40 research outputs found
Transport Properties of Topological Insulators: Band Bending, Bulk Metal-to-Insulator Transition, and Weak Anti-Localization
We reanalyze some of the critical transport experiments and provide a
coherent understanding of the current generation of topological insulators
(TIs). Currently TI transport studies abound with widely varying claims of the
surface and bulk states, often times contradicting each other, and a proper
understanding of TI transport properties is lacking. According to the simple
criteria given by Mott and Ioffe-Regel, even the best TIs are not true
insulators in the Mott sense, and at best, are weakly-insulating bad metals.
However, band-bending effects contribute significantly to the TI transport
properties including Shubnikov de-Haas oscillations, and we show that
utilization of this band-bending effect can lead to a Mott insulating bulk
state in the thin regime. In addition, by reconsidering previous results on the
weak anti-localization (WAL) effect with additional new data, we correct a
misunderstanding in the literature and generate a coherent picture of the WAL
effect in TIs
Ferromagnetic Anomalous Hall Effect in Cr-doped BiSe Thin Films via Surface-State Engineering
The anomalous Hall effect (AHE) is a non-linear Hall effect appearing in
magnetic conductors, boosted by internal magnetism beyond what is expected from
the ordinary Hall effect. With the recent discovery of the quantized version of
the AHE, the quantum anomalous Hall effect (QAHE), in Cr- or V-doped
topological insulator (TI) (Sb,Bi)Te thin films, the AHE in magnetic
TIs has been attracting significant interest. However, one of the puzzles in
this system has been that while Cr- or V-doped (Sb,Bi)Te and V-doped
BiSe exhibit AHE, Cr-doped BiSe has failed to exhibit even
ferromagnetic AHE, the expected predecessor to the QAHE, though it is the first
material predicted to exhibit the QAHE. Here, we have successfully implemented
ferromagnetic AHE in Cr-doped BiSe thin films by utilizing a surface
state engineering scheme. Surprisingly, the observed ferromagnetic AHE in the
Cr-doped BiSe thin films exhibited only positive slope regardless of
the carrier type. We show that this sign problem can be explained by the
intrinsic Berry curvature of the system as calculated from a tight-binding
model combined with a first-principles method.Comment: 24 pages, 5 figure
Solution to the hole-doping problem and tunable quantum Hall effect in BiSe thin films
BiSe, one of the most widely studied topological insulators
(TIs), is naturally electron-doped due to n-type native defects. However, many
years of efforts to achieve p-type BiSe thin films have failed so
far. Here, we provide a solution to this long-standing problem, showing that
the main culprit has been the high density of interfacial defects. By
suppressing these defects through an interfacial engineering scheme, we have
successfully implemented p-type BiSe thin films down to the
thinnest topological regime. On this platform, we present the first tunable
quantum Hall effect (QHE) study in BiSe thin films, and reveal not
only significantly asymmetric QHE signatures across the Dirac point but also
the presence of competing anomalous states near the zeroth Landau level. The
availability of doping tunable BiSe thin films will now make it
possible to implement various topological quantum devices, previously
inaccessible.Comment: 40 Pages, 8 Figures, 2 Tables, Accepted to Nano Letter
Restoring Pristine Bi2Se3 Surface with an Effective Se Decapping Process
High quality thin films of topological insulators (TI) such as Bi2Se3 have
been successfully synthesized by molecular beam epitaxy (MBE). Although the
surface of MBE films can be protected by capping with inert materials such as
amorphous Se, restoring an atomically clean pristine surface after decapping
has never been demonstrated, which prevents in-depth investigations of the
intrinsic properties of TI thin films with ex-situ tools. Using high resolution
scanning tunneling microscopy/spectroscopy (STM/STS), we demonstrate a simple
and highly reproducible Se decapping method that allows recovery of the
pristine surface of extremely high quality Bi2Se3 thin films grown and capped
with Se in a separate MBE system then exposed to atmosphere during transfer
into the STM system. The crucial step of our decapping process is the removal
of the surface contaminants on top of amorphous Se before thermal desorption of
Se at a mild temperature (~210 {\deg}C). This effective Se decapping process
opens up the possibility of ex-situ characterizations of pristine surfaces of
interesting selenide materials and beyond using cutting-edge techniques.Comment: 10 pages, 3 figure
Observation of inverse spin Hall effect in bismuth selenide
Bismuth Selenide (Bi2Se3) is a topological insulator exhibiting helical spin
polarization and strong spin-orbit coupling. The spin-orbit coupling links the
charge current to spin current via the spin Hall effect (SHE). We demonstrate a
Bi2Se3 spin detector by injecting the pure spin current from a magnetic
permalloy layer to a Bi2Se3 thin film and detect the inverse SHE in Bi2Se3. The
spin Hall angle of Bi2Se3 is found to be 0.0093 and the spin diffusion length
in Bi2Se3 to be 6.2 nm at room temperature. Our results suggest that
topological insulators with strong spin-orbit coupling can be used in
functional spintronic devices
Emergence of decoupled surface transport channels in bulk insulating Bi2Se3 thin films
In ideal topological insulator (TI) films the bulk state, which is supposed
to be insulating, should not provide any electric coupling between the two
metallic surfaces. However, transport studies on existing TI films show that
the topological states on opposite surfaces are electrically tied to each other
at thicknesses far greater than the direct coupling limit where the surface
wavefunctions overlap. Here, we show that as the conducting bulk channels are
suppressed, the parasitic coupling effect diminishes and the decoupled surface
channels emerge as expected for ideal TIs. In Bi2Se3 thin films with fully
suppressed bulk states, the two surfaces, which are directly coupled below ~10
QL, become gradually isolated with increasing thickness and are completely
decoupled beyond ~20 QL. On such a platform, it is now feasible to implement
transport devices whose functionality relies on accessing the individual
surface layers without any deleterious coupling effects.Comment: Accepted for publication in Physical Review Letter
Topological-Metal to Band-Insulator Transition in (Bi1-xInx)2Se3 Thin Films
By combining transport and photo emission measurements on (Bi1-xInx)2Se3 thin
films, we report that this system transforms from a topologically non-trivial
metal into a topologically trivial band insulator through three quantum phase
transitions. At x = 3-7%, there is a transition from a topologically
non-trivial metal to a trivial metal. At x = 15%, the metal becomes a
variable-range-hopping insulator. Finally, above x = 25%, the system becomes a
true band insulator with its resistance immeasurably large even at room
temperature. This material provides a new venue to investigate topologically
tunable physics and devices with seamless gating/tunneling insulators.Comment: 15 pages, 3 figure
Topological Surface States Originated Spin-Orbit Torques in Bi2Se3
Three dimensional topological insulator bismuth selenide (Bi2Se3) is expected
to possess strong spin-orbit coupling and spin-textured topological surface
states, and thus exhibit a high charge to spin current conversion efficiency.
We evaluate spin-orbit torques in Bi2Se3/Co40Fe40B20 devices at different
temperatures by spin torque ferromagnetic resonance measurements. As
temperature decreases, the spin-orbit torque ratio increases from ~ 0.047 at
300 K to ~ 0.42 below 50 K. Moreover, we observe a significant out-of-plane
torque at low temperatures. Detailed analysis indicates that the origin of the
observed spin-orbit torques is topological surface states in Bi2Se3. Our
results suggest that topological insulators with strong spin-orbit coupling
could be promising candidates as highly efficient spin current sources for
exploring next generation of spintronic applications
Signature of a topological phase transition in the Josephson supercurrent through a topological insulator
Topological insulators (TIs) hold great promise for realizing zero-energy
Majorana states in solid-state systems. Recently, several groups reported
experimental data suggesting that signatures of Majorana modes in topological
insulator Josephson junctions (TIJJs) have -- indeed -- been observed. To
verify this claim, one needs to study the topological properties of low-energy
Andreev-bound states (ABS) in TIs of which the Majorana modes are a special
case. It has been shown theoretically that topologically non-trivial low-energy
ABS are also present in TIJJs with doped topological insulators up to some
critical level of doping at which the system undergoes a topological phase
transition. Here, we present first experimental evidence for this topological
transition in the bulk band of a doped TI. Our theoretical calculations, and
numerical modeling link abrupt changes in the critical current of top-gated
TIJJs to moving the chemical potential in the charge-accumulation region on the
surface of the doped TI across a band-inversion point. We demonstrate that the
critical-current changes originate from a shift of the spatial location of
low-energy ABS from the surface to the boundary between topologically-trivial
and band-inverted regions after the transition. The appearance of a decay
channel for surface ABS is related to the vanishing of the band effective mass
in the bulk and thus exemplifies the topological character of surface ABS as
boundary modes. Importantly, the mechanism suggest a means of manipulating
Majorana modes in future experiments.Comment: 26 pages (including Supplementary Materials), 3 figure
Plasmon-phonon interactions in topological insulator rings
The great potential of Dirac electrons for plasmonics and photonics has been
readily recognized after their discovery in graphene, followed by applications
to smart optical devices. Dirac carriers are also found in topological
insulators (TI) --quantum systems having an insulating gap in the bulk and
intrinsic Dirac metallic states at the surface--. Here, we investigate the
plasmonic response of ring structures patterned in BiSe TI films, which
we investigate through terahertz (THz) spectroscopy. The rings are observed to
exhibit a bonding and an antibonding plasmon modes, which we tune in frequency
by varying their diameter. We develop an analytical theory based on the THz
conductivity of unpatterned films, which accurately describes the strong
plasmon-phonon hybridization and Fano interference experimentally observed as
the bonding plasmon is swiped across the promineng 2\,THz phonon exhibited by
this material. This work opens the road for the investigation of plasmons in
topological insulators and for their application in tunable THz devices.Comment: 9 pages, 2 figures. To be published in Advanced Optical Material