25,726 research outputs found
Time-dependent density-functional theory approach to nonlinear particle-solid interactions in comparison with scattering theory
An explicit expression for the quadratic density-response function of a
many-electron system is obtained in the framework of the time-dependent
density-functional theory, in terms of the linear and quadratic
density-response functions of noninteracting Kohn-Sham electrons and functional
derivatives of the time-dependent exchange-correlation potential. This is used
to evaluate the quadratic stopping power of a homogeneous electron gas for slow
ions, which is demonstrated to be equivalent to that obtained up to second
order in the ion charge in the framework of a fully nonlinear scattering
approach. Numerical calculations are reported, thereby exploring the range of
validity of quadratic-response theory.Comment: 14 pages, 3 figures. To appear in Journal of Physics: Condensed
Matte
Magnetoresistance, Micromagnetism, and Domain Wall Scattering in Epitaxial hcp Co Films
Large negative magnetoresistance (MR) observed in transport measurements of
hcp Co films with stripe domains were recently reported and interpreted in
terms of a novel domain wall (DW) scattering mechanism. Here detailed MR
measurements, magnetic force microscopy, and micromagnetic calculations are
combined to elucidate the origin of MR in this material. The large negative
room temperature MR reported previously is shown to be due to ferromagnetic
resistivity anisotropy. Measurements of the resistivity for currents parallel
(CIW) and perpendicular to DWs (CPW) have been conducted as a function of
temperature. Low temperature results show that any intrinsic effect of DWs
scattering on MR of this material is very small compared to the anisotropic MR.Comment: 5 pages, 5 Figures, submitted to PR
Electrical transport through single-molecule junctions: from molecular orbitals to conduction channels
We present an atomistic theory of electronic transport through single organic
molecules that reproduces the important features of the current-voltage
characteristics observed in recent experiments. We trace these features to
their origin in the electronic structure of the molecules and their local
atomic environment. We demonstrate how conduction channels arise from the
molecular orbitals and elucidate which specific properties of the individual
orbitals determine their contribution to the current.Comment: Revtex4, 4 pages, 4 figures. Version with color figures in
http://www-tfp.physik.uni-karlsruhe.de/~cuevas/Publications.htm
Speculations on Primordial Magnetic Helicity
We speculate that above or just below the electroweak phase transition
magnetic fields are generated which have a net helicity (otherwise said, a
Chern-Simons term) of order of magnitude , where is the
baryon or lepton number today. (To be more precise requires much more knowledge
of B,L-generating mechanisms than we currently have.) Electromagnetic helicity
generation is associated (indirectly) with the generation of electroweak
Chern-Simons number through B+L anomalies. This helicity, which in the early
universe is some 30 orders of magnitude greater than what would be expected
from fluctuations alone in the absence of B+L violation, should be reasonably
well-conserved through the evolution of the universe to around the times of
matter dominance and decoupling, because the early universe is an excellent
conductor. Possible consequences include early structure formation; macroscopic
manifestations of CP violation in the cosmic magnetic field (measurable at
least in principle, if not in practice); and an inverse-cascade dynamo
mechanism in which magnetic fields and helicity are unstable to transfer to
larger and larger spatial scales. We give a quasi-linear treatment of the
general-relativistic MHD inverse cascade instability, finding substantial
growth for helicity of the assumed magnitude out to scales , where is roughly the B+L to photon ratio and
is the magnetic correlation length. We also elaborate further on an
earlier proposal of the author for generation of magnetic fields above the EW
phase transition.Comment: Latex, 23 page
TANAMI - Tracking Active Galactic Nuclei with Austral Milliarcsecond Interferometry
We present a summary of the observation strategy of TANAMI (Tracking Active
Galactic Nuclei with Austral Milliarcsecond Interferometry), a monitoring
program to study the parsec-scale structure and dynamics of relativistic jets
in active galactic nuclei (AGN) of the Southern Hemisphere with the Australian
Long Baseline Array (LBA) and the trans-oceanic antennas Hartebeesthoek, TIGO,
and O'Higgins. TANAMI is focusing on extragalactic sources south of -30 degrees
declination with observations at 8.4 GHz and 22 GHz every ~2 months at
milliarcsecond resolution. The initial TANAMI sample of 43 sources has been
defined before the launch of the Fermi Gamma Ray Space Telescope to include the
most promising candidates for bright gamma-ray emission to be detected with its
Large Area Telescope (LAT). Since November 2008, we have been adding new
sources to the sample, which now includes all known radio- and gamma-ray bright
AGN of the Southern Hemisphere. The combination of VLBI and gamma-ray
observations is crucial to understand the broadband emission characteristics of
AGN and the nature of relativistic jets.Comment: Conference proceedings "2009 Fermi Symposium" eConf Proceedings
C09112
Calculation of the Density of States Using Discrete Variable Representation and Toeplitz Matrices
A direct and exact method for calculating the density of states for systems
with localized potentials is presented. The method is based on explicit
inversion of the operator . The operator is written in the discrete
variable representation of the Hamiltonian, and the Toeplitz property of the
asymptotic part of the obtained {\it infinite} matrix is used. Thus, the
problem is reduced to the inversion of a {\it finite} matrix
Impurity spin relaxation in S=1/2 XX chains
Dynamic autocorrelations (\alpha=x,z) of an
isolated impurity spin in a S=1/2 XX chain are calculated. The impurity spin,
defined by a local change in the nearest-neighbor coupling, is either in the
bulk or at the boundary of the open-ended chain. The exact numerical
calculation of the correlations employs the Jordan-Wigner mapping from spin
operators to Fermi operators; effects of finite system size can be eliminated.
Two distinct temperature regimes are observed in the long-time asymptotic
behavior. At T=0 only power laws are present. At high T the x correlation
decays exponentially (except at short times) while the z correlation still
shows an asymptotic power law (different from the one at T=0) after an
intermediate exponential phase. The boundary impurity correlations follow power
laws at all T. The power laws for the z correlation and the boundary
correlations can be deduced from the impurity-induced changes in the properties
of the Jordan-Wigner fermion states.Comment: Final version to be published in Phys. Rev. B. Three references
added, extended discussion of relation to previous wor
Cosmological post-Newtonian expansions to arbitrary order
We prove the existence of a large class of one parameter families of
solutions to the Einstein-Euler equations that depend on the singular parameter
\ep=v_T/c (0<\ep < \ep_0), where is the speed of light, and is a
typical speed of the gravitating fluid. These solutions are shown to exist on a
common spacetime slab M\cong [0,T)\times \Tbb^3, and converge as \ep
\searrow 0 to a solution of the cosmological Poisson-Euler equations of
Newtonian gravity. Moreover, we establish that these solutions can be expanded
in the parameter \ep to any specified order with expansion coefficients that
satisfy \ep-independent (nonlocal) symmetric hyperbolic equations
A Unified Conformal Model for Fundamental Interactions without Dynamical Higgs Field
A Higgsless model for strong, electro-weak and gravitational interactions is
proposed. This model is based on the local symmetry group SU(3)xSU(2)xU(1)xC
where C is the local conformal symmetry group. The natural minimal conformally
invariant form of total lagrangian is postulated. It contains all Standard
Model fields and gravitational interaction. Using the unitary gauge and the
conformal scale fixing conditions we can eliminate all four real components of
the Higgs doublet in this model. However the masses of vector mesons, leptons
and quarks are automatically generated and are given by the same formulas as in
the conventional Standard Model. The gravitational sector is analyzed and it is
shown that the model admits in the classical limit the Einsteinian form of
gravitational interactions. No figures.Comment: 25 pages, preprin
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