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Register a new userQuantum interference in macroscopic crystals of non-metallic Bi$_2$Se$_3$
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Photoemission experiments have shown that Bi$_2$Se$_3$ is a topological insulator. By controlled doping, we have obtained crystals of Bi$_2$Se$_3$ with non-metallic conduction. At low temperatures, we uncover a novel type of magnetofingerprint signal which involves the spin degrees of freedom. Given the mm-sized crystals, the observed amplitude is 200-500$times$ larger than expected from universal conductance fluctuations. The results point to very long phase breaking lengths in an unusual conductance channel in these non-metallic samples. We discuss the nature of the in-gap conducting states and their relation to the topological surface states.
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Achieving true bulk insulating behavior in Bi$_2$Se$_3$, the archetypal topological insulator with a simplistic one-band electronic structure and sizable band gap, has been prohibited by a well-known self-doping effect caused by selenium vacancies, whose extra electrons shift the chemical potential into the bulk conduction band. We report a new synthesis method for achieving stoichiometric Bi$_2$Se$_3$ crystals that exhibit nonmetallic behavior in electrical transport down to low temperatures. Hall effect measurements indicate the presence of both electron- and hole-like carriers, with the latter identified with surface state conduction and the achievement of ambipolar transport in bulk Bi$_2$Se$_3$ crystals without gating techniques. With carrier mobilities surpassing the highest values yet reported for topological surface states in this material, the achievement of ambipolar transport via upward band bending is found to provide a key method to advancing the potential of this material for future study and applications.
Shubnikov-de-Haas oscillations were studied under high magnetic field in Bi$_2$Se$_3$ nanostructures grown by Chemical Vapor Transport, for different bulk carrier densities ranging from $3times10^{19}text{cm}^{-3}$ to $6times10^{17}text{cm}^{-3}$. The contribution of topological surface states to electrical transport can be identified and separated from bulk carriers and massive two-dimensional electron gas. Band bending is investigated, and a crossover from upward to downward band bending is found at low bulk density, as a result of a competition between bulk and interface doping. These results highlight the need to control electrical doping both in the bulk and at interfaces in order to study only topological surface states.
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.
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.
We performed x-ray magnetic circular dichroism (XMCD) measurements on heterostructures comprising topological insulators (TIs) of the (Bi,Sb)$_2$(Se,Te)$_3$ family and the magnetic insulator EuS. XMCD measurements allow us to investigate element-selective magnetic proximity effects at the very TI/EuS interface. A systematic analysis reveals that there is neither significant induced magnetism within the TI nor an enhancement of the Eu magnetic moment at such interface. The induced magnetic moments in Bi, Sb, Te, and Se sites are lower than the estimated detection limit of the XMCD measurements of $sim!10^{-3}$ $mu_mathrm{B}$/at.
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