Ultrafast spin-polarized electron dynamics in the unoccupied topological surface state of Bi2Se3

Ultrafast evolution of bulk, surface and surface resonance states in photoexcited [Formula: see text]

We use circular dichroism (CD) in time- and angle-resolved photoemission spectroscopy (trARPES) to measure the femtosecond charge dynamics in the topological insulator (TI) [Formula: see text]. We detect clear CD signatures from topological surface states (TSS) and surface resonance (SR) states. In time-resolved measurements, independently from the pump polarization or intensity, the CD shows a dynamics which provides access to the unexplored electronic evolution in unoccupied states of [Formula: see text]. In particular, we are able to disentangle the unpolarized electron dynamics in the bulk states from the spin-textured TSS and SR states on the femtosecond timescale. Our study demonstrates that photoexcitation mainly involves the bulk states and is followed by sub-picosecond transport to the surface. This provides essential details on intra- and interband scattering in the relaxation process of TSS and SR states. Our results reveal the significant role of SRs in the subtle ultrafast interaction between bulk and surface states of TIs.

Electrostatic formation of the Majorana quasiparticles in the quantum dot-nanoring structure

Zero-energy Majorana quasiparticles can be induced at the edges of low dimensional systems. Non-Abelian statistics of these states make them valid candidates for the realisation of topological quantum computer. From the practical point of view, it is crucial to obtain a system in which an on demand creation and manipulation of this type of bound states is feasible. In this article, we show such a possibility in a setup comprising a quantum nanoring in which we specify a quantum dot region via electrostatic means. The presence of quantum dot can lead to the emergence of Andreev and Majorana bound states in the investigated system. We study the differences between those two types of bound states and the possibility of their manipulation. Moreover, the exact calculation method for spectral function has been proposed, which can be used to study the influence of bound states on the band structure of the proposed system. Using this method, it can be shown that the Majorana bound states, induced at the edge of the system, present themselves as a dispersionless zero-energy flat-band.

Band-Resolved Imaging of Photocurrent in a Topological Insulator

We study the microscopic origins of photocurrent generation in the topological insulator Bi_{2}Se_{3} via time- and angle-resolved photoemission spectroscopy. We image the unoccupied band structure as it evolves following a circularly polarized optical excitation and observe an asymmetric electron population in momentum space, which is the spectroscopic signature of a photocurrent. By analyzing the rise times of the population we identify which occupied and unoccupied electronic states are coupled by the optical excitation. We conclude that photocurrents can only be excited via resonant optical transitions coupling to spin-orbital textured states. Our work provides a microscopic understanding of how to control photocurrents in systems with spin-orbit coupling and broken inversion symmetry.

Observation of chiral surface excitons in a topological insulator Bi2Se3

The protected electron states at the boundaries or on the surfaces of topological insulators (TIs) have been the subject of intense theoretical and experimental investigations. Such states are enforced by very strong spin-orbit interaction in solids composed of heavy elements. Here, we study the composite particles-chiral excitons-formed by the Coulomb attraction between electrons and holes residing on the surface of an archetypical 3D TI, [Formula: see text] Photoluminescence (PL) emission arising due to recombination of excitons in conventional semiconductors is usually unpolarized because of scattering by phonons and other degrees of freedom during exciton thermalization. On the contrary, we observe almost perfectly polarization-preserving PL emission from chiral excitons. We demonstrate that the chiral excitons can be optically oriented with circularly polarized light in a broad range of excitation energies, even when the latter deviate from the (apparent) optical band gap by hundreds of millielectronvolts, and that the orientation remains preserved even at room temperature. Based on the dependences of the PL spectra on the energy and polarization of incident photons, we propose that chiral excitons are made from massive holes and massless (Dirac) electrons, both with chiral spin textures enforced by strong spin-orbit coupling. A theoretical model based on this proposal describes quantitatively the experimental observations. The optical orientation of composite particles, the chiral excitons, emerges as a general result of strong spin-orbit coupling in a 2D electron system. Our findings can potentially expand applications of TIs in photonics and optoelectronics.