755 research outputs found
Radiation-Induced "Zero-Resistance State" and the Photon Assisted Transport
We demonstrate that the radiation induced "zero-resistance state" observed in
a two-dimensional electron gas is a result of the non-trivial structure of the
density of states of the systems and the photon assisted transport. A toy model
of a structureless quantum tunneling junction where the system has oscillatory
density of states catches most of the important features of the experiments. We
present a generalized Kubo-Greenwood conductivity formula for the photon
assisted transport in a general system, and show essentially the same nature of
the transport anomaly in a uniform system.Comment: 4 pages, 3 figures. Please send comment to [email protected]. This
version added a paragraph to discuss the implication of negative conductanc
Measuring the cosmological 21-cm dipole with 21-cm global experiments
A measurement of the 21-cm global signal would be a revealing probe of the
Dark Ages, the era of first star formation, and the Epoch of Reionization. It
has remained elusive owing to bright galactic and extra-galactic foreground
contaminants, coupled with instrumental noise, ionospheric effects, and beam
chromaticity. The simultaneous detection of a consistent 21-cm dipole signal
alongside the 21-cm global signal would provide confidence in a claimed
detection. We use simulated data to investigate the possibility of using
drift-scan dipole antenna experiments to achieve a detection of both monopole
and dipole. We find that at least two antennae located at different latitudes
are required to localise the dipole. In the absence of foregrounds, a total
integration time of hours is required to detect the dipole. With
contamination by simple foregrounds, we find that the integration time required
increases to hours. We show that the extraction of the 21-cm dipole
from more realistic foregrounds requires a more sophisticated foreground
modelling approach. Finally, we motivate a global network of dipole antennae
that could reasonably detect the dipole in hours of integration
time.Comment: 12 pages, 15 figure
Fractional and unquantized dc voltage generation in THz-driven semiconductor superlattices
We consider the spontaneous creation of a dc voltage across a strongly
coupled semiconductor superlattice subjected to THz radiation. We show that the
dc voltage may be approximately proportional either to an integer or to a half-
integer multiple of the frequency of the applied ac field, depending on the
ratio of the characteristic scattering rates of conducting electrons. For the
case of an ac field frequency less than the characteristic scattering rates, we
demonstrate the generation of an unquantized dc voltage.Comment: 6 pages, 3 figures, RevTEX, EPSF. Revised version v3: corrected typo
Bloch oscillations, Zener tunneling and Wannier-Stark ladders in the time-domain
We present a time-domain analysis of carrier dynamics in a semiconductor
superlattice with two minibands. Integration of the density-matrix equations of
motion reveals a number of new features: (i) for certain values of the applied
static electric field strong interband transitions occur; (ii) in static fields
the complex time-dependence of the density-matrix displays a sequence of stable
plateaus in the low field regime, and (iii) for applied fields with a periodic
time-dependence the dynamic response can be understood in terms of the
quasienergy spectra.Comment: 4 pages, 6 PostScript figures available from [email protected], REVTEX
3.
X-ray absorption study of Ti-activated sodium aluminum hydride
Ti K-edge x-ray absorption near edge spectroscopy (XANES) was used to explore
the Ti valence and coordination in Ti-activated sodium alanate. An empirical
relationship was established between the Ti valence and the Ti K-edge onset
based on a set of standards. This relationship was used to estimate oxidation
states of the titanium catalyst in 2 mol% and 4 mol% Ti-doped NaAlH4. These
results demonstrate that the formal titanium valence is zero in doped sodium
alanate and nearly invariant during hydrogen cycling. A qualitative comparison
of the edge fine structure suggests that the Ti is present on the surface in
the form of amorphous TiAl3.Comment: 3 pages, 4 figures, submitted to Appl. Phys. Let
Recommended from our members
Ruin and Deficit Under Claim Arrivals with the Order Statistics Property
We consider an insurance risk model with extended flexibility,
under which claims arrive according to a point process with an order
statistics (OS) property, their amounts may have any joint distri-
bution and the premium income is accumulated following any non-
decreasing, possibly discontinuous real valued function. We generalize the definition of an OS point process, assuming it is generated by an arbitrary cdf allowing jump discontinuities, which corresponds to an arbitrary (possibly discontinuous) claim arrival cumulative intensity function. The latter feature is appealing for insurance applications since it allows to consider clusters of claims arriving instantaneously. Under these general assumptions, a closed form expression for the joint distribution of the time to ruin and the deficit at ruin is derived, which remarkably involves classical Appell polynomials. Corollaries of our main result generalize previous non-ruin formulas e.g., those obtained by Ignatov and Kaishev (2000, 2004, 2006) and Lef`evre and Loisel (2009) for the case of stationary Poisson claim arrivals and by Lef`evre and Picard (2011, 2014), for OS claim arrivals
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Operational risk and insurance: a ruin probabilistic reserving approach
A new methodology for financial and insurance operational risk capital estimation is proposed. It is based on using the finite time probability of (non-)ruin as an operational risk measure, within a general risk model. It allows for inhomogeneous operational loss frequency (dependent inter-arrival times) and dependent loss severities which may have any joint discrete or continuous distribution. Under the proposed methodology, operational risk capital assessment is viewed not as a one off exercise, performed at some moment of time, but as dynamic reserving, following a certain risk capital accumulation function. The latter describes the accumulation of risk capital with time and may be any nondecreasing, mpositive real function hHtL. Under these reasonably general assumptions, the probability of mnon-ruin is explicitly expressed using closed form expressions, derived by Ignatov and Kaishev (2000, 2004, 2007) and Ignatov, Kaishev and Krachunov (2001) and by setting it to a high enough preassigned mvalue, say 0.99, it is possible to obtain not just a value for the capital charge but a (dynamic) risk capital accumulation strategy, hHtL. In view of its generality, the proposed methodology is capable of accommodating any (heavy tailed) mdistributions, such as the Generalized Pareto Distribution, the Lognormal distribution the g-and-h mdistribution and the GB2 distribution. Applying this methodology on numerical examples, we demonstrate that dependence in the loss severities may have a dramatic effect on the estimated risk capital. In addition, we show also that one and the same high enough survival probability may be achieved by mdifferent risk capital accumulation strategies one of which may possibly be preferable to accumulating capital just linearly, as has been assumed by Embrechts et al. (2004). The proposed methodology takes into account also the effect of insurance on operational losses, in which case it is proposed to take the probability of joint survival of the financial institution and the insurance provider as a joint operational risk measure. The risk capital allocation strategy is then obtained in such a way that the probability of joint survival is equal to a preassigned high enough value, say 99.9
Interaction between dust grains near a conducting wall
The effect of the conducting electrode on the interaction of dust grains in a
an ion flow is discussed. It is shown that two grains levitating above the
electrode at the same height may attract one another. This results in the
instability of a dust layer in a plasma sheath.Comment: 9 pages. 3 figures. Submitted to Plasma Physics Report
Superlattice with hot electron injection: an approach to a Bloch oscillator
A semiconductor superlattice with hot electron injection into the miniband is
considered. The injection changes the stationary distribution function and
results in a qualitative change of the frequency behaviour of the differential
conductivity. In the regime with Bloch oscillating electrons and injection into
the upper part of the miniband the region of negative differential conductivity
is shifted from low frequencies to higher frequencies. We find that the dc
differential conductivity can be made positive and thus the domain instability
can be suppressed. At the same time the high-frequency differential
conductivity is negative above the Bloch frequency. This opens a new way to
make a Bloch oscillator operating at THz frequencies.Comment: RevTeX, 8 pages, 2 figures, to be published in Phys. Rev. B, 15
Januar 200
Recommended from our members
Ruin and deficit at ruin under an extended order statistics risk process
We consider an insurance risk model with extended flexibility, under which claims arrive according to a point process with an order statistics (OS) property, their amounts may have any joint distribution and the premium income is accumulated following any nondecreasing, possibly discontinuous real valued function. We generalize the definition of an OS point process, assuming it is generated by an arbitrary cdf, allowing jump discontinuities which corresponds to an arbitrary (possibly discontinuous) claim arrival cumulative intensity function. The latter feature is appealing for insurance applications since it allows to consider clusters of claims arriving instantaneously. Under these general assumptions, a closed form expression for the joint distribution of the time to ruin and the deficit at ruin is derived, which remarkably involves classical Appell polynomials. Corollaries of our main result generalize previous non-ruin formulas e.g., those obtained by Ignatov and Kaishev (2000, 2004, 2006) and Lef`evre and Loisel (2009) for the case of stationary Poisson claim arrivals and by Lef`evre and Picard (2011, 2014), for OS claim arrivals
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