Ultrafast measurements of mode-specific deformation potentials of Bi2_2Te3_3 and Bi2_2Se3_3

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 Bi$_2$Te$_3$ and Bi$_2$Se$_3$, following the excitation of coherent A$_{1g}$ optical phonons. We extract and compare the deformation potentials coupling the surface electronic states to local A$_{1g}$-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 Bi$_2$Te$_3$ and Bi$_2$Se$_3$ 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 führt. 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 heißen Elektronen-Lebensdauern in den unbesetzten QWSs folgt der Theorie der Fermi-Flüssigkeiten. Um alle beobachteten Bevölkerungszerfälle quantitativ zu beschreiben, muß die quantisierte elektronische Struktur berücksichtigt 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 heißer 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 für 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 Schließen der CDW- Bandlücke am Fermi-Niveau und die transiente Rückkehr 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