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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 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.


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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