13,870 research outputs found
The multi-frequency angular power spectrum of the epoch of reionization 21 cm signal
Observations of redshifted 21cm radiation from HI at high redshifts is an
important future probe of reionization. We consider the Multi-frequency Angular
Power Spectrum (MAPS) to quantify the statistics of the HI signal as a joint
function of the angular multipole l and frequency separation \Delta\nu. The
signal at two different frequencies is expected to get decorrelated as
\Delta\nu is increased, and quantifying this decorrelation is particularly
important in deciding the frequency resolution for future HI observations. This
is also expected to play a very crucial role in extracting the signal from
foregrounds as the signal is expected to decorrelate much faster than the
foregrounds (which are largely continuum sources) with increasing \Delta\nu. In
this paper we develop formulae relating the MAPS to different components of the
three dimensional HI power spectrum taking into account HI peculiar velocities.
We show that the flat-sky approximation provides a very good representation
over the angular scales of interest, and a final expression which is very
simple to calculate and interpret. We present results considering two models
for the HI distribution, namely, (i) DM: where the HI traces the dark matter
and (ii) PR: where the effects of patchy reionization are incorporated through
two parameters. We find that while the DM signal is largely featureless, the PR
signal peaks at the angular scales of the individual bubbles, and the signal is
considerably enhanced for large bubble size. For most cases of interest at l
\sim 100 the signal is uncorrelated beyond \Delta\nu \sim 1 MHz or even less,
whereas it occurs around \sim 0.1 MHz at l \sim 10^3. The \Delta\nu dependence
also carries an imprint of the bubble size and the bias, and is expected to be
an important probe of the reionization scenario (abridged).Comment: Accepted for publication in MNRAS. Revised to match the accepted
versio
Conduction Mechanism in a Molecular Hydrogen Contact
We present first principles calculations for the conductance of a hydrogen
molecule bridging a pair of Pt electrodes. The transmission function has a wide
plateau with T~1 which extends across the Fermi level and indicates the
existence of a single, robust conductance channel with nearly perfect
transmission. Through a detailed Wannier function analysis we show that the H2
bonding state is not involved in the transport and that the plateau forms due
to strong hybridization between the H2 anti-bonding state and states on the
adjacent Pt atoms. The Wannier functions furthermore allow us to derive a
resonant-level model for the system with all parameters determined from the
fully self-consistent Kohn-Sham Hamiltonian.Comment: 5 pages, 4 figure
On the Observability of "Invisible" / "Nearly Invisible" Charginos
It is shown that if the charginos decay into very soft leptons or hadrons +
due to degeneracy/ near- degeneracy with the LSP or the sneutrino,
the observability of the recently proposed signal via the single photon (+ soft
particles) + channel crucially depends on the magnitude of the \SNU
mass due to destructive interferences in the matrix element squared. If the
\SNU's and, consequently, left-sleptons are relatively light, the size of the
signal, previously computed in the limit \MSNU \to \infty only, is
drastically reduced. We present the formula for the signal cross section in a
model independent way and discuss the observability of the signal at LEP 192
and NLC energies.Comment: 27 pages, Late
Calculation of the Self-energy of Open Quantum Systems
We propose an easy method of calculating the self-energy of semi-infinite
leads attached to a mesoscopic system.Comment: 6 pages, 2 figures, published in J. Phys. Soc. Jp
Effects of Disorder and Momentum Relaxation on the Intertube Transport of Incommensurate Carbon Nanotube Ropes and Multiwall Nanotubes
We study theoretically the electrical transport between aligned carbon
nanotubes in nanotube ropes, and between shells in multiwall carbon nanotubes.
We focus on transport between two metallic nanotubes (or shells) of different
chiralities with mismatched Fermi momenta and incommensurate periodicities. We
perform numerical calculations of the transport properties of such systems
within a tight-binding formalism. For clean (disorder-free) nanotubes the
intertube transport is strongly suppressed as a result of momentum
conservation. For clean nanotubes, the intertube transport is typically
dominated by the loss of momentum conservation at the contacts. We discuss in
detail the effects of disorder, which also breaks momentum conservation, and
calculate the effects of localised scatterers of various types. We show that
physically relevant disorder potentials lead to very dramatic enhancements of
the intertube conductance. We show that recent experimental measurements of the
intershell transport in multiwall nanotubes are consistent with our theoretical
results for a model of short-ranged correlated disorder.Comment: References adde
Microscopic non-equilibrium theory of quantum well solar cells
We present a microscopic theory of bipolar quantum well structures in the
photovoltaic regime, based on the non-equilibrium Green's function formalism
for a multi band tight binding Hamiltonian. The quantum kinetic equations for
the single particle Green's functions of electrons and holes are
self-consistently coupled to Poisson's equation, including inter-carrier
scattering on the Hartree level. Relaxation and broadening mechanisms are
considered by the inclusion of acoustic and optical electron-phonon interaction
in a self consistent Born approximation of the scattering self energies.
Photogeneration of carriers is described on the same level in terms of a self
energy derived from the standard dipole approximation of the electron-photon
interaction. Results from a simple two band model are shown for the local
density of states, spectral response, current spectrum, and current-voltage
characteristics for generic single quantum well systems.Comment: 10 pages, 6 figures; corrected typos, changed caption Fig. 1,
replaced Fig.
Large orbital magnetic moments in carbon nanotubes generated by resonant transport
The nonequilibrium Green's function method is used to study the ballistic
transport in metallic carbon nanotubes when a current is injected from the
electrodes with finite bias voltages. We reveal, both analytically and
numerically, that large loop currents circulating around the tube are induced,
which come from a quantum mechanical interference and are much larger than the
current along the tube axis when the injected electron is resonant with a
time-reversed pair of degenerate states, which are, in fact, inherent in the
zigzag and chiral nanotubes. This results in large orbital magnetic moments,
making the nanotube a molecular solenoid.Comment: 5 pages, 4 figures; typos correcte
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