27 research outputs found
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
Quantum spin Hall edge states and interlayer coupling in twisted-bilayer WTe
The quantum spin Hall (QSH) effect, characterized by topologically protected
spin-polarized edge states, was recently demonstrated in monolayers of the
transition metal dichalcogenide (TMD) WTe. However, the robustness of this
topological protection remains largely unexplored in van der Waals
heterostructures containing one or more layers of a QSH insulator. In this
work, we use scanning tunneling microscopy and spectroscopy (STM/STS) to
explore the topological nature of twisted bilayer (tBL) WTe which is
produce from folded monolayers, as well as, tear-and-stack fabrication. At the
tBL bilayer edge, we observe the characteristic spectroscopic signature of the
QSH edge state that is absent in topologically trivial as-grown bilayer. For
small twist angles, a rectangular moir\'e pattern develops, which results in
local modifications of the band structure. Using first principles calculations,
we quantify the interactions in tBL WTe and its topological edge states as
function of interlayer distance and conclude that it is possible to tune the
topology of WTe bilayers via the twist angle as well as interlayer
interactions
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
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
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Strong and Weak 3D Topological Insulators Probed by Surface Science Methods
The contributions of surface science methods to discover and improve 3D topological insulator materials are reviewed herein, illustrated with examples from the authors’ own work. In particular, it is demonstrated that spin-polarized angular-resolved photoelectron spectroscopy is instrumental to evidence the spin-helical surface Dirac cone, to tune its Dirac point energy toward the Fermi level, and to discover novel types of topological insulators such as dual ones or switchable ones in phase change materials. Moreover, procedures are introduced to spatially map potential fluctuations by scanning tunneling spectroscopy and to identify topological edge states in weak topological insulators. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei
Suspended dry pick-up and flip-over assembly for van der Waals heterostructures with ultra-clean surfaces
Van der Waals heterostructures are an excellent platform for studying
intriguing interface phenomena, such as moir\'e and proximity effects. Surface
science techniques like scanning tunneling microscopy (STM) have proven a
powerful tool to study such heterostructures but have so far been hampered
because of their high sensitivity to surface contamination. Here, we report a
dry polymer-based assembly technique to fabricate van der Waals
heterostructures with atomically clean surfaces. The key features of our
suspended dry pick-up and flip-over technique are 1) the heterostructure
surface never comes into contact with polymers, 2) it is entirely solvent-free,
3) it is entirely performed in a glovebox, and 4) it only requires temperatures
below 130. By performing ambient atomic force microscopy and
atomically-resolved scanning tunneling microscopy on example heterostructures,
we demonstrate that we can fabricate air-sensitive heterostructures with
ultra-clean interfaces and surfaces. Due to the lack of polymer melting, the
technique is further compatible with heterostructure assembly under ultra-high
vacuum conditions, which promises ultimate heterostructure quality