24,762 research outputs found
Exactly Solvable Model for Helix-Coil-Sheet Transitions in Protein Systems
In view of the important role helix-sheet transitions play in protein
aggregation, we introduce a simple model to study secondary structural
transitions of helix-coil-sheet systems using a Potts model starting with an
effective Hamiltonian. This energy function depends on four parameters that
approximately describe entropic and enthalpic contributions to the stability of
a polypeptide in helical and sheet conformations. The sheet structures involve
long-range interactions between residues which are far in sequence, but are in
contact in real space. Such contacts are included in the Hamiltonian. Using
standard statistical mechanical techniques, the partition function is solved
exactly using transfer matrices. Based on this model, we study thermodynamic
properties of polypeptides, including phase transitions between helix, sheet,
and coil structures.Comment: Updated version with correction
Color Magnitude Relation and Morphology of Low-Redshift ULIRGs in SDSS
We present color-magnitude and morphological analysis of 54 low-redshift
ULIRGs, a subset of the IRAS 1Jy sample (Kim & Sanders, 1998), in the SDSS. The
ULIRGs are on average 1 magnitude brighter in M0.1r than the SDSS galaxies
within the same redshift range. The majority of the ULIRGs (~87%) have the
colors typical of the blue cloud, and only 4 sources (~7%) are located in the
red sequence. While ULIRGs are popularly thought to be precursors to a QSO
phase, we find few (~6%) in the "green valley" where the majority of the X-ray
and IR selected AGNs are found, and none of which harbors an AGN. For the 14
previously spectroscopic identified AGNs (~28%), we perform PSF subtractions
and find that on average the central point sources contribute less than one
third to the total luminosity, and that their high optical luminosities and
overall blue colors are apparently the result of star formation activity of the
host galaxies. Visual inspection of the SDSS images reveals a wide range of
disturbed morphologies. A detailed morphology analysis using Gini and M20
coefficients shows that slightly less than one half (~42% in g band) of the
ULIRGs are located in the region where most local mergers are found. The
heterogeneous distribution of ULIRGs in the G-M20 space is qualitatively
consistent with the results found by numerical simulations of disk-disk
mergers. Our study also shows that the measured morphological parameters are
systematically affected by the SNR and thus the merging galaxies can appear at
various regions in the G-M20 space. In general, our results reinforce the view
that ULIRGs contain young stellar populations and are mergers in progress. Our
study provides a uniform comparison sample for studying ULIRGs at higher
redshifts such as Spitzer mid-IR selected ULIRGs at z=1~2 and submm galaxies.Comment: 42 pages, 11 figures, ApJ accepte
Optical and transport gaps in gated bilayer graphene
We discuss the effect of disorder on the band gap measured in bilayer
graphene in optical and transport experiments. By calculating the optical
conductivity and density of states using a microscopic model in the presence of
disorder, we demonstrate that the gap associated with transport experiments is
smaller than that associated with optical experiments. Intrinsic bilayer
graphene has an optical conductivity in which the energy of the peaks
associated with the interband transition are very robust against disorder and
thus provide an estimate of the band gap. In contrast, extraction of the band
gap from the optical conductivity of extrinsic bilayer graphene is almost
impossible for significant levels of disorder due to the ambiguity of the
transition peaks. The density of states contains an upper bound on the gap
measured in transport experiments, and disorder has the effect of reducing this
gap which explains why these experiments have so far been unable to replicate
the large band gaps seen in optical measurements.Comment: 5 pages, 5 figures, RevTeX. Published versio
The Scatter in the Relationship between Redshift and the Radio-to-Submm Spectral Index
We derive the scatter in the relationship between redshift and radio-to-submm
spectral index, alpha^{350}_{1.4}, using the observed spectral energy
distributions of 17 low redshift star forming galaxies. A mean galaxy model is
derived, along with the rms scatter in alpha^{350}_{1.4}. The scatter is
roughly constant with redshift. Constant rms scatter, combined with the
flattening of the mean alpha^{350}_{1.4} -- z relationship with increasing
redshift, leads to increasing uncertainty for redshift estimates at high
redshifts. Normalizing by the dust temperature in the manner proposed by Blain
decreases the scatter in alpha^{350}_{1.4} for most of the sample, but does not
remove outliers, and free-free absorption at rest frequencies above 1.4 GHz is
not likely to be a dominant cause for scatter in the alpha^{350}_{1.4} -- z
relationship. We re-derive the cumulative redshift distribution of the 14 field
galaxies in a recent submm and radio source sample of Smail et al.. The most
likely median redshift for the distribution is 2.7, with a conservative lower
limit of z = 2, as was also found by Smail et al. based on the original
alpha^{350}_{1.4} -- z models. The normalization and shape of the redshift
distribution for the faint submm sources are consistent with those expected for
forming elliptical galaxies.Comment: Added Erratum, standard AAS LATEX forma
The Solar pp and hep Processes in Effective Field Theory
The strategy of modern effective field theory is exploited to pin down
accurately the flux factors for the and processes in the Sun.
The technique used is to combine the high accuracy established in few-nucleon
systems of the "standard nuclear physics approach" (SNPA) and the systematic
power counting of chiral perturbation theory (ChPT) into a consistent effective
field theory framework. Using highly accurate wave functions obtained in the
SNPA and working to \nlo3 in the chiral counting for the current, we make
totally parameter-free and error-controlled predictions for the and
processes in the Sun.Comment: 5 pages, aipproc macros are included. Talk given at International
Nuclear Physics Conference 2001, Berkeley, California, July 30 - August 3,
200
Deformation of glass forming metallic liquids: Configurational changes and their relation to elastic softening
The change in the configurational enthalpy of metallic glass forming liquids induced by mechanical deformation and its effect on elastic softening is assessed. The acoustically measured shear modulus is found to decrease with increasing configurational enthalpy by a dependence similar to one obtained by softening via thermal annealing. This establishes that elastic softening is governed by a unique functional relationship between shear modulus and configurational enthalpy
Controlled and combined remote implementations of partially unknown quantum operations of multiqubits using GHZ states
We propose and prove protocols of controlled and combined remote
implementations of partially unknown quantum operations belonging to the
restricted sets [An Min Wang: PRA, \textbf{74}, 032317(2006)] using GHZ states.
We detailedly describe the protocols in the cases of one qubit, respectively,
with one controller and with two senders. Then we extend the protocols to the
cases of multiqubits with many controllers and two senders. Because our
protocols have to demand the controller(s)'s startup and authorization or two
senders together working and cooperations, the controlled and combined remote
implementations of quantum operations definitely can enhance the security of
remote quantum information processing and potentially have more applications.
Moreover, our protocol with two senders is helpful to farthest arrive at the
power of remote implementations of quantum operations in theory since the
different senders perhaps have different operational resources and different
operational rights in practice.Comment: 26 pages, the submitted versio
Current-driven dynamics of chiral ferromagnetic domain walls
In most ferromagnets the magnetization rotates from one domain to the next
with no preferred handedness. However, broken inversion symmetry can lift the
chiral degeneracy, leading to topologically-rich spin textures such as
spin-spirals and skyrmions via the Dzyaloshinskii-Moriya interaction (DMI).
Here we show that in ultrathin metallic ferromagnets sandwiched between a heavy
metal and an oxide, the DMI stabilizes chiral domain walls (DWs) whose spin
texture enables extremely efficient current-driven motion. We show that spin
torque from the spin Hall effect drives DWs in opposite directions in
Pt/CoFe/MgO and Ta/CoFe/MgO, which can be explained only if the DWs assume a
N\'eel configuration with left-handed chirality. We directly confirm the DW
chirality and rigidity by examining current-driven DW dynamics with magnetic
fields applied perpendicular and parallel to the spin spiral. This work
resolves the origin of controversial experimental results and highlights a new
path towards interfacial design of spintronic devices
Neutral Gas Properties and Ly Escape in Extreme Green Pea Galaxies
Mechanisms regulating the escape of Ly photons and ionizing radiation
remain poorly understood. To study these processes we analyze VLA 21cm
observations of one Green Pea (GP), J160810+352809 (hereafter J1608), and HST
COS spectra of 17 GP galaxies at . All are highly ionized: J1608 has the
highest [O III] /[O II] for star-forming galaxies in
SDSS, and the 17 GPs have [O III]/[O II] . We set an upper limit on
J1608's HI mass of , near or below average compared to
similar mass dwarf galaxies. In the COS sample, eight GPs show Ly
absorption components, six of which also have Ly emission. The HI
column densities derived from Ly absorption are high, cm, well above the LyC optically thick limit. Using
low-ionization absorption lines, we measure covering fractions
(f_{\mbox{cov}}) of , and find that f_{\mbox{cov}} strongly
anti-correlates with Ly escape fraction. Low covering fractions may
facilitate Ly and LyC escape through dense neutral regions. GPs with
f_{\mbox{cov}}\sim1 all have low neutral gas velocities, while GPs with lower
f_{\mbox{cov}}=0.2-0.6 have a larger range of velocities. Conventional
mechanical feedback may help establish low f_{\mbox{cov}} in some cases,
whereas other processes may be important for GPs with low velocities. Finally,
we compare f_{\mbox{cov}} with proposed indicators of LyC escape. Ionizing
photon escape likely depends on a combination of neutral gas geometry and
kinematics, complicating the use of emission-line diagnostics for identifying
LyC emitters.Comment: 21 pages, 11 figures, accepted for publication in Ap
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