolved photoemission measures the transient population of initially empty electronic states following an optical pump pulse, and therefore allows us to access electron dynamics directly in the time domain. Previous studies have focused on carrier cooling in bismuth chalcogenides which are intrinsically n type [5][6][7] and can be p doped by Mg [8]. These studies showed that the electron dynamics of TSSs is dominated by the bulk conduction band, but did not provide scattering rates between TSS and the conduction or valence band.…”

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confidence: 99%

Bulk and surface electron dynamics in ap-type topological insulatorSnSb2Te4

et al. 2014

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Time-resolved two-photon photoemission was used to study the electronic structure and dynamics at the surface of SnSb 2 Te 4 , a p-type topological insulator. The Dirac point is found 0.32 ± 0.03 eV above the Fermi level. Electrons from the conduction band minimum are scattered on a time scale of 43 ± 4 fs to the Dirac cone. From there they decay to the partly depleted valence band with a time constant of 78 ± 5 fs. The significant interaction of the Dirac states with bulk bands is attributed to their bulk penetration depth of ∼3 nm as found from density functional theory calculations. While topological insulators (TIs) are bulk insulators, they exhibit a spin-polarized metallic topological surface state (TSS) with linear dispersion (Dirac cone) [1,2]. The helical spin structure of the Dirac cone was predicted to constrain intraband scattering in the absence of spin-flipping events, resulting in long carrier lifetimes [3]. Evidence for the suppression of elastic spin-flipping scattering events was given by Fourier-transformed scanning tunneling spectroscopy [4]. Time-resolved photoemission measures the transient population of initially empty electronic states following an optical pump pulse, and therefore allows us to access electron dynamics directly in the time domain. Previous studies have focused on carrier cooling in bismuth chalcogenides which are intrinsically n type [5][6][7] and can be p doped by Mg [8]. These studies showed that the electron dynamics of TSSs is dominated by the bulk conduction band, but did not provide scattering rates between TSS and the conduction or valence band. Such information could be related to results obtained from transport measurements and would be important for device applications.Complex ternary Sb 2 Te 3 -based alloys possess a layer structure with a van der Waals gap between Te layers and were recently proposed to exhibit a topological surface state [9][10][11]. However, intrinsic p doping of antimony-containing materials does not permit to access the Dirac point by conventional angle-resolved photoelectron spectroscopy [12] even after doping by alkali-metal atoms [13]. Angle-resolved two-photon photoemission (2PPE) uses a pump-probe process to access the unoccupied electronic states as indicated by the arrows in Fig. 1(a). Here, we show that SnSb 2 Te 4 is a p-doped TI and obtain energy and dispersion of the TSS as well as bulk conduction and valence bands by 2PPE. The results agree very well with results from density functional theory (DFT) calculations. Time-resolved 2PPE is used to measure the transient population dynamics. It is dominated by refilling from the conduction band and scattering to the valence band, which is partly depleted at the present doping level. The strong interaction between bulk and surface is attributed to the large penetration depth of the TSS. Two-photon photoemission experiments used the fundamental (1.63 eV, IR pump) and the third harmonic (4.89 eV, UV probe) of a Ti:sapphire oscillator with a repetition rate of 90 MHz. The width of the cro...

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“…Once the dressing field of the pump pulse disappears (t > 500 fs), the sidebands disappear, leaving a heated Dirac cone. The dynamics of this non-equilibrium heated distribution of electrons has been discussed in a number of Tr-ARPES experiments [25][26][27][28] . Here we will focus on the Tr-ARPES spectra taken at t = 0 to ascertain the relative contribution of Floquet-Bloch and Volkov states.…”

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confidence: 99%

Selective scattering between Floquet–Bloch and Volkov states in a topological insulator

Mahmood¹,

Chan²,

Alpichshev³

et al. 2016

Nature Phys

336

273

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The coherent optical manipulation of solids is emerging as a promising way to engineer novel quantum states of matter 1-5 . The strong time-periodic potential of intense laser light can be used to generate hybrid photon-electron states. Interaction of light with Bloch states leads to Floquet-Bloch states, which are essential in realizing new photo-induced quantum phases [6][7][8] . Similarly, dressing of free-electron states near the surface of a solid generates Volkov states, which are used to study nonlinear optics in atoms and semiconductors 9 . The interaction of these two dynamic states with each other remains an open experimental problem. Here we use time-and angle-resolved photoemission spectroscopy (Tr-ARPES) to selectively study the transition between these two states on the surface of the topological insulator Bi 2 Se 3 . We find that the coupling between the two strongly depends on the electron momentum, providing a route to enhance or inhibit it. Moreover, by controlling the light polarization we can negate Volkov states to generate pure Floquet-Bloch states. This work establishes a systematic path for the coherent manipulation of solids via light-matter interaction.The manipulation of solids using ultrafast optical pulses has opened up a new paradigm in condensed matter physics by allowing the study of emergent physical properties that are otherwise inaccessible in equilibrium 1,2,10 . An important example is provided by the Floquet-Bloch states 11 , which emerge in solids owing to a coherent interaction between Bloch states inside the solid and a periodic driving potential. This is a consequence of the Floquet theorem 12 , which states that a Hamiltonian periodic in time with period T has eigenstates that are evenly spaced by the drive energy (2π/T ). Floquet-Bloch states have generated a lot of interest recently both for realizing exotic states of matter such as a Floquet Chern insulator 7 , as well as understanding non-equilibrium periodic thermodynamics 13,14 . Experimental observation of these states requires the measurement of the transient electronic band structure of a crystal as it is perturbed by light. As has recently been demonstrated 15 in the topological insulator Bi 2 Se 3 , time-and angleresolved photoemission spectroscopy (Tr-ARPES) is a key tool that can achieve this. Characteristic signatures of Floquet-Bloch states in the Tr-ARPES spectra include replicas of the original band structure that are separated by the driving photon energy 15 .In addition to Floquet-Bloch states, light can also generate other coherent phenomena in solids [16][17][18] . In particular, it can dress freeelectron states near the surface of a solid (Fig. 1a), as the surface can provide the momentum conservation necessary for a photon to interact with a free electron. This dressing was first observed in time-resolved photoemission experiments 17 and has subsequently been referred to as laser-assisted photoemission (LAPE). LAPE is typically understood [19][20][21] by invoking the Volkov solution, which is an e...

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Ultrafast Surface Carrier Dynamics in the Topological Insulator Bi₂Te₃

Cited by 224 publications

References 30 publications

Ultrafast photodoping and effective Fermi-Dirac distribution of the Dirac particles in Bi2Se3

Ultrafast photodoping and effective Fermi-Dirac distribution of the Dirac particles in Bi2Se3

Bulk and surface electron dynamics in ap-type topological insulatorSnSb2Te4

Selective scattering between Floquet–Bloch and Volkov states in a topological insulator

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Ultrafast Surface Carrier Dynamics in the Topological Insulator Bi2Te3

Cited by 224 publications

References 30 publications

Ultrafast photodoping and effective Fermi-Dirac distribution of the Dirac particles in Bi2Se3

Ultrafast photodoping and effective Fermi-Dirac distribution of the Dirac particles in Bi2Se3

Bulk and surface electron dynamics in ap-type topological insulatorSnSb2Te4

Selective scattering between Floquet–Bloch and Volkov states in a topological insulator

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Ultrafast Surface Carrier Dynamics in the Topological Insulator Bi₂Te₃