397 research outputs found
Reversible Fluorination of Graphene: towards a Two-Dimensional Wide Bandgap Semiconductor
We report the synthesis and evidence of graphene fluoride, a two-dimensional
wide bandgap semiconductor derived from graphene. Graphene fluoride exhibits
hexagonal crystalline order and strongly insulating behavior with resistance
exceeding 10 G at room temperature. Electron transport in graphene
fluoride is well described by variable-range hopping in two dimensions due to
the presence of localized states in the band gap. Graphene obtained through the
reduction of graphene fluoride is highly conductive, exhibiting a resistivity
of less than 100 k at room temperature. Our approach provides a new
path to reversibly engineer the band structure and conductivity of graphene for
electronic and optical applications.Comment: 7 pages, 5 figures, revtex, to appear in PR
Network-mediated encoding of circadian time: The suprachiasmatic nucleus (SCN) from genes to neurons to circuits, and back
The transcriptional architecture of intracellular circadian clocks is similar across phyla, but in mammals interneuronal mechanisms confer a higher level of circadian integration. The suprachiasmatic nucleus (SCN) is a unique model to study these mechanisms, as it operates as a âŒ24 h clock not only in the living animal, but also when isolated in culture. This âclock in a dishâ can be used to address fundamental questions, such as how intraneuronal mechanisms are translated by SCN neurons into circuit-level emergent properties and how the circuit decodes, and responds to, light input. This review addresses recent developments in understanding the relationship between electrical activity, [Ca(2+)](i), and intracellular clocks. Furthermore, optogenetic and chemogenetic approaches to investigate the distinct roles of neurons and glial cells in circuit encoding of circadian time will be discussed, as well as the epigenetic and circuit-level mechanisms that enable the SCN to translate light input into coherent daily rhythms
The Quasar-LBG Two-point Angular Cross-correlation Function at z ~ 4 in the COSMOS Field
In order to investigate the origin of quasars, we estimate the bias factor
for low-luminosity quasars at high redshift for the first time. In this study,
we use the two-point angular cross-correlation function (CCF) for both
low-luminosity quasars at and Lyman-break galaxies
(LBGs). Our sample consists of both 25 low-luminosity quasars (16 objects are
spectroscopically confirmed low-luminosity quasars) in the redshift range
and 835 color-selected LBGs with at
in the COSMOS field. We have made our analysis for the following two
quasar samples; (1) the spectroscopic sample (the 16 quasars confirmed by
spectroscopy), and (2) the total sample (the 25 quasars including 9 quasars
with photometric redshifts). The bias factor for low-luminosity quasars at
is derived by utilizing the quasar-LBG CCF and the LBG
auto-correlation function. We then obtain the upper limits of the bias
factors for low-luminosity quasars, that are 5.63 and 10.50 for the total and
the spectroscopic samples, respectively. These bias factors correspond to the
typical dark matter halo masses, log
and , respectively. This result is not inconsistent with the predicted
bias for quasars which is estimated by the major merger models.Comment: 13 pages, 9 figures, Accepted for publication in Ap
Magnetic Structure of Nano-Graphite Moebius Ribbon
We consider the electronic and magnetic properties of nanographite ribbon
with zigzag edges under the periodic or Moebius boundary conditions. The zigzag
nano-graphite ribbons possess edge localized states at the Fermi level which
cause a ferrimagnetic spin polarization localized at the edge sites even in the
very weak Coulomb interaction. The imposition of the Moebius boundary condition
makes the system non-AB-bipartite lattice, and depress the spin polarization,
resulting in the formation of a magnetic domain wall. The width of the magnetic
domain depends on the Coulomb interaction and narrows with increasing U/t.Comment: 4 pages; 6 figures; published at J. Phys. Soc. Jpn. Vol. 72 No. 5 pp.
998-1001 (2003
Constraining the coalescence rate of supermassive black-hole binaries using pulsar timing
Pulsar timing observations are used to place constraints on the rate of
coalescence of supermassive black-hole (SMBH) binaries as a function of mass
and redshift. In contrast to the indirect constraints obtained from other
techniques, pulsar timing observations provide a direct constraint on the
black-hole merger rate. This is possible since pulsar timing is sensitive to
the gravitational waves (GWs) emitted by these sources in the final stages of
their evolution. We find that upper bounds calculated from the recently
published Parkes Pulsar Timing Array data are just above theoretical
predictions for redshifts below 10. In the future, with improved timing
precision and longer data spans, we show that a non-detection of GWs will rule
out some of the available parameter space in a particular class of SMBH binary
merger models. We also show that if we can time a set of pulsars to 10ns timing
accuracy, for example, using the proposed Square Kilometre Array, it should be
possible to detect one or more individual SMBH binary systems
Investigation of the spin-glass regime between the antiferromagnetic and superconducting phases in FeSeTe
Using bulk magnetization along with elastic and inelastic neutron scattering
techniques, we have investigated the phase diagram of
FeSeTe and the nature of magnetic correlations in three
nonsuperconducting samples of FeSeTe,
FeSeTe and FeSeTe. A cusp
and hysteresis in the temperature dependence of the magnetization for the
and 0.3 samples indicates spin-glass (SG) ordering below K. Neutron scattering measurements indicate that the spin-glass behavior is
associated with short-range spin density wave (SDW) ordering characterized by a
static component and a low-energy dynamic component with a characteristic
incommensurate wave vector of and an anisotropy
gap of 2.5 meV. Our high -resolution data also show that the
systems undergo a glassy structural distortion that coincides with the
short-range SDW order
Numerical Galaxy Catalog -I. A Semi-analytic Model of Galaxy Formation with N-body simulations
We construct the Numerical Galaxy Catalog (GC), based on a semi-analytic
model of galaxy formation combined with high-resolution N-body simulations in a
-dominated flat cold dark matter (CDM) cosmological model.
The model includes several essential ingredients for galaxy formation, such as
merging histories of dark halos directly taken from N-body simulations,
radiative gas cooling, star formation, heating by supernova explosions
(supernova feedback), mergers of galaxies, population synthesis, and extinction
by internal dust and intervening HI clouds. As the first paper in a series
using this model, we focus on basic photometric, structural and kinematical
properties of galaxies at present and high redshifts. Two sets of model
parameters are examined, strong and weak supernova feedback models, which are
in good agreement with observational luminosity functions of local galaxies in
a range of observational uncertainty. Both models agree well with many
observations such as cold gas mass-to-stellar luminosity ratios of spiral
galaxies, HI mass functions, galaxy sizes, faint galaxy number counts and
photometric redshift distributions in optical pass-bands, isophotal angular
sizes, and cosmic star formation rates. In particular, the strong supernova
feedback model is in much better agreement with near-infrared (K'-band) faint
galaxy number counts and redshift distribution than the weak feedback model and
our previous semi-analytic models based on the extended Press-Schechter
formalism. (Abridged)Comment: 26 pages including 27 figures, accepted for publication in ApJ,
full-resolution version is available at
http://grape.astron.s.u-tokyo.ac.jp/~yahagi/nugc
Constraints on the faint end of the quasar luminosity function at z~5 in the COSMOS field
We present the result of our low-luminosity quasar survey in the redshift
range of 4.5 < z < 5.5 in the COSMOS field. Using the COSMOS photometric
catalog, we selected 15 quasar candidates with 22 < i' < 24 at z~5, that are ~
3 mag fainter than the SDSS quasars in the same redshift range. We obtained
optical spectra for 14 of the 15 candidates using FOCAS on the Subaru Telescope
and did not identify any low-luminosity type-1 quasars at z~5 while a
low-luminosity type-2 quasar at z~5.07 was discovered. In order to constrain
the faint end of the quasar luminosity function at z~5, we calculated the
1sigma confidence upper limits of the space density of type-1 quasars. As a
result, the 1sigma confidence upper limits on the quasar space density are Phi<
1.33*10^{-7} Mpc^{-3} mag^{-1} for -24.52 < M_{1450} < -23.52 and Phi<
2.88*10^{-7} Mpc^{-3} mag^{-1} for -23.52 < M_{1450} < -22.52. The inferred
1sigma confidence upper limits of the space density are then used to provide
constrains on the faint-end slope and the break absolute magnitude of the
quasar luminosity function at z~5. We find that the quasar space density
decreases gradually as a function of redshift at low luminosity (M_{1450} ~
-23), being similar to the trend found for quasars with high luminosity
(M_{1450}<-26). This result is consistent with the so-called downsizing
evolution of quasars seen at lower redshifts.Comment: 8 pages, 9 figures, 1 table, accepted for publication in Ap
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