1,622 research outputs found
Nanostructured graphene for spintronics
Zigzag edges of the honeycomb structure of graphene exhibit magnetic
polarization making them attractive as building blocks for spintronic devices.
Here, we show that devices with zigzag edged triangular antidots perform
essential spintronic functionalities, such as spatial spin-splitting or spin
filtering of unpolarized incoming currents. Near-perfect performance can be
obtained with optimized structures. The device performance is robust against
substantial disorder. The gate-voltage dependence of transverse resistance is
qualitatively different for spin-polarized and spin-unpolarized devices, and
can be used as a diagnostic tool. Importantly, the suggested devices are
feasible within current technologies.Comment: 6 pages, 5 figures, publishe
Graphene on graphene antidot lattices: Electronic and transport properties
Graphene bilayer systems are known to exhibit a band gap when the layer
symmetry is broken, by applying a perpendicular electric field. The resulting
band structure resembles that of a conventional semiconductor with a parabolic
dispersion. Here, we introduce a novel bilayer graphene heterostructure, where
single-layer graphene is placed on top of another layer of graphene with a
regular lattice of antidots. We dub this class of graphene systems GOAL:
graphene on graphene antidot lattice. By varying the structure geometry, band
structure engineering can be performed to obtain linearly dispersing bands
(with a high concomitant mobility), which nevertheless can be made gapped with
the perpendicular field. We analyze the electronic structure and transport
properties of various types of GOALs, and draw general conclusions about their
properties to aid their design in experiments.Comment: 13 pages, 10 figures, submitte
Solar-cycle variation of the sound-speed asphericity from GONG and MDI data 1995-2000
We study the variation of the frequency splitting coefficients describing the
solar asphericity in both GONG and MDI data, and use these data to investigate
temporal sound-speed variations as a function of both depth and latitude during
the period from 1995-2000 and a little beyond. The temporal variations in even
splitting coefficients are found to be correlated to the corresponding
component of magnetic flux at the solar surface. We confirm that the
sound-speed variations associated with the surface magnetic field are
superficial. Temporally averaged results show a significant excess in sound
speed around 0.92 solar radii and latitude of 60 degrees.Comment: To be published in MNRAS, accepted July 200
Three-dimensional stability of the solar tachocline
The three-dimensional, hydrodynamic stability of the solar tachocline is
investigated based on a rotation profile as a function of both latitude and
radius. By varying the amplitude of the latitudinal differential rotation, we
find linear stability limits at various Reynolds numbers by numerical
computations. We repeated the computations with different latitudinal and
radial dependences of the angular velocity. The stability limits are all higher
than those previously found from two-dimensional approximations and higher than
the shear expected in the Sun. It is concluded that any part of the tachocline
which is radiative is hydrodynamically stable against small perturbations.Comment: 6 pages, 8 figures, accepted by Astron. & Astrophy
Deeply penetrating banded zonal flows in the solar convection zone
Helioseismic observations have detected small temporal variations of the
rotation rate below the solar surface corresponding to the so-called `torsional
oscillations' known from Doppler measurements of the surface. These appear as
bands of slower and faster than average rotation moving equatorward. Here we
establish, using complementary helioseismic observations over four years from
the GONG network and from the MDI instrument on board SOHO, that the banded
flows are not merely a near-surface phenomenon: rather they extend downward at
least 60 Mm (some 8% of the total solar radius) and thus are evident over a
significant fraction of the nearly 200 Mm depth of the solar convection zone.Comment: 4 pages, 4 figures To be published in ApJ Letters (accepted 3/3/2000
The Helioseismic and Magnetic Imager (HMI) Vector Magnetic Field Pipeline: Optimization of the Spectral Line Inversion Code
The Very Fast Inversion of the Stokes Vector (VFISV) is a Milne-Eddington
spectral line inversion code used to determine the magnetic and thermodynamic
parameters of the solar photosphere from observations of the Stokes vector in
the 6173 A Fe I line by the Helioseismic and Magnetic Imager (HMI) onboard the
Solar Dynamics Observatory (SDO). We report on the modifications made to the
original VFISV inversion code in order to optimize its operation within the HMI
data pipeline and provide the smoothest solution in active regions. The changes
either sped up the computation or reduced the frequency with which the
algorithm failed to converge to a satisfactory solution. Additionally, coding
bugs which were detected and fixed in the original VFISV release, are reported
here.Comment: Accepted for publication in Solar Physic
Does the Sun Shrink with Increasing Magnetic Activity?
We have analyzed the full set of SOHO/MDI f- and p-mode oscillation
frequencies from 1996 to date in a search for evidence of solar radius
evolution during the rising phase of the current activity cycle. Like Antia et
al. (2000), we find that a significant fraction of the f-mode frequency changes
scale with frequency; and that if these are interpreted in terms of a radius
change, it implies a shrinking sun. Our inferred rate of shrinkage is about 1.5
km/y, which is somewhat smaller than found by Antia et al. We argue that this
rate does not refer to the surface, but rather to a layer extending roughly
from 4 to 8 Mm beneath the visible surface. The rate of shrinking may be
accounted for by an increasing radial component of the rms random magnetic
field at a rate that depends on its radial distribution. If it were uniform,
the required field would be ~7 kG. However, if it were inwardly increasing,
then a 1 kG field at 8 Mm would suffice.
To assess contribution to the solar radius change arising above 4Mm, we
analyzed the p-mode data. The evolution of the p-mode frequencies may be
explained by a magnetic^M field growing with activity. The implications of the
near-surface magnetic field changes depend on the anisotropy of the random
magnetic field. If the field change is predominantly radial, then we infer an
additional shrinking at a rate between 1.1-1.3 km/y at the photosphere. If on
the other hand the increase is isotropic, we find a competing expansion at a
rate of 2.3 km/y. In any case, variations in the sun's radius in the activity
cycle are at the level of 10^{-5} or less, hence have a negligible contribution
to the irradiance variations.Comment: 10 pages (ApJ preprint style), 4 figures; accepted for publication in
Ap
Global-scale equatorial Rossby waves as an essential component of solar internal dynamics
The Sun's complex dynamics is controlled by buoyancy and rotation in the
convection zone and by magnetic forces in the atmosphere and corona. While
small-scale solar convection is well understood, the dynamics of large-scale
flows in the solar convection zone is not explained by theory or simulations.
Waves of vorticity due to the Coriolis force, known as Rossby waves, are
expected to remove energy out of convection at the largest scales. Here we
unambiguously detect and characterize retrograde-propagating vorticity waves in
the shallow subsurface layers of the Sun at angular wavenumbers below fifteen,
with the dispersion relation of textbook sectoral Rossby waves. The waves have
lifetimes of several months, well-defined mode frequencies below 200 nHz in a
co-rotating frame, and eigenfunctions of vorticity that peak at the equator.
Rossby waves have nearly as much vorticity as the convection at the same
scales, thus they are an essential component of solar dynamics. We find a
transition from turbulence-like to wave-like dynamics around the Rhines scale
of angular wavenumber of twenty; this might provide an explanation for the
puzzling deficit of kinetic energy at the largest spatial scales.Comment: This is the submitted version of the paper published in Nature
Astronomy. 23 pages, 8 figures, 1 tabl
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