39,363 research outputs found
Modeling two-state cooperativity in protein folding
A protein model with the pairwise interaction energies varying as local
environment changes, i.e., including some kinds of collective effect between
the contacts, is proposed. Lattice Monte Carlo simulations on the
thermodynamical characteristics and free energy profile show a well-defined
two-state behavior and cooperativity of folding for such a model. As a
comparison, related simulations for the usual G\={o} model, where the
interaction energies are independent of the local conformations, are also made.
Our results indicate that the evolution of interactions during the folding
process plays an important role in the two-state cooperativity in protein
folding.Comment: 5 figure
Electronic bandstructure and optical gain of lattice matched III-V dilute nitride bismide quantum wells for 1.55 m optical communication systems
Dilute nitride bismide GaNBiAs is a potential semiconductor alloy for near-
and mid-infrared applications, particularly in 1.55 m optical
communication systems. Incorporating dilute amounts of Bismuth (Bi) into GaAs
reduces the effective bandgap rapidly, while significantly increasing the
spin-orbit-splitting energy. Additional incorporation of dilute amounts of
Nitrogen (N) helps to attain lattice matching with GaAs, while providing a
route for flexible bandgap tuning. Here we present a study of the electronic
bandstructure and optical gain of the lattice matched
GaNBiAs/GaAs quaternary alloy quantum well (QW) based on the
16-band kp model. We have taken into consideration the interactions
between the N and Bi impurity states with the host material based on the band
anticrossing (BAC) and valence band anticrossing (VBAC) model. The optical gain
calculation is based on the density matrix theory. We have considered different
lattice matched GaNBiAs QW cases and studied their energy dispersion curves,
optical gain spectrum, maximum optical gain and differential gain; and compared
their performances based on these factors. The thickness and composition of
these QWs were varied in order to keep the emission peak fixed at 1.55 m.
The well thickness has an effect on the spectral width of the gain curves. On
the other hand, a variation in the injection carrier density has different
effects on the maximum gain and differential gain of QWs of varying
thicknesses. Among the cases studied, we found that the 6.3 nm thick
GaNBiAs lattice matched QW was most suited for 1.55
m (0.8 eV) GaAs-based photonic applications.Comment: Accepted in AIP Journal of Applied Physic
High energy neutrino early afterglows from gamma-ray bursts revisited
The high energy neutrino emission from gamma-ray bursts (GRBs) has been
expected in various scenarios. In this paper, we study the neutrino emission
from early afterglows of GRBs, especially under the reverse-forward shock model
and late prompt emission model. In the former model, the early afterglow
emission occurs due to dissipation made by an external shock with the
circumburst medium (CBM). In the latter model, internal dissipation such as
internal shocks produces the shallow decay emission in early afterglows. We
also discuss implications of recent Swift observations for neutrino signals in
detail. Future neutrino detectors such as IceCube may detect neutrino signals
from early afterglows, especially under the late prompt emission model, while
the detection would be difficult under the reverse-forward shock model.
Contribution to the neutrino background from the early afterglow emission may
be at most comparable to that from the prompt emission unless the outflow
making the early afterglow emission loads more nonthermal protons, and it may
be important in the very high energies. Neutrino-detections are inviting
because they could provide us with not only information on baryon acceleration
but also one of the clues to the model of early afterglows. Finally, we compare
various predictions for the neutrino background from GRBs, which are testable
by future neutrino-observations.Comment: 18 pages, 12 figures, accepted for publication in PR
Competing Quantum Orderings in Cuprate Superconductors: A Minimal Model
We present a minimal model for cuprate superconductors. At the unrestricted
mean-field level, the model produces homogeneous superconductivity at large
doping, striped superconductivity in the underdoped regime and various
antiferromagnetic phases at low doping and for high temperatures. On the
underdoped side, the superconductor is intrinsically inhomogeneous and global
phase coherence is achieved through Josephson-like coupling of the
superconducting stripes. The model is applied to calculate experimentally
measurable ARPES spectra.Comment: 5 pages, 4 eps included figure
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