19 research outputs found
Ultrafast measurements of mode-specific deformation potentials of BiTe and BiSe
Quantifying electron-phonon interactions for the surface states of
topological materials can provide key insights into surface-state transport,
topological superconductivity, and potentially how to manipulate the surface
state using a structural degree of freedom. We perform time-resolved x-ray
diffraction (XRD) and angle-resolved photoemission (ARPES) measurements on
BiTe and BiSe, following the excitation of coherent A
optical phonons. We extract and compare the deformation potentials coupling the
surface electronic states to local A-like displacements in these two
materials using the experimentally determined atomic displacements from XRD and
electron band shifts from ARPES.We find the coupling in BiTe and
BiSe to be similar and in general in agreement with expectations from
density functional theory. We establish a methodology that quantifies the
mode-specific electron-phonon coupling experimentally, allowing detailed
comparison to theory. Our results shed light on fundamental processes in
topological insulators involving electron-phonon coupling
Ultraschnelle Elektronendynamik in Niedrig-Dimensionalen Materialien
This work investigates the ultrafast electron dynamics in low-dimensional
materials using femtosecond time- and angle-resolved photoemission techniques.
In low-dimensional materials, the electronic wave functions are confined to
one and two dimensions, (1D) and (2D). This electron confinement can lead to
(i) quantum size effects, where the physical properties of a solid state
material depend decisively on the size of the considered system, and to (ii)
many-body phenomena, where electron electron correlation and coupling to other
excitations such as phonons can result in the formation of broken-symmetry
ground states like charge density wave (CDW) or superconducting phases.
Besides the fundamental scientific significance, low-dimensional materials
become increasingly important with the advent of nanotechnology. In this work,
several well-defined prototypical quasi-2D and quasi-1D model systems have
been studied.
The quantized band structure and electron dynamics of the occupied and
unoccupied quantum well states (QWSs) in the quasi-2D model system Pb/Si(111)
have been investigated with two-photon photoemission (2PPE) and photoemission
spectroscopy (PES) directly in the time domain. The general trend of the hot
electron lifetimes in the unoccupied QWS is confirmed to be governed by Fermi
liquid theory. To describe all observed population decays quantitatively, the
quantized electronic system has to be taken into account since the
simultaneous population decay and build-up in two adjacent QWSs favors inter-
subband scattering from the higher lying into the lower lying state. The
investigation of the dynamics of the highest occupied QWS in Pb/Si(111) with
time-resolved PES revealed an ultrafast electronic stabilization due to the
excitation of electron-hole pairs that induce a sudden change of the screening
of the ion cores and displacively excite a coherent phonon mode in the film
direction. Potential anisotropies of the hot electron lifetimes of self-
assembling metallic quasi-1D nanowires in 4x1-In/Si(111) were investigated
using time-resolved 2PPE and a novel position-sensitive electron time-of-
flight technique, which was developed as part of this work for efficient
angle-resolved photoemission studies of low-dimensional materials with
anisotropic band structure. Finally, collective excitations of the electron
and lattice system in the prototypical quasi-1D CDW compound TbTe3 have been
explored with time- and angle-resolved PES. The temperature-, fluence- and
k-dependence of the transient band structure in the CDW nesting region at the
Fermi level reveals an oscillation of the valence band, which is assigned to
the excitation of the amplitude mode of the CDW phase of TbTe3. At the highest
fluences investigated, the charge-ordered CDW melts within 100 fs, evidenced
by the ultrafast closing of the CDW gap in the band structure at the Fermi
level and the transient recurrence of a nearly free electron dispersion.Diese Arbeit untersucht die ultraschnelle Elektronendynamik in niedrig-
dimensionalen Materialien mit Femtosekunden zeit- und winkelaufgelösten
Photoemissionstechniken. In niedrig-dimensionalen Materialien sind die
elektronischen Wellenfunktionen auf ein und zwei Dimensionen beschränkt, (1D)
und (2D). Diese Einschränkung der Elektronen kann dazu führen, dass es (i) zu
größenbedingten Quanteneffekten kommt, bei denen die physikalischen
Eigenschaften eines Festkörpers entscheidend von der Systemgröße abhängen, und
(ii) zu Vielteilchen-Phänomenen, bei denen Elektronen-Elektronen-Korrelation
und die Ankopplung an andere Anregungen wie Phononen zur Bildung von
Grundzuständen mit gebrochener Symmetrie wie etwa Ladungsdichtewellen (charge
density waves, CDWs) oder supraleitenden Phasen fĂĽhrt. Neben der grundlegenden
wissenschaftlichen Bedeutung gewinnen niedrig-dimensionale Materialien mit dem
Aufkommen der Nanotechnologie zunehmend an Bedeutung. Mehrere wohldefinierte,
prototypische quasi-2D und quasi-1D Modellsysteme wurden in dieser Arbeit
untersucht.
Die quantisierte Bandstruktur und Elektronendynamik der besetzten und
unbesetzten Quantentrogzustände (quantum well states, QWSs) in dem quasi-2D
Modellsystem Pb/Si(111) wurde mittels Zwei-Photonen Photoemissionspektroskopie
(2PPE) und Photoemissionspektroskopie (PES) direkt in der Zeitdomäne
untersucht. Die allgemeine Tendenz der heiĂźen Elektronen-Lebensdauern in den
unbesetzten QWSs folgt der Theorie der Fermi-FlĂĽssigkeiten. Um alle
beobachteten Bevölkerungszerfälle quantitativ zu beschreiben, muß die
quantisierte elektronische Struktur berĂĽcksichtigt werden, da der
gleichzeitige Populationszerfall und -aufbau in zwei benachbarten QWSs Inter-
Subband Streuung vom höher liegenden in den tiefer liegenden Zustand nahe
legt. Die Untersuchung der Dynamik des höchsten besetzten QWSs in Pb/Si(111)
mittels zeitaufgelöster PES ergab eine ultraschnelle elektronische
Stabilisierung durch die Anregung von Elektronen-Loch Paaren, die eine
plötzliche Änderung der Abschirmung der Ionenkerne induzieren und eine
kohärente Phononen-Mode in der Filmrichtung anregen. Potenzielle Anisotropien
der Lebensdauer heiĂźer Elektronen in sich selbst-organisierenden metallischen
quasi-1D Nanodrähten in 4x1-In/Si(111) mittels zeit- und winkelaufgelöster
2PPE untersucht. Dazu wurde ein neuentwickeltes Spektrometer fĂĽr positions-
empfindliche Elektronen Flugzeit-Spektroskopie angewandt, das im Rahmen dieser
Arbeit für effiziente winkelaufgelöste Photoemissionsstudien von niedrig-
dimensionalen Materialien mit anisotroper Bandstruktur entwickelt wurde. Die
kollektiven Anregungen des Elektronen- und Gittersystems in dem quasi-1D CDW-
Material TbTe3 sind mit zeit- und winkelaufgelöster PES analysiert worden. Die
Temperatur-, Fluenz- und k-Abhängigkeit der transienten Bandstruktur im
Verschachtelungsbereich der CDW am Fermi-Niveau offenbart eine Schwingung des
Valenzbandes, die der Anregung der Amplituden-Mode der CDW-Phase von TbTe3
zugeschrieben wird. Bei der höchsten untersuchten Fluenz schmilzt die CDW-
Phase innerhalb von 100 fs, was durch das ultraschnelle SchlieĂźen der CDW-
BandlĂĽcke am Fermi-Niveau und die transiente RĂĽckkehr einer quasi-freien
Elektronendispersion angezeigt wird
Ultrafast electron dynamics in low-dimensional materials
This work investigates the ultrafast electron dynamics in low-dimensional materials using femtosecond time- and angle-resolved photoemission techniques. In low-dimensional materials, the electronic wave functions are confined to one and two dimensions, (1D) and (2D). This electron confinement can lead to (i) quantum size effects, where the physical properties of a solid state material depend decisively on the size of the considered system, and to (ii) many-body phenomena, where electron electron correlation and coupling to other excitations such as phonons can result in the formation of broken-symmetry ground states like charge density wave (CDW) or superconducting phases. Besides the fundamental scientific significance, low-dimensional materials become increasingly important with the advent of nanotechnology. In this work, several well-defined prototypical quasi-2D and quasi-1D model systems have been studied.
The quantized band structure and electron dynamics of the occupied and unoccupied quantum well states (QWSs) in the quasi-2D model system Pb/Si(111) have been investigated with two-photon photoemission (2PPE) and photoemission spectroscopy (PES) directly in the time domain. The general trend of the hot electron lifetimes in the unoccupied QWS is confirmed to be governed by Fermi liquid theory. To describe all observed population decays quantitatively, the quantized electronic system has to be taken into account since the simultaneous population decay and build-up in two adjacent QWSs favors inter-subband scattering from the higher lying into the lower lying state. The investigation of the dynamics of the highest occupied QWS in Pb/Si(111) with time-resolved PES revealed an ultrafast electronic stabilization due to the excitation of electron-hole pairs that induce a sudden change of the screening of the ion cores and displacively excite a coherent phonon mode in the film direction. Potential anisotropies of the hot electron lifetimes of self-assembling metallic quasi-1D nanowires in 4x1-In/Si(111) were investigated using time-resolved 2PPE and a novel position-sensitive electron time-of-flight technique, which was developed as part of this work for efficient angle-resolved photoemission studies of low-dimensional materials with anisotropic band structure. Finally, collective excitations of the electron and lattice system in the prototypical quasi-1D CDW compound TbTe3 have been explored with time- and angle-resolved PES. The temperature-, fluence- and k-dependence of the transient band structure in the CDW nesting region at the Fermi level reveals an oscillation of the valence band, which is assigned to the excitation of the amplitude mode of the CDW phase of TbTe3. At the highest fluences investigated, the charge-ordered CDW melts within 100 fs, evidenced by the ultrafast closing of the CDW gap in the band structure at the Fermi level and the transient recurrence of a nearly free electron dispersion