9,643 research outputs found
Composite fermion dynamics in half-filled Landau levels of graphene
We report on exact-diagonalization studies of correlated many-electron states
in the half-filled Landau levels of graphene, including pseudospin (valley)
degeneracy. We demonstrate that the polarized Fermi sea of non-interacting
composite fermions remains stable against a pairing transition in the lowest
two Landau levels. However, it undergoes spontaneous depolarization, which is
unprotected owing to the lack of single-particle pseudospin splitting. These
results suggest the absence of the Pfaffian phase in graphene.Comment: 3 pages, 4 figures (revision: reference update
Neutral Fermion Excitations in the Moore-Read state at \nu=5/2
We present evidence supporting the weakly paired Moore-Read phase in the
half-filled second Landau level, focusing on some of the qualitative features
of its excitations. Based on numerical studies, we show that systems with odd
particle number at the flux can be interpreted as a neutral
fermion mode of one unpaired fermion, which is gapped. The mode is found to
have two distinct minima, providing a signature that could be observed by
photoluminescence. In the presence of two quasiparticles the same neutral
fermion excitation is shown to be gapless, confirming expectations for
non-Abelian statistics of the Ising model with degenerate fusion channels 1 and
.Comment: 4 pages, 4 figures; v2: final published versio
New Zealandâs Performance Based Research Funding (PBRF) model undermines Maori research
The Performance Based Research Funding (PBRF) model was instigated in 2002 to increase âthe quality of research through peer assessment and performance indicatorsâ in New Zealand (Ministry of Education 2002: 17). It is used to allocate funding between universities, departments and researchers according to the putative quality and quantity of their research outputs over the preceding 6 years. PBRF is expected to incentivise improved research excellence and efficiency, and allow government to invest research funds where greatest returns will result. This is potentially a huge gain for MÄori. However, âby changing the conditions of knowledge production, research assessment exercises may also alter the shape and direction of disciplines by diverting and channelling researchersâ intellectual attention and political engagement, influencing what they study, how they do it, and how they report and writeâ (Middleton 2009: 194). Indeed, universities repeatedly encourage researchers to focus on activities that will improve their PBRF rankings. We believe that an unintended consequence of PBRF is the creation of significant barriers to increasing the volume, scope and quality of environmental research for MÄori
Skyrmions in the Moore-Read state at nu=5/2
We study charged excitations of the non-abelian Moore-Read liquid at filling
factor nu=5/2, allowing for spin depolarization. Using a combination of
numerical studies, and taking account of non-zero well widths, we find that at
sufficiently low Zeeman energy it is energetically favourable for charge e/4
quasiholes to bind into "skyrmions" of charge e/2. We show that skyrmion
formation is further promoted by disorder, and argue that this can lead to a
depolarized nu=5/2 ground state in realistic experimental situations. We
comment on the consequences for the activated transport.Comment: 4 pages, 3 figure
The mass-metallicity relation for high-redshift damped Ly-alpha galaxies
We used our database of ESO VLT-UVES spectra of quasars to build up a sample
of 67 Damped Lyman-alpha (DLA) systems with redshifts 1.7<zabs<3.7. For each
system, we measured average metallicities relative to Solar, [X/H] (with either
X=Zn, S or Si), and the velocity widths of low-ionization line profiles, W1. We
find that there is a tight correlation between the two quantities, detected at
the 5sigma significance level. The existence of such a correlation, over more
than two orders of magnitude spread in metallicity, is likely to be the
consequence of an underlying mass-metallicity relation for the galaxies
responsible for DLA absorption lines. The best-fit linear relation is
[X/H]=1.35(\pm 0.11)\log W1 -3.69(\pm 0.18)$ with W1 expressed in km/s. While
the slope of this velocity-metallicity relation is the same within
uncertainties between the higher and the lower redshift bins of our sample,
there is a hint of an increase of the intercept point of the relation with
decreasing redshift. This suggests that galaxy halos of a given mass tend to
become more metal-rich with time. Moreover, the slope of this relation is
consistent with that of the luminosity-metallicity relation for local galaxies.
The DLA systems having the lowest metallicities among the DLA population would
therefore, on average, correspond to the galaxies having the lowest masses. In
turn, these galaxies should have the lowest luminosities among the DLA galaxy
population. This may explain the recent result that the few DLA systems with
detected Ly-alpha emission have higher than average metallicities.Comment: proceedings of IAU Colloquium No. 199, 2005, ``Probing Galaxies
through Quasar Absorption Lines'', P.R. Williams, C. Shu, B. Menard, ed
Breakup of Air Bubbles in Water: Memory and Breakdown of Cylindrical Symmetry
Using high-speed video, we have studied air bubbles detaching from an
underwater nozzle. As a bubble distorts, it forms a thin neck which develops a
singular shape as it pinches off. As in other singularities, the minimum neck
radius scales with the time until breakup. However, because the air-water
interfacial tension does not drive breakup, even small initial cylindrical
asymmetries are preserved throughout the collapse. This novel, non-universal
singularity retains a memory of the nozzle shape, size and tilt angle. In the
last stages, the air appears to tear instead of pinch.Comment: Submitted to Phys. Rev. Lett. 4 pages, 4 figures. Revised for
resubmissio
Gravitational Lensing of the SDSS High-Redshift Quasars
We predict the effects of gravitational lensing on the color-selected
flux-limited samples of z~4.3 and z>5.8 quasars, recently published by the
Sloan Digital Sky Survey (SDSS). Our main findings are: (i) The lensing
probability should be 1-2 orders of magnitude higher than for conventional
surveys. The expected fraction of multiply-imaged quasars is highly sensitive
to redshift and the uncertain slope of the bright end of the luminosity
function, beta_h. For beta_h=2.58 (3.43) we find that at z~4.3 and i*<20.0 the
fraction is ~4% (13%) while at z~6 and z*<20.2 the fraction is ~7% (30%). (ii)
The distribution of magnifications is heavily skewed; sources having the
redshift and luminosity of the SDSS z>5.8 quasars acquire median magnifications
of med(mu_obs)~1.1-1.3 and mean magnifications of ~5-50. Estimates of
the quasar luminosity density at high redshift must therefore filter out
gravitationally-lensed sources. (iii) The flux in the Gunn-Peterson trough of
the highest redshift (z=6.28) quasar is known to be f_lambda<3 10^-19
erg/sec/cm^2/Angstrom. Should this quasar be multiply imaged, we estimate a 40%
chance that light from the lens galaxy would have contaminated the same part of
the quasar spectrum with a higher flux. Hence, spectroscopic studies of the
epoch of reionization need to account for the possibility that a lens galaxy,
which boosts the quasar flux, also contaminates the Gunn-Peterson trough. (iv)
Microlensing by stars should result in ~1/3 of multiply imaged quasars in the
z>5.8 catalog varying by more than 0.5 magnitudes over the next decade. The
median equivalent width would be lowered by ~20% with respect to the intrinsic
value due to differential magnification of the continuum and emission-line
regions.Comment: 27 pages, 10 figures. Expansion on the discussion in
astro-ph/0203116. Replaced with version accepted for publication in Ap
Disponibilidade de micronutrientes em classes dominantes de solos do trĂłpico Ășmido brasileiro. I. Zinco.
bitstream/item/107836/1/BP55.pd
The Transverse Proximity Effect: A Probe to the Environment, Anisotropy, and Megayear Variability of QSOs
The transverse proximity effect is the expected decrease in the strength of
the Lya forest absorption in a QSO spectrum when another QSO lying close to the
line of sight enhances the photoionization rate above that due to the average
cosmic ionizing background. We select three QSOs from the Early Data Release of
the Sloan Digital Sky Survey that have nearby foreground QSOs, with proper line
of sight tangential separations of 0.50, 0.82, and 1.10 h^{-1} Mpc. We estimate
that the ionizing flux from the foreground QSO should increase the
photoionization rate by a factor (94, 13, 13) in these three cases, which would
be clearly detectable in the first QSO and marginally so in the other two. We
do not detect the transverse proximity effect. Three possible explanations are
provided: an increase of the gas density in the vicinity of QSOs, time
variability, and anisotropy of the QSO emission. We find that the increase of
gas density near QSOs can be important if they are located in the most massive
halos present at high redshift, but is not enough to fully explain the absence
of the transverse proximity effect. Anisotropy requires an unrealistically
small opening angle of the QSO emission. Variability demands that the
luminosity of the QSO with the largest predicted effect was much lower 10^6
years ago, whereas the transverse proximity effect observed in the HeII Lya
absorption in QSO 0302-003 by Jakobsen et al. (2003) implies a lifetime longer
than 10^7 years. A combination of all three effects may better explain the lack
of Lya absorption reduction. A larger sample of QSO pairs may be used to
diagnose the environment, anisotropy and lifetime distribution of QSOs.Comment: 27 pages, 13 figures, accepted by Ap
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