424 research outputs found
Independent tuning of electronic properties and induced ferromagnetism in topological insulators with heterostructure approach
The quantum anomalous Hall effect (QAHE) has been recently demonstrated in
Cr- and V-doped three-dimensional topological insulators (TIs) at temperatures
below 100 mK. In those materials, the spins of unfilled d-electrons in the
transition metal dopants are exchange coupled to develop a long-range
ferromagnetic order, which is essential for realizing QAHE. However, the
addition of random dopants does not only introduce excess charge carriers that
require readjusting the Bi/Sb ratio, but also unavoidably introduces
paramagnetic spins that can adversely affect the chiral edge transport in QAHE.
In this work, we show a heterostructure approach to independently tune the
electronic and magnetic properties of the topological surface states in
(BixSb1-x)2Te3 without resorting to random doping of transition metal elements.
In heterostructures consisting of a thin (BixSb1-x)2Te3 TI film and yttrium
iron garnet (YIG), a high Curie temperature (~ 550 K) magnetic insulator, we
find that the TI surface in contact with YIG becomes ferromagnetic via
proximity coupling which is revealed by the anomalous Hall effect (AHE). The
Curie temperature of the magnetized TI surface ranges from 20 to 150 K but is
uncorrelated with the Bi fraction x in (BixSb1-x)2Te3. In contrast, as x is
varied, the AHE resistivity scales with the longitudinal resistivity. In this
approach, we decouple the electronic properties from the induced ferromagnetism
in TI. The independent optimization provides a pathway for realizing QAHE at
higher temperatures, which is important for novel spintronic device
applications.Comment: Accepted by Nano Letter
Band structure of topological insulators from noise measurements in tunnel junctions
The unique properties of spin-polarized surface or edge states in topological
insulators (TIs) make these quantum coherent systems interesting from the point
of view of both fundamental physics and their implementation in low power
spintronic devices. Here we present such a study in TIs, through tunneling and
noise spectroscopy utilizing TI/AlO/Co tunnel junctions with bottom TI
electrodes of either BiTe or BiSe. We demonstrate that features
related to the band structure of the TI materials show up in the tunneling
conductance and even more clearly through low frequency noise measurements. The
bias dependence of 1/f noise reveals peaks at specific energies corresponding
to band structure features of the TI. TI tunnel junctions could thus simplify
the study of the properties of such quantum coherent systems, that can further
lead to the manipulation of their spin-polarized properties for technological
purposes
Unconventional Planar Hall Effect in Exchange-Coupled Topological Insulator-Ferromagnetic Insulator Heterostructures
The Dirac electrons occupying the surface states (SSs) of topological
insulators (TIs) have been predicted to exhibit many exciting magneto-transport
phenomena. Here we report on the first experimental observation of an
unconventional planar Hall effect (PHE) and an electrically gate-tunable
hysteretic planar magnetoresistance (PMR) in EuS/TI heterostructures, in which
EuS is a ferromagnetic insulator (FMI) with an in-plane magnetization. In such
exchange-coupled FMI/TI heterostructures, we find a significant (suppressed)
PHE when the in-plane magnetic field is parallel (perpendicular) to the
electric current. This behavior differs from previous observations of the PHE
in ferromagnets and semiconductors. Furthermore, as the thickness of the 3D TI
films is reduced into the 2D limit, in which the Dirac SSs develop a
hybridization gap, we find a suppression of the PHE around the charge neutral
point indicating the vital role of Dirac SSs in this phenomenon. To explain our
findings, we outline a symmetry argument that excludes linear-Hall mechanisms
and suggest two possible non-linear Hall mechanisms that can account for all
the essential qualitative features in our observations.Comment: 17 pages, 4 figures, accepted by Phys. Rev.
Proximity Driven Enhanced Magnetic Order at Ferromagnetic Insulator / Magnetic Topological Insulator Interface
Magnetic exchange driven proximity effect at a magnetic insulator /
topological insulator (MI/TI) interface provides a rich playground for novel
phenomena as well as a way to realize low energy dissipation quantum devices.
Here we report a dramatic enhancement of proximity exchange coupling in the MI
/ magnetic-TI EuS / SbVTe hybrid heterostructure, where V
doping is used to drive the TI (SbTe) magnetic. We observe an
artificial antiferromagnetic-like structure near the MI/TI interface, which may
account for the enhanced proximity coupling. The interplay between the
proximity effect and doping provides insights into controllable engineering of
magnetic order using a hybrid heterostructure.Comment: 5 pages, 4 figure
Revealing the Empty-State Electronic Structure of Single-Unit-Cell FeSe/SrTiO
We use scanning tunneling spectroscopy to investigate the filled and empty
electronic states of superconducting single-unit-cell FeSe deposited on
SrTiO(001). We map the momentum-space band structure by combining
quasiparticle interference imaging with decay length spectroscopy. In addition
to quantifying the filled-state bands, we discover a -centered electron
pocket 75 meV above the Fermi energy. Our density functional theory
calculations show the orbital nature of empty states at and suggest
that the Se height is a key tuning parameter of their energies, with broad
implications for electronic properties.Comment: 5 pages, 5 figure
Spin chirality fluctuation in two-dimensional ferromagnets with perpendicular anisotropy
Non-coplanar spin textures with scalar spin chirality can generate effective
magnetic field that deflects the motion of charge carriers, resulting in
topological Hall effect (THE), a powerful probe of the ground state and
low-energy excitations of correlated systems. However, spin chirality
fluctuation in two-dimensional ferromagnets with perpendicular anisotropy has
not been considered in prior studies. Herein, we report direct evidence of
universal spin chirality fluctuation by probing the THE above the transition
temperatures in two different ferromagnetic ultra-thin films, SrRuO and V
doped SbTe. The temperature, magnetic field, thickness, and carrier
type dependences of the THE signal, along with our Monte-Carlo simulations,
unambiguously demonstrate that the spin chirality fluctuation is a universal
phenomenon in two-dimensional Ising ferromagnets. Our discovery opens a new
paradigm of exploring the spin chirality with topological Hall transport in
two-dimensional magnets and beyondComment: accepted by nature material
Zero-field dissipationless chiral edge transport and the nature of dissipation in the quantum anomalous Hall state
The quantum anomalous Hall (QAH) effect is predicted to possess, at zero
magnetic field, chiral edge channels that conduct spin polarized current
without dissipation. While edge channels have been observed in previous
experimental studies of the QAH effect, their dissipationless nature at a zero
magnetic field has not been convincingly demonstrated. By a comprehensive
experimental study of the gate and temperature dependences of local and
nonlocal magnetoresistance, we unambiguously establish the dissipationless edge
transport. By studying the onset of dissipation, we also identify the origin of
dissipative channels and clarify the surprising observation that the critical
temperature of the QAH effect is two orders of magnitude smaller than the Curie
temperature of ferromagnetism.Comment: main text+supporting materials. This is the accepted version for PRL.
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