84 research outputs found
Chern Number Tunable Quantum Anomalous Hall Effect in Monolayer Transitional Metal Oxides via Manipulating Magnetization Orientation
Although much effort has been made to explore quantum anomalous Hall effect
(QAHE) in both theory and experiment, the QAHE systems with tunable Chern
numbers are yet limited. Here, we theoretically propose that NiAsO and
PdSbO, monolayer transitional metal oxides, can realize QAHE with tunable
Chern numbers via manipulating their magnetization orientations. When the
magnetization lies in the \textit{x-y} plane and all mirror symmetries are
broken, the low-Chern-number (i.e., ) phase emerges. When the
magnetization exhibits non-zero \textit{z}-direction component, the system
enters the high-Chern-number (i.e., ) phase, even in the
presence of canted magnetization. The global band gap can approach the
room-temperature energy scale in monolayer PdSbO (23.4 meV), when the
magnetization is aligned to \textit{z}-direction. By using Wannier-based
tight-binding model, we establish the phase diagram of magnetization induced
topological phase transition. Our work provides a high-temperature QAHE system
with tunable Chern number for the practical electronic application
Valley-polarized quantum anomalous Hall effect in van der Waals heterostructures based on monolayer jacutingaite family materials
We numerically study the general valley polarization and anomalous Hall
effect in van der Waals (vdW) heterostructures based on monolayer jacutingaite
family materials PtAX (A = Hg, Cd, Zn; X = S, Se, Te). We perform a
systematic study on the atomic, electronic, and topological properties of vdW
heterostructures composed of monolayer PtAX and two-dimensional
ferromagnetic insulators. We show that four kinds of vdW heterostructures
exhibit valley-polarized quantum anomalous Hall phase, i.e.,
PtHgS/NiBr, PtHgSe/CoBr,
PtHgSe/NiBr, and PtZnS/CoBr, with a maximum
valley splitting of 134.2 meV in PtHgSe/NiBr and sizable
global band gap of 58.8 meV in PtHgS/NiBr. Our findings
demonstrate an ideal platform to implement applications on topological
valleytronics
Second-Order Topological Insulator in van der Waals Heterostructures of CoBr/PtHgSe/CoBr
Second-order topological insulator, which has (d-2)-dimensional topological
hinge or corner states, has been observed in three-dimensional materials, but
has yet not been observed in two-dimensional system. In this Letter, we
theoretically propose the realization of second-order topological insulator in
the van der Waals heterostructure of CoBr/PtHgSe/CoBr.
PtHgSe is a large gap topological insulator. With
in-plane exchange field from neighboring CoBr, a large band gap above 70
meV opens up at the edge. The corner states, which are robust against edge
disorders and irregular shapes, are confirmed in the nanoflake. We further show
that the second-order topological states can also be realized in the
heterostructure of jacutingaite family topological insulators.
We believe that our work will be beneficial for the experimental realization of
second-order topological insulators in van der Waals layered materials
Prompt-aligned Gradient for Prompt Tuning
Thanks to the large pre-trained vision-language models (VLMs) like CLIP, we
can craft a zero-shot classifier by "prompt", e.g., the confidence score of an
image being "[CLASS]" can be obtained by using the VLM provided similarity
measure between the image and the prompt sentence "a photo of a [CLASS]".
Therefore, prompt shows a great potential for fast adaptation of VLMs to
downstream tasks if we fine-tune the prompt-based similarity measure. However,
we find a common failure that improper fine-tuning may not only undermine the
prompt's inherent prediction for the task-related classes, but also for other
classes in the VLM vocabulary. Existing methods still address this problem by
using traditional anti-overfitting techniques such as early stopping and data
augmentation, which lack a principled solution specific to prompt. We present
Prompt-aligned Gradient, dubbed ProGrad, to prevent prompt tuning from
forgetting the the general knowledge learned from VLMs. In particular, ProGrad
only updates the prompt whose gradient is aligned (or non-conflicting) to the
"general direction", which is represented as the gradient of the KL loss of the
pre-defined prompt prediction. Extensive experiments demonstrate the stronger
few-shot generalization ability of ProGrad over state-of-the-art prompt tuning
methods. Codes are available at https://github.com/BeierZhu/Prompt-align.Comment: Accepted by ICCV202
Anderson Localization from Berry-Curvature Interchange in Quantum Anomalous Hall System
We theoretically investigate the localization mechanism of the quantum
anomalous Hall effect (QAHE) in the presence of spin-flip disorders. We show
that the QAHE keeps quantized at weak disorders, then enters a Berry-curvature
mediated metallic phase at moderate disorders, and finally goes into the
Anderson insulating phase at strong disorders. From the phase diagram, we find
that at the charge neutrality point although the QAHE is most robust against
disorders, the corresponding metallic phase is much easier to be localized into
the Anderson insulating phase due to the \textit{interchange} of Berry
curvatures carried respectively by the conduction and valence bands. At the
end, we provide a phenomenological picture related to the topological charges
to better understand the underlying physical origin of the QAHE Anderson
localization.Comment: 6 pages, 4 figure
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