2,752 research outputs found
Transport in Graphene superimposed by a moving Electrical Superlattice Potential
We calculate dc-conductivities of ballistic graphene undulated by a overlying
moving unidirectional electrical superlattice (SL) potential whose SL-velocity
is smaller than the electron velocity. We obtain no dependence of the
conductivity on the velocity along the direction of the superlattice
wavevector. In the orthogonal direction however, the dependence is strong on
the velocity especially at voltages where a new Dirac point emerges for zero
velocity. It is shown that the infinite graphene system can serve as an ideal
motion detector at potentials where the first new Dirac point emerges. There
the conductivity is zero at vanishing SL velocities and jumps to infinity when
the SL starts moving. For finite systems at voltages where the number of new
Dirac points is of the order of the ratio of the electron velocity by the
SL-velocity, the modifications to the conductivity of a moving SL is at least
of similar magnitude as the conductivity of the stagnant SL.Comment: 10 pages, 3 figures, Appendix B added, version published in PR
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
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
Lindemann Parameters for solid Membranes focused on Carbon Nanotubes
Temperature fluctuations in the normal direction of planar crystals such as
graphene are quite violent and may be expected to influence strongly their
melting properties. In particular, they will modify the Lindemann melting
criterium. We calculate this modification in a self-consistent Born
approximation. The result is applied to graphene and its wrapped version
represented by single-walled carbon nanotubes (SWNTs). It is found that the
out-of-plane fluctuations dominate over the in-plane fluctuations. This makes
strong restrictions to possible Lindemann parameters. Astonishing we find that
these large out-of-plane fluctuations have only a small influence upon the
melting temperature.Comment: 6 pages, 1 figure, typos corrected, version published in PR
Magnetic field induced 3D to 1D crossover in type II superconductors
We review and analyze magnetization and specific heat investigations on
type-II superconductors which uncover remarkable evidence for the magnetic
field induced fnite size effect and the associated 3D to 1D crossover which
enhances thermal fluctuations.Comment: 26 pages, 19 figure
Modification of the rho meson detected by low-mass electron-positron pairs in central Pb-Au collisions at 158 A GeV/c
We present a measurement of pair production in central Pb-Au
collisions at 158 GeV/. As reported earlier, a significant excess of the
pair yield over the expectation from hadron decays is observed. The
improved mass resolution of the present data set, recorded with the upgraded
CERES experiment at the CERN-SPS, allows for a comparison of the data with
different theoretical approaches. The data clearly favor a substantial
in-medium broadening of the spectral function over a density-dependent
shift of the pole mass. The in-medium broadening model implies that
baryon induced interactions are the key mechanism to in-medium modifications of
the -meson in the hot fireball at SPS energy.Comment: Revised versio
Event-by-event fluctuations at SPS
Results on event-by-event fluctuations of the mean transverse momentum and
net charge in Pb-Au collisions, measured by the CERES Collaboration at
CERN-SPS, are presented. We discuss the centrality and beam energy dependence
and compare our data to cascade calculations.Comment: 4 pages, 4 figures, proceedings to INPC2004 Goteborg, Swede
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