1,667 research outputs found
The slimming effect of advection on black-hole accretion flows
At super-Eddington rates accretion flows onto black holes have been described
as slim (aspect ratio ) or thick (H/R >1) discs, also known as
tori or (Polish) doughnuts. The relation between the two descriptions has never
been established, but it was commonly believed that at sufficiently high
accretion rates slim discs inflate, becoming thick. We wish to establish under
what conditions slim accretion flows become thick. We use analytical equations,
numerical 1+1 schemes, and numerical radiative MHD codes to describe and
compare various accretion flow models at very high accretion rates.We find that
the dominant effect of advection at high accretion rates precludes slim discs
becoming thick. At super-Eddington rates accretion flows around black holes can
always be considered slim rather than thick.Comment: 8 pages, 5 figures. Astronomy & Astrophysics, in pres
Ferromagnetism and interlayer exchange coupling in short period (Ga,Mn)As/GaAs superlattices
Magnetic properties of (Ga,Mn)As/GaAs superlattices are investigated. The
structures contain magnetic (Ga,Mn)As layers, separated by thin layers of
non-magnetic GaAs spacer. The short period GaMnAs/GaAs
superlattices exhibit a paramagnetic-to-ferromagnetic phase transition close to
60K, for thicknesses of (Ga,Mn)As down to 23 \AA. For
GaMnAs/GaAs superlattices of similar dimensions, the Curie
temperature associated with the ferromagnetic transition is found to oscillate
with the thickness of non magnetic spacer. The observed oscillations are
related to an interlayer exchange interaction mediated by the polarized holes
of the (Ga,Mn)As layers.Comment: REVTeX 4 style; 4 pages, 2 figure
Spatially controlled formation of superparamagnetic (Mn,Ga)As nanocrystals in high temperature annealed (Ga,Mn)As/GaAs superlattices
The annealing-induced formation of (Mn,Ga)As nanocrystals in (Ga,Mn)As/GaAs
superlattices was studied by X-ray diffraction, transmission electron
microscopy and magnetometry. The superlattice structures with 50 A thick
(Ga,Mn)As layers separated by 25, 50 and 100 A thick GaAs spacers were grown by
molecular beam epitaxy at low temperature (250 C), and then annealed at high
temperatures of: 400, 560 and 630 C. The high temperature annealing causes
decomposition to GaMnAs ternary alloy and formation of (Mn,Ga)As nanocrystals
inside the GaAs matrix. The nanocrystals are confined in the planes that were
formerly occupied by (Ga,Mn)As layers for up to the 560 C of annealing and
diffuse throughout the GaAs spacer layers at 630 C annealing. The corresponding
magnetization measurements show the evolution of the magnetic properties of
as-grown and annealed samples from ferromagnetic, through superparamagnetic to
the combination of both.Comment: 14 pages, 3 figure
Ripples Make Waves: Binding Structured Activity and Plasticity in Hippocampal Networks
Establishing novel episodic memories and stable spatial representations depends on an exquisitely choreographed, multistage process involving the online encoding and offline consolidation of sensory information, a process that is largely dependent on the hippocampus. Each step is influenced by distinct neural network states that influence the pattern of activation across cellular assemblies. In recent years, the occurrence of hippocampal sharp wave ripple (SWR) oscillations has emerged as a potentially vital network phenomenon mediating the steps between encoding and consolidation, both at a cellular and network level by promoting the rapid replay and reactivation of recent activity patterns. Such events facilitate memory formation by optimising the conditions for synaptic plasticity to occur between contingent neural elements. In this paper, we explore the ways in which SWRs and other network events can bridge the gap between spatiomnemonic processing at cellular/synaptic and network levels in the hippocampus
Effect of annealing on carrier density and Curie temperature in epitaxial (Ga,Mn)As thin films
We report a clear correspondence between changes in the Curie temperature and
carrier density upon annealing in epitaxially grown (Ga,Mn)As layers with
thicknesses in the range between 5 nm and 20 nm. The changes are dependent on
the layer thickness, indicating that the (Ga,Mn)As - GaAs interface has
importance for the physical properties of the (Ga,Mn)As layer. The
magnetoresistance shows additional features when compared to thick (Ga,Mn)As
layers, that are at present of unknown origin.Comment: 9 pages, 3 figure
Ferromagnetic GaMnAs/GaAs superlattices - MBE growth and magnetic properties
We have studied the magnetic properties of (GaMnAs)m/(GaAs)n superlattices
with magnetic GaMnAs layers of thickness between 8 and 16 molecular layers (ML)
(23-45 \AA), and with nonmagnetic GaAs spacers from 4 ML to 10 ML (11-28 \AA).
While previous reports state that GaMnAs layers thinner than 50 \AA are
paramagnetic in the whole Mn composition range achievable using MBE growth (up
to 8% Mn), we have found that short period superlattices exhibit a
paramagnetic-to-ferromagnetic phase transition with a transition temperature
which depends on both the thickness of the magnetic GaMnAs layer and the
nonmagnetic GaAs spacer. The neutron scattering experiments have shown that the
magnetic layers in superlattices are ferromagnetically coupled for both thin
(below 50 \AA) and thick (above 50 \AA) GaMnAs layers.Comment: Proceedings of 4th International Workshop on Molecular Beam Epitaxy
and Vapour Phase Epitaxy Growth Physics and Technology, September 23 - 28
(2001), Warszawa, Poland, to appear in Thin Solid Films. 24 pages, 8 figure
High-Energy Limit of Massless Dirac Fermions in Multilayer Graphene using Magneto-Optical Transmission Spectroscopy
We have investigated the absorption spectrum of multilayer graphene in high
magnetic fields. The low energy part of the spectrum of electrons in graphene
is well described by the relativistic Dirac equation with a linear dispersion
relation. However, at higher energies (>500 meV) a deviation from the ideal
behavior of Dirac particles is observed. At an energy of 1.25 eV, the deviation
from linearity is 40 meV. This result is in good agreement with the theoretical
model, which includes trigonal warping of the Fermi surface and higher-order
band corrections. Polarization-resolved measurements show no observable
electron-hole asymmetry.Comment: 4 pages,3 figure
Termination dependent topological surface states of the natural superlattice phase BiSe
We describe the topological surface states of BiSe, a compound in the
infinitely adaptive Bi-BiSe natural superlattice phase series,
determined by a combination of experimental and theoretical methods. Two
observable cleavage surfaces, terminating at Bi or Se, are characterized by
angle resolved photoelectron spectroscopy and scanning tunneling microscopy,
and modeled by ab-initio density functional theory calculations. Topological
surface states are observed on both surfaces, but with markedly different
dispersions and Kramers point energies. BiSe therefore represents the
only known compound with different topological states on differently terminated
surfaces.Comment: 5 figures references added Published in PRB:
http://link.aps.org/doi/10.1103/PhysRevB.88.08110
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