130 research outputs found
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Dynamic deformability of individual PbSe nanocrystals during superlattice phase transitions
The behavior of individual nanocrystals during superlattice phase transitions can profoundly affect the structural perfection and electronic properties of the resulting superlattices. However, details of nanocrystal morphological changes during superlattice phase transitions are largely unknown due to the lack of direct observation. Here, we report the dynamic deformability of PbSe semiconductor nanocrystals during superlattice phase transitions that are driven by ligand displacement. Real-time high-resolution imaging with liquid-phase transmission electron microscopy reveals that following ligand removal, the individual PbSe nanocrystals experience drastic directional shape deformation when the spacing between nanocrystals reaches 2 to 4 nm. The deformation can be completely recovered when two nanocrystals move apart or it can be retained when they attach. The large deformation, which is responsible for the structural defects in the epitaxially fused nanocrystal superlattice, may arise from internanocrystal dipole-dipole interactions
Seeing is believing: The colour of silver alloys and the global silver circulation in the Chinese Ming and Qing dynasties
This research investigates the correlation between colour perception and the circulation of silver in China during the Ming (1368–1644 CE) and Qing (1644–1911 CE) dynasties. The primary aim is to deepen our understanding of how silver alloys were perceived and experienced in this historical context while also situating our study within the broader context of the global silver trade. During the Ming and Qing dynasties, silver possessed immense historical significance as a precious commodity. We argue that copper had a more substantial influence on the final colour of silver alloys compared to lead. Furthermore, employing a colourimetric model, our Monte‐Carlo simulation demonstrates that over 70% of silver from Mesoamerica to China could be discerned by nonexperts using only their unaided vision, largely due to the elevated copper content. Crucially, our simulation experiment reveals differing effects of copper and lead on the colour of silver alloys. The latter demonstrates minimal change until reaching a threshold of 15%, signifying that lead is a suitable and cost‐effective substitute for silver. These findings suggest that the detection of silver purity was less demanding than previously assumed, opening up opportunities for arbitrage
Controlling photonic spin Hall effect via exceptional points
The photonic spin Hall effect (SHE), featured by a spin-dependent transverse
shift of an impinging optical beam driven by its polarization handedness, has
many applications including precise metrology and spin-based nanophotonic
devices. It is highly desirable to control and enhance the photonic SHE.
However, such a goal remains elusive, due to the weak spin-orbit interaction of
light, especially for systems with optical loss. Here we reveal a flexible way
to modulate the photonic SHE via exceptional points, by exploiting the
transverse shift in a parity-time (PT) symmetric system with balanced gain and
loss. The underlying physics is associated with the near-zero value and abrupt
phase jump of the reflection coefficients at exceptional points. We find that
the transverse shift is zero at exceptional points, but it is largely enhanced
in their vicinity. In addition, the transverse shift switches its sign across
the exceptional point, resulting from spontaneous PT-symmetry breaking. Due to
the sensitivity of transverse shift at exceptional points, our work also
indicates that the photonic SHE can enable a precise way to probe the location
of exceptional point in photonic systems.Comment: 14 pages, 4 figure
Breakdown of effective-medium theory beyond the critical angle
Effective-medium theory pertains to the theoretical modelling of
homogenization, which aims to replace an inhomogeneous structure of
subwavelength-scale constituents with a homogeneous effective medium. The
effective-medium theory is fundamental to various realms, including
electromagnetics and material science, since it can largely decrease the
complexity in the exploration of light-matter interactions by providing simple
acceptable approximation. Generally, the effective-medium theory is thought to
be applicable to any all-dielectric system with deep-subwavelength
constituents, under the condition that the effective medium does not have a
critical angle, at which the total internal reflection occurs. Here we reveal a
fundamental breakdown of the effective-medium theory that can be applied in
very general conditions: showing it for deep-subwavelength all-dielectric
multilayers even without critical angle. Our finding relies on an exotic
photonic spin Hall effect, which is shown to be ultra-sensitive to the stacking
order of deep-subwavelength dielectric layers, since the spin-orbit interaction
of light is dependent on slight phase accumulations during the wave
propagation. Our results indicate that the photonic spin Hall effect could
provide a promising and powerful tool for measuring structural defects for
all-dielectric systems even in the extreme nanometer scale.Comment: 17 pages, 4 figure
Observation of spin-tensor induced topological phase transitions of triply degenerate points with a trapped ion
Triply degenerate points (TDPs), which correspond to new types of topological
semimetals, can support novel quasiparticles possessing effective integer spins
while preserving Fermi statistics. Here by mapping the momentum space to the
parameter space of a three-level system in a trapped ion, we experimentally
explore the transitions between different types of TDPs driven by
spin-tensor--momentum couplings. We observe the phase transitions between TDPs
with different topological charges by measuring the Berry flux on a loop
surrounding the gap-closing lines, and the jump of the Berry flux gives the
jump of the topological charge (up to a factor) across the transitions.
For the Berry flux measurement, we employ a new method by examining the
geometric rotations of both spin vectors and tensors, which lead to a
generalized solid angle equal to the Berry flux. The controllability of
multi-level ion offers a versatile platform to study high-spin physics and our
work paves the way to explore novel topological phenomena therein.Comment: 9 pages, 10 figure
Uncertainty-inspired Open Set Learning for Retinal Anomaly Identification
Failure to recognize samples from the classes unseen during training is a
major limit of artificial intelligence (AI) in real-world implementation of
retinal anomaly classification. To resolve this obstacle, we propose an
uncertainty-inspired open-set (UIOS) model which was trained with fundus images
of 9 common retinal conditions. Besides the probability of each category, UIOS
also calculates an uncertainty score to express its confidence. Our UIOS model
with thresholding strategy achieved an F1 score of 99.55%, 97.01% and 91.91%
for the internal testing set, external testing set and non-typical testing set,
respectively, compared to the F1 score of 92.20%, 80.69% and 64.74% by the
standard AI model. Furthermore, UIOS correctly predicted high uncertainty
scores, which prompted the need for a manual check, in the datasets of rare
retinal diseases, low-quality fundus images, and non-fundus images. This work
provides a robust method for real-world screening of retinal anomalies
Anti-mold, self-cleaning superhydrophobic bamboo fiber/polypropylene composites with mechanical durability
Bamboo fiber/polypropylene composites (BPCs) have been widely used in buildings, interior decoration, and automobile components. However, pollutants and fungi can interact with the hydrophilic bamboo fibers on the surface of Bamboo fiber/polypropylene composites, degrading their appearance and mechanical properties. To improve their anti-fouling and anti-mildew properties, a superhydrophobic modified Bamboo fiber/polypropylene composite (BPC-TiO2-F) was fabricated by introducing titanium dioxide (TiO2) and poly(DOPAm-co-PFOEA) onto the surface of a Bamboo fiber/polypropylene composite. The morphology of BPC-TiO2-F was analyzed by XPS, FTIR, and SEM. The results showed that TiO2 particles covered on Bamboo fiber/polypropylene composite surface via complexation between phenolic hydroxyl groups and Ti atoms. Low-surface-energy fluorine-containing poly(DOPAm-co-PFOEA) was introduced onto the Bamboo fiber/polypropylene composite surface, forming a rough micro/nanostructure that endowed BPC-TiO2-F with superhydrophobicity (water contact angle = 151.0° ± 0.5°). The modified Bamboo fiber/polypropylene composite exhibited excellent self-cleaning properties, and a model contaminant, Fe3O4 powder, was rapidly removed from the surface by water drops. BPC-TiO2-F showed excellent anti-mold performance, and no mold was on its surface after 28 days. The superhydrophobic BPC-TiO2-F had good mechanical durability and could withstand sandpaper abrasion with a weight load of 50 g, finger wiping for 20 cycles, and tape adhesion abrasion for 40 cycles. BPC-TiO2-F showed good self-cleaning properties, mildew resistance, and mechanical resistance, giving it promising applications for automotive upholstery and building decoration
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