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A large Rashba effect is essential for future applications in spintronics. Particularly attractive is understanding and controlling nonequilibrium properties of ferroelectric Rashba semiconductors. Here, time- and angle-resolved photoemission is utilized to access the ultrafast dynamics of bulk and surface transient Rashba states after femtosecond optical excitation of GeTe. A complex thermalization pathway is observed, wherein three different timescales can be clearly distinguished: intraband thermalization, interband equilibration, and electronic cooling. These dynamics exhibit an unconventional temperature dependence: while the cooling phase speeds up with increasing sample temperature, the opposite happens for interband thermalization. It is demonstrated how, due to the Rashba effect, an interdependence of these timescales on the relative strength of both electron-electron and electron-phonon interactions is responsible for the counterintuitive temperature dependence, with spin-selection constrained interband electron-electron scatterings found both to dominate dynamics away from the Fermi level, and to weaken with increasing temperature. These findings are supported by theoretical calculations within the Boltzmann approach explicitly showing the opposite behavior of all relevant electron-electron and electron-phonon scattering channels with temperature, thus confirming the microscopic mechanism of the experimental findings. The present results are important for future applications of ferroelectric Rashba semiconductors and their excitations in ultrafast spintronics. Nanyang Technological University Published version J.S.-B. acknowledges financial support from the Impuls- und Vernetzungsfonds der Helmholtz-Gemeinschaft under grant No. HRSF-0067. I.W. and M.B. acknowledge financial support from the Nanyang Technological University, NAP-SUG. G.S. acknowledges financial support by the Austrian Science Fund (FWF), Projects No. P30960-N27 and I 4493-N. Open access funding enabled and organized by Projekt DEAL.

Kürzlich wurde entdeckt, wie sich zwei Klassen topologischer Isolatoren ineinander überführen lassen. Die Art und Weise, wie das geschieht, verspricht neue funktionelle Eigenschaften in den Materialien. So lie en sich elektrische oder spintronische Kanäle mit Spannungspulsen ein und ausschalten

Structure inversion asymmetry is an inherent feature of quantum confined heterostructures with non equivalent interfaces. It leads to a spin splitting of the electron states and strongly affects the electronic band structure. The effect is particularly large in topological insulators because the topological surface states are extremely sensitive to the interfaces. Here, the first experimental observation and theoretical explication of this effect are reported for topological crystalline insulator quantum wells made of Pb1 amp; 8722;xSnxSe confined by Pb1 amp; 8722;yEuySe barriers on one side and by vacuum on the other. This provides a well defined structure asymmetry controlled by the surface condition. The electronic structure is mapped out by angle resolved photoemission spectroscopy and tight binding calculations, evidencing that the spin splitting decisively depends on hybridization and, thus, quantum well width. Most importantly, the topological boundary states are not only split in energy but also separated in space unlike conventional Rashba bands that are splitted only in momentum. The splitting can be strongly enhanced to very large values by control of the surface termination due to the charge imbalance at the polar quantum well surface. The findings thus, open up a wide parameter space for tuning of such systems for device applications

Topological materials promise new functionalities, which are revealed with the help of angle resolved photoemission. Herein, the search for the magnetic bandgap at the Dirac point as a precondition for the quantum anomalous Hall effect is reviewed and its opening for the topological insulator heterostructure MnBi2Te4 Bi2Te3 is demonstrated. Essential preconditions are explained and the reasons why nonmagnetic gaps occur when Se replaces Te. Angle resolved photoelectron spectroscopy ARPES probes the quantum mechanical final state, and this allows investigation of spin manipulation by light using spin resolved ARPES and the dependence of the charge carrier lifetime on the peculiar spin texture of topological states. It is shown that ARPES data do not support SmB6 as the first strongly correlated topological insulator and an alternative, trivial explanation for the results of ARPES and electrical transport experiments is formulated. Epitaxially grown topological crystalline insulators are, due to their dependence on crystal symmetries, more versatile in the control of individual bulk band inversions. It is shown that this leads to topological quantum phase transitions and associated novel functionalities. Finally, the surface and bulk band connectivity of a type II 3D Weyl semimetal is investigated and an outlook is given for the scientific field

SmB6 has recently attracted considerable interest as a candidate for the first strongly correlated topological insulator. Such materials promise entirely new properties such as correlation-enhanced bulk bandgaps or a Fermi surface from spin excitations. Whether SmB6 and its surface states are topological or trivial is still heavily disputed however, and a solution is hindered by major disagreement between angle-resolved photoemission (ARPES) and scanning tunneling microscopy (STM) results. Here, a combined ARPES and STM experiment is conducted. It is discovered that the STM contrast strongly depends on the bias voltage and reverses its sign beyond 1 V. It is shown that the understanding of this contrast reversal is the clue to resolving the discrepancy between ARPES and STM results. In particular, the scanning tunneling spectra reflect a low-energy electronic structure at the surface, which supports a trivial origin of the surface states and the surface metallicity of SmB6 .