arXiv subjects

Hong Dang Nguyen

Publications and source records attributed to Hong Dang Nguyen.

3 recordsLinked to original sources

arXiv2026

Soft and chiral phonons in chiral phase of K3NiO2

Raman scattering measurements confirmed the theoretical prediction that the structural phase transition from the achiral tetragonal to the chiral tetragonal phase, which occurs near 400 K, is induced by a doubly degenerate soft phonon at the Z point of the Brillouin zone. In the low-temperature chiral phase, the soft mode activates in Raman spectra, splits into two components with A1 and B1 symmetries and harden with cooling according to Cochran law. Circularly polarized Raman scattering did not reveal the angular momentum of these singly degenerate phonons at the Gamma point, which is consistent with theory. We also calculated the phonon branches in the whole Brillouin zone for both crystalline phases and compared the results with the phonons observable in the Raman spectra. The calculations revealed that some phonons with nonzero k have angular momentum in the chiral phase. A pronounced circular motion of atoms can be observed, for example, in a Dirac-type topological phonon at the M-point of the Brillouin zone with a frequency of 168 cm-1.

cond-mat.mtrl-sci↗

arXiv2026

Dielectric, magnetic and lattice dynamics properties of double perovskite (Ca0.5Mn1.5)MnWO6

Recent dielectric and magnetic studies of (Ca0.5Mn1.5)MnWO6 ceramics [A.A. Belik, Chem. Mater. 36, 7604 (2024)] have classified this material as a rare hybrid multiferroic, with both antiferromagnetic and (anti)ferroelectric ordering occurring at the same temperature of 22 K. The pronounced dielectric anomaly observed at this temperature indicated that the structural change is primarily induced by a phonon soft mode and not by a spin arrangement, as is usually the case in type II multiferroics. However, our comprehensive investigation involving new ceramic samples as well as the sample from the above-mentioned reference does not support this conclusion. Low-temperature polarization measurements revealed no evidence of either ferroelectric or antiferroelectric order in both sample series. The dielectric permittivity exhibits only a slight change at the antiferromagnetic transition, and phonon modes observed in IR and Raman spectra show no indication of a symmetry change at low temperatures. In the new samples the Neel temperature is shifted to TN = 18 K. XRD, SEM, EDS and WDS analyses confirmed the composition (Ca0.5Mn1.5)MnWO6 of both ceramics, but also indicated a small amount (percentage points) of MnO and CaO impurities in the sample from the previous publication and Mn3O4, CaWO4 secondary phases (<4%) in the new ceramics. The differences in dielectric and magnetic properties of the two samples can therefore be explained by their different chemical purity. The small dielectric anomaly of the new sample at the antiferromagnetic transition temperature is explained by a spin-phonon coupling. We conclude that (Ca0.5Mn1.5)MnWO6 is not a multiferroic, but a paraelectric antiferromagnet.

cond-mat.mtrl-sci↗

arXiv2025

Hierarchical Neural Collapse Detection Transformer for Class Incremental Object Detection

Recently, object detection models have witnessed notable performance improvements, particularly with transformer-based models. However, new objects frequently appear in the real world, requiring detection models to continually learn without suffering from catastrophic forgetting. Although Incremental Object Detection (IOD) has emerged to address this challenge, these existing models are still not practical due to their limited performance and prolonged inference time. In this paper, we introduce a novel framework for IOD, called Hier-DETR: Hierarchical Neural Collapse Detection Transformer, ensuring both efficiency and competitive performance by leveraging Neural Collapse for imbalance dataset and Hierarchical relation of classes' labels.

cs.CV↗