235 research outputs found
Universal theory of spin-momentum-orbital-site locking
Spin textures, i.e., the distribution of spin polarization vectors in
reciprocal space, exhibit diverse patterns determined by symmetry constraints,
resulting in a variety of spintronic phenomena. Here, we propose a universal
theory to comprehensively describe the nature of spin textures by incorporating
three symmetry flavors of reciprocal wavevector, atomic orbital and atomic
site. Such approach enables us to establish a complete classification of spin
textures constrained by the little co-group and predict unprecedentedly
reported spin texture types, such as Zeeman-type spin splitting in
antiferromagnets and quadratic spin texture. To examine the impact of atomic
orbitals and sites, we predict orbital-dependent spin texture and anisotropic
spin-momentum-site locking effects and corresponding material candidates
validated through first-principles calculations. Our theory not only covers all
possible spin textures in crystal solids described by magnetic space groups,
but also introduces new possibilities for designing innovative spin textures by
the coupling of multiple degrees of freedom
Chiral Dirac-like fermion in spin-orbit-free antiferromagnetic semimetals
Dirac semimetal is a phase of matter, whose elementary excitation is
described by the relativistic Dirac equation. In the limit of zero mass, its
parity-time symmetry enforces the Dirac fermion in the momentum space, which is
composed of two Weyl fermions with opposite chirality, to be non-chiral.
Inspired by the flavor symmetry in particle physics, we theoretically propose a
massless Dirac-like equation yet linking two Weyl fields with the identical
chirality by assuming SU(2) isospin symmetry, independent of the space-time
rotation exchanging the two fields. Dramatically, such symmetry is hidden in
certain solid-state spin-1/2 systems with negligible spin-orbit coupling, where
the spin degree of freedom is decoupled with the lattice. Therefore, the
existence of the corresponding quasiparticle, dubbed as flavor Weyl fermion,
cannot be explained by the conventional (magnetic) space group framework. The
four-fold degenerate flavor Weyl fermion manifests linear dispersion and a
Chern number of 2, leading to a robust network of topologically protected Fermi
arcs throughout the Brillouin zone. For material realization, we show that the
transition-metal chalcogenide CoNb3S6 with experimentally confirmed collinear
antiferromagnetic order is ideal for flavor Weyl semimetal under the
approximation of vanishing spin-orbit coupling. Our work reveals a counterpart
of the flavor symmetry in magnetic electronic systems, leading to further
possibilities of emergent phenomena in quantum materials.Comment: 27 pages and 5 figure
Sequential Action-Induced Invariant Representation for Reinforcement Learning
How to accurately learn task-relevant state representations from
high-dimensional observations with visual distractions is a realistic and
challenging problem in visual reinforcement learning. Recently, unsupervised
representation learning methods based on bisimulation metrics, contrast,
prediction, and reconstruction have shown the ability for task-relevant
information extraction. However, due to the lack of appropriate mechanisms for
the extraction of task information in the prediction, contrast, and
reconstruction-related approaches and the limitations of bisimulation-related
methods in domains with sparse rewards, it is still difficult for these methods
to be effectively extended to environments with distractions. To alleviate
these problems, in the paper, the action sequences, which contain
task-intensive signals, are incorporated into representation learning.
Specifically, we propose a Sequential Action--induced invariant Representation
(SAR) method, in which the encoder is optimized by an auxiliary learner to only
preserve the components that follow the control signals of sequential actions,
so the agent can be induced to learn the robust representation against
distractions. We conduct extensive experiments on the DeepMind Control suite
tasks with distractions while achieving the best performance over strong
baselines. We also demonstrate the effectiveness of our method at disregarding
task-irrelevant information by deploying SAR to real-world CARLA-based
autonomous driving with natural distractions. Finally, we provide the analysis
results of generalization drawn from the generalization decay and t-SNE
visualization. Code and demo videos are available at
https://github.com/DMU-XMU/SAR.git
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