3,519 research outputs found
Robustness of edge states in graphene quantum dots
We analyze the single particle states at the edges of disordered graphene
quantum dots. We show that generic graphene quantum dots support a number of
edge states proportional to circumference of the dot over the lattice constant.
Our analytical theory agrees well with numerical simulations. Perturbations
breaking electron-hole symmetry like next-nearest neighbor hopping or edge
impurities shift the edge states away from zero energy but do not change their
total amount. We discuss the possibility of detecting the edge states in an
antidot array and provide an upper bound on the magnetic moment of a graphene
dot.Comment: Added figure 6, extended discussion (version as accepted by Physical
Review B
Disparity among low first ionization potential elements
The elemental composition of the solar wind differs from the solar
photospheric composition. Elements with low first ionization potential (FIP)
appear enhanced compared to O in the solar wind relative to the respective
photospheric abundances. This so-called FIP effect is different in the slow
solar wind and the coronal hole wind. However, under the same plasma
conditions, for elements with similar FIPs such as Mg, Si, and Fe, comparable
enhancements are expected. We scrutinize the assumption that the FIP effect is
always similar for different low FIP elements, namely Mg, Si, and Fe. We
investigate the dependency of the FIP effect of low FIP elements on the O7+/O6+
charge state ratio depending on time and solar wind type. We order the observed
FIP ratios with respect to the O7+/O6+ charge state ratio into bins and analyze
separately the respective distributions of the FIP ratio of Mg, Si, and Fe for
each O7+/O6+ charge state ratio bin. We observe that the FIP effect shows the
same qualitative yearly behavior for Mg and Si, while Fe shows significant
differences during the solar activity maximum and its declining phase. In each
year, the FIP effect for Mg and Si always increases with increasing O7+/O6+
charge state ratio, but for high O7+/O6+ charge state ratios the FIP effect for
Fe shows a qualitatively different behavior. During the years 2001-2006,
instead of increasing with the O7+/O6+ charge state ratio, the Fe FIP ratio
exhibits a broad peak. Also, the FIP distribution per O7+/O6+ charge state bin
is significantly broader for Fe than for Mg and Si. These observations support
the conclusion that the elemental fractionation is only partly determined by
FIP. In particular, the qualitative difference behavior with increasing O7+/O6+
charge state ratio between Fe on the one hand and Mg and Si on the other hand
is not yet well explained by models of fractionation
An elliptic expansion of the potential field source surface model
Context. The potential field source surface model is frequently used as a
basis for further scientific investigations where a comprehensive coronal
magnetic field is of importance. Its parameters, especially the position and
shape of the source surface, are crucial for the interpretation of the state of
the interplanetary medium. Improvements have been suggested that introduce one
or more additional free parameters to the model, for example, the current sheet
source surface (CSSS) model.
Aims. Relaxing the spherical constraint of the source surface and allowing it
to be elliptical gives modelers the option of deforming it to more accurately
match the physical environment of the specific period or location to be
analyzed.
Methods. A numerical solver is presented that solves Laplace's equation on a
three-dimensional grid using finite differences. The solver is capable of
working on structured spherical grids that can be deformed to create elliptical
source surfaces.
Results. The configurations of the coronal magnetic field are presented using
this new solver. Three-dimensional renderings are complemented by
Carrington-like synoptic maps of the magnetic configuration at different
heights in the solar corona. Differences in the magnetic configuration computed
by the spherical and elliptical models are illustrated.Comment: 11 pages, 7 figure
Evolution of an equatorial coronal hole structure and the released coronal hole wind stream: Carrington rotations 2039 to 2050
The Sun is a highly dynamic environment that exhibits dynamic behavior on
many different timescales. In particular, coronal holes exhibit temporal and
spatial variability. Signatures of these coronal dynamics are inherited by the
coronal hole wind streams that originate in these regions and can effect the
Earth's magnetosphere. Both the cause of the observed variabilities and how
these translate to fluctuations in the in situ observed solar wind is not yet
fully understood. During solar activity minimum the structure of the magnetic
field typically remains stable over several Carrington rotations (CRs). But how
stable is the solar magnetic field? Here, we address this question by analyzing
the evolution of a coronal hole structure and the corresponding coronal hole
wind stream emitted from this source region over 12 consecutive CRs in 2006. To
this end, we link in situ observations of Solar Wind Ion Composition
Spectrometer (SWICS) onboard the Advanced Composition Explorer (ACE) with
synoptic maps of Michelson Doppler imager (MDI) on the Solar and Heliospheric
Observatory (SOHO) at the photospheric level through a combination of ballistic
back-mapping and a potential field source surface (PFSS) approach. Together,
these track the evolution of the open field line region that is identified as
the source region of a recurring coronal hole wind stream.
We find that the shape of the open field line region and to some extent also
the solar wind properties are influenced by surrounding more dynamic closed
loop regions. We show that the freeze-in order can change within a coronal hole
wind stream on small timescales and illustrate a mechanism that can cause
changes in the freeze-in order. The inferred minimal temperature profile is
variable even within coronal hole wind and is in particular most variable in
the outer corona
Quantized conductance at the Majorana phase transition in a disordered superconducting wire
Superconducting wires without time-reversal and spin-rotation symmetries can
be driven into a topological phase that supports Majorana bound states. Direct
detection of these zero-energy states is complicated by the proliferation of
low-lying excitations in a disordered multi-mode wire. We show that the phase
transition itself is signaled by a quantized thermal conductance and electrical
shot noise power, irrespective of the degree of disorder. In a ring geometry,
the phase transition is signaled by a period doubling of the magnetoconductance
oscillations. These signatures directly follow from the identification of the
sign of the determinant of the reflection matrix as a topological quantum
number.Comment: 7 pages, 4 figures; v3: added appendix with numerics for long-range
disorde
Graphene Rings in Magnetic Fields: Aharonov-Bohm Effect and Valley Splitting
We study the conductance of mesoscopic graphene rings in the presence of a
perpendicular magnetic field by means of numerical calculations based on a
tight-binding model. First, we consider the magnetoconductance of such rings
and observe the Aharonov-Bohm effect. We investigate different regimes of the
magnetic flux up to the quantum Hall regime, where the Aharonov-Bohm
oscillations are suppressed. Results for both clean (ballistic) and disordered
(diffusive) rings are presented. Second, we study rings with smooth mass
boundary that are weakly coupled to leads. We show that the valley degeneracy
of the eigenstates in closed graphene rings can be lifted by a small magnetic
flux, and that this lifting can be observed in the transport properties of the
system.Comment: 12 pages, 9 figure
Visual Experience Shapes Orthographic Representations in the Visual Word Form Area
Current neurocognitive research suggests that the efficiency of visual word recognition rests on abstract memory representations of written letters and words stored in the visual word form area (VWFA) in the left ventral occipitotemporal cortex. These representations are assumed to be invariant to visual characteristics such as font and case. In the present functional MRI study, we tested this assumption by presenting written words and varying the case format of the initial letter of German nouns (which are always capitalized) as well as German adjectives and adverbs (both usually in lowercase). As evident from a Word Type × Case Format interaction, activation in the VWFA was greater to words presented in unfamiliar case formats relative to familiar case formats. Our results suggest that neural representations of written words in the VWFA are not fully abstract and still contain information about the visual format in which words are most frequently perceived
Energy balance closure for the LITFASS-2003 experiment
In the first part, this paper synthesises the main results from a series of previous studies on the closure of the local energy balance at low-vegetation sites during the LITFASS-2003 experiment. A residual of up to 25% of the available energy has been found which cannot be fully explained either by the measurement uncertainty of the single components of the surface energy balance or by the length of the flux-averaging period. In the second part, secondary circulations due to heterogeneities in the surface characteristics (roughness, thermal and moisture properties) are discussed as a possible cause for the observed energy balance non-closure. This hypothesis seems to be supported from the fluxes derived from area-averaging measurement techniques (scintillometers, aircraft)
A generalized approach to model the spectra and radiation dose rate of solar particle events on the surface of Mars
For future human missions to Mars, it is important to study the surface
radiation environment during extreme and elevated conditions. In the long term,
it is mainly Galactic Cosmic Rays (GCRs) modulated by solar activity that
contributes to the radiation on the surface of Mars, but intense solar
energetic particle (SEP) events may induce acute health effects. Such events
may enhance the radiation level significantly and should be detected as
immediately as possible to prevent severe damage to humans and equipment.
However, the energetic particle environment on the Martian surface is
significantly different from that in deep space due to the influence of the
Martian atmosphere. Depending on the intensity and shape of the original solar
particle spectra as well as particle types, the surface spectra may induce
entirely different radiation effects. In order to give immediate and accurate
alerts while avoiding unnecessary ones, it is important to model and well
understand the atmospheric effect on the incoming SEPs including both protons
and helium ions. In this paper, we have developed a generalized approach to
quickly model the surface response of any given incoming proton/helium ion
spectra and have applied it to a set of historical large solar events thus
providing insights into the possible variety of surface radiation environments
that may be induced during SEP events. Based on the statistical study of more
than 30 significant solar events, we have obtained an empirical model for
estimating the surface dose rate directly from the intensities of a power-law
SEP spectra
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