P2023BU006

BL-9B

Ultrafast Spin-Dependent Dynamics
in a Carrier-Tuned Topological Insulator
K. Sumidaa, K. Kunitomob, M. Kakokib, K. A. Kokhc, O. E. Tereshchenkoc,J. Reimannd, J. Güdded, U. Höferd, K. Miyamotoa, T. Okudaa,e, and A. Kimurab,ea

Hiroshima Synchrotron Radiation Center, Hiroshima University, 2-313 Kagamiyama,
Higashi-Hiroshima 739-0046, Japanb
Graduate School of Advanced Science and Engineering, Hiroshima University, 1-3-1 Kagamiyama,
Higashi-Hiroshima 739-8526, JapancPhysics Department, Novosibirsk State University, ul. Pirogova 2, 630090 Novosibirsk, Russia
dDepartment of Physics and Materials Sciences Center, Philipps-University, D-35032 Marburg, GermanyeInternational Institute for Sustainability with Knotted Chiral Meta Matter (SKCM2), 1-3-1 Kagamiyama,
Higashi-Hiroshima 739-8526, Japan
Keywords: Topological insulator, Time-resolved ARPES, Spin-resolved ARPES, Ultrafast carrier dynamics, Spin texture.

Topological insulators (TIs) characterized by spin-polarized Dirac-cone-like band structures on theirsurfaces have been extensively investigated over the last decade. As a hallmark of TIs caused by strong spinorbit coupling, the spin-momentum-locked topological surface state (TSS) offers a pure spin current on theirsurfaces. The spin-polarized surface electrons of TIs are robust against nonmagnetic impurities and defects.
So far, many TIs, such as Bi2Se3, Bi2Te3, and Sb2Te3, have been theoretically predicted and experimentally
verified by angle-resolved photoemission spectroscopy (ARPES) with spin-resolution.
Recently, the ultrafast optical response of TSS has attracted much attention. Time-resolved ARPES
utilizing femtosecond-laser reveals several characteristics and functionalities, e.g., the emergence of Floquet
state [1], population inversion [2], and the surface photovoltage effect [3]. However, the ultrafast carrier
dynamics derived from the unique spin texture of TSS have hardly been investigated.
In this work, we investigated the spindependent ultrafast surface carrier dynamics ofa ternary topological insulator (Sb,Bi)2Te3,
where the Dirac point is adjusted to the Fermi
energy (EF), by a combination of laser-based
time-resolved ARPES and synchrotron-based
spin-resolved ARPES techniques. The pumpprobe time-resolved ARPES measurements
were performed at Philipps University ofMarburg. We used two different pump energies:
mid-infrared (MIR) at 0.30 eV and visible (VIS)
at 2.58 eV [Figs. 1(a) and 1(d)]. We used anultraviolet (UV) at 5.16 eV light as the probe.
The spin-resolved ARPES measurements,utilizing very low energy electron diffraction
type spin-polarimeters, were performed atESPRESSO end-station (BL-9B) of Hiroshima
Synchrotron Radiation Center [4].
Figure 1(b) shows the ARPES image
FIGURE 1. (a) Schematic illustration of time-resolved
recorded at temporal overlap (t = 0 ps) with theARPES experiment using MIR-pump and UV-probe. (b)MIR-pump. We can recognize the Dirac-cone% direction
ARPES image of (Sb0.57Bi0.43)2Te3 along Γ" − Klike TSS and bulk conduction band above EF.
recorded at t = 0 ps. (c) Photoemission intensity profile as aWe also notice that the photoemission intensities
function of delay time. (d-f) Same as (a-c) but using VIS-pump.

at the TSS in a certain energy region around 0.05 eV are very weak [see the red arrow in Fig. 1(b)], while
those around 0.15 eV are pronounced. To illustrate the temporal evolution of the bands, Figure 1(c) displays
the momentum-integrated photoemission intensity profile as a function of delay time. We find a dip structurearound 0.05 eV from 0.0 to 0.5 ps. After 0.5 ps, the intensity around 0.05 eV drastically increases because
the photo-excited electrons at higher energy states relax through intra-band and inter-band scatterings. Thesefindings suggest that the direct transition is prohibited around 0.05 eV in the MIR-pump scheme. To compare
the ultrafast carrier dynamics, we also performed time-resolved ARPES using the VIS-pump [Figs. 1(e) and1(f)]. However, in sharp contrast to the MIR-pump scheme, no dip structures are visible at t = 0 ps.
Since the forbidden transition observed in the MIR-pump scheme likely stems from the unique spin
texture of the TSS, we proceeded with the spin-resolved measurement of the occupied bands below EF.
Figure 2(a) shows the spin-resolved ARPES image captured at 17.0 eV. Red and blue colors correspond topositive and negative in-plane spin-polarization, respectively. From EF to -0.3 eV, the spin-polarized lower
portion of TSS is evident. Additionally, M-shaped spin-split Rashba bands are observed around -0.5 eV.
Since the directions of in-plane spin-polarization at the upper and lower TSSs are reversed with respect tothe Dirac point, the forbidden transitions in the “resonant” MIR-pump scheme are most likely caused by theintrinsic spin texture of TSS, as depicted in Fig. 2(b). In the VIS-pump scheme, on the other hand, the directtransition to the upper TSS is allowed because the transition occurs from the unpolarized deeper-lying bulk
continuum states. Finally, we discuss why the direct transition is permitted around 0.15 eV at the TSS in theMIR-pump scheme [Fig. 1(b)]. The upper panels of Fig. 2(c) show the constant energy contours recorded att ~ 0 ps with VIS-pump. At 0.05 eV, an isotropic shape contour is seen. On the other hand, at 0.15 eV, thecontour shape undergoes deformation from circular to hexagonal due to the warping effect. On the warpedbands, it is known that not only in-plane but also out-of-plane spin components emerge [the lower panels ofFig. 2(c)] [5]. In such a case, the direct transition would be partially allowed.
In conclusion, we have experimentally demonstrated the spin-dependent forbidden transition in the TI by
using time-resolved ARPES and spin-resolved ARPES. Our findings pave the way for ultrafast optospintronic applications using the TSS of TIs.

% direction recorded at 17.0 eV. Gray
FIGURE 2. (a) In-plane spin-polarization mapping of (Sb0.57Bi0.43)2Te3 along Γ" − Kcircles represent the fitting results of the TSS shown in Fig. 1(e). (b) Schematic illustration of spin-dependent direct transition
using MIR- and VIS-pump schemes. (c) Upper panel: constant energy contours at 0.05 and 0.15 eV recorded at t ~ 0 ps with
VIS-pump. Lower panel: Schematic illustrations of spin texture of isotropic (left) and anisotropic TSS (right). Light green andorange colors correspond to the in-plane and out-of-plane spin components, respectively.

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