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Yuan H.Y.,University of Science and Technology of China | Wang X.R.,Hong Kong University of Science and Technology | Wang X.R.,HKUST Shenzhen Research Institute
Applied Physics Letters | Year: 2017

Magnon-photon coupling in antiferromagnets has many attractive features that do not exist in ferro- or ferrimagnets. We show quantum-mechanically that, in the absence of an external field, one of the two degenerated spin wave bands couples with photons, while the other does not. The photon mode anticrosses with the coupled spin waves when their frequencies are close to each other. Similar to its ferromagnetic counterpart, the magnon-photon coupling strength is proportional to the square root of the number of spins N in antiferromagnets. An external field removes the spin wave degeneracy, and both spin wave bands couple to the photons, resulting in two anticrossings between the magnons and photons. Two transmission peaks were observed near the anticrossing frequency. The maximum damping that allows clear discrimination of the two transmission peaks is proportional to N, and it is well below the damping of antiferromagnetic insulators. Therefore, the strong magnon-photon coupling can be realized in antiferromagnets and the coherent information transfer between the photons and magnons is possible. © 2017 Author(s).


Yuan H.Y.,Hong Kong University of Science and Technology | Wang X.R.,HKUST Shenzhen Research Institute
Physical Review B - Condensed Matter and Materials Physics | Year: 2014

We theoretically and numerically study magnetic domain wall (DW) pinning at a notch in a magnetic nanowire. Based on the static DW equation, a general relationship between an external field and a DW structure for a given notch geometry is found. By estimating the field below which this relationship holds, we obtain the depinning field theoretically. Our theoretical estimate of the depinning field compares well with simulation results. Furthermore, our theory explains well why the depinning field of a transverse wall of one chirality is larger than that of the opposite chirality. © 2014 American Physical Society.


Wang X.S.,Hong Kong University of Science and Technology | Wang X.S.,HKUST Shenzhen Research Institute | Wang X.R.,Hong Kong University of Science and Technology | Wang X.R.,HKUST Shenzhen Research Institute
Physical Review B - Condensed Matter and Materials Physics | Year: 2014

Spin waves (or magnons) interact with magnetic domain walls (DWs) in a complicated way that a DW can propagate either along or against magnon flow. However, thermally activated magnons always drive a DW to the hotter region of a nanowire of magnetic insulators under a temperature gradient. We theoretically illustrate why it is surely so by showing that DW entropy is always larger than that of a domain as long as material parameters do not depend on spin textures. Equivalently, the total free energy of the wire can be lowered when the DW moves to the hotter region. The larger DW entropy is related to the increase of magnon density of states at low energy originated from the gapless magnon bound states. © 2014 American Physical Society.


Hu R.,HKUST Shenzhen Research Institute | Hu R.,Hong Kong University of Science and Technology | Leung N.L.C.,Hong Kong University of Science and Technology | Tang B.Z.,HKUST Shenzhen Research Institute | And 2 more authors.
Chemical Society Reviews | Year: 2014

Macromolecules with aggregation-induced emission (AIE) attributes are a class of luminescent materials that display enhanced emission when they are aggregated. They have attracted much attention because of their good solubility, processability, high emission efficiency in the aggregated states, etc. A large variety of AIE macromolecules have been developed, showing exponential growth of research interest in this field. This review summarizes the design principles and recent synthetic advancements, topological structures, as well as the frontiers of functionalities and potential applications of AIE macromolecules, especially fluorescence sensing, biological applications and optoelectronic applications, with an emphasis on the recent progress. New luminogenic systems without conventional chromophores displaying aggregated state emission are discussed. The highly dense clusters of heteroatoms with lone pair electrons in these systems may serve as the chromophore and are cited as "heterodox clusters". It is expected that the mechanistic insights into the AIE phenomena, based on the restriction of intramolecular motions and structure rigidification, can guide the future design of AIE materials with fascinating structures and functionalities. © 2014 The Royal Society of Chemistry.


Chen Y.,Hong Kong University of Science and Technology | Chen Y.,HKUST Shenzhen Research Institute | Fu A.K.Y.,Hong Kong University of Science and Technology | Fu A.K.Y.,HKUST Shenzhen Research Institute | And 2 more authors.
Cellular Signalling | Year: 2012

Precise regulation of synapse formation, maintenance and plasticity is crucial for normal cognitive function, and synaptic failure has been suggested as one of the hallmarks of neurodegenerative diseases. In this review, we describe the recent progress in our understanding of how the receptor tyrosine kinase Ephs and their ligands ephrins regulate dendritic spine morphogenesis, synapse formation and maturation, as well as synaptic plasticity. In particular, we discuss the emerging evidence implicating that deregulation of Eph/ephrin signaling contributes to the aberrant synaptic functions associated with cognitive impairment in Alzheimer's disease. Understanding how Eph/ephrin regulates synaptic function may therefore provide new insights into the development of therapeutic agents against neurodegenerative diseases. © 2011 Elsevier Inc..


Hu B.,Hong Kong University of Science and Technology | Hu B.,HKUST Shenzhen Research Institute | Wang X.R.,Hong Kong University of Science and Technology | Wang X.R.,HKUST Shenzhen Research Institute
Physical Review Letters | Year: 2013

The stability of the well-known Walker propagating domain wall (DW) solution of the Landau-Lifshitz-Gilbert equation is analytically investigated. Surprisingly, a propagating DW is always dressed with spin waves so that the Walker rigid-body propagating DW mode does not occur in reality. In the low field region only stern spin waves are emitted while both stern and bow waves are generated under high fields. In a high enough field, but below the Walker breakdown field, the Walker solution could be convective or absolute unstable if the transverse magnetic anisotropy is larger than a critical value, corresponding to a significant modification of the DW profile and DW propagating speed. © 2013 American Physical Society.


Peng Y.,Hong Kong University of Science and Technology | Wang F.,Hong Kong University of Science and Technology | Wang Z.,Hong Kong University of Science and Technology | Alsayed A.M.,Solvay Group | And 4 more authors.
Nature Materials | Year: 2015

The microscopic kinetics of ubiquitous solid-solid phase transitions remain poorly understood. Here, by using single-particle-resolution video microscopy of colloidal films of diameter-tunable microspheres, we show that transitions between square and triangular lattices occur via a two-step diffusive nucleation pathway involving liquid nuclei. The nucleation pathway is favoured over the direct one-step nucleation because the energy of the solid/liquid interface is lower than that between solid phases. We also observed that nucleation precursors are particle-swapping loops rather than newly generated structural defects, and that coherent and incoherent facets of the evolving nuclei exhibit different energies and growth rates that can markedly alter the nucleation kinetics. Our findings suggest that an intermediate liquid should exist in the nucleation processes of solid-solid transitions of most metals and alloys, and provide guidance for better control of the kinetics of the transition and for future refinements of solid-solid transition theory.


Wang Z.,Hong Kong University of Science and Technology | Wang Z.,Zhejiang University | Wang Z.,HKUST Shenzhen Research Institute | Chen S.,Hong Kong University of Science and Technology | And 10 more authors.
Journal of the American Chemical Society | Year: 2013

There is a great demand for long-term cellular tracers because of their great importance in monitoring biological processes, pathological pathways, therapeutic effects, etc. Herein we report a new type of fluorescence "turn-on" probe for tracing live cells over a long period of time. We synthesized the fluorogenic probe by attaching a large number of tetraphenylethene (TPE) labels to a chitosan (CS) chain. The resultant TPE-CS bioconjugate shows a unique aggregation-induced emission (AIE) behavior. It is nonfluorescent when dissolved but becomes highly emissive when its molecules are aggregated. The AIE aggregates can be readily internalized by HeLa cells. The cellular staining by the TPE-CS aggregates is so indelible that it enables cell tracing for as long as 15 passages. The internalized AIE aggregates are kept inside the cellular compartments and do not contaminate other cell lines in the coculture systems, permitting the differentiation of specific cancerous cells from normal healthy cells. © 2013 American Chemical Society.


Chen S.,Hong Kong University of Science and Technology | Hong Y.,Hong Kong University of Science and Technology | Liu Y.,Hong Kong University of Science and Technology | Liu J.,Hong Kong University of Science and Technology | And 9 more authors.
Journal of the American Chemical Society | Year: 2013

Intracellular pH (pHi) is an important parameter associated with cellular behaviors and pathological conditions. Sensing pHi and monitoring its changes in live cells are essential but challenging due to the lack of effective probes. We herein report a pH-sensitive fluorogen for pH i sensing and tracking. The dye is a tetraphenylethene-cyanine adduct (TPE-Cy). It is biocompatible and cell-permeable. Upon diffusing into cells, it responds sensitively to pHi in the entire physiological range, visualizing the acidic and basic compartments with intense red and blue emissions, respectively. The ratiometric signal of the red and blue channels can thus serve as an indicator for local proton concentration. The utility of TPE-Cy in pHi imaging and monitoring is demonstrated with the use of confocal microscopy, ratiometric analysis, and flow cytometry. © 2013 American Chemical Society.


Liu Y.,Hong Kong University of Science and Technology | Zhao J.,Hong Kong University of Science and Technology | Li Z.,Hong Kong University of Science and Technology | Mu C.,Hong Kong University of Science and Technology | And 7 more authors.
Nature Communications | Year: 2014

Although the field of polymer solar cell has seen much progress in device performance in the past few years, several limitations are holding back its further development. For instance, current high-efficiency (>9.0%) cells are restricted to material combinations that are based on limited donor polymers and only one specific fullerene acceptor. Here we report the achievement of high-performance (efficiencies up to 10.8%, fill factors up to 77%) thick-film polymer solar cells for multiple polymer:fullerene combinations via the formation of a near-ideal polymer:fullerene morphology that contains highly crystalline yet reasonably small polymer domains. This morphology is controlled by the temperature-dependent aggregation behaviour of the donor polymers and is insensitive to the choice of fullerenes. The uncovered aggregation and design rules yield three high-efficiency (>10%) donor polymers and will allow further synthetic advances and matching of both the polymer and fullerene materials, potentially leading to significantly improved performance and increased design flexibility. & © 2014 Macmillan Publishers Limited. All rights reserved.

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