dc.description.abstract
This thesis investigates the excitation and dynamics of charge carriers in two narrow-gap
semiconductors featuring spin-polarized surface states, which are promising candidates
for spintronic applications. On the topological insulator Bi2Se3, we investigated the
origin of helicity-dependent photocurrents by two-photon photoemission. On BiTeI, we
studied the electron- and hole-dynamics in the surface state and the conduction band
minimum (CBM), which both feature a giant Rashba-splitting, with time- and angleresolved
photoemission spectroscopy.
In the case of Bi2Se3, circular polarized pulses of 1.7 eV photon energy are used to excite
electrons resonantly from the occupied topological surface state (TSS) to the unoccupied
TSS. The excited electrons are photoemitted with s-polarized pulses of 4.7 eV, conserving
the momentum distribution. The dichroic contrast created during the excitation is
identified to be predominantly governed by the interaction of the light with the initial
state, as corroborated by off-resonantly excited spectra, model calculations performed
by our collaboration partners, and time-resolved data. Anti-symmetric patterns, as expected
from the commonly assumed coupling of the photon angular momentum to the
parallel component of the electron spin, can only be observed for electrons stemming
from the CBM, but not from the TSS. The TSS, in contrast, shows clear three-fold
symmetric patterns. Residual asymmetries, probably originating from experimental inaccuracies,
are large enough to explain the helicity-dependent photocurrents observed
in previous transport measurements.
On BiTeI, the electronic system is probed with 6.2 eV photon energy, giving access to
the carriers around the Fermi level. We confirm the reported electronic structure
of the Te-terminated surface and find only minor influences on the electronic structure
by the dopants manganese and vanadium. We observe a complex interplay of surface
and bulk dynamics after photoexcitation with pulses of 1.5 eV photon energy. Electronelectron
scattering is found to be effective across all subsystems. The dynamics above the Fermi level are governed by bulk interactions. Heat is dissipated from the hot electron gas by
electron-phonon coupling. The long thermalization times between electrons and lattice
suggest a phonon bottleneck, created by long lifetimes of optical phonons. The dynamics
below the Fermi level at binding energies up to 50 meV are sped up by drift currents induced
by the positively charged surface. Within the Rashba-split surface state (RSS), we find
a temperature-dependent coupling of the photoholes to the surface plasmon. Furthermore,
we refute previous reports of ballistic spin-dependent transport upon excitation
with circular polarized pump pulses.
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