221 research outputs found
Efficient algorithm for current spectral density calculation in single-electron tunneling and hopping
This write-up describes an efficient numerical method for the Monte Carlo
calculation of the spectral density of current in the multi-junction
single-electron devices and hopping structures. In future we plan to expand
this write-up into a full-size paper.Comment: 4 page
Sub-electron Charge Relaxation via 2D Hopping Conductors
We have extended Monte Carlo simulations of hopping transport in completely
disordered 2D conductors to the process of external charge relaxation. In this
situation, a conductor of area shunts an external capacitor
with initial charge . At low temperatures, the charge relaxation process
stops at some "residual" charge value corresponding to the effective threshold
of the Coulomb blockade of hopping. We have calculated the r.m.s value
of the residual charge for a statistical ensemble of capacitor-shunting
conductors with random distribution of localized sites in space and energy and
random , as a function of macroscopic parameters of the system. Rather
unexpectedly, has turned out to depend only on some parameter
combination: for negligible Coulomb interaction
and for substantial interaction. (Here
is the seed density of localized states, while is the
dielectric constant.) For sufficiently large conductors, both functions
follow the power law , but with different
exponents: for negligible and
for significant Coulomb interaction. We have been able to derive this law
analytically for the former (most practical) case, and also explain the scaling
(but not the exact value of the exponent) for the latter case. In conclusion,
we discuss possible applications of the sub-electron charge transfer for
"grounding" random background charge in single-electron devices.Comment: 12 pages, 5 figures. In addition to fixing minor typos and updating
references, the discussion has been changed and expande
A Numerical Study of Transport and Shot Noise at 2D Hopping
We have used modern supercomputer facilities to carry out extensive Monte
Carlo simulations of 2D hopping (at negligible Coulomb interaction) in
conductors with the completely random distribution of localized sites in both
space and energy, within a broad range of the applied electric field and
temperature , both within and beyond the variable-range hopping region. The
calculated properties include not only dc current and statistics of localized
site occupation and hop lengths, but also the current fluctuation spectrum.
Within the calculation accuracy, the model does not exhibit noise, so
that the low-frequency noise at low temperatures may be characterized by the
Fano factor . For sufficiently large samples, scales with conductor
length as , where , and
parameter is interpreted as the average percolation cluster length. At
relatively low , the electric field dependence of parameter is
compatible with the law which follows from directed
percolation theory arguments.Comment: 17 pages, 8 figures; Fixed minor typos and updated reference
A Numerical Study of Coulomb Interaction Effects on 2D Hopping Transport
We have extended our supercomputer-enabled Monte Carlo simulations of hopping
transport in completely disordered 2D conductors to the case of substantial
electron-electron Coulomb interaction. Such interaction may not only suppress
the average value of hopping current, but also affect its fluctuations rather
substantially. In particular, the spectral density of current
fluctuations exhibits, at sufficiently low frequencies, a -like increase
which approximately follows the Hooge scaling, even at vanishing temperature.
At higher , there is a crossover to a broad range of frequencies in which
is nearly constant, hence allowing characterization of the current
noise by the effective Fano factor F\equiv S_I(f)/2e \left. For
sufficiently large conductor samples and low temperatures, the Fano factor is
suppressed below the Schottky value (F=1), scaling with the length of the
conductor as . The exponent is significantly
affected by the Coulomb interaction effects, changing from when such effects are negligible to virtually unity when they are
substantial. The scaling parameter , interpreted as the average
percolation cluster length along the electric field direction, scales as when Coulomb interaction effects are negligible
and when such effects are substantial, in
good agreement with estimates based on the theory of directed percolation.Comment: 19 pages, 7 figures. Fixed minor typos and updated reference
Nanoantenna-enhanced ultrafast nonlinear spectroscopy of a single gold nanoparticle
Optical nanoantennas are a novel tool to investigate previously unattainable
dimensions in the nanocosmos. Just like their radio-frequency equivalents,
nanoantennas enhance the light-matter interaction in their feed gap. Antenna
enhancement of small signals promises to open a new regime in linear and
nonlinear spectroscopy on the nanoscale. Without antennas especially the
nonlinear spectroscopy of single nanoobjects is very demanding. Here, we
present for the first time antenna-enhanced ultrafast nonlinear optical
spectroscopy. In particular, we utilize the antenna to determine the nonlinear
transient absorption signal of a single gold nanoparticle caused by mechanical
breathing oscillations. We increase the signal amplitude by an order of
magnitude which is in good agreement with our analytical and numerical models.
Our method will find applications in linear and nonlinear spectroscopy of
nanoobjects, ranging from single protein binding events via nonlinear tensor
elements to the limits of continuum mechanics
Strong pulses detected from a rotating radio transient J18191458
We analyze individual pulses detected from RRAT J18191458. From April 2007
to April 2010, we carried out observations using the Nanshan 25-m radio
telescope of Urumqi Observatory at a central frequency of 1541.25 MHz. We
obtain a dispersion measure pc cm^{-3} by analyzing all the
423 detected bursts. The tri-band pattern of arrival time residuals is
confirmed by a single pulse timing analysis. Twenty-seven bimodal bursts
located in the middle residual band are detected, and, profiles of two typical
bimodal bursts and two individual single-peak pulses are presented. We
determine the statistical properties of SNR and W of bursts in different
residual bands. The W variation with SNR shows that the shapes of bursts
are quite different from each other. The cumulative probability distribution of
intensity for a possible power law with index is inferred
from the number of those bursts with and high intensities.Comment: 6 pages, 8 figures, 1 table, accepted for publication in A&
Chandra and XMM-Newton observations of Tololo 0109-383
We present and discuss Chandra and XMM-Newton observations of the Seyfert 2
galaxy and Compton-thick absorbed source, Tololo~0109-383. The hard X-ray
emission (i.e. above 2 keV), is dominated by a spatially unresolved
reflection component, as already discovered by previous ASCA and BeppoSAX
observ ations. The soft X-ray emission is partly (15%) extended over
about 1 kpc. Below 2 keV, the spectrum is very steep and two emission lines,
probably due to recombination to He-like ions of oxygen and neon, are clearly
present. Combining X-rays and optical information taken from the literature, we
propose an overall scenario for the nuclear regions of this source.Comment: 6 pages, 6 figures, A&A 399, 51
A correlation of the cosmic microwave sky with large scale structure
We cross correlate the large-scale cosmic microwave background (CMB) sky
measured by WMAP with two probes of large-scale structure at z ~ 1. The hard
X-ray background, measured by the HEAO-1 satellite, is positively correlated
with the WMAP data at the 2.5-3.0 sigma level. The number counts of radio
galaxies in the NVSS survey are also correlated at a slightly weaker level
(2.-2.5 sigma). These correlations appear to arise from both hemispheres on the
sky and are resilient to changes in the levels of masking of the Galaxy and
point sources, suggesting that foregrounds are not responsible for the signal.
The implication is that some of the observed CMB fluctuations arise at low
redshifts. The level of the correlations is consistent with that expected for
the cosmological constant (Omega_Lambda = 0.72) concordance model resulting
from the integrated Sachs-Wolfe effect. Thus, we may be observing dark energy's
effect on the growth of structure.Comment: 8 pages, 3 postscript figure
Polarization Signal of Distant Clusters and Reconstruction of Primordial Potential Fluctuations
We examine the polarization signal of the cosmic microwave background
radiation associated with distant clusters.
The polarization is induced by the Thomson scattering of microwave photons
with ionized gas of clusters and contains information of quadrupole temperature
anisotropies observed at the clusters. The three-dimensional map of the signal
are expressed in terms of the spin-weighted harmonics for its angular
dependence. Its radial dependence is expanded perturbatively with respect to
the distances (equivalently redshifts) to the clusters.
The independent information that we can extract out from the map is clarified
explicitly.Comment: 14 pages, 4 figures, to appear in Phys.Rev.
- …