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Register a new userLarge linear magnetoresistance from neutral defects in Bi$_2$Se$_3$
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The chalcogenide Bi$_2$Se$_3$ can attain the three dimensional (3D) Dirac semimetal state under the influence of strain and microstrain. Here we report the presnece of large linear magnetoresistance in such a Bi$_2$Se$_3$ crystal. The magnetoresistance has quadratic form at low fields which crossovers to linear above 4 T. The temperature dependence of magnetoresistance scales with carrier mobility and the crossover field scales with inverse of mobility. Our analysis suggest that the linear magnetoresistance in our system has a classical origin and arises from the scattering of high mobility 3D Dirac electrons from crystalline inhomogeneities. We observe that the charged selenium vacancies are strongly screened by high mobility Dirac electrons and the neutral crystalline defects are the main scattering center for transport mechanism. Our analysis suggests that both the resistivity and the magnetoresistance have their origin in scattering of charge carriers from neutral defects.
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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.
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.
Proximity-induced magnetic effects on the surface Dirac spectra of topological insulators are investigated by scanning tunneling spectroscopic (STS) studies of bilayer structures consisting of undoped Bi2Se3 thin films on top of Cr-doped Bi2Se3 layers. For thickness of the top Bi2Se3 layer equal to or smaller than 3 quintuple layers (QL), a spatially inhomogeneous surface spectral gap Delta opens up below T_c^{2D}, which is much higher than the bulk Curie temperature T_c^{3D}. The mean value and spatial homogeneity of the gap Delta generally increase with increasing c-axis magnetic field (H) and increasing Cr doping level (x), suggesting that the physical origin of this surface gap is associated with proximity-induced c-axis ferromagnetism. On the other hand, the temperature (T) dependence of Delta is non-monotonic, showing an initial increase below T_c^{2D} followed by a dip and then reaching maximum at T << T_c^{3D}. These phenomena may be attributed to proximity magnetism induced by two types of contributions with different temperature dependence: a 3D contribution from the bulk magnetism that dominates at low T, and a 2D contribution associated with the RKKY interactions mediated by surface Dirac fermions, which dominates at T_c^{3D} << T < T_c^{2D}. Additionally, spatially localized sharp resonant spectra are found along the boundaries of gapped and gapless regions. These spectral resonances are long-lived at H = 0 and become suppressed under strong c-axis magnetic fields, and are attributed to magnetic impurity-induced topological defects in the spin texture of surface Dirac fermions.
We report the observation of quantum Hall effect (QHE) in a Bi$_2$Se$_3$ single crystal having carrier concentration ($n$) $sim1.13times10^{19}$cm$^{-3}$, three dimensional Fermi surface and bulk transport characteristics. The plateaus in Hall resistivity coincide with minima of Shubnikov de Haas oscillations in resistivity. Our results demonstrate that the presence of perfect two dimensional transport is not an essential condition for QHE in Bi$_2$Se$_3$. The results of high resolution x-ray diffraction (HRXRD), energy-dispersive x-ray spectroscopy (EDX), and residual resistivity measurements show the presence of enhanced crystalline defects and microstrain. We propose that the formation of localized state at the edge of each Landau level due to resonance between the bulk and defect band of Bi$_2$Se$_3$ causes the quantum Hall effect.
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