Femtosecond dynamics of spin-polarized electrons in topological insulators

A faster control of spins is a major request for the new generation of computing and spintronic systems. In this framework, since many years, ultrashort light pulses have been utilized to trigger and detect the spin dynamics of electrons in magnetic materials and multilayers. Recently, three-dimensional topological insulators (TIs) have received attention in the field of spintronics due to their spectacular features, in particular, the existence, within the insulating gap of bulk states, of spin-polarized surface states (Dirac-cone) that are protected from backscattering by time-reversal symmetry. We have studied the sub-picosecond dynamics in the spin-polarized unoccupied electronic structure of Bi$_2$Te$_3$, employing circular-polarized light in time and angle resolved photoemission spectroscopy (trARPES). Exploiting the noncollinear optical parametric amplification (NOPA) besides several nonlinear optical processes resulted in tunable ultrashort visible pump pulses with 30 fs length and 1.8 eV energy and ultraviolet probe pulses with about 6 eV energy and 60 fs duration. The stable optical setup and the high repetition rate of an Yb-laser source grants a high signal-to-noise ratio in our photoemission process. The obtained 65 fs time resolution, along with 30 meV energy resolution of the time-of-flight (TOF) energy analyzer, provides us with an exciting possibility to explore the ultrafast electronic dynamics in the unoccupied band structures. Furthermore, circular dichroism (CD) allows access to the spin state of the photoemitted electrons. We found a signature of femtosecond unpolarized bulk bands dynamics in the presence of spin-polarized electrons of the surface states. This observation aided to distinguish the bulk and surface contributions in the spin-electronic current

Coherent phonons and the interplay between charge density wave and Mott phases in 1<i>T</i>-TaSe₂

1$T$-TaSe$_{2}$ is host to coexisting strongly-correlated phases including charge density waves (CDWs) and an unusual Mott transition at low temperature. Here, we investigate coherent phonon oscillations in 1$T$-TaSe$_{2}$ using a combination of time- and angle-resolved photoemission spectroscopy (TR-ARPES) and time-resolved reflectivity (TRR). Perturbation by a femtosecond laser pulse triggers a modulation of the valence band binding energy at the $\Gamma$-point, related to the Mott gap, that is consistent with the in-plane CDW amplitude mode frequency. By contrast, TRR measurements show a modulation of the differential reflectivity comprised of multiple frequencies belonging to the distorted CDW lattice modes. Comparison of the temperature dependence of coherent and spontaneous phonons across the CDW transition shows that the amplitude mode intensity is more easily suppressed during perturbation of the CDW state by the optical excitation compared to other modes. Our results clearly identify the relationship of the in-plane CDW amplitude mode with the Mott phase in 1$T$-TaSe$_{2}$ and highlight the importance of lattice degrees of freedom.Comment: 7 pages, 4 figures, supplemental materia

Excitonic and lattice contributions to the charge density wave in 1T-TiSe2 revealed by a phonon bottleneck

Understanding collective electronic states such as superconductivity and charge density waves is pivotal for fundamental science and applications. The layered transition metal dichalcogenide 1T-TiSe2 hosts a unique charge density wave (CDW) phase transition whose origins are still not fully understood. Here, we present ultrafast time- and angle-resolved photoemission spectroscopy (TR-ARPES) measurements complemented by time-resolved reflectivity (TRR) which allows us to establish the contribution of excitonic and electron-phonon interactions to the CDW. We monitor the energy shift of the valence band (VB) and coupling to coherent phonons as a function of laser fluence. The VB shift, directly related to the CDW gap closure, exhibits a markedly slower recovery dynamics at fluences above Fth = 60 microJ cm-2. This observation coincides with a shift in the relative weight of coherently coupled phonons to higher frequency modes in time-resolved reflectivity (TRR), suggesting a phonon bottleneck. Using a rate equation model, the emergence of a high-fluence bottleneck is attributed to an abrupt reduction in coupled phonon damping and an increase in exciton dissociation rate linked to the loss of CDW superlattice phonons. Thus, our work establishes the important role of both excitonic and phononic interactions in the CDW phase transition and the advantage of combining complementary femtosecond techniques to understand the complex interactions in quantum materials.Comment: 11 pages, 4 figure

Excimer-Laser Bestrahlung von Eisen, Aluminium und Silizium in Stickstoff- und Methan-Atmosphäre

Die Materialbearbeitung durch Laserbestrahlung ist ein expandierendes Fachgebiet mit attraktiven technischen Anwendungen. Unter allen lasergestützten Behandlungsmethoden kann die Bestrahlung von Metallen und Halbleitern in kontrollierten reaktiven Gasatmosphären erfolgreich zu wichtigen Modifikationen der Oberfläche der bestrahlten Materialien führen. Wenn das reaktive Gas Stickstoff oder Kohlenstoff enthält, wird der entsprechende Prozess Laser-Nitrieren bzw. Laser-Karbidisieren genannt. In der vorliegenden Arbeit wurden Eisen-, Kohlenstoff-, Aluminium- und Siliziumproben in kontrollierter Stickstoff- (N2) und Methan-(CH4)-Atmosphäre mit einem gepulsten XeCl-Excimer Laser (Wellenlänge=308 nm, Pulsdauer=55 ns FWHM) behandelt, um Stickstoff und Kohlenstoff in die Proben einzubringen. Die durch den Laser verursachten Veränderungen werden als Funktion der verschiedenen experimentellen Parameter (Gasdruck, Laserfluenz, Pulszahl) analysiert und die Mechanismen des Massentransports werden detailiert untersucht. Sowohl feste Lösungen (Fe(N), Fe(C)) als auch stöchiometrische Phasen (AlN, SiC) können an der Oberfläche hergestellt werden, die Schichten mit verbesserter Haftung an das darunterliegende Substrat bilden. Die experimentelle Analyse wurde mit einer Vielzahl von Techniken (Rutherford-Rückstreu-Spektroskopie, Resonante Kernreaktionsanalyse, Mößbauer-Spektroskopie, X-Ray Absorption Fine Structure, Röntgen-Diffraktion, Mikrohärte-Messung) durchgeführt, um damit Informationen über die Elementverteilung, die Phasenbildung, die lokale atomare Umgebung und die mechanischen Eigenschaften der behandelten Proben zu erhalten. Durch die Untersuchung der Veränderungen der Materialien nach der Laserbehandlung können außerdem wertvolle Einsichten in die Mechanismen des Masseneintrages und der Wechselwirkung zwischen Plasma und Oberfläche während der Laserbestrahlung erhalten werden

Ultrafast spin dynamics in metallic layers and strongly correlated oxides

The spin dynamics triggered by an ultrashort optical excitation can lead to a variety of behaviors depending on the specific spin and electronic structure of the material. In metallic films, electron-quasiparticles (phonons and magnons) interactions takes place on sub-picosecond timescale and demagnetization is established within 100 fs. In half-metal oxides, spin dynamics is much slower (100 ps) due to the inhibition of spin-flip processes. Furthermore, the dynamics of magnetic anisotropies can be exploited to control the magnetization in ferromagnets. Optically-induced reversible switching of the magnetization has been recently demonstrated in thin magnetic layers on the 100 picoseconds timescale

Ultrafast evolution of bulk, surface and surface resonance states in photoexcited hboxBi2hboxTe3hbox {Bi}_{2}hbox {Te}_{3}

open8openHedayat, Hamoon; Bugini, Davide; Yi, Hemian; Chen, Chaoyu; Zhou, Xingjiang; Cerullo, Giulio; Dallera, Claudia; Carpene, EttoreHedayat, Hamoon; Bugini, Davide; Yi, Hemian; Chen, Chaoyu; Zhou, Xingjiang; Cerullo, Giulio; Dallera, Claudia; Carpene, Ettor