2,654 research outputs found
Strong magnetoresistance induced by long-range disorder
We calculate the semiclassical magnetoresistivity of
non-interacting fermions in two dimensions moving in a weak and smoothly
varying random potential or random magnetic field. We demonstrate that in a
broad range of magnetic fields the non-Markovian character of the transport
leads to a strong positive magnetoresistance. The effect is especially
pronounced in the case of a random magnetic field where becomes
parametrically much larger than its B=0 value.Comment: REVTEX, 4 pages, 2 eps figure
Zero-frequency anomaly in quasiclassical ac transport: Memory effects in a two-dimensional metal with a long-range random potential or random magnetic field
We study the low-frequency behavior of the {\it ac} conductivity
of a two-dimensional fermion gas subject to a smooth random
potential (RP) or random magnetic field (RMF). We find a non-analytic
correction to , which corresponds to a
long-time tail in the velocity correlation function. This contribution
is induced by return processes neglected in Boltzmann transport theory. The
prefactor of this -term is positive and proportional to for
RP, while it is of opposite sign and proportional to in the weak RMF
case, where is the mean free path and the disorder correlation length.
This non-analytic correction also exists in the strong RMF regime, when the
transport is of a percolating nature. The analytical results are supported and
complemented by numerical simulations.Comment: 12 pages, RevTeX, 7 figure
Analyzing Machupo virus-receptor binding by molecular dynamics simulations
In many biological applications, we would like to be able to computationally
predict mutational effects on affinity in protein-protein interactions.
However, many commonly used methods to predict these effects perform poorly in
important test cases. In particular, the effects of multiple mutations,
non-alanine substitutions, and flexible loops are difficult to predict with
available tools and protocols. We present here an existing method applied in a
novel way to a new test case; we interrogate affinity differences resulting
from mutations in a host-virus protein-protein interface. We use steered
molecular dynamics (SMD) to computationally pull the machupo virus (MACV) spike
glycoprotein (GP1) away from the human transferrin receptor (hTfR1). We then
approximate affinity using the maximum applied force of separation and the area
under the force-versus-distance curve. We find, even without the rigor and
planning required for free energy calculations, that these quantities can
provide novel biophysical insight into the GP1/hTfR1 interaction. First, with
no prior knowledge of the system we can differentiate among wild type and
mutant complexes. Moreover, we show that this simple SMD scheme correlates well
with relative free energy differences computed via free energy perturbation.
Second, although the static co-crystal structure shows two large
hydrogen-bonding networks in the GP1/hTfR1 interface, our simulations indicate
that one of them may not be important for tight binding. Third, one viral site
known to be critical for infection may mark an important evolutionary
suppressor site for infection-resistant hTfR1 mutants. Finally, our approach
provides a framework to compare the effects of multiple mutations, individually
and jointly, on protein-protein interactions.Comment: 33 pages, 8 figures, 5 table
Comment on "Antilocalization in a 2D Electron Gas in a Random Magnetic Field"
In a recent Letter, Taras-Semchuk and Efetov reconsider the problem of
electron localization in a random magnetic field in two dimensions. They claim
that due to the long-range nature of the vector potential correlations an
additional term appears in the effective field theory (-model) of the
problem, leading to delocalization at the one-loop level. This calls into
question the results of earlier analytical studies, where the random magnetic
field problem was mapped onto the conventional unitary-class -model,
implying that the leading quantum correction is of two-loop order and of a
localizing nature. We show in this Comment, however, that the new term in fact
does not exist and was erroneously obtained by Taras-Semchuk and Efetov because
of an inconsistent treatment violating gauge invariance.Comment: 1 page, 2 figure
Re-entrant ferroelectricity in liquid crystals
The ferroelectric (Sm C) -- antiferroelectric (Sm C) -- reentrant
ferroelectric (re Sm C) phase temperature sequence was observed for system
with competing synclinic - anticlinic interactions. The basic properties of
this system are as follows (1) the Sm C phase is metastable in temperature
range of the Sm C stability (2) the double inversions of the helix
handedness at Sm C -- Sm C and Sm C% -- re-Sm C phase
transitions were found (3) the threshold electric field that is necessary to
induce synclinic ordering in the Sm C phase decreases near both Sm
C -- Sm C and Sm C -- re-Sm C phase boundaries, and it has
maximum in the middle of the Sm C stability region. All these properties
are properly described by simple Landau model that accounts for nearest
neighboring layer steric interactions and quadrupolar ordering only.Comment: 10 pages, 5 figures, submitted to PR
Local Isoelectronic Reactivity of Solid Surfaces
The quantity w^N(r) = ( 1/ k^2 T_el)[partial n(r, T_el) / partial
T_el]_(v(r),N) is introduced as a convenient measure of the local isoelectronic
reactivity of surfaces. It characterizes the local polarizability of the
surface and it can be calculated easily. The quantity w^N(r) supplements the
charge transfer reactivity measured e.g. by the local softness to which it is
closely related. We demonstrate the applicability and virtues of the function
w^N(r) for the example of hydrogen dissociation and adsorption on Pd(100).Comment: RevTeX, 13 pages, 3 figures, to appear in Phys. Rev. Let
Magnetic Photon Splitting: Computations of Proper-time Rates and Spectra
The splitting of photons in the presence of an intense magnetic field has
recently found astrophysical applications in polar cap models of gamma-ray
pulsars and in magnetar scenarios for soft gamma repeaters. Numerical
computation of the polarization-dependent rates of this third order QED process
for arbitrary field strengths and energies below pair creation threshold is
difficult: thus early analyses focused on analytic developments and simpler
asymptotic forms. The recent astrophysical interest spurred the use of the
S-matrix approach by Mentzel, Berg and Wunner to determine splitting rates. In
this paper, we present numerical computations of a full proper-time expression
for the rate of splitting that was obtained by Stoneham, and is exact up to the
pair creation threshold. While the numerical results derived here are in accord
with the earlier asymptotic forms due to Adler, our computed rates still differ
by as much as factors of 3 from the S-matrix re-evaluation of Wilke and Wunner,
reflecting the extreme difficulty of generating accurate S-matrix numerics for
fields below about \teq{4.4\times 10^{13}}Gauss. We find that our proper-time
rates appear very accurate, and exceed Adler's asymptotic specializations
significantly only for photon energies just below pair threshold and for
supercritical fields, but always by less than a factor of around 2.6. We also
provide a useful analytic series expansion for the scattering amplitude valid
at low energies.Comment: 13 pages, AASTeX format, including 3 eps figures, ApJ in pres
Crop moisture estimation over the southern Great Plains with dual polarization 1.66 centimeter passive microwave data from Nimbus 7
Moisture content of snow-free, unfrozen soil is inferred using passive microwave brightness temperatures from the scanning multichannel microwave radiometer (SMMR) on Nimbus-7. Investigation is restricted to the two polarizations of the 1.66 cm wavelength sensor. Passive microwave estimates of soil moisture are of two basic categories; those based upon soil emissivity and those based upon the polarization of soil emission. The two methods are compared and contrasted through the investigation of 54 potential functions of polarized brightness temperatures and, in some cases, ground-based temperature measurements. Of these indices, three are selected for the estimated emissivity, the difference between polarized brightness temperatures, and the normalized polarization difference. Each of these indices is about equally effective for monitoring soil moisture. Using an antecedent precipitation index (API) as ground control data, temporal and spatial analyses show that emissivity data consistently give slightly better soil moisture estimates than depolarization data. The difference, however, is not statistically significant. It is concluded that polarization data alone can provide estimates of soil moisture in areas where the emissivity cannot be inferred due to nonavailability of surface temperature data
Potential Energy Surface for H_2 Dissociation over Pd(100)
The potential energy surface (PES) of dissociative adsorption of H_2 on
Pd(100) is investigated using density functional theory and the full-potential
linear augmented plane wave (FP-LAPW) method. Several dissociation pathways are
identified which have a vanishing energy barrier. A pronounced dependence of
the potential energy on ``cartwheel'' rotations of the molecular axis is found.
The calculated PES shows no indication of the presence of a precursor state in
front of the surface. Both results indicate that steering effects determine the
observed decrease of the sticking coefficient at low energies of the H_2
molecules. We show that the topology of the PES is related to the dependence of
the covalent H(s)-Pd(d) interactions on the orientation of the H_2 molecule.Comment: RevTeX, 8 pages, 5 figures in uufiles forma
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