ИСТИНА

Three-dimensional topological insulators (TIs) are characterized by spin-polarized Dirac-cone surface states that are protected from backscattering by time-reversal symmetry. Within the past few years, TIs have attracted a lot of interest due to their unique electronic structure with spin-polarized topological surface states (TSSs), which may pave the way for these materials to have a great potential in multiple applications. However, to enable consideration of TIs as building blocks for novel devices, stability of TSSs toward oxidation should be tested. We investigate the behavior of the topological surface states on Bi2X3 (X = Se, Te) by valence-band and core level photoemission in a wide range of water and oxygen pressures both in situ (from 10 8 to 0.1 mbar) and ex situ (at 1 bar). We find that no chemical reactions occur in pure oxygen and in pure water. Water itself does not chemically react with both Bi2Se3 and Bi2Te3 surfaces and only leads to slight p-doping. In dry air, the oxidation of the Bi2Te3 surface occurs on the time scale of months, in the case of Bi2Se3 surface of cleaved crystal, not even on the time scale of years. The presence of water, however, promotes the oxidation in air, and we suggest the underlying reactions supported by density functional calculations. All in all, the surface reactivity is found to be negligible, which allows expanding the acceptable ranges of conditions for preparation, handling and operation of future Bi2X3-based devices. Among the family of TIs with a tetradymite structure, Sb2Te3 is of p-type and appears to be the least explored material since its TSS is unoccupied in the ground state, a property that allows the use of optical excitations to generate spin currents relevant for spintronics. Here, we report relatively fast surface oxidation of Sb2Te3 under ambient conditions. We show that the clean surface reacts rapidly with molecular oxygen and slowly with water, and that humidity plays an important role during oxide layer growth. In humid air, we show that Sb2Te3 oxidizes on a time scale of minutes to hours, and much faster than other tetradymite TIs. The high surface reactivity revealed by our experiments is of critical importance and must be taken into account for the production and exploitation of novel TI-based devices using Sb2Te3 as a working material. Control of the spin polarization of topological surface states (TSSs) using femtosecond light pulses opens novel perspectives for the generation and manipulation of dissipationless surface spin currents on ultrafast time scales. Using time-, spin-, and angle-resolved spectroscopy, we directly monitor the ultrafast response of the spin polarization of photoexcited TSSs to circularly polarized femtosecond pulses of infrared light. We achieve all- optical switching of the transient out-of-plane spin polarization, which relaxes in about 1.2 ps. Our observations establish the feasibility of ultrafast optical control of spin-polarized Dirac fermions in TIs and pave the way for optospintronic applications at ultimate speeds.