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Yu liu

Publications and source records attributed to Yu liu.

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StreetTree: A Large-Scale Global Benchmark for Fine-Grained Tree Species Classification

The fine grained classification of street trees is a crucial task for urban planning, streetscape management, and the assessment of urban ecosystem services. However, progress in this field has been hindered by the lack of large scale, geographically diverse, and publicly available benchmark datasets specifically designed for street trees. To address this critical gap, we introduce StreetTree, the world's first large scale benchmark dataset dedicated to fine grained street tree classification. The dataset contains over 12 million images covering more than 8,300 common street tree species, collected from urban streetscapes across 133 countries spanning five continents, and supplemented with expert verified observational data. StreetTree poses challenges for pretrained vision models under complex urban environments including high inter species visual similarity, long tailed natural distributions, significant intra class variations caused by seasonal changes, and diverse imaging conditions such as lighting, occlusions from buildings, and varying camera angles. In addition, we provide a hierarchical taxonomy (order, family, genus, and species) to support research in hierarchical classification and representation learning. Through extensive experiments with various vision models, we establish solid baselines and reveal the limitations of existing methods in handling such real world complexities. We believe that StreetTree will serve as a key resource for driving new advancements at the intersection of computer vision and urban science.

cs.CV

Ferroelectrically Controlled Chirality Switching of Weyl Quasiparticles

Weyl quasiparticles, as gapless low-energy excitations with nontrivial chirality, have garnered extensive interest in recent years. However, archieving effective and reversible control over their chirality (topological charge) remains a major challeng due to topological protection. In this work, we propose a ferroelectric mechanism to switch the chirality of Weyl phonons, where the reversal of ferroelectric polarization is intrinsically coupled to a simultaneous reversal of the chirality of Weyl points. This enables electric-field-driven control over the topological properties of phonon excitations. Through a comprehensive symmetry analysis of polar space groups, we identify 27 groups capable of hosting symmetry-protected Weyl phonons with chiral charges $C = 1$, $2$, and $3$, whose chirality can be reversed via polarization switching. The first-principles calculations are performed to screen feasible material candidates for each type of chirality, yielding a set of prototypical ferroelectric compounds that realize the proposed mechanism. As a representative example, K$_2$ZnBr$_4$ hosts the minimal configuration of two pairs of Weyl phonons. Upon polarization reversal, the chirality of all Weyl points is inverted, accompanied by a reversal of associated topological features such as Berry curvature and surface arcs. These findings provide a viable pathway for dynamic, electrical control of topological band crossings and open new avenues for chirality-based phononic applications.

cond-mat.mtrl-sci