Bulk and surface electron dynamics in ap-type topological insulatorSnSb2Te4

Abstract

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...

Colloquium: Topological band theory

“…Theoretically predicted single surface Dirac cones have been observed in several of these materials via ARPES as well as pump-probe spectroscopy of unoccupied surface states. (Niesner et al 2012;Niesner et al 2014) The crystals of this series are built by stacking layers. We highlight the materials flexibility by considering the example of GemBi2nTe(m+3n).…”

Spectroscopy and dynamics of unoccupied electronic states of the topological insulatorsSb2Te3andSb2Reimann1, Güdde2, Kuroda3 et al. 2014Phys. Rev. BSelf Cite605171View full textAdd to dashboardCite

“…Because this technique uses short pump pulses to populate initially unoccupied states before the excited electrons are photoemitted by time-delayed probe pulses, it is particularly well suited for the investigation of p-doped topological insulators. It can not only explore the electronic structure above the Fermi energy, but makes it also possible to study the dynamics of excited electrons in the TSS directly in the time domain [9][10][11][12][13][14][15] The experiments were carried out in a μ-metal shielded UHV chamber at a base pressure of 3×10 −11 mbar. The optical setup is described in detail in Ref.…”

Phase relations in the Tl5Te3-Tl9SbTe6-Tl9TbTe6 system

“…Only one endothermic effect was observed for Tl 5 Te 3 (723 K) and Tl 9 SbTe 6 (790 K) while two effects for Tl 9 TbTe 6 (780 and 1110 K) showed the completion of the synthesis.…”

“…[1][2][3]. Moreover, some of such compounds have attracted scientific and technological interest as topological insulators [4,5].…”

“…1). The basic structural component of Tl 5 Te 3 compound is octahedron with a thallium atom, Tl(2), in its center. These octahedra connected by vertices form a frame [10][11][12][13].…”