2,827 research outputs found
Modelling two-dimensional Crystals with Defects under Stress: Superelongation of Carbon Nanotubes at high Temperatures
We calculate analytically the phase diagram of a two-dimensional square
crystal and its wrapped version with defects under external homogeneous stress
as a function of temperature using a simple elastic lattice model that allows
for defect formation. The temperature dependence turns out to be very weak. The
results are relevant for recent stress experiments on carbon nanotubes. Under
increasing stress, we find a crossover regime which we identify with a cracking
transition that is almost independent of temperature. Furthermore, we find an
almost stress-independent melting point. In addition, we derive an enhanced
ductility with relative strains before cracking between 200-400%, in agreement
with carbon nanotube experiments. The specific values depend on the Poisson
ratio and the angle between the external force and the crystal axes. We give
arguments that the results for carbon nanotubes are not much different to the
wrapped square crystal.Comment: 12 pages, 6 eps figures, section VI added discussing the
modifications of our model when applied to tube
Optical Phonon Lineshapes and Transport in Metallic Carbon Nanotubes under High Bias Voltage
We calculate the current-voltage characteristic of metallic nanotubes at high
bias voltage showing that a bottleneck exists for short nanotubes in contrast
to large ones. We attribute this to a redistribution of lower-lying acoustic
phonons caused by phonon-phonon scattering with hot optical phonons. The
current-voltage characteristic and the electron and phonon distribution
functions are derived analytically, and serve to obtain in a self-contained way
the frequency shift and line broadening of the zone center optical phonons due
to the electron-phonon coupling at high bias. We obtain a positive frequency
shift from the zero bias shift and no broadening of the optical phonon mode at
very high voltages, in agreement with recent experiments.Comment: 16 pages, 8 figures, minor changes, pusblished in PR
Beyond the random phase approximation in the Singwi-Sj\"olander theory of the half-filled Landau level
We study the Chern-Simons system and consider a self-consistent
field theory of the Singwi-Sj\"olander type which goes beyond the random phase
approximation (RPA). By considering the Heisenberg equation of motion for the
longitudinal momentum operator, we are able to show that the zero-frequency
density-density response function vanishes linearly in long wavelength limit
independent of any approximation. From this analysis, we derive a consistency
condition for a decoupling of the equal time density-density and
density-momentum correlation functions. By using the Heisenberg equation of
motion of the Wigner distribution function with a decoupling of the correlation
functions which respects this consistency condition, we calculate the response
functions of the system. In our scheme, we get a density-density
response function which vanishes linearly in the Coulomb case for
zero-frequency in the long wavelength limit. Furthermore, we derive the
compressibility, and the Landau energy as well as the Coulomb energy. These
energies are in better agreement to numerical and exact results, respectively,
than the energies calculated in the RPA.Comment: 9 Revtex pages, 4 eps figures, typos correcte
Phonon-induced superconductivity at high temperatures in electrical graphene superlattices
We discuss the BCS theory for electrons in graphene with a superimposed
electrical unidirectional superlattice (SL) potential. New Dirac points emerge
together with van Hove singularities (VHSs) linking them. We obtain a
superconducting transition temperature Tc for chemical potentials close to the
VHSs assuming that acoustic phonon coupling should be the dominant mechanism.
Pairing of two onsite electrons with one electron close to the K and the other
close to the −K point is the most stable pair formation. The resulting order
parameter is almost constant over the entire SL
Phase Diagram of Vortices in High-T_c Superconductors with a Melting Line in the deep H_{c2} Region
We use a simple elastic Hamiltonian for the vortex lattice in a weak impurity
background which includes defects in the form of integer-valued fields to
calculate the free energy of a vortex lattice in the deep H_{c2} region. The
phase diagram in this regime is obtained by applying the variational approach
of M{\'e}zard and Parisi developed for random manifolds. We find a first-order
line between the Bragg-glass and vortex-glass phase as a continuation of the
melting line. In the liquid phase, we obtain an almost vertical third-order
glass transition line near the critical temperature in the H-T plane.
Furthermore, we find an almost vertical second-order phase transition line in
the Bragg-glass as well as the vortex-glass phases which crosses the
first-order Bragg-glass, vortex-glass transition line. We calculate the jump of
the temperature derivate of the induction field across this second-order line
as well as the entropy and magnetic field jumps across the first-order line.Comment: 19 pages, 5 figures, a discussion is added at the end of section VI
relating our results with the results of Refs. 4, 10, typos corrected,
version published in PR
Uncoupling of EGFR–RAS signaling and nuclear localization of YBX1 in colorectal cancer
The transcription factor YBX1 can act as a mediator of signals transmitted via
the EGFR–RAS–MAPK axis. YBX1 expression has been associated with tumor
progression and prognosis in multiple types of cancer. Immunohistochemical
studies have revealed dependency between YBX1 expression and individual EGFR
family members. We analyzed YBX1 and EGFR family proteins in a colorectal
cancer (CRC) cohort and provide functional analyses of YBX1 in the context of
EGFR–RAS–MAPK signaling. Immunohistochemistry for YBX1 and EGFR family
receptors with two antibodies for YBX1 and EGFR were performed and related to
clinicopathological data. We employed Caco2 cells expressing an inducible
KRASV12 gene to determine effects on localization and levels of YBX1. Mouse
xenografts of Caco2-KRASV12 cells were used to determine YBX1 dynamics in a
tissue context. The two different antibodies against YBX1 showed discordant
immunohistochemical stainings in cell culture and clinical specimens.
Expression of YBX1 and EGFR family members were not correlated in CRC.
Analysis of Caco2 xenografts displayed again heterogeneity of YBX1 staining
with both antibodies. Our results suggest that YBX1 is controlled via complex
regulatory mechanisms involving tumor stroma interaction and signal
transduction processes. Our study highlights that YBX1 antibodies have
different specificities, advocating their use in a combined manner
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