E. Golias

Nonmagnetic band gap at the Dirac point of the magnetic topological insulator (Bi1−xMnx)2Se3

Magnetic doping is expected to open a band gap at the Dirac point of topological insulators by breaking time-reversal symmetry and to enable novel topological phases. Epitaxial (Bi1−xMnx)2Se3 is a prototypical magnetic topological insulator with a pronounced surface band gap of ∼100 meV. We show that this gap is neither due to ferromagnetic order in the bulk or at the surface nor to the local magnetic moment of the Mn, making the system unsuitable for realizing the novel phases. We further show that Mn doping does not affect the inverted bulk band gap and the system remains topologically nontrivial. We suggest that strong resonant scattering processes cause the gap at the Dirac point and support this by the observation of in-gap states using resonant photoemission. Our findings establish a mechanism for gap opening in topological surface states which challenges the currently known conditions for topological protection.

Ultrafast spin-polarization control of Dirac fermions in topological insulators

Topological insulators are promising materials for future spintronic applications due to the unique properties of their spin-polarized surface states. Using time-, spin-, and angle-resolved photoemission spectroscopy, the authors establish the utilization of photoexcited topological surface states as unique channels in which to drive fully controllable spin-polarized electrical currents on ultrafast time scales. Using circularly polarized femtosecond laser pulses of infrared light, the authors show that the dynamics of the generated currents exhibits two distinct timescales for the spin and charge degrees of freedom on the surface, and identify the underlying mechanisms giving rise to this behavior. Moreover, they find that the generated currents are completely reversible with the helicity of the circular-light polarization, paving the way for novel applications of topological insulators in spintronics at ultimate speeds.

Absence of giant spin splitting in the two-dimensional electron liquid at the surface of SrTiO3 (001)

We reinvestigate the putative giant spin splitting at the surface of SrTiO

Subpicosecond spin dynamics of excited states in the topological insulator Bi2Te3

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Topological quantum phase transition from mirror to time reversal symmetry protected topological insulator

reported by Santander-Syro

Laser-induced persistent photovoltage on the surface of a ternary topological insulator at room temperature

et~al.

Evolution of the remnant Fermi-surface state in the lightly doped correlated spin-orbit insulator Sr2-xLaxIrO4

[Nature Mat. 13, 1085 (2014)]. Our spin- and angle-resolved photoemission experiments on (001) oriented surfaces supporting a two-dimensional electron liquid with high carrier density show no detectable spin polarization in the photocurrent. We demonstrate that this result excludes a giant spin splitting while it is fully consistent with the unconventional Rashba-like splitting seen in band structure calculations that reproduce the experimentally observed ladder of quantum confined subbands.

Mapping the band structure of GeSbTe phase change alloys around the Fermi level

Using time-, spin-, and angle-resolved photoemission, we investigate the ultrafast spin dynamics of hot electrons on the surface of the topological insulator Bi2Te3 following optical excitation by femtosecond-infrared pulses. We observe two surface-resonance states above the Fermi level coexisting with a transient population of Dirac fermions that relax in similar to 2 ps. One state disperses up to similar to 0.4 eV just above the bulk continuum, and the other one at similar to 0.8 eV inside a projected bulk band gap. At the onset of the excitation, both states exhibit a reversed spin texture with respect to that of the transient Dirac bands, in agreement with our one-step photoemission calculations. Our data reveal that the high-energy state undergoes spin relaxation within similar to 0.5 ps, a process that triggers the subsequent spin dynamics of both the Dirac cone and the low-energy state, which behave as two dynamically locked electron populations. We discuss the origin of this behavior by comparing the relaxation times observed for electrons with opposite spins to the ones obtained from a microscopic Boltzmann model of ultrafast band cooling introduced into the photoemission calculations. Our results demonstrate that the nonequilibrium surface dynamics is governed by electron-electron rather than electron-phonon scattering, with a characteristic time scale unambiguously determined by the complex spin texture of excited states above the Fermi level. Our findings reveal the critical importance of detecting momentum and energy-resolved spin textures with femtosecond resolution to fully understand the subpicosecond dynamics of transient electrons on the surface of topological insulators.

Disentangling bulk from surface contributions in the electronic structure of black phosphorus

Topological insulators constitute a new phase of matter protected by symmetries. Time-reversal symmetry protects strong topological insulators of the Z2 class, which possess an odd number of metallic surface states with dispersion of a Dirac cone. Topological crystalline insulators are merely protected by individual crystal symmetries and exist for an even number of Dirac cones. Here, we demonstrate that Bi-doping of Pb1−xSnxSe (111) epilayers induces a quantum phase transition from a topological crystalline insulator to a Z2 topological insulator. This occurs because Bi-doping lifts the fourfold valley degeneracy and induces a gap at