17,369 research outputs found
The geometry of Hrushovski constructions, I. The uncollapsed case
An intermediate stage in Hrushovski's construction of flat strongly minimal
structures in a relational language L produces omega-stable structures of rank
omega. We analyze the pregeometries given by forking on the regular type of
rank omega in these structures. We show that varying L can affect the (local)
isomorphism type of the pregeometry, but not its finite subpregeometries. A
sequel will compare these to the pregeometries of the strongly minimal
structures.Comment: 31 page
On the size of the Fe II emitting region in the AGN Akn 120
We present a reverberation analysis of the strong, variable optical Fe II
emission bands in the spectrum of Akn 120, a low-redshift AGN which is one of
the best candidates for such a study. On time scales of several years the Fe II
line strengths follow the variations in the continuum strength. However, we are
unable to measure a clear reverberation lag time for these Fe II lines on any
time scale. This is due to the very broad and flat-topped nature of the Fe II
cross correlation functions, as compared to the H-beta response which is much
more sharply localized in time. Although there is some suggestion in the light
curve of a 300-day response time, our statistical analysis does not pick up
such a feature. We conclude that the optical Fe II emission does not come from
a photoionization-powered region similar in size to the H-beta emitting region,
but we cannot say for sure where it does come from. Our results are generally
consistent either with emission from a photoionized region several times larger
than the H-beta zone, or with emission from gas heated by some other means,
perhaps responding only indirectly to the continuum variations.Comment: Accepted for publication in the Ap
Deriving bases for Abelian functions
We present a new method to explicitly define Abelian functions associated
with algebraic curves, for the purpose of finding bases for the relevant vector
spaces of such functions. We demonstrate the procedure with the functions
associated with a trigonal curve of genus four. The main motivation for the
construction of such bases is that it allows systematic methods for the
derivation of the addition formulae and differential equations satisfied by the
functions. We present a new 3-term 2-variable addition formulae and a complete
set of differential equations to generalise the classic Weierstrass identities
for the case of the trigonal curve of genus four.Comment: 35page
Evidence for a Fractional Quantum Hall Nematic State in Parallel Magnetic Fields
We report magneto-transport measurements for the fractional quantum Hall
state at filling factor 5/2 as a function of applied parallel magnetic
field (). As is increased, the 5/2 state becomes increasingly
anisotropic, with the in-plane resistance along the direction of
becoming more than 30 times larger than in the perpendicular direction.
Remarkably, the resistance anisotropy ratio remains constant over a relatively
large temperature range, yielding an energy gap which is the same for both
directions. Our data are qualitatively consistent with a fractional quantum
Hall \textit{nematic} phase
Observation of An Anisotropic Wigner Crystal
We report a new correlated phase of two-dimensional charged carriers in high
magnetic fields, manifested by an anisotropic insulating behavior at low
temperatures. It appears near Landau level filling factor in hole
systems confined to wide GaAs quantum wells when the sample is tilted in
magnetic field to an intermediate angle. The parallel field component
() leads to a crossing of the lowest two Landau levels, and an
elongated hole wavefunction in the direction of . Under these
conditions, the in-plane resistance exhibits an insulating behavior, with the
resistance along more than 10 times smaller than the resistance
perpendicular to . We interpret this anisotropic insulating phase as a
two-component, striped Wigner crystal
Anisotropic composite fermions and fractional quantum Hall effect
We study the role of anisotropy on the transport properties of composite
fermions near Landau level filling factor in two-dimensional holes
confined to a GaAs quantum well. By applying a parallel magnetic field, we tune
the composite fermion Fermi sea anisotropy and monitor the relative change of
the transport scattering time at along the principal directions.
Interpreted in a simple Drude model, our results suggest that the scattering
time is longer along the longitudinal direction of the composite fermion Fermi
sea. Furthermore, the measured energy gap for the fractional quantum Hall state
at decreases when anisotropy becomes significant. The decrease,
however, might partly stem from the charge distribution becoming bilayer-like
at very large parallel magnetic fields
Garden of Roses
https://digitalcommons.library.umaine.edu/mmb-ps/2747/thumbnail.jp
Multicomponent fractional quantum Hall states with subband and spin degrees of freedom
In wide GaAs quantum wells where two electric subbands are occupied we apply
a parallel magnetic field or increase the electron density to cause a crossing
of the two Landau levels of these subbands and with opposite spins. Near
the crossing, the fractional quantum Hall states in the filling factor range
exhibit a remarkable sequence of pseudospin polarization transitions
resulting from the interplay between the spin and subband degrees of freedom.
The field positions of the transitions yield a new and quantitative measure of
the composite Fermions' discrete energy level separations. Surprisingly, the
separations are smaller when the electrons have higher spin-polarization
Short-Timescale monitoring of the X-ray, UV and broad double-peak emission line of the nucleus of NGC 1097
Recent studies have suggested that the short-timescale ( days)
variability of the broad (10,000 km s) double-peaked H
profile of the LINER nucleus of NGC1097 could be driven by a variable X-ray
emission from a central radiatively inefficient accretion flow (RIAF). To test
this scenario, we have monitored the NGC1097 nucleus in X-ray and UV continuum
with Swift and the H flux and profile in the optical spectrum using
SOAR and Gemini-South from 2012 August to 2013 February. During the monitoring
campaign, the H flux remained at a very low level --- 3 times lower
than the maximum flux observed in previous campaigns and showing only limited
() variability. The X-ray variations were small, only
throughout the campaign, while the UV did not show significant variations. We
concluded that the timescale of the H profile variation is close to the
sampling interval of the optical observations, which results in only marginal
correlation between the X-ray and H fluxes. We have caught the AGN in
NGC1097 in a very low activity state, in which the ionizing source was very
weak and capable of ionizing just the innermost part of the gas in the disk.
Nonetheless, the data presented here still support the picture in which the gas
that emits the broad double-peaked Balmer lines is illuminated/ionized by a
source of high-energy photons which is located interior to the inner radius of
the line-emitting part of the disk.Comment: The paper contains 14 pages, 7 figures and is accepted for
publication at the Astrophysical Journa
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