arXiv subjects

Yu-Chun Wang

Publications and source records attributed to Yu-Chun Wang.

5 recordsLinked to original sources

arXiv2025

Passivation-Free Ga-Polar AlGaN/GaN Recessed-Gate HEMTs on Sapphire with 2.8 W/mm POUT and 26.8% PAE at 94 GHz

In this work, we demonstrate a passivation-free Ga-polar recessed-gate AlGaN/GaN HEMT on a sapphire substrate for W-band operation, featuring a 5.5 nm Al0.35Ga0.65N barrier under the gate and a 31 nm Al0.35Ga0.65N barrier in the gate access regions. The device achieves a drain current density of 1.8 A/mm, a peak transconductance of 750 mS/mm, and low gate leakage with a high on/off ratio of 10^7. Small-signal characterization reveals a current-gain cutoff frequency of 127 GHz and a maximum oscillation frequency of 203 GHz. Continuous-wave load-pull measurements at 94 GHz demonstrate an output power density of 2.8 W/mm with 26.8% power-added efficiency (PAE), both of which represent the highest values reported for Ga-polar GaN HEMTs on sapphire substrates and are comparable to state-of-the-art Ga-polar GaN HEMTs on SiC substrates. Considering the low cost of sapphire, the simplicity of the epitaxial design, and the reduced fabrication complexity relative to N-polar devices, this work highlights the potential of recessed-gate Ga-polar AlGaN/GaN HEMTs on sapphire as a promising candidate for next-generation millimeter-wave power applications.

physics.app-ph↗

arXiv2025

Absence of transport altermagnetic spin-splitting effect in RuO2

Altermagnets, which exhibit the advantages of both antiferromagnets and ferromagnets, have attracted significant attention recently. Among them, ruthenium dioxide (RuO2), a prototypical altermagnet candidate, is under intensive debate on its magnetic order and altermagnetic characters. In this work, we provide a comprehensive study of the spin-to-charge conversion in epitaxial RuO2 thin films with various orientations and fabrication methods. By utilizing thermal spin injections from a ferrimagnetic insulator, we unambiguously reveal a negative spin Hall angle for RuO2, which is opposite to all the previous reports using ferromagnetic metals. Most importantly, we observe robust anisotropic spin-to-charge conversion in RuO2, with voltage ratios of 30% for the (100)- and (110)-orientations and 40% for the (101)-orientations. The ratio remains consistent across RuO2 films fabricated by sputtering, pulsed laser deposition, and molecular-beam epitaxy. These results conclusively show a robust and anisotropic spin Hall effect in RuO2 with the absence of altermagnetic spin-splitting contributions. Our study provides crucial insights and advances the understanding of spin-to-charge conversions in emerging materials with low crystal symmetries.

cond-mat.mtrl-sci↗

arXiv2024

Versatile Control of Nonlinear Topological States in Non-Hermitian Systems

The non-Hermitian skin effect (NHSE) and nonlinearity can both delocalize topological modes (TMs) from the interface. However, the NHSE requires precise parameter tuning, while nonlinearity in Hermitian systems results in partial delocalization with limited mode capacity. To overcome these limitations, we propose a non-Hermitian nonlinear topological interface model that integrates Hermitian and non-Hermitian lattices with nonreciprocal hopping and nonlinearity. This system enables the complete delocalization of TMs across the entire lattice without fine-tuning, while allowing precise control over the wavefunction profile and spatial distribution through the intrinsic configuration and intensity of the nonlinearity. Using the spectral localizer, we demonstrate the topological protection and robustness of these extended non-Hermitian TMs against disorder. Furthermore, we show that under external pumping, localized excitations evolve into predefined profiles and generate long-range patterns, an effect unattainable in Hermitian systems. These findings reveal how the interplay of nonlinearity and NHSE shapes topological states, paving the way for compact topological devices.

quant-ph↗

arXiv2021

Disentangled Representation with Dual-stage Feature Learning for Face Anti-spoofing

As face recognition is widely used in diverse security-critical applications, the study of face anti-spoofing (FAS) has attracted more and more attention. Several FAS methods have achieved promising performances if the attack types in the testing data are the same as training data, while the performance significantly degrades for unseen attack types. It is essential to learn more generalized and discriminative features to prevent overfitting to pre-defined spoof attack types. This paper proposes a novel dual-stage disentangled representation learning method that can efficiently untangle spoof-related features from irrelevant ones. Unlike previous FAS disentanglement works with one-stage architecture, we found that the dual-stage training design can improve the training stability and effectively encode the features to detect unseen attack types. Our experiments show that the proposed method provides superior accuracy than the state-of-the-art methods on several cross-type FAS benchmarks.

cs.CV↗

arXiv2015

Textual Analysis for Studying Chinese Historical Documents and Literary Novels

We analyzed historical and literary documents in Chinese to gain insights into research issues, and overview our studies which utilized four different sources of text materials in this paper. We investigated the history of concepts and transliterated words in China with the Database for the Study of Modern China Thought and Literature, which contains historical documents about China between 1830 and 1930. We also attempted to disambiguate names that were shared by multiple government officers who served between 618 and 1912 and were recorded in Chinese local gazetteers. To showcase the potentials and challenges of computer-assisted analysis of Chinese literatures, we explored some interesting yet non-trivial questions about two of the Four Great Classical Novels of China: (1) Which monsters attempted to consume the Buddhist monk Xuanzang in the Journey to the West (JTTW), which was published in the 16th century, (2) Which was the most powerful monster in JTTW, and (3) Which major role smiled the most in the Dream of the Red Chamber, which was published in the 18th century. Similar approaches can be applied to the analysis and study of modern documents, such as the newspaper articles published about the 228 incident that occurred in 1947 in Taiwan.

cs.CL↗