Session W31: Focus Session: Topological Insulators: Synthesis & Characterization - Quantum Transport & Nanostructures

Evidence for a Dirac Spectrum in the Topological Insulator Bi$_2$Te$_2$Se from High-Field Shubnikov-de Haas Oscillations

The transport properties of surface states in the 3D topological insulators based on bismuth have been observed in a number of experiments. However, there is still no direct evidence for the Dirac dispersion predicted for these states. We have measured the Shubnikov-de Haas (SdH) oscillations in Bi$_2$Te$_2$Se in intense dc fields. At B$>$40 T, we can reach the n=1 surface Landau Level. In the index plot (of 1/B versus n), the relatively large oscillation amplitudes in our crystals (as large as $17\%$ of the total conductance) enables us to resolve, in the high-field limit, the $\frac{1}{2}$-shift predicted from the Berry phase in the Dirac spectrum. In addition, the linearity of the index plot shows that the surface Lande g-factor is quite small (less than 5) in Bi$_2$Te$_2$Se, in contrast with recent inferences based on low-B SdH experiments. \\ Supported by NSF DMR 0819860   

Thin films of topological insulators in a parallel or tilted magnetic field

In thin films of topological insulators, the surface states on the opposite edges are coupled by a tunneling coupling $t$. We discuss the energy spectrum and the transport properties of the system in a magnetic field. When an electron tunnels between the edges, the Lorentz force due to the in-plane magnetic field $\mathbf{B =(0,B_y,0)}$ changes the in-plane electron momentum by $\Delta p_x\propto B_y$. As a result, the Fermi circles on the opposite edges shift by $\Delta p_x$ in the momentum space. We propose that this effect can be detected by measuring the tunneling conductance $\sigma_{zz}(B_y)$ between the edges of the system. We show that $\sigma_{zz}(B_y)$ has special signatures due to the helical spin configuration of the surface Dirac cones. In case of a tilted field $\mathbf{B=(0,B_y,B_z)}$, the perpendicular component $B_z$ quantizes the in-plane motion to the Landau levels, while the in-plane component $B_y$ spatially shifts the wave functions on the different edges. As the overlap between the wave functions changes, the tunneling amplitude $t$ is renormalized and acquires dependence on both $B_y$ and $B_z$. This effect can be observed as the dependence of the interlayer conductance and in-plane conductivity on the tilt angle $\theta$ of the magnetic field tan$\theta=B_y/B_z$.   

Clear Revelation of Topological Surface States in Bi$_{2}$Se$_{3}$ Thin Films by \textit{in situ} Al Passivation

We report that \textit{in situ} aluminum (Al) passivation of Bi$_{2}$Se$_{3}$ can inhibit the degradation process and clearly reveal the massless, Dirac-like topological surface states. In this work, an 8 quintuple layers Bi$_{2}$Se$_{3}$ film was passivated with 2 nm Al, immediately after the MBE growth, which prevents native oxide (BiO$_{x})$ formation, isolates the film from ambient $n$-type doping or contaminations in the subsequent fabrication process. Dual evidences from both Shubnikov-de Hass (SdH) oscillations and weak antilocalization (WAL) effect, originated from the $\pi $ Berry phase of the nontrivial surface states, were clearly revealed in the sample with \textit{in situ} Al passivation. In contrast, we show that the two dimensional carrier density was increased 39.2{\%} for the un-passivated control samples. Also, the SdH oscillations were completely absent and a large deviation from the WAL was observed   

High Field Magnetoresistance in ultra pure Bi2Se3

The longitudinal and transverse components of magnetoresistance were measured in bulk crystals of undoped, high purity Bismuth Selenide in pulsed magnetic fields of up to 60 Tesla. Data is presented from samples with a range of carrier concentrations extending into the quantum limit. Measurements were also performed at multiple angles along the plane containing the current direction to investigate the angular dependence of the linear behavior of the magnetoresistance in this material.   

Surface-dominated conduction in a 6nm-thick Bi2Se3 thin

We report a direct observation of surface dominated conduction in an intrinsic Bi$_{2}$Se$_{3}$ thin film with a thickness of 6 quintuple layers (QLs) grown on lattice-matched CdS (0001) substrates by molecular beam epitaxy (MBE). Shubnikov-de Haas (SdH) oscillations from the topological surface states suggest that the Fermi level falls inside the bulk band gap and is 53 +/-5 meV above the Dirac point, in agreement with 70 +/- 20 meV obtained from scanning tunneling spectroscopies (STS). Our results demonstrate a great potential of producing genuine topological insulator devices using Dirac Fermions of the surface states.   

Magnetoconductance in high-mobility topological insulator Bi2Se3 devices

We report the fabrication and measurement of gate-tunable high mobility exfoliated ($<$100nm thick) Bi$_{2}$Se$_{3}$ devices. We measure electronic transport of these devices in magnetic fields up to 35T, and find a complex pattern of quantum oscillations consistent with both the surface and the bulk channels. We study the dependence of the oscillations on the magnetic field angle and gate voltage and discuss models for coexistence of surface and bulk oscillations.   

Magnetic Field Signatures of Topological States in 3D Time-Reversal Invariant Insulators

While the topological behavior of Bi$_2$Se$_3$ has been identified experimentally\footnote{Y.L. Chen et al., \emph{Science} {\bf 325}, 178 (2009).} \footnote{P. Roushan et al., \emph{Nature} {\bf 460}, 7259 (2010).}, characterization by electron transport has been difficult due to high bulk transport caused by inadvertent doping of the crystal.\footnote{N. P. Butch et al., \emph{Phys. Rev. B.} {\bf 81}, 24 (2010).} We perform self-consistent quantum transport calculations to show that patterned surfaces offer a unique environment in which the system may be characterized by resultant magnetic field distributions. We compare doped and undoped Bi$_2$Se$_3$ samples with normal metals to show a qualitative difference in current flow around the patterned surface. We find that the surface to bulk conductance ratio can be inferred from the magnetic field in patterned systems due to the spatial separation of bulk and surface currents created by the corrugation, which applies even in heavily doped systems. The magnetic field is sufficiently large so as to be observed using ultracold atom microscopy.   

Quantum Hall effect from Dirac fermions of the 3D topological insulator HgTe

Three dimensional (3D) topological insulators (TI) exhibit two dimensional (2D) topologically protected conducting surface states created by strong spin-orbit coupling. Time reversal invariance (TRI) of those states manifest itself in spin-momentum locking and a dispersion relation of massless Dirac fermions. We present our results of magneto-transport of the 3D TI HgTe [1]. Our samples are 70 nm HgTe films strained on slightly Zn doped CdTe substrates. Tensile strain due to the lattice mismatch between the HgTe film and the substrate lifts the heavy hole - light hole degeneracy, which results in TI states at the Brillouin zone center. We observe evidence for a quantized Hall (QH) resistance developing at temperatures below T=50K. The observed effect is confirmed to derive from Dirac fermions of the two TI surfaces as shown through a non-zero Berry's phase by an extrapolation of the filling factors of the QH plateaus to the large magnetic field limit. We have also confirmed the 2D character of the probed states through tilted magnetic field measurements. If time allows, we will discuss our results for very high magnetic fields and dilution refrigeration temperatures experiments. Work supported by the Gordon and Betty Moore Foundation. [1] C. Br\"une et.al., PRL 106, 126803 (2011)   

Topological insulator Bi$_{2}$Te$_{3}$ nanowire field effect devices

Bismuth telluride (Bi$_{2}$Te$_{3})$ has been studied extensively as one of the best thermoelectric materials and recently shown to be a prototype topological insulator with nontrivial conducting surface states. We have grown Bi$_{2}$Te$_{3}$ nanowires by a two-step solution phase reaction and characterized their material and structural properties by XRD, TEM, XPS and EDS. We fabricate both backgated (on SiO$_{2}$/Si) and top-gated (with ALD high-k gate dielectric such as Al$_{2}$O$_{3}$ or HfO$_{2})$ field effect devices on such nanowires with diameters $\sim $50nm. Ambipolar field effect and a resistance modulation of up to 600{\%} at low temperatures have been observed. The 4-terminal resistance shows insulating behavior (increasing with decreasing temperature) from 300~K to 50K, then saturates in a plateau for temperatures below 50K, consistent with the presence of metallic surface state. Aharonov--Bohm (AB) oscillations are observed in the magneto-resistance with a magnetic field parallel to the nanowire, providing further evidence of the presence of surface state conduction Finally, a prominent weak anti-localization (WAL) feature that weakens with increasing magnetic field and/or temperature is observed in the magneto-resistance with a magnetic field perpendicular to the nanowire.   

Analysis of quantum interference in mesoscopic channels of epitaxial Bi$_2$Se$_3$

Predictions of topologically protected surface states lead to expectations of longer scattering lengths from the surface channel in candidate topological insulators such as Bi$_2$Se$_3$. In this context, we probe coherent transport in mesoscopic channels of MBE-grown Bi$_2$Se$_3$ at temperatures down to 0.5K and magnetic fields up to 6T. The magnetoresistance reveals two types of quantum corrections superimposed upon a classical background: low-field weak antilocalization and an aperiodic, reproducible fingerprint. Analysis and comparison of the quantum corrections data are used to extract important length scales and provide insights into the origin of these corrections. The channel length and temperature dependence of the magnetofingerprint is consistent with the theory of universal conductance fluctuations for diffusive systems that are two dimensional in phase coherent phenomena. Periodic oscillations in the autocorrelation of the fingerprint, persistent to high fields and high temperatures, point towards the presence of dominant scattering centers. Work supported by NSF-MRSEC and ONR.   

Synthesis and Transport Measurements of Catalyst-Free Topological Insulator Bi$_{2}$Se$_{3}$ Nanostructures

The semiconductor bismuth selenide (Bi$_{2}$Se$_{3}$) was predicted to be a topological insulator (TI) with a single Dirac cone, which was observed using Angle-Resolved-Photo-Emission-Spectroscopy in 2008. TI's are materials which exhibit electrically insulating properties in the bulk, but they have metallic surface states. The surface states are topologically protected from perturbations, defects, and impurities. Nano-sized structures might be well suited for the study of surface states because the surface effects are likely to dominate over bulk properties due to the high surface-to-volume ratio. So far, nanowires and nanoribbons of TI s have been synthesized using metal catalysts, such as gold, iron, or nickel. However, it has been found that these catalysts dope the nanostructures, which has the potential to modify their properties. We show catalyst-free growth of nanowires and nanoribbons of Bi$_{2}$Se$_{3}$ using the Vapor-Liquid-Solid method. Analysis by EDAX and TEM imaging suggest high purity samples. We have fabricated devices from these nanostructures and present electron transport measurements.   

Topological Insulator Nanoribbon Synthesis by Metal-Organic Chemical Vapor Deposition

We report a method for metal-organic chemical vapor deposition (MOCVD) synthesis of Bi$_2$Se$_3$ topological insulator nanoribbons. We use gold nanoparticles to catalyze nanoribbon growth on silicon substrates. Trimethyl Bismuth and Diethyl Selenium are used as the metal-organic precursors. The growth parameters can be varied to control the morphology from narrow nanoribbons to wide platelets. Resulting nanostructures are characterized by electron diffraction, energy dispersive X-ray spectroscopy, and low-temperature transport measurements. We also investigate the synthesis of ternary compounds using this growth method.   

Electrical, Thermal, and Thermoelectric Characterizations of Vapor Solid Bi$_{2}$Te$_{3}$ Nanoplates

Single-crystal nanoplates of Bi$_{2}$Te$_{3}$ synthesized by the vapor solid method are characterized by electrical, thermal, and thermoelectrical measurements. The Bi$_{2}$Te$_{3}$ domains investigated are less than 12 nm thick and are suspended to remove substrate doping effects. A room temperature thermal conductivity of 1.5 Wm$^{-1}$K$^{-1}$ was measured, lower than the 1.8--3.3 Wm$^{-1}$K$^{-1}$ range reported for bulk crystals with different carrier types and concentrations. The room temperature electrical conductivity was measured at 1.5$\times $10$^{5}$ Sm$^{-1}$. Applying the Wiedemann-Franz Law, the electron contribution to the total thermal conductivity is nearly 40 {\%} at room temperature. The electrical conductivity is similar to that reported for bulk single crystals at an electron concentration of 3.5$\times $10$^{19}$ cm$^{-3}$.~ However, the room-temperature Seebeck coefficient of -66 $\mu $VK$^{-1}$ indicates $n$-type doping and is lower than that reported for $n$-type Bi$_{2}$Te$_{3}$ single crystals at an electron concentration as high as 14.6$\times $10$^{19}$ cm$^{-3}$. Consequently, the figure of merit is only 0.11 at room temperature, a factor of 7.9 lower than the highest ZT reported for $n$-type single crystals at the optimized doping level.   

Hybrid Bismuth Selenide Nanostructures Synthesized by Chemical Vapor Deposition

The recent demonstration of theoretically predicted topologically ordered states in real bismuth based chemical compounds such as bismuth antimony, bismuth selenide, bismuth telluride etc opened the field of topological insulators (materials exhibiting insulator properties in bulk, but metallic behavior on the surface) for a plethora of possible applications. Topological insulator nanostructures in particular are of great interest due to their large surface to volume ratio. We will report on the CVD synthesis of various hybrid 1D and 2D nanostructures of the bismuth-selenium complex, and their morphological and structural properties (investigated by SEM and TEM imaging coupled with EDAX and XRD spectroscopy). Optical and transport properties will be also presented and related to possible spintronics and no dissipation electronics applications.   

Electronic transport of Sb-doped Bi$_{2}$Se$_{3}$ topological insulator nanoribbons

Vapor-liquid-solid (VLS) grown nanoribbons, having large surface / volume ratio and high crystal quality, provide a unique opportunity to study topological insulator materials by electronic transport. However, clear observation of the surface states is often hindered due to materials' imperfections. Bulk impurities and environmental doping effects are known to contribute to dominant background transport signal, so that appropriate doping and surface protection are necessary to reduce the excess carriers. We report that Antimony (Sb), known to be an effective compensational dopant for bulk crystals, can be incorporated into Bi$_{2}$Se$_{3}$ nanoribbons and it reduces the bulk electron contribution significantly. With a Zinc Oxide protective layer, the carrier density of thin ribbons reaches below 10$^{12}$cm$^{-2}$. This talk will include magnetotransport studies and temperature dependant transport of nanoribbons as well.