27 research outputs found
Mesoscopic Transport of Quantum Anomalous Hall Effect in Sub-Micron Size Regime
The quantum anomalous Hall (QAH) effect has been demonstrated in
two-dimensional topological insulator systems incorporated with ferromagnetism.
However, a comprehensive understanding of mesoscopic transport in sub-micron
QAH devices has yet been established. Here we fabricated miniaturized QAH
devices with channel widths down to 600 nm, where the QAH features are still
preserved. A back-scattering channel is formed in narrow QAH devices through
percolative hopping between 2D compressible puddles. Large resistance
fluctuations are observed in narrow devices near the coercive field, which is
associated with collective interference between intersecting paths along domain
walls when the device geometry is smaller than the phase coherence length
. Through measurement of size-dependent breakdown current, we confirmed
that the chiral edge states are confined at the physical boundary with its
width on the order of Fermi wavelength.Comment: 7 pages, 5 figure
Universal conductance fluctuations in a MnBiTe thin film
Quantum coherence of electrons can produce striking behaviors in mesoscopic
conductors, including weak localization and the Aharonov-Bohm effect. Although
magnetic order can also strongly affect transport, the combination of coherence
and magnetic order has been largely unexplored. Here, we examine quantum
coherence-driven universal conductance fluctuations in the antiferromagnetic,
canted antiferromagnetic, and ferromagnetic phases of a thin film of the
topological material MnBiTe. In each magnetic phase we extract a charge
carrier phase coherence length of about 100 nm. The conductance
magnetofingerprint is repeatable when sweeping applied magnetic field within
one magnetic phase, but changes when the applied magnetic field crosses the
antiferromagnetic/canted antiferromagnetic magnetic phase boundary.
Surprisingly, in the antiferromagnetic and canted antiferromagnetic phase, but
not in the ferromagnetic phase, the magnetofingerprint depends on the direction
of the field sweep. To explain these observations, we suggest that conductance
fluctuation measurements are sensitive to the motion and nucleation of magnetic
domain walls in MnBiTe
Measured potential profile in a quantum anomalous Hall system suggests bulk-dominated current flow
Ideally, quantum anomalous Hall systems should display zero longitudinal
resistance. Yet in experimental quantum anomalous Hall systems elevated
temperature can make the longitudinal resistance finite, indicating dissipative
flow of electrons. Here, we show that the measured potentials at multiple
locations within a device at elevated temperature are well-described by
solution of Laplace's equation, assuming spatially-uniform conductivity,
suggesting non-equilibrium current flows through the two-dimensional bulk.
Extrapolation suggests that at even lower temperatures current may still flow
primarily through the bulk rather than, as had been assumed, through edge
modes. An argument for bulk current flow previously applied to quantum Hall
systems supports this picture.Comment: 6 pages, 4 figures, plus supplemental material
Distinguishing two-component anomalous Hall effect from topological Hall effect in magnetic topological insulator MnBi2Te4
In transport, the topological Hall effect (THE) is widely interpreted as a
sign of chiral spin textures, like magnetic skyrmions. However, the
co-existence of two anomalous Hall effects (AHE) could give rise to similar
non-monotonic features or "humps", making it difficult to distinguish between
the two. Here we demonstrate that the "artifact" two-component anomalous Hall
effect can be clearly distinguished from the genuine topological Hall effect by
three methods: 1. Minor loops 2. Temperature dependence 3. Gate dependence. One
of the minor loops is a single loop that cannot fit into the full AHE loop
under the assumption of AHE+THE. In addition, by increasing the temperature or
tuning the gate bias, the emergence of humps is accompanied by a polarity
change of the AHE. Using these three methods, one can find the humps are from
another AHE loop with a different polarity. Our material is a magnetic
topological insulator MnBi2Te4 grown by molecular beam epitaxy, where the
presence of the secondary phase MnTe2 on the surface contributes to the extra
positive AHE component. Our work may help future researchers to exercise
cautions and use these three methods to examine carefully in order to ascertain
genuine topological Hall effect
Proximity-induced quasi-one-dimensional superconducting quantum anomalous Hall state: a promising scalable top-down approach towards localized Majorana modes
In this work, ~100 nm wide quantum anomalous Hall insulator (QAHI)
nanoribbons are etched from a two-dimensional QAHI film. One part of the
nanoribbon is covered with superconducting Nb, while the other part is
connected to an Au lead via two-dimensional QAHI regions. Andreev reflection
spectroscopy measurements were performed, and multiple in-gap conductance peaks
were observed in three different devices. In the presence of an increasing
magnetic field perpendicular to the QAHI film, the multiple in-gap peak
structure evolves into a single zero-bias conductance peak (ZBCP). Theoretical
simulations suggest that the measurements are consistent with the scenario that
the increasing magnetic field drives the nanoribbons from a multi-channel
occupied regime to a single channel occupied regime, and that the ZBCP may be
induced by zero energy Majorana modes as previously predicted [24]. Although
further experiments are needed to clarify the nature of the ZBCP, we provide
initial evidence that quasi-1D QAHI nanoribbon/superconductor heterostructures
are new and promising platforms for realizing zero-energy Majorana modes
Giant Hall Switching by Surface-State-Mediated Spin-Orbit Torque in a Hard Ferromagnetic Topological Insulator
Topological insulators (TI) can apply highly efficient spin-orbit torque
(SOT) and manipulate the magnetization with their unique topological surface
states, and their magnetic counterparts, magnetic topological insulators (MTI)
offer magnetization without shunting and are one of the highest in SOT
efficiency. Here, we demonstrate efficient SOT switching of a hard MTI, V-doped
(Bi,Sb)2Te3 (VBST) with a large coercive field that can prevent the influence
of an external magnetic field and a small magnetization to minimize stray
field. A giant switched anomalous Hall resistance of 9.2 is realized,
among the largest of all SOT systems. The SOT switching current density can be
reduced to , and the switching ratio can be enhanced to
60%. Moreover, as the Fermi level is moved away from the Dirac point by both
gate and composition tuning, VBST exhibits a transition from
edge-state-mediated to surface-state-mediated transport, thus enhancing the SOT
effective field to and the spin Hall angle to
at 5 K. The findings establish VBST as an extraordinary candidate
for energy-efficient magnetic memory devices
Association analyses of East Asian individuals and trans-ancestry analyses with European individuals reveal new loci associated with cholesterol and triglyceride levels
Large-scale meta-analyses of genome-wide association studies (GWAS) have identified >175 loci associated with fasting cholesterol levels, including total cholesterol (TC), high-density lipoprotein cholesterol (HDL-C), low-density lipoprotein cholesterol (LDL-C), and triglycerides (TG). With differences in linkage disequilibrium (LD) structure and allele frequencies between ancestry groups, studies in additional large samples may detect new associations. We conducted staged GWAS meta-analyses in up to 69,414 East Asian individuals from 24 studies with participants from Japan, the Philippines, Korea, China, Singapore, and Taiwan. These meta-analyses identified (P < 5 × 10-8) three novel loci associated with HDL-C near CD163-APOBEC1 (P = 7.4 × 10-9), NCOA2 (P = 1.6 × 10-8), and NID2-PTGDR (P = 4.2 × 10-8), and one novel locus associated with TG near WDR11-FGFR2 (P = 2.7 × 10-10). Conditional analyses identified a second signal near CD163-APOBEC1. We then combined results from the East Asian meta-analysis with association results from up to 187,365 European individuals from the Global Lipids Genetics Consortium in a trans-ancestry meta-analysis. This analysis identified (log10Bayes Factor ≥6.1) eight additional novel lipid loci. Among the twelve total loci identified, the index variants at eight loci have demonstrated at least nominal significance with other metabolic traits in prior studies, and two loci exhibited coincident eQTLs (P < 1 × 10-5) in subcutaneous adipose tissue for BPTF and PDGFC. Taken together, these analyses identified multiple novel lipid loci, providing new potential therapeutic targets
Meta-analysis of genome-wide association studies in East Asian-ancestry populations identifies four new loci for body mass index
Recent genetic association studies have identified 55 genetic loci associated with obesity or body mass index (BMI). The vast majority, 51 loci, however, were identified in European-ancestry populations. We conducted a meta-analysis of associations between BMI and ∼2.5 million genotyped or imputed single nucleotide polymorphisms among 86 757 individuals of Asian ancestry, followed by in silico and de novo replication among 7488–47 352 additional Asian-ancestry individuals. We identified four novel BMI-associated loci near the KCNQ1 (rs2237892, P = 9.29 × 10−13), ALDH2/MYL2 (rs671, P = 3.40 × 10−11; rs12229654, P = 4.56 × 10−9), ITIH4 (rs2535633, P = 1.77 × 10−10) and NT5C2 (rs11191580, P = 3.83 × 10−8) genes. The association of BMI with rs2237892, rs671 and rs12229654 was significantly stronger among men than among women. Of the 51 BMI-associated loci initially identified in European-ancestry populations, we confirmed eight loci at the genome-wide significance level (P < 5.0 × 10−8) and an additional 14 at P < 1.0 × 10−3 with the same direction of effect as reported previously. Findings from this analysis expand our knowledge of the genetic basis of obesity
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Topological Spintronics Based on Magnetic Skyrmions and Magnetic Topological Insulators
Spintronics harnesses spin degrees of freedom for information storage and processing, offering notable advantages of non-volatility, low power consumption, and fast speed. Topology, as invariant geometric and physical properties under continuous deformation, endows spintronics with higher efficiency and robustness against external perturbations. Protected by real-space topology, magnetic skyrmions are swirling topological spin structures with particle-like properties and potential candidates for high-density, non-volatile storages. As an electrical readout of magnetic skyrmions, the topological Hall effect (THE) is a non-monotonic feature in the Hall signal. However, in the presence of an anomalous Hall effect (AHE), the THE can be easily confused with the non-monotonic co-existence of two AHEs, or artifact of “THE”. Here, we develop systematic methodologies for distinguishing between the two. Genuine THE occurs in the transition region of the AHE, while artifact of “THE” may occur well beyond the saturation of the “AHE component”. Minor loops of genuine THE with AHE are always within the full loop, while minor loops of artifact of “THE” may reveal a single loop that cannot fit into the “AHE component”. The temperature or gate dependence of artifact of “THE” may also be accompanied by a polarity change of the “AHE component”. Our methods may help future researchers ascertain genuine THE for applications of magnetic skyrmions.
Protected by k-space topology, magnetic topological insulators (MTI) can apply highly efficient spin-orbit torque (SOT) and manipulate the magnetization with their unique topological surface states. Here, we demonstrate efficient SOT switching of a hard MTI, V-doped (Bi,Sb)2Te3 (VBST) with a large coercive field that can prevent the influence of an external magnetic field. A giant switched anomalous Hall resistance of 9.2 kΩ is realized, among the largest of all SOT systems. The SOT switching current density can be reduced to 2.8×105 A/cm2. Moreover, as the Fermi level is moved away from the Dirac point by both gate and composition tuning, VBST exhibits a transition from edge-state-mediated to surface-state-mediated transport, thus enhancing the SOT effective field to 1.56±0.12 T/(106 A/cm2) and the interfacial charge-to-spin conversion efficiency to 3.9±0.3 nm-1. The findings establish VBST as an extraordinary candidate for energy-efficient magnetic memory devices