709 research outputs found
Chaos and Complexity of quantum motion
The problem of characterizing complexity of quantum dynamics - in particular
of locally interacting chains of quantum particles - will be reviewed and
discussed from several different perspectives: (i) stability of motion against
external perturbations and decoherence, (ii) efficiency of quantum simulation
in terms of classical computation and entanglement production in operator
spaces, (iii) quantum transport, relaxation to equilibrium and quantum mixing,
and (iv) computation of quantum dynamical entropies. Discussions of all these
criteria will be confronted with the established criteria of integrability or
quantum chaos, and sometimes quite surprising conclusions are found. Some
conjectures and interesting open problems in ergodic theory of the quantum many
problem are suggested.Comment: 45 pages, 22 figures, final version, at press in J. Phys. A, special
issue on Quantum Informatio
Active Galactic Nuclei under the scrutiny of CTA
Active Galactic Nuclei (hereafter AGN) produce powerful outflows which offer
excellent conditions for efficient particle acceleration in internal and
external shocks, turbulence, and magnetic reconnection events. The jets as well
as particle accelerating regions close to the supermassive black holes
(hereafter SMBH) at the intersection of plasma inflows and outflows, can
produce readily detectable very high energy gamma-ray emission. As of now, more
than 45 AGN including 41 blazars and 4 radiogalaxies have been detected by the
present ground-based gamma-ray telescopes, which represents more than one third
of the cosmic sources detected so far in the VHE gamma-ray regime. The future
Cherenkov Telescope Array (CTA) should boost the sample of AGN detected in the
VHE range by about one order of magnitude, shedding new light on AGN population
studies, and AGN classification and unification schemes. CTA will be a unique
tool to scrutinize the extreme high-energy tail of accelerated particles in
SMBH environments, to revisit the central engines and their associated
relativistic jets, and to study the particle acceleration and emission
mechanisms, particularly exploring the missing link between accretion physics,
SMBH magnetospheres and jet formation. Monitoring of distant AGN will be an
extremely rewarding observing program which will inform us about the inner
workings and evolution of AGN. Furthermore these AGN are bright beacons of
gamma-rays which will allow us to constrain the extragalactic infrared and
optical backgrounds as well as the intergalactic magnetic field, and will
enable tests of quantum gravity and other "exotic" phenomena.Comment: 28 pages, 23 figure
Chaos in Quantum Dots: Dynamical Modulation of Coulomb Blockade Peak Heights
The electrostatic energy of an additional electron on a conducting grain
blocks the flow of current through the grain, an effect known as the Coulomb
blockade. Current can flow only if two charge states of the grain have the same
energy; in this case the conductance has a peak. In a small grain with
quantized electron states, referred to as a quantum dot, the magnitude of the
conductance peak is directly related to the magnitude of the wavefunction near
the contacts to the dot. Since dots are generally irregular in shape, the
dynamics of the electrons is chaotic, and the characteristics of Coulomb
blockade peaks reflects those of wavefunctions in chaotic systems. Previously,
a statistical theory for the peaks was derived by assuming these wavefunctions
to be completely random. Here we show that the specific internal dynamics of
the dot, even though it is chaotic, modulates the peaks: because all systems
have short-time features, chaos is not equivalent to randomness. Semiclassical
results are derived for both chaotic and integrable dots, which are
surprisingly similar, and compared to numerical calculations. We argue that
this modulation, though unappreciated, has already been seen in experiments.Comment: 4 pages, 3 postscript figs included (2 color), uses epsf.st
Semiclassical Theory of Coulomb Blockade Peak Heights in Chaotic Quantum Dots
We develop a semiclassical theory of Coulomb blockade peak heights in chaotic
quantum dots. Using Berry's conjecture, we calculate the peak height
distributions and the correlation functions. We demonstrate that the
corrections to the corresponding results of the standard statistical theory are
non-universal and can be expressed in terms of the classical periodic orbits of
the dot that are well coupled to the leads. The main effect is an oscillatory
dependence of the peak heights on any parameter which is varied; it is
substantial for both symmetric and asymmetric lead placement. Surprisingly,
these dynamical effects do not influence the full distribution of peak heights,
but are clearly seen in the correlation function or power spectrum. For
non-zero temperature, the correlation function obtained theoretically is in
good agreement with that measured experimentally.Comment: 5 color eps figure
Modelling avalanches in martensites
Solids subject to continuous changes of temperature or mechanical load often
exhibit discontinuous avalanche-like responses. For instance, avalanche
dynamics have been observed during plastic deformation, fracture, domain
switching in ferroic materials or martensitic transformations. The statistical
analysis of avalanches reveals a very complex scenario with a distinctive lack
of characteristic scales. Much effort has been devoted in the last decades to
understand the origin and ubiquity of scale-free behaviour in solids and many
other systems. This chapter reviews some efforts to understand the
characteristics of avalanches in martensites through mathematical modelling.Comment: Chapter in the book "Avalanches in Functional Materials and
Geophysics", edited by E. K. H. Salje, A. Saxena, and A. Planes. The final
publication is available at Springer via
http://dx.doi.org/10.1007/978-3-319-45612-6_
Discovery of very-high-energy emission from RGB J2243+203 and derivation of its redshift upper limit
Very-high-energy (VHE; 100 GeV) gamma-ray emission from the blazar RGB
J2243+203 was discovered with the VERITAS Cherenkov telescope array, during the
period between 21 and 24 December 2014. The VERITAS energy spectrum from this
source can be fit by a power law with a photon index of , and a
flux normalization at 0.15 TeV of . The integrated
\textit{Fermi}-LAT flux from 1 GeV to 100 GeV during the VERITAS detection is
, which is an order of
magnitude larger than the four-year-averaged flux in the same energy range
reported in the 3FGL catalog, (). The detection with VERITAS
triggered observations in the X-ray band with the \textit{Swift}-XRT. However,
due to scheduling constraints \textit{Swift}-XRT observations were performed 67
hours after the VERITAS detection, not simultaneous with the VERITAS
observations. The observed X-ray energy spectrum between 2 keV and 10 keV can
be fitted with a power-law with a spectral index of , and the
integrated photon flux in the same energy band is . EBL model-dependent upper limits
of the blazar redshift have been derived. Depending on the EBL model used, the
upper limit varies in the range from z to z
VERITAS and Multiwavelength Observations of the BL Lacertae Object 1ES 1741+196
We present results from multiwavelength observations of the BL Lacertae
object 1ES 1741+196, including results in the very-high-energy -ray
regime using the Very Energetic Radiation Imaging Telescope Array System
(VERITAS). The VERITAS time-averaged spectrum, measured above 180 GeV, is
well-modelled by a power law with a spectral index of
. The integral flux above 180
GeV is
m s, corresponding to 1.6% of the Crab Nebula flux on average.
The multiwavelength spectral energy distribution of the source suggests that
1ES 1741+196 is an extreme-high-frequency-peaked BL Lacertae object. The
observations analysed in this paper extend over a period of six years, during
which time no strong flares were observed in any band. This analysis is
therefore one of the few characterizations of a blazar in a non-flaring state.Comment: 8 pages, 5 figures. Accepted for publication in MNRA
A search for spectral hysteresis and energy-dependent time lags from X-ray and TeV gamma-ray observations of Mrk 421
Blazars are variable emitters across all wavelengths over a wide range of
timescales, from months down to minutes. It is therefore essential to observe
blazars simultaneously at different wavelengths, especially in the X-ray and
gamma-ray bands, where the broadband spectral energy distributions usually
peak.
In this work, we report on three "target-of-opportunity" (ToO) observations
of Mrk 421, one of the brightest TeV blazars, triggered by a strong flaring
event at TeV energies in 2014. These observations feature long, continuous, and
simultaneous exposures with XMM-Newton (covering X-ray and optical/ultraviolet
bands) and VERITAS (covering TeV gamma-ray band), along with contemporaneous
observations from other gamma-ray facilities (MAGIC and Fermi-LAT) and a number
of radio and optical facilities. Although neither rapid flares nor significant
X-ray/TeV correlation are detected, these observations reveal subtle changes in
the X-ray spectrum of the source over the course of a few days. We search the
simultaneous X-ray and TeV data for spectral hysteresis patterns and time
delays, which could provide insight into the emission mechanisms and the source
properties (e.g. the radius of the emitting region, the strength of the
magnetic field, and related timescales). The observed broadband spectra are
consistent with a one-zone synchrotron self-Compton model. We find that the
power spectral density distribution at Hz from the
X-ray data can be described by a power-law model with an index value between
1.2 and 1.8, and do not find evidence for a steepening of the power spectral
index (often associated with a characteristic length scale) compared to the
previously reported values at lower frequencies.Comment: 45 pages, 15 figure
Search for composite and exotic fermions at LEP 2
A search for unstable heavy fermions with the DELPHI detector at LEP is
reported. Sequential and non-canonical leptons, as well as excited leptons and
quarks, are considered. The data analysed correspond to an integrated
luminosity of about 48 pb^{-1} at an e^+e^- centre-of-mass energy of 183 GeV
and about 20 pb^{-1} equally shared between the centre-of-mass energies of 172
GeV and 161 GeV. The search for pair-produced new leptons establishes 95%
confidence level mass limits in the region between 70 GeV/c^2 and 90 GeV/c^2,
depending on the channel. The search for singly produced excited leptons and
quarks establishes upper limits on the ratio of the coupling of the excited
fermio
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