144 research outputs found
Controlling the efficiency of spin injection into graphene by carrier drift
Electrical spin injection from ferromagnetic metals into graphene is hindered
by the impedance mismatch between the two materials. This problem can be
reduced by the introduction of a thin tunnel barrier at the interface. We
present room temperature non-local spin valve measurements in
cobalt/aluminum-oxide/graphene structures with an injection efficiency as high
as 25%, where electrical contact is achieved through relatively transparent
pinholes in the oxide. This value is further enhanced to 43% by applying a DC
current bias on the injector electrodes, that causes carrier drift away from
the contact. A reverse bias reduces the AC spin valve signal to zero or
negative values. We introduce a model that quantitatively predicts the behavior
of the spin accumulation in the graphene under such circumstances, showing a
good agreement with our measurements.Comment: 4 pages, 3 color figure
Linear scaling between momentum and spin scattering in graphene
Spin transport in graphene carries the potential of a long spin diffusion
length at room temperature. However, extrinsic relaxation processes limit the
current experimental values to 1-2 um. We present Hanle spin precession
measurements in gated lateral spin valve devices in the low to high (up to
10^13 cm^-2) carrier density range of graphene. A linear scaling between the
spin diffusion length and the diffusion coefficient is observed. We measure
nearly identical spin- and charge diffusion coefficients indicating that
electron-electron interactions are relatively weak and transport is limited by
impurity potential scattering. When extrapolated to the maximum carrier
mobilities of 2x10^5 cm^2/Vs, our results predict that a considerable increase
in the spin diffusion length should be possible
Towards a Full Census of the Obscure(d) Vela Supercluster using MeerKAT
Recent spectroscopic observations of a few thousand partially obscured
galaxies in the Vela constellation revealed a massive overdensity on
supercluster scales straddling the Galactic Equator (l 272.5deg) at km/s. It remained unrecognised because it is located just beyond the
boundaries and volumes of systematic whole-sky redshift and peculiar velocity
surveys - and is obscured by the Milky Way. The structure lies close to the
apex where residual bulkflows suggest considerable mass excess. The uncovered
Vela Supercluster (VSCL) conforms of a confluence of merging walls, but its
core remains uncharted. At the thickest foreground dust column densities (|b| <
6 deg) galaxies are not visible and optical spectroscopy is not effective. This
precludes a reliable estimate of the mass of VSCL, hence its effect on the
cosmic flow field and the peculiar velocity of the Local Group. Only systematic
HI-surveys can bridge that gap. We have run simulations and will present
early-science observing scenarios with MeerKAT 32 (M32) to complete the census
of this dynamically and cosmologically relevant supercluster. M32 has been put
forward because this pilot project will also serve as precursor project for HI
MeerKAT Large Survey Projects, like Fornax and Laduma. Our calculations have
shown that a survey area of the fully obscured part of the supercluster, where
the two walls cross and the potential core of the supercluster resides, can be
achieved on reasonable time-scales (200 hrs) with M32.Comment: 10 pages, 3 figures, accepted for publication, Proceedings of
Science, workshop on "MeerKAT Science: On the Pathway to the SKA", held in
Stellenbosch 25-27 May 201
Electronic spin transport in graphene field effect transistors
Spin transport experiments in graphene, a single layer of carbon atoms,
indicate spin relaxation times that are significantly shorter than the
theoretical predictions. We investigate experimentally whether these short spin
relaxation times are due to extrinsic factors, such as spin relaxation caused
by low impedance contacts, enhanced spin flip processes at the device edges or
the presence of an aluminium oxide layer on top of graphene in some samples.
Lateral spin valve devices using a field effect transistor geometry allowed for
the investigation of the spin relaxation as a function of the charge density,
going continuously from metallic hole to electron conduction (charge densities
of cm) via the Dirac charge neutrality point (). The results are quantitatively described by a one dimensional spin
diffusion model where the spin relaxation via the contacts is taken into
account. Spin valve experiments for various injector/detector separations and
spin precession experiments reveal that the longitudinal (T) and the
transversal (T) relaxation times are similar. The anisotropy of the spin
relaxation times and , when the spins are injected
parallel or perpendicular to the graphene plane, indicates that the effective
spin orbit fields do not lie exclusively in the two dimensional graphene plane.
Furthermore, the proportionality between the spin relaxation time and the
momentum relaxation time indicates that the spin relaxation mechanism is of the
Elliott-Yafet type. For carrier mobilities of 2-5 cm2^/Vs and
for graphene flakes of 0.1-2 m in width, we found spin relaxation times of
the order of 50-200 ps, times which appear not to be determined by the
extrinsic factors mentioned above.Comment: 11 pages, 13 figure
Misalignment between cold gas and stellar components in early-type galaxies
Recent work suggests blue ellipticals form in mergers and migrate quickly from the blue cloud of star-forming galaxies to the red sequence of passively evolving galaxies, perhaps as a result of black hole feedback. Such rapid reddening of stellar populations implies that large gas reservoirs in the pre-merger star-forming pair must be depleted on short time-scales. Here we present pilot observations of atomic hydrogen gas in four blue early-type galaxies that reveal increasing spatial offsets between the gas reservoirs and the stellar components of the galaxies, with advancing post-starburst age. Emission line spectra show associated nuclear activity in two of the merged galaxies, and in one case radio lobes aligned with the displaced gas reservoir. These early results suggest that a kinetic process (possibly feedback from black hole activity) is driving the quick truncation of star formation in these systems, rather than a simple exhaustion of gas suppl
Three-dimensional modeling of the HI kinematics of NGC 2915
The nearby blue compact dwarf, NGC 2915, has its stellar disc embedded in a
large, extended (~ 22 B-band scale-lengths) HI disc. New high-resolution HI
synthesis observations of NGC 2915 have been obtained with the Australia
Telescope Compact Array. These observations provide evidence of extremely
complex HI kinematics within the immediate vicinity of the galaxy's
star-forming core. We identify and quantify double-peaked HI line profiles near
the centre of the galaxy and show that the HI energetics can be accounted for
by the mechanical energy output of the central high-mass stellar population
within time-scales of 10^6-10^7 yr. Full three-dimensional models of the HI
data cube are generated and compared to the observations to test various
physical scenarios associated with the high-mass star-forming core of NGC 2915.
Purely circular HI kinematics are ruled out together with the possibility of a
high-velocity-dispersion inter-stellar medium at inner radii. Radial velocities
of ~ 30 km/s are required to describe the central-most HI kinematics of the
system. Our results lend themselves to the simple physical scenario in which
the young stellar core of the galaxy expels the gas outwards from the centre of
the disc, thereby creating a central HI under-density. These kinematics should
be thought of as being linked to a central HI outflow rather than a large-scale
galactic blow-out or wind.Comment: 11 pages, 6 figures, accepted for publication in MNRA
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