Ultrafast photocurrents at the surface of the three-dimensional topological insulator Bi2Se3\mathrm{Bi}_2\mathrm{Se}_3

Topological insulators constitute a new and fascinating class of matter with insulating bulk yet metallic surfaces that host highly mobile charge carriers with spin-momentum locking. Remarkably, the direction and magnitude of surface currents can be controlled with tailored light beams, but the underlying mechanisms are not yet well understood. To directly resolve the "birth" of such photocurrents we need to boost the time resolution to the scale of elementary scattering events ($\sim$ 10 fs). Here, we excite and measure photocurrents in the three-dimensional model topological insulator $\mathrm{Bi}_2\mathrm{Se}_3$ with a time resolution as short as 20 fs by sampling the concomitantly emitted broadband THz electromagnetic field from 1 to 40 THz. Remarkably, the ultrafast surface current response is dominated by a charge transfer along the Se-Bi bonds. In contrast, photon-helicity-dependent photocurrents are found to have orders of magnitude smaller magnitude than expected from generation scenarios based on asymmetric depopulation of the Dirac cone. Our findings are also of direct relevance for optoelectronic devices based on topological-insulator surface currents

Thickness dependence of electron-electron interactions in topological p-n junctions

Electron-electron interactions in topological p-n junctions consisting of vertically stacked topological insulators are investigated. n-type Bi2Te3 and p-type Sb2Te3 of varying relative thicknesses are deposited using molecular beam epitaxy and their electronic properties measured using low-temperature transport. The screening factor is observed to decrease with increasing sample thickness, a finding which is corroborated by semi-classical Boltzmann theory. The number of two-dimensional states determined from electron-electron interactions is larger compared to the number obtained from weak-antilocalization, in line with earlier experiments using single layers.Comment: 38 pages, 5 figures, 1 tabl

Electrical resistance of individual defects at a topological insulator surface

Three-dimensional topological insulators host surface states with linear dispersion, which manifest as a Dirac cone. Nanoscale transport measurements provide direct access to the transport properties of the Dirac cone in real space and allow the detailed investigation of charge carrier scattering. Here, we use scanning tunnelling potentiometry to analyse the resistance of different kinds of defects at the surface of a (Bi0.53Sb0.47)2Te3 topological insulator thin film. The largest localized voltage drop we find to be located at domain boundaries in the topological insulator film, with a resistivity about four times higher than that of a step edge. Furthermore, we resolve resistivity dipoles located around nanoscale voids in the sample surface. The influence of such defects on the resistance of the topological surface state is analysed by means of a resistor network model. The effect resulting from the voids is found to be small compared to the other defects

Capacitance‐Voltage Measurements of (Bi1‐xSbx)2Te3 Field Effect Devices

Capacitance-voltage (C-V) traces in n-type-(Bi1-xSbx)(2)Te-3/oxide/metal capacitor structures using an AC capacitance bridge are investigated. By tuning the top gate voltage (V-tg) from positive to negative values, the system at the interface is tuned from accumulation, via depletion into inversion. The results show the typical low-frequency and high frequency C-V traces, depending on measuring frequency, temperature, and illumination intensity and reflecting their sensitive dependence on recombination/generation rates. Superimposed a strong hysteresis under inversion is also observed which is ascribed to the presence of conventional localized surface states (LSS) which coexist with topological surface states (TSS)

Lifting the spin-momentum locking in ultra-thin topological insulator films

Three-dimensional (3D) topological insulators (TIs) are known to carry 2D Dirac-like topological surface states in which spin-momentum locking prohibits backscattering. When thinned down to a few nanometers, the hybridization between the topological surface states at the top and bottom surfaces results in a topological quantum phase transition, which can lead to the emergence of a quantum spin Hall phase. Here, we study the thickness-dependent transport properties across the quantum phase transition on the example of (Bi$_{0.16}$Sb$_{0.84}$)$_2$Te$_3$ films, with a four-tip scanning tunnelling microscope. Our findings reveal an exponential drop of the conductivity below the critical thickness. The steepness of this drop indicates the presence of spin-conserving backscattering between the top and bottom surface states, effectively lifting the spin-momentum locking and resulting in the opening of a gap at the Dirac point. Our experiments provide crucial steps towards the detection of quantum spin Hall states in transport measurements