62 research outputs found
Topological Semimetal-Insulator Quantum Phase Transition in Zintl Compounds Ba2X (X=Si, Ge)
By first-principles calculations, we find that Ba2X(X=Si, Ge) hosts a
topological semimetal phase with one nodal ring in the kx=0 plane, which is
protected by the glide mirror symmetry when spin-orbit coupling (SOC) is
ignored. The corresponding drumheadlike surface flat band appears on the (100)
surface in surface Green function calculation. Furthermore, a
topological-semimetal-to-insulator transition (TSMIT) is found. The nodal line
semimetal would evolve into topological insulator as SOC is turned on. The
topologically protected metallic surface states emerge around the Gamma=0
point, which could be tuned into topologically-trivial insulator state by more
than 3% hydrostatic strain. These results reveal a new category of materials
showing quantum phase transition between topological semimetal and insulator,
and tunability through elastic strain engineering.Comment: 14 pages. 4 figure
The Non-Perturbative Quantum Nature of the Dislocation-Phonon Interaction
Despite the long history of dislocation-phonon interaction studies, there are
many problems that have not been fully resolved during this development. These
include an incompatibility between a perturbative approach and the long-range
nature of a dislocation, the relation between static and dynamic scattering,
and the nature of dislocation-phonon resonance. Here by introducing a fully
quantized dislocation field, the "dislon"[1], a phonon is renormalized as a
quasi-phonon, with shifted quasi-phonon energy, and accompanied by a finite
quasi-phonon lifetime that is reducible to classical results. A series of
outstanding legacy issues including those above can be directly explained
within this unified phonon renormalization approach. In particular, a
renormalized phonon naturally resolves the decades-long debate between dynamic
and static dislocation-phonon scattering approaches.Comment: 5 pages main text, 3 figures, 10 pages supplemental material
Tunable THz Surface Plasmon Polariton based on Topological Insulator-Layered Superconductor Hybrid Structure
We theoretically investigate the surface plasmon polariton (SPP) at the
interface between 3D strong topological insulator (TI) and layered
superconductor-magnetic insulator structure. The tunability of SPP through
electronic doping can be enhanced when the magnetic permeability of the layered
structure becomes higher. When the interface is gapped by superconductivity or
perpendicular magnetism, SPP dispersion is further distorted, accompanied by a
shift of group velocity and penetration depth. Such a shift of SPP reaches
maximum when the magnitude of Fermi level approaches the gap value, and may
lead to observable effects. The tunable SPP at the interface between layered
superconductor and magnetism materials in proximity to TI surface may provide
new insight in the detection of Majorana Fermions.Comment: 6 pages, 4 figure
VDC: Versatile Data Cleanser for Detecting Dirty Samples via Visual-Linguistic Inconsistency
The role of data in building AI systems has recently been emphasized by the
emerging concept of data-centric AI. Unfortunately, in the real-world, datasets
may contain dirty samples, such as poisoned samples from backdoor attack, noisy
labels in crowdsourcing, and even hybrids of them. The presence of such dirty
samples makes the DNNs vunerable and unreliable.Hence, it is critical to detect
dirty samples to improve the quality and realiability of dataset. Existing
detectors only focus on detecting poisoned samples or noisy labels, that are
often prone to weak generalization when dealing with dirty samples from other
domains.In this paper, we find a commonality of various dirty samples is
visual-linguistic inconsistency between images and associated labels. To
capture the semantic inconsistency between modalities, we propose versatile
data cleanser (VDC) leveraging the surpassing capabilities of multimodal large
language models (MLLM) in cross-modal alignment and reasoning.It consists of
three consecutive modules: the visual question generation module to generate
insightful questions about the image; the visual question answering module to
acquire the semantics of the visual content by answering the questions with
MLLM; followed by the visual answer evaluation module to evaluate the
inconsistency.Extensive experiments demonstrate its superior performance and
generalization to various categories and types of dirty samples.Comment: 22 pages,5 figures,17 table
Boosting Backdoor Attack with A Learnable Poisoning Sample Selection Strategy
Data-poisoning based backdoor attacks aim to insert backdoor into models by
manipulating training datasets without controlling the training process of the
target model. Existing attack methods mainly focus on designing triggers or
fusion strategies between triggers and benign samples. However, they often
randomly select samples to be poisoned, disregarding the varying importance of
each poisoning sample in terms of backdoor injection. A recent selection
strategy filters a fixed-size poisoning sample pool by recording forgetting
events, but it fails to consider the remaining samples outside the pool from a
global perspective. Moreover, computing forgetting events requires significant
additional computing resources. Therefore, how to efficiently and effectively
select poisoning samples from the entire dataset is an urgent problem in
backdoor attacks.To address it, firstly, we introduce a poisoning mask into the
regular backdoor training loss. We suppose that a backdoored model training
with hard poisoning samples has a more backdoor effect on easy ones, which can
be implemented by hindering the normal training process (\ie, maximizing loss
\wrt mask). To further integrate it with normal training process, we then
propose a learnable poisoning sample selection strategy to learn the mask
together with the model parameters through a min-max optimization.Specifically,
the outer loop aims to achieve the backdoor attack goal by minimizing the loss
based on the selected samples, while the inner loop selects hard poisoning
samples that impede this goal by maximizing the loss. After several rounds of
adversarial training, we finally select effective poisoning samples with high
contribution. Extensive experiments on benchmark datasets demonstrate the
effectiveness and efficiency of our approach in boosting backdoor attack
performance
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
Spin Transitions and Compressibility of ε‐Fe7N3 and γ′‐Fe4N: Implications for Iron Alloys in Terrestrial Planet Cores
Iron nitrides are possible constituents of the cores of Earth and other terrestrial planets. Pressure‐induced magnetic changes in iron nitrides and effects on compressibility remain poorly understood. Here we report synchrotron X‐ray emission spectroscopy (XES) and X‐ray diffraction (XRD) results for ε‐Fe7N3 and γ′‐Fe4N up to 60 GPa at 300 K. The XES spectra reveal completion of high‐ to low‐spin transition in ε‐Fe7N3 and γ′‐Fe4N at 43 and 34 GPa, respectively. The completion of the spin transition induces stiffening in bulk modulus of ε‐Fe7N3 by 22% at ~40 GPa, but has no resolvable effect on the compression behavior of γ′‐Fe4N. Fitting pressure‐volume data to the Birch‐Murnaghan equation of state yields V0 = 83.29 ± 0.03 (Å3), K0 = 232 ± 9 GPa, K0′ = 4.1 ± 0.5 for nonmagnetic ε‐Fe7N3 above the spin transition completion pressure, and V0 = 54.82 ± 0.02 (Å3), K0 = 152 ± 2 GPa, K0′ = 4.0 ± 0.1 for γ′‐Fe4N over the studied pressure range. By reexamining evidence for spin transition and effects on compressibility of other candidate components of terrestrial planet cores, Fe3S, Fe3P, Fe7C3, and Fe3C based on previous XES and XRD measurements, we located the completion of high‐ to low‐spin transition at ~67, 38, 50, and 30 GPa at 300 K, respectively. The completion of spin transitions of Fe3S, Fe3P, and Fe3C induces elastic stiffening, whereas that of Fe7C3 induces elastic softening. Changes in compressibility at completion of spin transitions in iron‐light element alloys may influence the properties of Earth’s and planetary cores.Key PointsSpin transition in ε‐Fe7N3 and γ′‐Fe4N at 300 K completes at 43 and 34 GPa, respectivelyThe completion of spin transition leads to stiffening in bulk modulus of ε‐Fe7N3, but not in γ′‐Fe4NEvidence for spin transitions in Fe‐light‐element alloys and their effects are reexaminedPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/163586/2/jgrb54505_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/163586/1/jgrb54505.pd
Dirac-Electrons-Mediated Magnetic Proximity Effect in Topological Insulator / Magnetic Insulator Heterostructures
The possible realization of dissipationless chiral edge current in a
topological insulator / magnetic insulator heterostructure is based on the
condition that the magnetic proximity exchange coupling at the interface is
dominated by the Dirac surface states of the topological insulator. Here we
report a polarized neutron reflectometry observation of Dirac electrons
mediated magnetic proximity effect in a bulk-insulating topological insulator
(BiSb)Te / magnetic insulator EuS heterostructure.
We are able to maximize the proximity induced magnetism by applying an
electrical back gate to tune the Fermi level of topological insulator to be
close to the charge neutral point. A phenomenological model based on
diamagnetic screening is developed to explain the suppressed proximity induced
magnetism at high carrier density. Our work paves the way to utilize the
magnetic proximity effect at the topological insulator/magnetic insulator
hetero-interface for low-power spintronic applications.Comment: 5 pages main text with 4 figures; 2 pages supplemental materials;
suggestions and discussions are welcome
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