110 research outputs found
Imaging Pauli repulsion in scanning tunneling microscopy
A scanning tunneling microscope (STM) has been equipped with a nanoscale
force sensor and signal transducer composed of a single D2 molecule that is
confined in the STM junction. The uncalibrated sensor is used to obtain
ultra-high geometric image resolution of a complex organic molecule adsorbed on
a noble metal surface. By means of conductance-distance spectroscopy and
corresponding density functional calculations the mechanism of the
sensor/transducer is identified. It probes the short-range Pauli repulsion and
converts this signal into variations of the junction conductance.Comment: 4 pages, 4 figures, accepted to Phys. Rev. Let
A Generalized Diffusion Tensor for Fully Anisotropic Diffusion of Energetic Particles in the Heliospheric Magnetic Field
The spatial diffusion of cosmic rays in turbulent magnetic fields can, in the
most general case, be fully anisotropic, i.e. one has to distinguish three
diffusion axes in a local, field-aligned frame. We reexamine the transformation
for the diffusion tensor from this local to a global frame, in which the Parker
transport equation for energetic particles is usually formulated and solved.
Particularly, we generalize the transformation formulas to allow for an
explicit choice of two principal local perpendicular diffusion axes. This
generalization includes the 'traditional' diffusion tensor in the special case
of isotropic perpendicular diffusion. For the local frame, we motivate the
choice of the Frenet-Serret trihedron which is related to the intrinsic
magnetic field geometry. We directly compare the old and the new tensor
elements for two heliospheric magnetic field configurations, namely the hybrid
Fisk and the Parker field. Subsequently, we examine the significance of the
different formulations for the diffusion tensor in a standard 3D model for the
modulation of galactic protons. For this we utilize a numerical code to
evaluate a system of stochastic differential equations equivalent to the Parker
transport equation and present the resulting modulated spectra. The computed
differential fluxes based on the new tensor formulation deviate from those
obtained with the 'traditional' one (only valid for isotropic perpendicular
diffusion) by up to 60% for energies below a few hundred MeV depending on
heliocentric distance.Comment: 8 pages, 6 figures, accepted in Ap
Density-functional theory of inhomogeneous electron systems in thin quantum wires
Motivated by current interest in strongly correlated quasi-one-dimensional
(1D) Luttinger liquids subject to axial confinement, we present a novel
density-functional study of few-electron systems confined by power-low external
potentials inside a short portion of a thin quantum wire. The theory employs
the 1D homogeneous Coulomb liquid as the reference system for a Kohn-Sham
treatment and transfers the Luttinger ground-state correlations to the
inhomogeneous electron system by means of a suitable local-density
approximation (LDA) to the exchange-correlation energy functional. We show that
such 1D-adapted LDA is appropriate for fluid-like states at weak coupling, but
fails to account for the transition to a ``Wigner molecules'' regime of
electron localization as observed in thin quantum wires at very strong
coupling. A detailed analyzes is given for the two-electron problem under axial
harmonic confinement.Comment: 8 pages, 7 figures, submitte
Control of spin in quantum dots with non-Fermi liquid correlations
Spin effects in the transport properties of a quantum dot with spin-charge
separation are investigated. It is found that the non-linear transport spectra
are dominated by spin dynamics. Strong spin polarization effects are observed
in a magnetic field. They can be controlled by varying gate and bias voltages.
Complete polarization is stable against interactions. When polarization is not
complete, it is power-law enhanced by non-Fermi liquid effects.Comment: 4 pages, 4 figure
Shot noise of a quantum dot with non-Fermi liquid correlations
The shot noise of a one-dimensional wire interrupted by two barriers shows
interesting features related to the interplay between Coulomb blockade effects,
Luttinger correlations and discrete excitations. At small bias the Fano factor
reaches the lowest attainable value, 1/2, irrespective of the ratio of the two
junction resistances. At larger voltages this asymmetry is power-law
renormalized by the interaction strength. We discuss how the measurement of
current and these features of the noise allow to extract the Luttinger liquid
parameter.Comment: 4 pages, 3 figures,to be published in Phys. Rev. B. For high
resolution image of Fig.1 see http://server1.fisica.unige.it/~braggio/doc.ht
Magnetic clouds in the solar wind: A numerical assessment study of analytical models
Magnetic clouds (MCs) are "magnetized plasma clouds" moving in the solar
wind. MCs transport magnetic flux and helicity away from the Sun. These
structures are not stationary but feature temporal evolution as they propagate
in the solar wind. Simplified analytical models are frequently used for the
description of MCs, and fit certain observational data well. The goal of the
present study is to investigate numerically the validity of an analytical model
which is widely used for the description of MCs, and to determine under which
conditions this model's implied assumptions cease to be valid. A numerical
approach is applied. Analytical solutions that have been derived in previous
studies are implemented in a \textbf{3-D magnetohydrodynamic} simulation code
as initial conditions. Initially, the analytical model represents the main
observational features of the MCs. However, these characteristics prevail only
if the structure moves with a velocity close to the velocity of the background
flow. In this case an MC's evolution can quite accurately be described using an
analytic, self-similar approach. The dynamics of the magnetic structures which
move with a velocity significantly above or below that of the velocity of the
solar wind is investigated in detail. Besides the standard case in which MCs
only expand and propagate in the solar wind, the case of an MC rotating around
its axis of symmetry is also considered, and the resulting influence on the
MC's dynamics is studied
Transport of interacting electrons through a double barrier in quantum wires
We generalize the fermionic renormalization group method to describe
analytically transport through a double barrier structure in a one-dimensional
system. Focusing on the case of weakly interacting electrons, we investigate
thoroughly the dependence of the conductance on the strength and the shape of
the double barrier for arbitrary temperature T. Our approach allows us to
systematically analyze the contributions to renormalized scattering amplitudes
from different characteristic scales absent in the case of a single impurity,
without restricting the consideration to the model of a single resonant level.
Both a sequential resonant tunneling for high T and a resonant transmission for
T smaller than the resonance width are studied within the unified treatment of
transport through strong barriers. For weak barriers, we show that two
different regimes are possible. Moderately weak impurities may get strong due
to a renormalization by interacting electrons, so that transport is described
in terms of theory for initially strong barriers. The renormalization of very
weak impurities does not yield any peak in the transmission probability;
however, remarkably, the interaction gives rise to a sharp peak in the
conductance, provided asymmetry is not too high.Comment: 18 pages, 8 figures; figures added, references updated, extended
discussio
The Origin, Early Evolution and Predictability of Solar Eruptions
Coronal mass ejections (CMEs) were discovered in the early 1970s when space-borne coronagraphs revealed that eruptions of plasma are ejected from the Sun. Today, it is known that the Sun produces eruptive flares, filament eruptions, coronal mass ejections and failed eruptions; all thought to be due to a release of energy stored in the coronal magnetic field during its drastic reconfiguration. This review discusses the observations and physical mechanisms behind this eruptive activity, with a view to making an assessment of the current capability of forecasting these events for space weather risk and impact mitigation. Whilst a wealth of observations exist, and detailed models have been developed, there still exists a need to draw these approaches together. In particular more realistic models are encouraged in order to asses the full range of complexity of the solar atmosphere and the criteria for which an eruption is formed. From the observational side, a more detailed understanding of the role of photospheric flows and reconnection is needed in order to identify the evolutionary path that ultimately means a magnetic structure will erupt
Short-term variability of the Sun-Earth system: an overview of progress made during the CAWSES-II period
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