103 research outputs found
On the possibility of a warped disc origin of the inclined stellar discs at the Galactic Centre
(Abridged) The Galactic Center (GC) hosts a population of young stars some of
which seem to form mutually inclined discs of clockwise and counter clockwise
rotating stars. We present a warped disc origin scenario for these stars
assuming that an initially flat accretion disc becomes warped due to the
Pringle instability, or due to Bardeen-Petterson effect, before it fragments to
stars. We show that this is plausible if the star formation efficiency
, and the viscosity parameter .
After fragmentation, we model the disc as a collection of concentric, circular,
mutually tilted rings, and construct warped disc models for mass ratios and
other parameters relevant to the GC environment, but also for more massive
discs. We take into account the disc's self-gravity and the torques exerted by
a surrounding star cluster. We show that a self-gravitating low-mass disc () precesses in integrity in the life-time of the stars, but
precesses freely when the torques from a non-spherical cluster are included. An
intermediate-mass disc () breaks into pieces which
precess independently in the self-gravity-only case, and become disrupted in
the presence of the star cluster torques. For a high-mass disc () the evolution is dominated by self-gravity and the disc is broken
but not dissolved. The time-scale after which the disc breaks scales almost
linearly with () for self-gravitating models. Typical values are
longer than the age of the stars for a low mass disc, and are in the range
yr for high and intermediate-mass discs respectively.
None of these models explain the rotation properties of the two GC discs, but a
comparison of them with the clockwise disc shows that the lowest mass model in
a spherical star cluster matches the data best.Comment: 16 pages, 19 figures, abstract abridged to meet arXiv requirements.
Accepted for publication in MNRA
Stability of spin droplets in realistic quantum Hall devices
We study the formation and characteristics of "spin droplets",i.e., compact
spin-polarized configurations in the highest occupied Landau level, in an
etched quantum Hall device at filling factors . The confining
potential for electrons is obtained with self-consistent electrostatic
calculations on a GaAs/AlGaAs heterostructure with experimental system
parameters. Real-space spin-density-functional calculations for electrons
confined in the obtained potential show the appearance of stable spin droplets
at . The qualitative features of the spin droplet are similar to
those in idealized (parabolic) quantum-dot systems. The universal stability of
the state against geometric deformations underline the applicability of spin
droplets in, e.g., spin-transport through quantum point contacts.Comment: 11 pages, 6 figure
Realistic modelling of quantum point contacts subject to high magnetic fields and with current bias at out of linear response regime
The electron and current density distributions in the close proximity of
quantum point contacts (QPCs) are investigated. A three dimensional Poisson
equation is solved self-consistently to obtain the electron density and
potential profile in the absence of an external magnetic field for gate and
etching defined devices. We observe the surface charges and their apparent
effect on the confinement potential, when considering the (deeply) etched QPCs.
In the presence of an external magnetic field, we investigate the formation of
the incompressible strips and their influence on the current distribution both
in the linear response and out of linear response regime. A spatial asymmetry
of the current carrying incompressible strips, induced by the large source
drain voltages, is reported for such devices in the non-linear regime.Comment: 16 Pages, 9 Figures, submitted to PR
Self-consistent Coulomb picture of an electron-electron bilayer system
In this work we implement the self-consistent Thomas-Fermi approach and a
local conductivity model to an electron-electron bilayer system. The presence
of an incompressible strip, originating from screening calculations at the top
(or bottom) layer is considered as a source of an external potential
fluctuation to the bottom (or top) layer. This essentially yields modifications
to both screening properties and the magneto-transport quantities. The effect
of the temperature, inter-layer distance and density mismatch on the density
and the potential fluctuations are investigated. It is observed that the
existence of the incompressible strips plays an important role simply due to
their poor screening properties on both screening and the magneto-resistance
(MR) properties. Here we also report and interpret the observed MR Hysteresis
within our model.Comment: 12 pages, 12 figures, submitted to PR
Interaction mediated asymmetries of the quantized Hall effect
Experimental and theoretical investigations on the integer quantized Hall
effect in gate defined narrow Hall bars are presented. At low electron mobility
the classical (high temperature) Hall resistance line RH(B) cuts through the
center of all Hall plateaus. In contrast, for our high mobility samples the
intersection point, at even filling factors \nu = 2; 4 ..., is clearly shifted
towards larger magnetic fields B. This asymmetry is in good agreement with
predictions of the screening theory, i. e. taking Coulomb interaction into
account. The observed effect is directly related to the formation of
incompressible strips in the Hall bar. The spin-split plateau at \nu= 1 is
found to be almost symmetric regardless of the mobility. We explain this within
the so-called effective g-model.Comment: 4 pages, 3 figure
Exchange-correlation enhancement of the Lande-g* factor in integer quantized Hall plateaus
We study the emergent role of many-body effects on a two dimensional electron
gas (2DEG) within the Thomas-Fermi-Poisson approximation, including both the
exchange and correlation interactions in the presence of a strong perpendicular
magnetic field. It is shown that, the indirect interactions widen the
odd-integer incompressible strips spatially, whereas the even-integer filling
factors almost remain unaffected.Comment: 8 pages,4 figure
Incompressible strips in dissipative Hall bars as origin of quantized Hall plateaus
We study the current and charge distribution in a two dimensional electron
system, under the conditions of the integer quantized Hall effect, on the basis
of a quasi-local transport model, that includes non-linear screening effects on
the conductivity via the self-consistently calculated density profile. The
existence of ``incompressible strips'' with integer Landau level filling factor
is investigated within a Hartree-type approximation, and non-local effects on
the conductivity along those strips are simulated by a suitable averaging
procedure. This allows us to calculate the Hall and the longitudinal resistance
as continuous functions of the magnetic field B, with plateaus of finite widths
and the well-known, exactly quantized values. We emphasize the close relation
between these plateaus and the existence of incompressible strips, and we show
that for B values within these plateaus the potential variation across the Hall
bar is very different from that for B values between adjacent plateaus, in
agreement with recent experiments.Comment: 13 pages, 11 figures, All color onlin
Simulations of Direct Collisions of Gas Clouds with the Central Black Hole
We perform numerical simulations of clouds in the Galactic Centre (GC)
engulfing the nuclear super-massive black hole and show that this mechanism
leads to the formation of gaseous accretion discs with properties that are
similar to the expected gaseous progenitor discs that fragmented into the
observed stellar disc in the GC. As soon as the cloud hits the black hole, gas
with opposite angular momentum relative to the black hole collides downstream.
This process leads to redistribution of angular momentum and dissipation of
kinetic energy, resulting in a compact gaseous accretion disc. A parameter
study using thirteen high resolution simulations of homogeneous clouds falling
onto the black hole and engulfing it in parts demonstrates that this mechanism
is able to produce gaseous accretion discs that could potentially be the
progenitor of the observed stellar disc in the GC. A comparison of simulations
with different equations of state (adiabatic, isothermal and full cooling)
demonstrates the importance of including a detailed thermodynamical
description. However the simple isothermal approach already yields good results
on the radial mass transfer and accretion rates, as well as disc eccentricities
and sizes. We find that the cloud impact parameter strongly influences the
accretion rate whereas the impact velocity has a small affect on the accretion
rate.Comment: 21 pages, 18 figures, Accepted for publication in MNRA
Quantum Hall effect in a high-mobility two-dimensional electron gas on the surface of a cylinder
The quantum Hall effect is investigated in a high-mobility two-dimensional
electron gas on the surface of a cylinder. The novel topology leads to a
spatially varying filling factor along the current path. The resulting
inhomogeneous current-density distribution gives rise to additional features in
the magneto-transport, such as resistance asymmetry and modified longitudinal
resistances. We experimentally demonstrate that the asymmetry relations
satisfied in the integer filling factor regime are valid also in the transition
regime to non-integer filling factors, thereby suggesting a more general form
of these asymmetry relations. A model is developed based on the screening
theory of the integer quantum Hall effect that allows the self-consistent
calculation of the local electron density and thereby the local current density
including the current along incompressible stripes. The model, which also
includes the so-called `static skin effect' to account for the current density
distribution in the compressible regions, is capable of explaining the main
experimental observations. Due to the existence of an
incompressible-compressible transition in the bulk, the system behaves always
metal-like in contrast to the conventional Landauer-Buettiker description, in
which the bulk remains completely insulating throughout the quantized Hall
plateau regime
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