2,632 research outputs found
Statistical study of the conductance and shot noise in open quantum-chaotic cavities: Contribution from whispering gallery modes
In the past, a maximum-entropy model was introduced and applied to the study
of statistical scattering by chaotic cavities, when short paths may play an
important role in the scattering process. In particular, the validity of the
model was investigated in relation with the statistical properties of the
conductance in open chaotic cavities. In this article we investigate further
the validity of the maximum-entropy model, by comparing the theoretical
predictions with the results of computer simulations, in which the Schroedinger
equation is solved numerically inside the cavity for one and two open channels
in the leads; we analyze, in addition to the conductance, the zero-frequency
limit of the shot-noise power spectrum. We also obtain theoretical results for
the ensemble average of this last quantity, for the orthogonal and unitary
cases of the circular ensemble and an arbitrary number of channels. Generally
speaking, the agreement between theory and numerics is good. In some of the
cavities that we study, short paths consist of whispering gallery modes, which
were excluded in previous studies. These cavities turn out to be all the more
interesting, as it is in relation with them that we found certain systematic
discrepancies in the comparison with theory. We give evidence that it is the
lack of stationarity inside the energy interval that is analyzed, and hence the
lack of ergodicity that gives rise to the discrepancies. Indeed, the agreement
between theory and numerical simulations is improved when the energy interval
is reduced to a point and the statistics is then collected over an ensemble. It
thus appears that the maximum-entropy model is valid beyond the domain where it
was originally derived. An understanding of this situation is still lacking at
the present moment.Comment: Revised version, minor modifications, 28 pages, 7 figure
A Growth Model for Multicellular Tumor Spheroids
Most organisms grow according to simple laws, which in principle can be
derived from energy conservation and scaling arguments, critically dependent on
the relation between the metabolic rate B of energy flow and the organism mass
m. Although this relation is generally recognized to be of the form B(m) = mp,
the specific value of the exponent p is the object of an ongoing debate, with
many mechanisms being postulated to support different predictions. We propose
that multicellular tumor spheroids provide an ideal experimental model system
for testing these allometric growth theories, especially under controlled
conditions of malnourishment and applied mechanical stress
Quantum State Tomography Using Successive Measurements
We describe a quantum state tomography scheme which is applicable to a system
described in a Hilbert space of arbitrary finite dimensionality and is
constructed from sequences of two measurements. The scheme consists of
measuring the various pairs of projectors onto two bases --which have no
mutually orthogonal vectors--, the two members of each pair being measured in
succession. We show that this scheme implies measuring the joint
quasi-probability of any pair of non-degenerate observables having the two
bases as their respective eigenbases. The model Hamiltonian underlying the
scheme makes use of two meters initially prepared in an arbitrary given quantum
state, following the ideas that were introduced by von Neumann in his theory of
measurement.Comment: 12 Page
Faecal pellet count method: some evaluations of dropping detectability for Capreolus capreolus Linnaeus, 1758 (Mammalia: Cervidae), Cervus elaphus Linnaeus, 1758 (Mammalia: Cervidae) and Lepus europaeus Pallas, 1778 (Mammalia: Leporidae).
A direct image of the obscuring disk surrounding an active galactic nucleus
Active galactic nuclei (AGN) are generally accepted to be powered by the
release of gravitational energy in a compact accretion disk surrounding a
massive black hole. Such disks are also necessary to collimate powerful radio
jets seen in some AGN. The unifying classification schemes for AGN further
propose that differences in their appearance can be attributed to the opacity
of the accreting material, which may obstruct our view of the central region of
some systems. The popular model for the obscuring medium is a parsec-scale disk
of dense molecular gas, although evidence for such disks has been mostly
indirect, as their angular size is much smaller than the resolution of
conventional telescopes. Here we report the first direct images of a pc-scale
disk of ionised gas within the nucleus of NGC 1068, the archetype of obscured
AGN. The disk is viewed nearly edge-on, and individual clouds within the
ionised disk are opaque to high-energy radiation, consistent with the unifying
classification scheme. In projection, the disk and AGN axes align, from which
we infer that the ionised gas disk traces the outer regions of the long-sought
inner accretion disk.Comment: 14 pages, LaTeX, PSfig, to appear in Nature. also available at
http://hethp.mpe-garching.mpg.de/Preprint
Impact of an external electron acceptor on phosphorus mobility between water and sediments
The present work assessed the impact of an external electron acceptor on phosphorus fluxes between water-sediment interface. Microcosm experiments simulating a sediment microbial fuel cell (SMFC) were carried out and phosphorus was extracted by an optimized combination of three methods. Despite the low voltage recorded, ∼96 mV (SMFC with carbon paper anode) and ∼146 mV (SMFC with stainless steel scourer anode), corresponding to a power density of 1.15 mW/m2 and 0.13 mW/m2, it was enough to produce an increase in the amounts of metal bound phosphorus (14% vs 11%), Ca-bound phosphorus (26% vs 23%) and refractory phosphorus (33% vs 28%). These results indicate an important role of electroactive bacteria in the phosphorus cycling and open a new perspective for preventing metal bound phosphorus dissolution from sediments.The authors are indebted and grateful to the Regional Department of Water Resources and Land Planning (Azores) and its staff. The authors also acknowledge the Grant SFRH/BPD/8052812011 from the Foundation for Science and Technology, Portugal, awarded to Gilberto Martins
Modelling the X-ray spectra of high velocity outflows from quasars
High velocity outflows from supermassive black holes have been invoked to
explain the recent identification of strong absorption features in the hard
X-ray spectra of several quasars. Here, Monte Carlo radiative transfer
calculations are performed to synthesise X-ray spectra from models of such
flows. It is found that simple, parametric bi-conical outflow models with
plausible choices for the wind parameters predict spectra that are in good
qualitative agreement with observations in the 2 - 10 keV band. The influence
on the spectrum of both the mass-loss rate and opening angle of the flow are
considered: the latter is important since photon leakage plays a significant
role in establishing an ionization gradient within the flow, a useful
discriminant between spherical and conical outflow for this and other
applications. Particular attention is given to the bright quasar PG1211+143 for
which constraints on the outflow geometry and mass-loss rate are discussed
subject to the limitations of the currently available observational data.Comment: 15 pages, 8 figures. Accepted for publication by MNRA
The Angular Clustering of Galaxy Pairs
We identify close pairs of galaxies from 278 deg^2 of Sloan Digital Sky
Survey commissioning imaging data. The pairs are drawn from a sample of 330,041
galaxies with 18 < r^* < 20. We determine the angular correlation function of
galaxy pairs, and find it to be stronger than the correlation function of
single galaxies by a factor of 2.9 +/- 0.4. The two correlation functions have
the same logarithmic slope of 0.77. We invert Limber's equation to estimate the
three-dimensional correlation functions; we find clustering lengths of r_0= 4.2
+/- 0.4 h^{-1} Mpc for galaxies and 7.8 +/- 0.7 h^{-1} Mpc for galaxy pairs.
These results agree well with the global richness dependence of the correlation
functions of galaxy systems.Comment: 12 pages. ApJ, in pres
The metallicity distributions in high-latitudes with SDSS
We present metallicities and their variations with different parameters for
36 high-latitude fields covering Galactic longitudes 0<l<360. The metallicities
for relatively short vertical distances (z<2.5 kpc) show systematic
fluctuations with Galactic longitude, similar to those of the thick-disc
scaleheight, which may be interpreted as indicating a common origin, viz., the
flare effect of the disc. This suggestion is supported by the metallicity
variations which we find as functions of radial distance. The metallicity
variation at larger vertical distances (6.5<z<9.5 kpc) is small but monotonic.
Three different vertical metallicity gradients could be detected:
d[M/H]/dz=-0.22(+/-0.03), d[M/H]/dz=-0.38 (+/-0.06), and d[M/H]/dz=-0.08
(+/-0.07) dex/kpc for the intervals z<3, 3<z<5, and 5<z<10 kpc, respectively.
Since our data cover the whole Galactic longitude interval, the resulting
metallicity gradients can be interpreted as properties of the larger-scale
Galaxy. The first gradient confirms the dissipational formation of the disc at
short z-distances. The steeper gradient corresponds to the transition region
between different population components of the Galaxy, and finally, the lowest
value provides an adequate description of the inner-halo metallicity gradient.Comment: 14 pages, 7 figures and 2 tables, accepted for publication in New
Astronom
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