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Within cell plasma membranes, unsaturated lipids are asymmetrically distributed over the inner and outer leaflets, offering an attractive local structural feature. However, the mechanism to keep lipid transmembrane asymmetry and the closely related transmembrane movement (flip-flop) for unsaturated lipids remain poorly understood. Here, we applied sum frequency generation vibrational spectroscopy to investigate this lipid transmembrane asymmetry upon mimicking the cell membrane homeostatic processes. On the one hand, unsaturated lipids were found to hinder the flip-flop process and preserve lipid transmembrane asymmetry in model cell membranes, owing to the steric hindrance caused by their bent tails. On the other hand, local unsaturated lipids in the mixed unsaturated/saturated lipid bilayer were conducive to the formation of the local asymmetry. Therefore, lipid unsaturation can be recognized as an intrinsic key factor to form and maintain lipid transmembrane asymmetry in cell membranes. [ABSTRACT FROM AUTHOR]

Polar metals, a family of materials with exclusive but coexisting electric polarization and metallicity, have attracted plenty of studies recently. Experimentally, how these two exclusive states interact with each other is still an open question thus far. Here, we report on the existence of ferroelectric metal states with switchable electric polarization and unexpected high carrier density in Ba0.5La0.5TiO3 films. A combination of atomic resolution scanning transmission electron microscopy, high-resolution x-ray diffraction, piezoresponse force microscopy, optical second harmonic generation, and electrical transport was utilized to investigate the crystal and electronic structures of Ba0.5La0.5TiO3 films. Unexpectedly, with the modulation of ferroelectricity, the density of conduction electrons can be tuned from 1019 to 1021 cm−3. Our results provide a way to design polar metals with coexisting switchable electric polarization and high-density conduction electrons. [ABSTRACT FROM AUTHOR]

Layered double hydroxides are one of the most effective electrocatalysts owing to their compositional and structural flexibility. However, their self-stacking leads to limited active sites and low conductivity. In this work, a three-dimensional (3D) core–shell architecture with spatially separated active sites has been fabricated based on layered ternary nickel cobalt iron hydroxide (NiCoFe-LTH) nanosheets and cobalt iron Prussian blue analog (CoFe-PBA) on nickel foam (NF) (i.e., CoFe-PBA@NiCoFe-LTH/NF) with CoFe-PBA as self-sacrificial templates that are partially in situ transformed into NiCoFe-LTH nanosheets. The CoFe-PBA@NiCoFe-LTH/NF has a well-defined core–shell 3D flower-like nanostructure in which NiCoFe-LTH nanosheets create the larger shell while CoFe-PBA nanocubes form the small core. This hybrid structure is evaluated as an electrocatalyst for oxygen evolution reaction (OER) and found to exhibit a low overpotential of 228 mV at 10 mA cm−2, a low Tafel slope of 36 mV dec−1, and good catalytic stability for 72 h in 1.0 M alkaline solution. The strong performance is attributed to the unique 3D core–shell flower-like nanosheet architecture that avoids the stacking of the 2D LTH, provides abundant spatially separated active sites, and enhances electron transport and stability. Furthermore, the OER mechanism and growth process of the electrocatalysts were systematically studied. These results suggest that such electrocatalysts with unique architecture are promising for efficient and durable OER. [ABSTRACT FROM AUTHOR]

A coupled-cavity linac (CCL) is widely used around the world in many fields. The radio frequency design is always a fundamental and crucial problem in CCL. In biperiodic structures, especially side-coupled structures, entirely different structures between accelerating cavities (ACs) and coupled cavities (CCs) will cause different frequency drops (δf) with the introduction of coupling slots. In order to achieve precise and consistent frequencies in each cell, the rule of individual cavity frequency drops should be figured out. A model of lumped resonant circuits is chosen to provide accurate prediction for normal mode frequencies. Through solving equivalent circuit equations, a parameter called frequency gap difference (ΔFg) is proposed, whose value is directly proportional to the frequency difference (df) between AC and CC and the ratio is one-to-one when AC frequency (fa) is close to CC frequency (fc). Based on the property of ΔFg, a simplified method on how to design and tune a triplet CCL is proposed; instead of tuning each individual cell, the new method only requires measuring the frequencies of the coupled normal modes, which considerably simplifies the tuning process and reduces the turn-around time. The particular design flow and tuning method have been successfully applied in the Nanjing University CCL Project. In addition, through comparing cavities with and without coupling slots, the value of frequency drops with different coupling coefficients is revealed and analyzed. [ABSTRACT FROM AUTHOR]