49 research outputs found
Current-induced magnetization switching in a magnetic topological insulator heterostructure
We present the current-induced switching of the internal magnetization
direction in a magnetic topological insulator/topological insulator
heterostructure in the quantum anomalous Hall regime. The switching process is
based on the bias current dependence of the coercive field, which is attributed
to the effect of the spin-orbit torque provided by the unpolarized bias
current. Increasing the bias current leads to a decrease in the magnetic order
in the sample. When the applied current is subsequently reduced, the magnetic
moments align with an externally applied magnetic field, resulting in
repolarization in the opposite direction. This includes a reversal of the spin
polarisation and hence a reversal of the chiral edge mode. Possible
applications in spintronic devices are discussed.Comment: 6 pages, 3 figures (5 pages and 5 figures in supplementary
information
Electrical resistance of individual defects at a topological insulator surface
Three-dimensional topological insulators host surface states with linear
dispersion, which manifest as a Dirac cone. Nanoscale transport measurements
provide direct access to the transport properties of the Dirac cone in real
space and allow the detailed investigation of charge carrier scattering. Here,
we use scanning tunnelling potentiometry to analyse the resistance of different
kinds of defects at the surface of a (Bi0.53Sb0.47)2Te3 topological insulator
thin film. The largest localized voltage drop we find to be located at domain
boundaries in the topological insulator film, with a resistivity about four
times higher than that of a step edge. Furthermore, we resolve resistivity
dipoles located around nanoscale voids in the sample surface. The influence of
such defects on the resistance of the topological surface state is analysed by
means of a resistor network model. The effect resulting from the voids is found
to be small compared to the other defects
Lifting the spin-momentum locking in ultra-thin topological insulator films
Three-dimensional (3D) topological insulators (TIs) are known to carry 2D
Dirac-like topological surface states in which spin-momentum locking prohibits
backscattering. When thinned down to a few nanometers, the hybridization
between the topological surface states at the top and bottom surfaces results
in a topological quantum phase transition, which can lead to the emergence of a
quantum spin Hall phase. Here, we study the thickness-dependent transport
properties across the quantum phase transition on the example of
(BiSb)Te films, with a four-tip scanning tunnelling
microscope. Our findings reveal an exponential drop of the conductivity below
the critical thickness. The steepness of this drop indicates the presence of
spin-conserving backscattering between the top and bottom surface states,
effectively lifting the spin-momentum locking and resulting in the opening of a
gap at the Dirac point. Our experiments provide crucial steps towards the
detection of quantum spin Hall states in transport measurements
Gate-induced decoupling of surface and bulk state properties in selectively-deposited BiTe nanoribbons
Three-dimensional topological insulators (TIs) host helical Dirac surface
states at the interface with a trivial insulator. In quasi-one-dimensional TI
nanoribbon structures the wave function of surface charges extends
phase-coherently along the perimeter of the nanoribbon, resulting in a
quantization of transverse surface modes. Furthermore, as the inherent
spin-momentum locking results in a Berry phase offset of of
self-interfering charge carriers an energy gap within the surface state
dispersion appears and all states become spin-degenerate. We investigate and
compare the magnetic field dependent surface state dispersion in selectively
deposited BiTe TI micro- and nanoribbon structures by analysing the
gate voltage dependent magnetoconductance at cryogenic temperatures. While in
wide microribbon devices the field effect mainly changes the amount of bulk
charges close to the top surface we identify coherent transverse surface states
along the perimeter of the nanoribbon devices responding to a change in top
gate potential. We quantify the energetic spacing in between these quantized
transverse subbands by using an electrostatic model that treats an initial
difference in charge carrier densities on the top and bottom surface as well as
remaining bulk charges. In the gate voltage dependent transconductance we find
oscillations that change their relative phase by at half-integer values
of the magnetic flux quantum applied coaxial to the nanoribbon, which is a
signature for a magnetic flux dependent topological phase transition in narrow,
selectively deposited TI nanoribbon devices.Comment: 11 pages, 5 figure
Probing edge state conductance in ultra-thin topological insulator films
Quantum spin Hall (QSH) insulators have unique electronic properties,
comprising a band gap in their two-dimensional interior and one-dimensional
spin-polarized edge states in which current flows ballistically. In scanning
tunneling microscopy (STM), the edge states manifest themselves as a localized
density of states. However, there is a significant research gap between the
observation of edge states in nanoscale spectroscopy, and the detection of
ballistic transport in edge channels which typically relies on transport
experiments with microscale lithographic contacts. Here, we study few-layer
films of the three-dimensional topological insulator
(BiSbTe, for which a topological transition to a
two-dimensional topological QSH insulator phase has been proposed. Indeed, an
edge state in the local density of states is observed within the band gap. Yet,
in nanoscale transport experiments with a four-tip STM, 2 and 3 quintuple layer
films do not exhibit a ballistic conductance in the edge channels. This
demonstrates that the detection of edge states in spectroscopy can be
misleading with regard to the identification of a QSH phase. In contrast,
nanoscale multi-tip transport experiments are a robust method for effectively
pinpointing ballistic edge channels, as opposed to trivial edge states, in
quantum materials
Metal-semimetal Schottky diode relying on quantum confinement
Quantum confinement in a semimetal thin film such as bismuth (Bi) can lead to a semimetal-to-semiconductor transition which allows for the use of semimetals as semiconductors when patterned at nanoscale lengths. Bi native oxide on Bi thin film grown by molecular beam epitaxy (MBE) is investigated using X-ray photoelectron spectroscopy (XPS) to measure the elemental composition of the oxide. Also, an in-situ argon plasma etch step is developed allowing for the direct coating of the surface of thin Bi films by a metal contact to form a Schottky junction. Model structures of rhombohedral [111] and [110] bismuth thin films are found from density functional theory (DFT) calculations. The electronic structure of the model thin films is investigated using a GW correction and the formation of an energy band gap due to quantum confinement is found. Electrical characterization of the fabricated Bi-metal Schottky diode confirms a band gap opening in Bi thin film for a film thickness of approximately 5 nm consistent with the theoretical calculations
In-plane magnetic field-driven symmetry breaking in topological insulator-based three-terminal junctions
Topological surface states of three-dimensional topological insulator
nanoribbons and their distinct magnetoconductance properties are promising for
topoelectronic applications and topological quantum computation. A crucial
building block for nanoribbon-based circuits are three-terminal junctions.
While the transport of topological surface states on a planar boundary is not
directly affected by an in-plane magnetic field, the orbital effect cannot be
neglected when the surface states are confined to the boundary of a nanoribbon
geometry. Here, we report on the magnetotransport properties of such
three-terminal junctions. We observe a dependence of the current on the
in-plane magnetic field, with a distinct steering pattern of the surface state
current towards a preferred output terminal for different magnetic field
orientations. We demonstrate that this steering effect originates from the
orbital effect, trapping the phase-coherent surface states in the different
legs of the junction on opposite sides of the nanoribbon and breaking the
left-right symmetry of the transmission across the junction. The reported
magnetotransport properties demonstrate that an in-plane magnetic field is not
only relevant but also very useful for the characterization and manipulation of
transport in three-dimensional topological insulator nanoribbon-based junctions
and circuits, acting as a topoelectric current switch.Comment: Main Text (8 pages, 5 figures) + Supplemental Material (13 pages, 10
figures
Integration of selectively grown topological insulator nanoribbons in superconducting quantum circuits
We report on the precise integration of nm-scale topological insulator
Josephson junctions into mm-scale superconducting quantum circuits via
selective area epitaxy and local stencil lithography. By studying dielectric
losses of superconducting microwave resonators fabricated on top of our
selective area growth mask, we verify the compatibility of this in situ
technique with microwave applications. We probe the microwave response of
on-chip microwave cavities coupled to topological insulator-shunted
superconducting qubit devices and observe a power dependence that indicates
nonlinear qubit behaviour. Our method enables integration of complex networks
of topological insulator nanostructures into superconducting circuits, paving
the way for both novel voltage-controlled Josephson and topological qubits.Comment: 11 pages, 6 figure
Stencil lithography of superconducting contacts on MBE-grown topological insulator thin films
Topological insulator (Bi0.06Sb0.94)2Te3 thin films grown by molecular beam epitaxy have been capped in-situ with a 2 nm Al film to conserve the pristine topological surface states. Subsequently, a shadow mask - structured by means of focus ion beam - was in-situ placed underneath the sample to deposit a thick layer of Al on well-defined microscopically small areas. The 2 nm thin Al layer fully oxidizes after exposure to air and in this way protects the TI surface from degradation. The thick Al layer remains metallic underneath a 3–4 nm thick native oxide layer and therefore serves as (super-) conducting contacts. Superconductor-Topological Insulator-Superconductor junctions with lateral dimensions in the nm range have then been fabricated via an alternative stencil lithography technique. Despite the in-situ deposition, transport measurements and transmission electron microscope analysis indicate a low transparency, due to an intermixed region at the interface between topological insulator thin film and metallic Al