Session D41: Topological Insulators: Engineered Structures I

Plasma Etching Effects on the Transport in Topological Insulator Bi$_2$Te$_3$ Nanoplates

Carrier transport in various topological insulators (TIs) such as Bi$_2$Se$_3$ and Bi$_2$Te$_3$ exhibits a novel linear magneto-resistance (LMR) [1] in addition to the more extensively studied weak anti-localization effect. The robustness against raising temperature and 2D nature of this LMR [1] allude to its connection with the topologically protected 2D surface transport in TI. In this work, we study how the plasma etching induced surface roughness or corrugation impacts the transport in TI Bi$_2$Te$_3$ nanoplates, to understand how the topological surface transport responds to controlled perturbation to material surface. Bi$_2$Te$_3$ nanoplates with varied thickness were grown using CVD method and hall bar devices were studied under different Argon plasma etching conditions. Our experiments show that plasma etching induces drastic change in the Hall coefficient but has relatively weak effect on the LMR. We will also discuss the data analyzed by the two band carrier m! a ngo-transport model which allows quantitative separation of the surface carrier concentration and mobility from the bulk carriers.   

Electrostatic Coupling Between the Surface States of a Topological Insulator

We report electronic transport measurements on nanofabricated topological insulator Bi$_{\mathrm{1.5}}$Sb$_{\mathrm{0.5}}$Te$_{\mathrm{1.7}}$Se$_{\mathrm{1.3}}$ exfoliated devices with electrostatic top- and bottom-gate electrodes. We observe independent, ambipolar modulation of the device resistance on both the top and bottom surfaces. On thin devices, the bottom-gate capacitively couples to the top surface, indicating poor bulk screening which allows for surface-to-surface electrostatic coupling. We explain the data through a capacitance model and extract information about the surface and bulk density of states. Additionally, we show that the ambipolarity of the surface state resistance persists up to room temperature.   

Helical mode and supercurrent measured on the topological surface states of Bi$_{2}$Te$_{3}$ nanoribbon field effect devices

Topological superconductivity can be proximity induced by coupling s-wave superconductors with spin-helical electron systems, such as the surface of 3D topological insulators (TIs), where the energy bands follow Dirac dispersion and the electronic states possess helical spin-momentum locking. We have grown Bi$_{2}$Te$_{3}$ nanoribbons (NRs) by vapor liquid solid method and characterized their crystalline structure by TEM and Raman spectroscopy. We fabricate backgated field effect devices where the chemical potential ($\mu )$ can be tuned from bulk bands to surface states and ambipolar field effect has been observed. The temperature dependence of the resistance and Shubnikov de Haas oscillations show suppressed bulk conduction with surface conduction dominating and a pi-Berry's phase. The Aharonov--Bohm oscillations (ABO), measured with a magnetic field parallel to the NR axis, have a period equal to one flux quanta with conductance maxima at half flux quanta (pi-ABO), for $\mu $ close to the charge neutrality point. Such pi-ABO is a direct evidence of the existence of 1D helical modes at half flux quanta. We have also fabricated Josephson junctions on our TI NR devices with inter-electrode separations up to 200 nm, and measured supercurrent with a proximity induced gap of 0.5meV at 0.25K.   

Topological insulator nanowires and nanowire hetero-junctions

The existing topological insulator materials (TIs) continue to present a number of challenges to complete understanding of the physics of topological spin-helical Dirac surface conduction channels, owing to a relatively large charge conduction in the bulk. One way to reduce the bulk contribution and to increase surface-to-volume ratio is by nanostructuring. Here we report on the synthesis and characterization of Sb$_2$Te$_3$, Bi$_2$Te$_3$ nanowires and nanotubes and Sb$_2$Te$_3$/Bi$_2$Te$_3$ heterojunctions electrochemically grown in porous anodic aluminum oxide (AAO) membranes with varied (from 50 to 150 nm) pore diameters. Stoichiometric rigid polycrystalline nanowires with controllable cross-sections were obtained using cell voltages in the 30 - 150 mV range. Transport measurements in up to 14 T magnetic fields applied along the nanowires show Aharonov-Bohm (A-B) quantum oscillations with periods corresponding to the nanowire diameters. All nanowires were found to exhibit sharp weak anti-localization (WAL) cusps, a characteristic signature of TIs. In addition to A-B oscillations, new quantization plateaus in magnetoresistance (MR) at low fields ($< 0.7~\textrm{T}$) were observed. The analysis of MR as well as $I-V$ characteristics of heterojunctions will be presented.   

Doping control and spatially resolved optoelectronics of Bi2Se3 nanowires and nanoribbons

Bi2Se3 has been predicted to be a 3D topological insulator with chiral surface states protected by time-reversal symmetry. Single crystalline Bi2Se3 nanowires and nanoribbons were synthesized via a vapor-liquid-solid approach. Carrier concentrations can be tuned in a wide range by varying Se vapor pressure during the growth. High carrier mobilities up to 200 cm$^{2}$/Vs at room temperature and 1000 cm$^{2}$/Vs at 2 K were achieved. A surface conduction channel was identified from the temperature dependent transport measurement. Magnetoresistance measurement showed a signature weak anti-localization peak of the chiral surface states. Scanning photocurrent microscopy (SPCM) study of these nanoribbons showed an alternating photocurrent polarity with a length scale of 1 um, which indicates a potential variation on the surface of these nanoribbons, despite the high crystallinity confirmed by transmission electron microscope. Kelvin probe microscopy was used to characterize such surface potential variation of these nanowires and nanoribbons. We will discuss the possible origin of this surface potential variation.   

Observation of Robust Surface States in Highly-Disordered Topological Insulator Nanotubes

We have studied electrical transport properties of candidate topological insulator (TI) Bismuth Telluride (Bi$_{\mathrm{2}}$Te$_{\mathrm{3}})$ nanotubes at low temperatures and high magnetic fields. Bi$_{\mathrm{2}}$Te$_{\mathrm{3}}$ nanotube samples were synthesized by solution phase method, with the outer diameters in the range of 90 $\sim$ 200 nm and wall thickness 10 $\sim$ 15 nm, and typical length of over 10 $\mu $m. Focused ion beam (FIB) assisted deposition and e-beam lithography were applied to fabricate Ohmic contacts. Thermal conductivity measurements show the nanotubes have similar carrier concentration to other metallic nanowires and ribbons, while the nanotubes have insulating behavior, which is due to disorder. For the highly disordered samples, strong quantum oscillations in magnetoresistance were observed in parallel field, with an h/e period associated with the outer surface of the nanotubes. Detailed analysis indicates that the oscillations are due to Anomalous Aharonov-Bohm Effect originating from Dirac-like TI surface states. The relationship between oscillation and disorder will be discussed.   

Unusual behavior of the surface states in the topological insulator-magnetic insulator heterostructure

Topological insulator is a new class of solids that possess non-trivial topology in electronic structures. Spin-polarized conducting states should develop at the interface between topological insulators and trivial insulators as dictated by the topological invariants associated with the time-reversal symmetry. As such, the conducting surface states are very robust to impurities but susceptible to magnetic impurities that destroy their spin-momentum helical structures. We study the behavior of the surface states when magnetic impurity layers are deposited on top of the topological insulator surface, (Sb2Te3-MnTe), using first-principles calculations. We find that the helical nature of the surface states persists even at the presence of magnetic impurity layers and that the energy gap at the Dirac point due to the magnetic layers exhibits unusual behavior as the density of magnetic impurities is changed. We derive a model Hamiltonian based on Anderson model to describe the interaction of the surface states and magnetic-impurity layers and explain the behavior of the surface states. We show that the coupling between the surface states, d-orbitals of the magnetic impurities, and the RKKY-type interactions in magnetic impurities determine the energy gap of the surface states as well as the magnetic ordering in the impurity layer.   

Spin Transfer Torque Generated by the Topological Insulator Bismuth Selenide

We measure large spin-transfer torques generated by in-plane currents in thin films of the topological insulator bismuth selenide at room temperature. We use spin-torque ferromagnetic resonance in Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$/Ni$_{\mathrm{81}}$Fe$_{\mathrm{19}}$ bilayers to determine that the spin-torque arising from the Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ and acting on the Ni$_{\mathrm{81}}$Fe$_{\mathrm{19}}$ layer possesses substantial vector components both in the sample plane and perpendicular to the plane. The out-of-plane torque is several times larger than expected from the Oersted field, and the efficiency of in-plane (anti-damping) spin torque generation per unit current density in the Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ is greater than has been observed in any other material.   

Spin-Transfer Torques in Dual-Gated Bismuth Selenide Topological Insulator Devices

Recent theoretical and experimental work on topological insulator / ferromagnet bilayers suggests that bismuth selenide can act as a source of spin current for applying a spin transfer torque to an adjacent magnetic layer. To help determine the mechanism of the in-plane and out-of-plane spin torques, we fabricate dual-gated bismuth selenide devices with a ferromagnetic permalloy nanowire positioned between the gates to act as an absorber of spin currents. We use the spin-torque ferromagnetic resonance technique to measure current-induced torques acting on the permalloy nanowire. We will attempt to distinguish between surface and bulk mechanisms for the torque by sweeping a uniform voltage applied to both gates to tune the carrier density. We will also study whether the surface spin current can be modified by applying different gate voltages to induce a large gradient in the electron chemical potential near the permalloy wire. Such a modification is expected as a consequence of locking between the orientations of the electron wavevector and spin in topological insulator surface states.   

Voltage driven magnetic bifurcations in nanomagnet-topological insulator composite structure

Multiplicity of magnetization dynamics in thin ferromagnetic insulator (FMI) deposited on topological insulator (TI) has been studied as an effect of electron flow through the interface. The intrinsic spin polarization of TI surface current evokes the magnetization precession, which in turn modifies the TI electron spin polarization and current intensity. The net effect of this self-consistent behavior of FMI magnetization and TI surface itinerant electrons results in auto oscillations, magnetization reversal or magnetic deviation from equilibrium state according to the applied DC voltage. These phenomena are also accompanied with strong anomalous Hall effect and they are separated by the threshold voltages of magnetization bifurcations. Comparisons with spin transfer torque and spin-Hall-based mechanisms of magnetization reversal/oscillation reveal significant advantage in power efficiency of this proposed electrical control of magnetization dynamics.   

Magnetization Switching via Giant Spin-Orbit Torque in a Magnetically Doped Topological Insulator Heterostructure

The magnetization switching induced by in-plane current in a Chromium-doped topological insulator bilayer heterostructure has been observed and is attributed to a giant spin-orbit toque. The critical current density of around 10$^{\mathrm{4}}$ A/cm$^{\mathrm{2}}$ for magnetization switching is nearly three orders of magnitude lower than in the traditional heavy metal/ferromagnetic heterostructures. The effective magnetic field arising from the spin-orbit torque is also increased by three orders. This giant spin-orbit torque and efficient current-induced magnetization switching may lead to innovative spintronics applications such as ultra-low power dissipation memory and logic devices.   

Transport Studies of Thin Film Magnetic Topological Insulator Nanostructures

Ferromagnetic order in a topological insulator breaks time-reversal symmetry, opening a gap in the surface states and giving rise to a number of exotic phenomena, including dissipationless chiral edge conduction along domain walls. Thin films of the topological insulator (Bi,Sb)$_2$Te$_3$ doped with chromium exhibit ferromagnetic ordering that is not mediated by bulk carriers, allowing the magnetism to persist in the bulk energy gap [1,2]. Here, we discuss fabrication and transport measurements of nanostructures based on these films. We further discuss the possibility of engineering magnetic domains in the film to study the chiral edge state along the domain wall. \\[4pt] [1] C.-Z. Chang {\it et al.}, Science {\bf 340}, 167 (2013). \\[0pt] [2] X. Kou {\it et al.}, Nano Lett. {\bf 13}, 4587 (2013).   

Magnetic proximity effect induced effects in topological insulator/YIG heterostructure

The broken time-reversal symmetry in topological insulator (TI) can lead to quantized anomalous Hall effect (QAHE). QAHE has recently been observed in TI doped with Cr which turns ferromagnetic at very low temperatures.Here we carry out an experimental study on induced ferromagnetism in heterostructures of a thin TI film (Bi2S$^{\mathrm{e3}})$ and an insulating magneticfilm (YIG). The YIG film is grown by pulsed laser deposition with an atomically flat surface and in-plane magnetic anisotopy, and Bi2S$^{\mathrm{e3}}$films of different thicknesses are grown on YIG ina molecular beam epitaxy system.Excellent crystallinity of TI films is confirmed by RHEED. Th topological surface states from the top TI surface are confirmedby ARPES. In the 3nm TI sample, a non-linear current-voltage is observed at all temperature, indicating the existence of the quantum confinement induced gap. In the 5 nm TI sample, the current-voltage characteristic is linear.The anomalousHall effect (AHE) is observedat low temperatures which clearly demonstrates the magnetic proximity induced magnetic momentin the surface of TI, andthemagnitudestrongly decreass a the temperature increases.Moreover, the positive magnetoresistance is affected bythe induced magnetic layer andthe weak anti-localization effect is clearly weakened. This and the AHE indicate a proximity effect between TI and YIG This research was supported by UC Lab Fees Program.   

Zero field conductance singularity in two terminal ferromagnet-topological insulator device

Spin-momentum interlocking of surface electronic states on 3D topological insulator (TI) grants the unique opportunity to generate electric current directed according to the spin polarization of injected electrons instead of the applied electric field. Such asymmetry in momentum distribution of injected electrons takes place in the vicinity of ferromagnetic contact but vanishes on the length of few mean free passes. We propose to use this property in two terminal devices consisting of two parallel ferromagnetic contacts deposited on the surface of 3D TI. When the injected spin polarization leads to electron momentum pointing towards the other electrode, it facilitate the direct transmission, resulting in a lower resistance; in contrast with a reversed bias, the spin-determined momentum points away from the other electrode, because of which the electrons could gain the right momentum only after multiple scatterings to approach the second electrode, thus resulting in a higher resistance. We stress that this asymmetry in the resistance keeps up to arbitrarily small applied voltage since it does not need the formation of space charge region that is essential in conventional diodes. The rectification ratio near zero voltage are estimated and potential application are discussed.