404 research outputs found
Fully Coherent X-ray Pulses from a Regenerative Amplifier Free Electron Laser
We propose and analyze a novel regenerative amplifier free electron laser
(FEL) to produce fully coherent x-ray pulses. The method makes use of
narrow-bandwidth Bragg crystals to form an x-ray feedback loop around a
relatively short undulator. Self-amplified spontaneous emission (SASE) from the
leading electron bunch in a bunch train is spectrally filtered by the Bragg
reflectors and is brought back to the beginning of the undulator to interact
repeatedly with subsequent bunches in the bunch train. The FEL interaction with
these short bunches not only amplifies the radiation intensity but also
broadens its spectrum, allowing for effective transmission of the x-rays
outside the crystal bandwidth. The spectral brightness of these x-ray pulses is
about two to three orders of magnitude higher than that from a single-pass SASE
FEL.Comment: 11 pages, 6 figure
Emergence of a Chern-insulating state from a semi-Dirac dispersion
A Chern insulator (quantum anomalous Hall insulator) phase is demonstrated to
exist in a typical semi-Dirac system, the TiO2/VO2 heterostructure. By
combining first-principles calculations with Wannier-based tight-binding model,
we calculate the Berry curvature distribution, finding a Chern number of -2 for
the valence bands, and demonstrate the existence of gapless chiral edge states,
ensuring quantization of the Hall conductivity to 2e^2/h. A new semi-Dirac
model, where each semi-Dirac cone is formed by merging three conventional Dirac
points, is proposed to reveal how the nontrivial topology with finite Chern
number is compatible with a semi-Dirac electronic spectrum.Comment: 12 pages, 3 figure
Nonnative Interactions in Coupled Folding and Binding Processes of Intrinsically Disordered Proteins
Proteins function by interacting with other molecules, where both native and nonnative interactions play important roles. Native interactions contribute to the stability and specificity of a complex, whereas nonnative interactions mainly perturb the binding kinetics. For intrinsically disordered proteins (IDPs), which do not adopt rigid structures when being free in solution, the role of nonnative interactions may be more prominent in binding processes due to their high flexibilities. In this work, we investigated the effect of nonnative hydrophobic interactions on the coupled folding and binding processes of IDPs and its interplay with chain flexibility by conducting molecular dynamics simulations. Our results showed that the free-energy profiles became rugged, and intermediate states occurred when nonnative hydrophobic interactions were introduced. The binding rate was initially accelerated and subsequently dramatically decreased as the strength of the nonnative hydrophobic interactions increased. Both thermodynamic and kinetic analysis showed that disordered systems were more readily affected by nonnative interactions than ordered systems. Furthermore, it was demonstrated that the kinetic advantage of IDPs (“fly-casting” mechanism) was enhanced by nonnative hydrophobic interactions. The relationship between chain flexibility and protein aggregation is also discussed
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