Effect of Rashba splitting on ultrafast carrier dynamics in BiTeI

Anna S. Ketterl, Beatrice Andres, Marco Polverigiani, Vladimir Voroshnin, Cornelius Gahl, Konstantin A. Kokh, Oleg E. Tereshchenko, Evgueni V. Chulkov, Alexander Shikin, and Martin Weinelt

Phys. Rev. B 103, 085406 – Published 3 February 2021

Abstract

Narrow-gap semiconductors with strong spin-orbit coupling such as bismuth tellurohalides have become popular candidates for spintronic applications. But driving spin-polarized photocurrents in these materials with circularly polarized light requires picosecond lifetimes of the photoexcited carriers and low spin-flip scattering rates. In search of these essential ingredients, we conducted an extensive study of the carrier dynamics on the Te-terminated surface of BiTeI, which exhibits a giant Rashba splitting of both surface and bulk states. We observe a complex interplay of surface and bulk dynamics after photoexcitation. Carriers are rapidly rearranged in momentum space by quasielastic phonon and defect scattering, while a phonon bottleneck leads to a slow equilibration between bulk electrons and lattice. The particular band dispersion opens an inelastic decay channel for hot carriers in the form of plasmon excitations, which are immanent to Rashba-split systems. These ultrafast scattering processes effectively redistribute excited carriers in momentum and energy space and thereby inhibit spin-polarized photocurrents.

Received 17 September 2020
Revised 13 January 2021
Accepted 14 January 2021

DOI:https://doi.org/10.1103/PhysRevB.103.085406

Physics Subject Headings (PhySH)

Carrier dynamics Quasiparticles & collective excitations Spin-orbit coupling Surface plasmons Surface states

Surfaces Two-dimensional electron system

Photoexcitation Time & angle resolved photoemission spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Anna S. Ketterl¹, Beatrice Andres¹, Marco Polverigiani¹, Vladimir Voroshnin ^2,3, Cornelius Gahl ¹, Konstantin A. Kokh^4,5, Oleg E. Tereshchenko^5,6, Evgueni V. Chulkov^2,7,8, Alexander Shikin², and Martin Weinelt ^1,*

¹Freie Universität Berlin, Fachbereich Physik, Arnimallee 14, 14195 Berlin, Germany
²Saint Petersburg State University, Institute of Physics, Uljanovskaya 1, 198504 Saint Petersburg, Russia
³Helmholtz-Zentrum Berlin für Materialien und Energie, Albert Einstein Straße 15, 12489 Berlin, Germany
⁴V. S. Sobolev Institute of Geology and Mineralogy, 630090 Novosibirsk, Russia
⁵Novosibirsk State University, 630090 Novosibirsk, Russia
⁶A. V. Rzhanov Institute of Semiconductor Physics, 630090 Novosibirsk, Russia
⁷Departamento de Física de Materiales UPV/EHU, Centro de Física de Materiales CFM-MPC, and Centro Mixto CSIC-UPV/EHU, 20080 San Sebastián/Donostia, Basque Country, Spain
⁸International Physics Center (DIPC), 20018 San Sebastián/Donostia, Basque Country, Spain

^*weinelt@physik.fu-berlin.de

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Vol. 103, Iss. 8 — 15 February 2021

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Images

Figure 1
Crystal and electronic structure of BiTeI. (a) Schematic of the trilayer crystal structure of BiTeI. (b) Dispersion of the RSS of the Te-terminated surface of BiTeI:Mn, recorded along the $k_{x}$ direction at 60 K sample temperature. The superimposed parabolas were fitted to the dispersion of the RSS. (c) The $k_{| |}$ maps at different energies reveal a small hexagonal warping of the RSS at the Fermi energy $E_{F}$ , the band bottom of the conduction band at binding energy $E - E_{F} = - 0.1$ eV, overlapping with the inner contour of the RSS, the crossing point of the RSS at $- 0.21$ eV, and the band bottom of the RSS at $- 0.31$ eV.
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Figure 2
Ultrafast carrier dynamics under photoexcitation. (a) False-color plot of the photoelectron intensity of the BiTeI:Mn sample, plotted over binding energy and pump-probe delay. The intensity was integrated over the entire accessible $k_{| |}$ range. The spectrum was recorded at a temperature of 60 K with a pump fluence of 150 $μ J / {cm}^{2}$ . (b) Exemplary delay spectrum at a single kinetic energy bin ( $Δ E = 12 meV$ ) at the highest indicated energy in (a), with the corresponding fit of thermalization time constant $τ_{1}$ and relaxation back to the initial distribution with time constant $τ_{2}$ . (c) Delay spectra and fits recorded with three different pump fluences at the energies indicated by dashed lines in (a). The points represent the measured data, while solid lines are corresponding fits. The traces around the Fermi level have been multiplied by a factor of 5 for clarity. (d) Time constant $τ_{2}$ , obtained from the fitting procedure explained in (b), from the data shown in (a), plotted over the respective energies.
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Figure 3
Evolution of the electronic temperature revealing a phonon bottle neck in electron relaxation. (a) Photoelectron intensity recorded on BiTeI:Mn plotted over the kinetic energy at five characteristic delays integrated over $k_{∥}$ . The sample was kept at room temperature and the applied pump fluence was 70 $μ J / {cm}^{2}$ . The spectra were fitted with a function comprising surface and bulk model DOS, multiplied with a Fermi function and convolved with a Gaussian to account for experimental resolution. (b) Electronic temperatures over pump-probe delay, extracted from the Fermi distribution in the fits of the DOS, as exemplarily shown in (a). The values have been corrected to fit the initial sample temperatures, as the global fits overestimate the electronic temperature. Light blue and red filled areas indicate error margins. They are not statistical, but denote the regions where reasonable agreement between fit and data can be achieved. (c) Shift of the chemical potential, extracted from the data in the same way as the temperatures displayed in (b).
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Figure 4
Temperature dependence of the hole dynamics revealing inelastic electron decay via plasmon excitation. (a) Decay constant $τ_{2}$ of the carriers below the Fermi level in pristine (a) BiTeI and (b) BiTeI $+$ 3% Mn. The decay constants were obtained from the fitting procedure explained in Fig. 2 of two different data sets measured on the same samples, but at different temperatures. The pump fluence was 410 and 150 $μ J / {cm}^{2}$ , respectively. Light blue and red filled areas indicate error margins. They are not statistical, but denote the regions where reasonable agreement between fit and data can be achieved. (c) Energy scheme of the band bending on the Te-terminated surface of BiTeI. (d) Schematic of the dispersion of the RSS (red) and surface plasmon dispersion (green). The blue arrows indicate the possible inelastic electron decay channel by plasmon emission.
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Figure 5
Missing pump-induced circular dichroism in the angular distribution (CDAD) of photoelectrons indicating the lack of photocurrents. Time-resolved photoelectron spectra recorded with right and left circularly polarized (rcp and lcp) pump pulses of 1.5 eV photon energy and 6.2 eV $p$ -polarized probe pulses at zero delay on a BiTeI:Mn sample at 60 K. (a) Sum of spectra recorded with rcp and lcp pump polarization, respectively. The RSS and conduction band are quite discernible. (b), (c) Difference spectra, i.e., pump-induced CDAD patterns, for two different cuts through the 3D data set. No dichroic signal emerges, when the polarization is carefully adjusted at the sample position.
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