Session X29: Topological Insulator - Bi-based Materials

Hidden landscapes in thin film topological insulators: between order and disorder, 2D and 3D, normal and topological phases

Topological insulator (TI) is one of the rare systems in the history of condensed matter physics that is initiated by theories and followed by experiments. Although this theory-driven advance helped move the field quite fast despite its short history, apparently there exist significant gaps between theories and experiments. Many of these discrepancies originate from the very fact that the worlds readily accessible to theories are often far from the real worlds that are available in experiments. For example, the very paradigm of topological protection of the surface states on Z2 TIs such as Bi2Se3, Bi2Te3, Sb2Te3, etc, is in fact valid only if the sample size is infinite and the crystal momentum is well-defined in all three dimensions. On the other hand, many widely studied forms of TIs such as thin films and nano-wires have significant confinement in one or more of the dimensions with varying level of disorders. In other words, many of the real world topological systems have some important parameters that are not readily captured by theories, and thus it is often questionable how far the topological theories are valid to real systems. Interestingly, it turns out that this very uncertainty of the theories provides additional control knobs that allow us to explore hidden topological territories. In this talk, I will discuss how these additional knobs in thin film topological insulators reveal surprising, at times beautiful, landscapes at the boundaries between order and disorder, 2D and 3D, normal and topological phases.   

Finite-size driven topological and metal-insulator transition in (Bi$_{1-x}$In$_{x})_{2}$Se$_{3\, }$thin films

In a topological insulator (TI), if one of its heavy elements is replaced by a light one, the spin-orbit coupling (SOC) strength decreases and eventually the TI transforms into a normal insulator beyond a critical level of substitution.This is the standard description of the topological phase transition (TPT). However, this notion of TPT, driven solely by the SOC (or something equivalent), is not complete for finite size samples considering that the thickness of the topological surface states diverges at the critical point. Here, on specially-engineered (Bi$_{x}$In$_{1-x})_{2}$Se$_{3}$ thin films, using systematic transport measurments we show that not only the SOC but also the finite sample size can induce TPT. This study sheds light on the role of spatial confinement as an extra tuning parameter controlling the topological critical point.   

Quantum transport of two-species Dirac fermions in dual-gated three-dimensional topological insulators

Topological insulators (TI) are a novel class of quantum matter with a gapped insulating bulk yet gapless spin helical Dirac fermion conducting surface states. Here, we report local and non-local electrical and magneto transport measurements in dual-gated $BiSbTeSe_2$ thin film TI devices, with conduction dominated by the spatially separated top and bottom surfaces, each hosting a single species of Dirac fermions with independent gate control over the carrier type and density. We observe many intriguing quantum transport phenomena in such a fully-tunable two-species topological Dirac gas, including a zero-magnetic-field minimum conductivity of ~$4e^{2}/h$ at the double Dirac point, a series of ambipolar two-component "half-integer" Dirac quantum Hall states and an electron-hole total filling factor $\nu$=0 state (with a zero-Hall plateau), exhibiting dissipationless (chiral) and dissipative (non-chiral) edge conduction respectively. Such a system paves the way to explore rich physics ranging from topological magnetoelectric effects to exciton condensation.   

Separation of quantum oscillations from bulk and topological surface states in metallic Bi$_2$Se$_{2.1}$Te$_{0.9}$

Shubnikov-de Haas (SdH) oscillations in metallic Bi$_2$Se$_{2.1}$Te$_{0.9}$ are studied in magnetic fields up to 35 Tesla. It is demonstrated that two characteristic frequencies determine the quantum oscillations of the conductivity. Angle dependent measurements and calculations of the Berry phase show that the two frequencies $F_1$ and $F_2$ describe oscillations from surface and bulk carriers, respectively. At low magnetic fields, only SdH oscillation from topological surface states can be detected whereas at high magnetic field the bulk oscillations dominate. The origin of the separation of bulk and surface SdH oscillations into different magnetic field ranges is revealed in the difference of the cyclotron masses $m_c$. The bulk $m_c$ is nearly three times larger than the surface cyclotron mass resulting in a stronger attenuation of the bulk oscillation amplitude upon decreasing magnetic field. This makes it possible to detect and characterize the surface SdH oscillations in the low-field range.   

Record surface state mobility and quantum Hall effect in topological insulator thin films via interface engineering

Thin films of topological insulators (TIs) with conduction dominated by high mobility topological surface state (TSS) channel have been difficult to achieve due to increased material defects, thus making it difficult to probe TIs in quantum regime. Here by utilizing a structurally matched buffer layer based on In$_{2}$Se$_{3}$, we have achieved Bi$_{2}$Se$_{3}$ films with low defect density resulting in `order of magnitude' improvement in mobilities and carrier densities. This has led to TSS dominated transport and first observation of quantum Hall effect in Bi$_{2}$Se$_{3}$.   

\textbf{Differentiation of surface and bulk conductivities in topological insulator via four-probe spectroscopy}

The direct measurement of the topological surface states (TSS) conductivity is often hard to achieve due to the pronounced contribution from the bulk conduction channel. Here, we show a new method to differentiate conductivities from the surface states and the coexisting bulk states in topological insulators (TI) using a four-probe transport spectroscopy in a multi-probe scanning tunneling microscopy system. In contrast to conventional models that assume two resistors in parallel to count for both the TSS and bulk conductance channels, we derive a scaling relation of measured resistance with respect to varying inter-probe spacing for two interconnected conduction channels, which allows quantitative determination of conductivities from both channels. Using this method, we demonstrate the separation of 2D and 3D conduction in TI by comparing the conductance scaling of Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$,$_{\mathrm{\thinspace }}$Bi$_{\mathrm{2}}$Te$_{\mathrm{2}}$Se, and Sb-doped Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ with that of a pure 2D conductance of graphene on SiC substrate. We also quantitatively show the effect of surface doping carriers on the 2D conductance enhancement in TI. The method offers an approach to understanding not just the topological insulators but also the 2D to 3D crossover of conductance in other complex systems.   

High magnetic field Shubnikov-de Haas oscillations in BiTeCl

We report high magnetic field Shubnikov-de Haas oscillations on single crystals of the bulk Rashba compound BiTeCl. Effective mass and scattering rate extracted from temperature and magnetic field dependence of the oscillations amplitude are in good agreement with our previous optical measurements. The present work will focus on the angular dependence study. First, we notice that oscillations were detected for all our measured angles between the magnetic field and the crystallographic z-axis, $0\leq\theta\leq120^{\circ}$. This is consistent with a 3D Fermi surface and, in good agreement with optical data, confirms that oscillations originate from bulk carriers. Second, the frequency has unusual angular dependence around $\theta=90^{\circ}$. We will show that this behavior is consistent with a torus shaped Fermi surface, providing direct evidence for a Rashba spin-split bulk conduction band. Moreover, we extract reliably the bulk Rashba parameters and the anisotropy of BiTeCl.   

Quantized steps and topological nature of universal conductance fluctuation in Bi$_{2}$Te$_{2}$Se

Here we report the experimental observation of universal conductance fluctuations (UCF) in Bi$_{2}$Te$_{2}$Se. Four aspects were addressed to support the UCF's topological nature of the electronic state. i) The irregular fluctuations are repeatable in different temperature and reversal magnetic fields. ii) All the UCF features coincide after the field is normalized to the perpendicular direction. This points to a two-dimensional electronic state. iii) A parallel field is applied to suppress the bulk coherent paths, while the UCF features stays similar. This excludes a quasi-2D bulk state. iv). The intrinsic UCF magnitude is extracted, which is close to the predicted values of a topological surface state. v). Quantized steps of the UCF magnitudes are observed when the magnetic field is modulated. (\textit{Sci.Rep.} 2012, 2,595; \textit{Appl. Phys. Expre.} 2014,7,065202; arxiv 2015)   

Edge states and integer quantum Hall effect in topological insulator thin films

The integer quantum Hall effect is a topological state of quantum matter in two dimensions, and has recently been observed in three-dimensional topological insulator thin films. In this report, I will talk about the Landau levels and edge states of surface Dirac fermions in topological insulators under a strong magnetic field. We examine the formation of the quantum plateaux of the Hall conductance and find two different patterns, in one pattern the filling number covers all integers while only odd integers in the other. We focus on the quantum plateau closest to zero energy and demonstrate the breakdown of the quantum spin Hall effect as a result of the interplay of magnetic field and structure inversion asymmetry. We also reveal that the edge states exist only for the integer Hall conductance while no edge-state solution can be found for the "half-integer" Hall conductance. The addition of top and bottom surface Dirac fermions always form well-defined edge states, and gives an integer quantum Hall effect. This work establishes an intuitive picture of the edge states to understand the integer quantum Hall effect for Dirac electrons in topological insulator thin films.   

Duo gating on a 3D topological insulator - independent tuning of both topological surface states

ABSTRACT: Topological insulators are associated with a trove of exciting physics, such as the ability to host robust anyons, Majorana Bound States, which can be used for quantum computation. For future Majorana devices\footnote{L. Fu, C.L. Kane,Phys. Rev. Lett {\bf 100}, 096407 (2008).} it is desirable to have the Fermi energy tuned as close as possible to the Dirac point of the topological surface state. Based on previous work on gating BSTS\footnote{Y. Xu et al.Nat. Phys.{\bf 10}, 956-963 (2014).} \footnote{R. Yoshimi et al., Nat. Comm. {\bf 6}, 6627 (2015).}, we report the experimental progress towards gate-tuning of the top and bottom topological surface states of BiSbTeSe$_2$ crystal flakes. When the Fermi level is moved across the Dirac point conduction is shown to change from electron dominated transport to hole dominated transport independently for either surface. In the high magnetic field, one can tune the system precisely between the different landau levels of both surfaces, thus a full gating map of the possible landau levels combination is established. In addition, we provide a simple capacitance model to explain the general hysteresis behaviors in topological insulator systems.   

Weak antilocalization in Bi$_{\mathrm{2-}}_{x}$In$_{x}$Te$_{\mathrm{3}}$ single crystals

Bi$_{\mathrm{2}}$Te$_{\mathrm{3}}$ has recently been identified as one of the most promising systems with which to realize a three-dimensional topological insulator. However, the bulk, stoichiometric Bi$_{\mathrm{2}}$Te$_{\mathrm{3}}$ single crystals often exhibit $p$-type metallic electrical conduction due to the Bi$_{\mathrm{Te}}$-type antisite defects, which overshadows the contribution of surface states. We have established that, upon group III (indium and/or thallium) doping, the Fermi level of Bi$_{\mathrm{2}}$Te$_{\mathrm{3}}$ can be lifted from the valence band into the band gap, and eventually shifted into the conduction band. Such doping progressively changes the electrical conduction of Bi$_{\mathrm{2-}}_{x}$A$_{x}$Te$_{\mathrm{3}}$ (A $=$ In, Tl, and $x \quad =$ 0 $-$ 0.30) single crystals from $p$-type to $n$-type. This is observed via measurements of both the Hall effect and the Seebeck coefficient performed in the (0001) basal plane in the temperature range of 2 $-$ 300 K. At low levels, the temperature dependent in-plane electrical resistivity maintains its metallic character as the density of holes decreases. Heavier doping content, $x \quad =$ 0.20 (0.10) for In (Tl), drives the electrical resistivity into a prominent non-metallic regime displaying the weak anti-localization type of magnetoresistance at the lowest temperatures for Bi$_{\mathrm{1.80}}$In$_{\mathrm{0.20}}$Te$_{\mathrm{3}}$. At the highest concentration, the samples revert back into the metallic state with electron dominated conduction. Thermal conductivity measurements of Bi$_{\mathrm{2-}}_{x}$A$_{x}$Te$_{\mathrm{3}}$ single crystals, as examined by the Debye-Callaway phonon conductivity model, reveal a generally stronger point defect scattering of phonons upon doping.   

Induced Superconductivity In Bi2Se3 Nanostructures By Anneal Doping Of Palladium

Utilizing thermal annealing at temperatures in excess of 100 Celsius we induce superconductivity in Bi2Se3 by palladium doping. Changes in the material structure are analyzed using a combination of AFM, optical microscopy and Raman spectroscopy. The absorption of Pd results in superconductivity in the material with a transition temperature below 1K. The differential conductance as a function of temperature and magnetic field reveals multiple transitions in the material at several applied currents.   

Chromium Doping of the Topological Insulator Bi$_{\mathrm{1.5}}$Sb$_{\mathrm{0.5}}$Te$_{\mathrm{1.7}}$Se$_{\mathrm{1.3}}$

A major materials science challenge is to minimize bulk conductivity in topological insulators so that topological surface state physics can be cleanly accessed. One solution to this problem has been the development of quaternary bismuth chalcogenides Bi$_{\mathrm{2-x}}$Sb$_{\mathrm{x}}$Te$_{\mathrm{3-y}}$Se$_{\mathrm{y}}$ (BSTS) which can be tuned to place the mid-gap Fermi level near the Dirac point associated with the topological surface state. This is ideal for accessing interesting topological physics including the exotic magnetoelectric effects associated with breaking time reversal symmetry. With this goal in mind, we have grown Cr-doped crystals of Bi$_{\mathrm{1.5}}$Sb$_{\mathrm{0.5}}$Te$_{\mathrm{1.7}}$Se$_{\mathrm{1.3}}$ to assess the impact of magnetic dopants on the electronic and magnetic properties of this material. For 4 percent Cr doping we find electronic structure modifications measured by angle-resolved ultraviolet photoelectron spectroscopy and observe magnetic ordering below 50 K in bulk magnetometry. Higher doping levels show evidence of phase segregation.