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Register a new userPhotocurrent measurements in topological insulator $text{Bi}_2text{Se}_3$ nanowires
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Circular photogalvanic currents are a promising new approach for spin-optoelectronics. To date, such currents have only been induced in topological insulator flakes or extended films. It is not clear whether they can be generated in nanodevices. In this paper, we demonstrate the generation of circular photogalvanic currents in $text{Bi}_2text{Se}_3$ nanowires. Each nanowire shows topological surface states. Here, we generate and distinguish the different photocurrent contributions via the driving light wave. We separate the circular photogalvanic currents from those due to thermal Seebeck effects, through controlling the laser light polarization. The results reveal a spin-polarized surface-Dirac electron flow in the nanowires arising from spin-momentum locking and spin-orbit effects. The second photocurrent contribution described in this letter is caused by the thermal Seebeck effect. By scanning the photocurrent, it can be spatially resolved; upon reversing the gradient direction along the nanowire, the photocurrent changes its sign, and close to the gold contacts, the amplitudes of the different photocurrent contributions are affected by the proximity to the contacts. In the center of the nanowires, where the effects from the gold contact/ topological insulator stacks vanish, the spin-polarized current remains constant along the nanowires. This opens up a new method of all-optical spin current generation in topological insulator nanowires and hybrid structures for nanoscale spin-orbitronics.
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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 three-dimensional topological insulator (TI), Bi$_2$Se$_3$. 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 meVs, 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 such 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.
The influence of individual impurities of Fe on the electronic properties of topological insulator Bi$_2$Se$_3$ is studied by Scanning Tunneling Microscopy. The microscope tip is used in order to remotely charge/discharge Fe impurities. The charging process is shown to depend on the impurity location in the crystallographic unit cell, on the presence of other Fe impurities in the close vicinity, as well as on the overall doping level of the crystal. We present a qualitative explanation of the observed phenomena in terms of tip-induced local band bending. Our observations evidence that the specific impurity neighborhood and the position of the Fermi energy with respect to the Dirac point and bulk bands have both to be taken into account when considering the electron scattering on the disorder in topological insulators.
We perform ab-initio calculations on Bi$_mathrm{{Se}}$ antisite defects in the surface of Bi$_2$Se$_3$, finding strong low-energy defect resonances with a spontaneous ferromagnetism, fixed to an out-of-plane orientation due to an exceptional large magnetic anisotropy energy. For antisite defects in the surface layer, we find semi-itinerant ferromagnetism and strong hybridization with the Dirac surface state, generating a finite energy gap. For deeper lying defects, such hybridization is largely absent, the magnetic moments becomes more localized, and no energy gap is present.
We use optical pump--THz probe spectroscopy at low temperatures to study the hot carrier response in thin Bi$_2$Se$_3$ films of several thicknesses, allowing us to separate the bulk from the surface transient response. We find that for thinner films the photoexcitation changes the transport scattering rate and reduces the THz conductivity, which relaxes within 10 picoseconds (ps). For thicker films, the conductivity increases upon photoexcitation and scales with increasing both the film thickness and the optical fluence, with a decay time of approximately 5 ps as well as a much higher scattering rate. These different dynamics are attributed to the surface and bulk electrons, respectively, and demonstrate that long-lived mobile surface photo-carriers can be accessed independently below certain film thicknesses for possible optoelectronic applications.
Scanning tunneling microscopy and spectroscopy have been used to investigate the femtosecond dynamics of Dirac fermions in the topological insulator Bi$_2$Se$_3$ ultrathin films. At two-dimensional limit, bulk electrons becomes quantized and the quantization can be controlled by film thickness at single quintuple layer level. By studying the spatial decay of standing waves (quasiparticle interference patterns) off steps, we measure directly the energy and film thickness dependence of phase relaxation length $l_{phi}$ and inelastic scattering lifetime $tau$ of topological surface-state electrons. We find that $tau$ exhibits a remarkable $(E-E_F)^{-2}$ energy dependence and increases with film thickness. We show that the features revealed are typical for electron-electron scattering between surface and bulk states.
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