P21S

Laser Spin-ARPES

Demonstration of phase-resolved spin-ARPES on
topological surface states inTowa Kousaa , T. Iwataa,b , S. Kumarc, A. Kimuraa,b ,K. Miyamotoc, T. Okudab,c and K. Kurodaa,ba

Graduate School of Advanced Science and Engineering, Hiroshima University, 1-3-1 Kagamiyama,
Higashi-Hiroshima 739-8526, Japan.
bInternational Institute for Sustainability with Knotted Chiral Meta Matter(WPI-SKCM2),
Hiroshima University, Higashi-Hiroshima 739-8526, Japan.
cHiroshima Synchrotoron Radiation Center, Hiroshima University, 2-313 Kagamiyama,
Higashi-Hiroshima 739-0046, Japan

Keywords: Spin-orbit coupling, Topological surface states, Ultrafast dynamics

Optical transition such as photoemission takes place in attosecond scale (1 as = 10-18 s). It is very
interesting to phenomena in the attosecond region from point of view of the pursuit of ultimate understandingof quantum mechanics and pioneering electronics that can be controlled on as-time-scale. Great developmentof ultrashort pulse laser technology over the past few decades has permitted to track the ultrafast dynamics
as the attosecond scale in the time domain [1-4]. However, the ultrashort pulse nature of the attosecond pulse
diminished energy resolution because of a trade-off relationship between the pulse width in time andfrequency domains. This means that it is difficult to experimentally trace the ultrafast photoemission
dynamics together with resolving valance band structures.
In this work, to overcome the above difficulty, we attempted phase-resolved spin-ARPES [5-7] by laserbased angle-resolved photoelectron spectroscopy with 3D spin detection. Using this technique, we accessed
phase information in the transition matrix of the photoelectron states and its kinetic energy dependence thatcan be converted to the time scale of photoemission dynamics in as-time-scale [8]. For this measurement, apicosecond laser was used to achieve a 20-meV energy resolution for spin-ARPES and 3D spin detection
was used for the delay phase determination[9]. In addition, we built a polarization rotation control systemthat allows us to perform the automatic spin-ARPES measurements collecting a number of data points.
We studied the spin polarization of the photoelectrons emitted from the topological surface state of thetypical topological insulator Bi2Te3. The topological surface state is ideal for this experiment because they
form fully spin-polarized electronic states that are locked to be a helical texture in momentum space. Let us
also note that the spin-polarization of the photoelectron is collected on the mirror plane [Fig.1(a)], andtherefore the orbital selection rule is allowed in the photoemission process. Owing to these simple
experimental geometry, one can selectivity excite either even or odd orbital function that is coupled todifferent spin wavefunction [10] and superimposes these two dipole matrixes with the different spin inphotoelectron final state, resulting in the rotation of the spin polarization axis [5-7]. This process is
schematically summarized in Fig1.(b).
Though this process, the spin-orbit coupled wavefunction in the topological surface states allows us toselectively excite the spin and control its direction by photon polarization. Furthermore, the spin direction
as a function of the photon polarization sensitivity depends on the relative phase between the two matrix
elements, which therefore allows us to extract the phase from our spin-ARPES data. The extracted relative
phase
between two transition is used for time conversion according to the Eisenbud-Wigner-Smith(EWS) model [1,11] :
=ℏ
where

is a time delay obtained in the EWS-model, and(1)is kinetic energy of the photoelectron.

Fig2.(a) represents a Fermi surface and ARPES of the topological surface states in Bi2Te3. Fig2.(b)represents the results of photoelectron spin interference performed at each of wavenumbers k1~k4 in Fig2.(a).
Also, we just give a brief description here, a slight correlation is observed between each phase and kinetic
energy, indicating that phase decomposition is possible. Using its data and Eq.(1), we obtained the result thatthe delay time= 1.29 10s.
..(b)(a). .(c)[Fig1.] (a) Electron orbitals and spins of topological surface states on the mirror plane. (b) Concept of photoelectron spininterference. (c) Concepts that tie our work to EWS model [5-7,11]. The wave packets of two transition are scattered differently
as they pass thorough the potential V, therefore
occurs between the two transitions. We noted thatis extracted for each
Ek andis calculated from Eq.(1).

.(b)

(a)

[Fig2.] (a) ARPES of topological surface states in Bi2Te3. Temperature is 40~50K. Photon energy is 6.3eV. Photon is liner
polarization. k1~k4 are wavenumber of each spin detection. Clean surface was obtained by cleaving in 7.3 10 Pa. The
mirror plane (Γ- M direction), the detection plane and the slit direction are all parallel. (b) Spin detection with polarization
rotation at binding energies corresponding to each wave number. Sseff = 0.27. The dots are experimental values, and the solidlines are model fittings [5-7].

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