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Register a new userToward the intrinsic limit of topological insulator Bi2Se3
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Combining high resolution scanning tunneling microscopy and first principle calculations, we identified the major native defects, in particular the Se vacancies and Se interstitial defects that are responsible for the bulk conduction and nanoscale potential fluctuation in single crystals of archetypal topological insulator Bi2Se3. Here it is established that the defect concentrations in Bi2Se3 are far above the thermodynamic limit, and that the growth kinetics dominate the observed defect concentrations. Furthermore, through careful control of the synthesis, our tunneling spectroscopy suggests that our best samples are approaching the intrinsic limit with the Fermi level inside the band gap without introducing extrinsic dopants.
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We investigate the ultrafast transient absorption spectrum of Bi2Se3 topological insulator. Bi2Se3 single crystal is grown through conventional solid-state reaction routevia self-flux method. The structural properties have been studied in terms of high-resolution Powder X-ray Diffraction (PXRD). Detailed Rietveld analysis of PXRD of the crystal showed that sample is crystallized in the rhombohedral crystal structure with a space group of R-3m, and the lattice parameters are a=b=4.14A and c=28.7010A. Scanning Electron Microscopy (SEM) result shows perfectly crystalline structure with layered type morphology which evidenced from surface XRD. Energy Dispersive Spectroscopy (EDS) analysis determined quantitative amounts of the constituent atoms, found to be very close to their stoichiometric ratio. Further the fluence dependent nonlinear behaviour is studied by means of ultrafast transient absorption spectroscopy. The ultrafast spectroscopy also predicts the capability of this single crystal to generate Terahertz (THz) radiations (T-rays).
We present a magneto-infrared spectroscopic study of thin Bi2Se3 single crystal flakes. Magneto-infrared transmittance and reflectance measurements are performed in the Faraday geometry at 4.2K in a magnetic field up to 17.5T. Thin Bi2Se3 flakes (much less than 1{mu}m thick) are stabilized on the Scotch tape, and the reduced thickness enables us to obtain appreciable far-infrared transmission through the highly reflective Bi2Se3 single crystals. A pronounced electron-phonon coupling is manifested as a Fano resonance at the {alpha} optical phonon mode in Bi2Se3, resulting from the quantum interference between the optical phonon mode and the continuum of the electronic states. However, the Fano resonance exhibits no systematic line broadening, in contrast to the earlier observation of a similar Fano resonance in Bi2Se3 using magneto-infrared reflectance spectroscopy.
The nonlinear Hall effect due to Berry curvature dipole (BCD) induces frequency doubling, which was recently observed in time-reversal-invariant materials. Here we report novel electric frequency doubling in the absence of BCD on a surface of the topological insulator Bi2Se3 under zero magnetic field. We observe that the frequency-doubling voltage transverse to the applied ac current shows a threefold rotational symmetry, whereas it forbids BCD. One of the mechanisms compatible with the symmetry is skew scattering, arising from the inherent chirality of the topological surface state. We introduce the Berry curvature triple, a high-order moment of the Berry curvature, to explain skew scattering under the threefold rotational symmetry. Our work paves the way to obtain a giant second-order nonlinear electric effect in high mobility quantum materials, as the skew scattering surpasses other mechanisms in the clean limit.
We measure the temperature-dependent carrier density and resistivity of the topological surface state of thin exfoliated Bi2Se3 in the absence of bulk conduction. When the gate-tuned chemical potential is near or below the Dirac point the carrier density is strongly temperature dependent reflecting thermal activation from the nearby bulk valence band, while above the Dirac point, unipolar n-type surface conduction is observed with negligible thermal activation of bulk carriers. In this regime linear resistivity vs. temperature reflects intrinsic electron-acoustic phonon scattering. Quantitative comparison with a theoretical transport calculation including both phonon and disorder effects gives the ratio of deformation potential to Fermi velocity D/hbarvF = 4.7 {AA}-1. This strong phonon scattering in the Bi2Se3 surface state gives intrinsic limits for the conductivity and charge carrier mobility at room temperature of ~550 {mu}S per surface and ~10,000 cm2/Vs.
We study the manipulation of the photoelectron spin-polarization in Bi$_2$Se$_3$ by spin- and angle-resolved photoemission spectroscopy. General rules are established that enable controlling the spin-polarization of photoemitted electrons via light polarization, sample orientation, and photon energy. We demonstrate the $pm$100% reversal of a single component of the measured spin-polarization vector upon the rotation of light polarization, as well as a full three-dimensional manipulation by varying experimental configuration and photon energy. While a material-specific density-functional theory analysis is needed for the quantitative description, a minimal two-atomic-layer model qualitatively accounts for the spin response based on the interplay of optical selection rules, photoelectron interference, and topological surface-state complex structure. It follows that photoelectron spin-polarization control is generically achievable in systems with a layer-dependent, entangled spin-orbital texture.
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