5,683 research outputs found
Chaos in the Random Field Ising Model
The sensitivity of the random field Ising model to small random perturbations
of the quenched disorder is studied via exact ground states obtained with a
maximum-flow algorithm. In one and two space dimensions we find a mild form of
chaos, meaning that the overlap of the old, unperturbed ground state and the
new one is smaller than one, but extensive. In three dimensions the
rearrangements are marginal (concentrated in the well defined domain walls).
Implications for finite temperature variations and experiments are discussed.Comment: 4 pages RevTeX, 6 eps-figures include
Critical Exponents of the Three Dimensional Random Field Ising Model
The phase transition of the three--dimensional random field Ising model with
a discrete () field distribution is investigated by extensive Monte
Carlo simulations. Values of the critical exponents for the correlation length,
specific heat, susceptibility, disconnected susceptibility and magnetization
are determined simultaneously via finite size scaling. While the exponents for
the magnetization and disconnected susceptibility are consistent with a first
order transition, the specific heat appears to saturate indicating no latent
heat. Sample to sample fluctuations of the susceptibilty are consistent with
the droplet picture for the transition.Comment: Revtex, 10 pages + 4 figures included as Latex files and 1 in
Postscrip
Helical motion and the origin of QPO in blazar-type sources
Recent observations and analysis of blazar sources provide strong evidence
for (i) the presence of significant periodicities in their lightcurves and (ii)
the occurrence of helical trajectories in their radio jets. In scenarios, where
the periodicity is caused by differential Doppler boosting effects along a
helical jet path, both of these facts may be naturally tied together. Here we
discuss four possible driving mechanisms for the occurrence of helical
trajectories: orbital motion in a binary system, Newtonian-driven jet
precession, internal jet rotation and motion along a global helical magnetic
field. We point out that for non-ballistic helical motion the observed period
may appear strongly shortened due to classical travel time effects. Finally,
the possible relevance of the above mentioned driving mechanisms is discussed
for Mkn~501, OJ 287 and AO 0235+16.Comment: 6 pages, 1 figure; presented at the 5th Microquasar Workshop,
Beijing, June 2004. Accepted for publication in the Chinese Journal of
Astronomy and Astrophysic
Non-thermal Processes in Black-Hole-Jet Magnetospheres
The environs of supermassive black holes are among the universe's most
extreme phenomena. Understanding the physical processes occurring in the
vicinity of black holes may provide the key to answer a number of fundamental
astrophysical questions including the detectability of strong gravity effects,
the formation and propagation of relativistic jets, the origin of the highest
energy gamma-rays and cosmic-rays, and the nature and evolution of the central
engine in Active Galactic Nuclei (AGN). As a step towards this direction, this
paper reviews some of the progress achieved in the field based on observations
in the very high energy domain. It particularly focuses on non-thermal particle
acceleration and emission processes that may occur in the rotating
magnetospheres originating from accreting, supermassive black hole systems.
Topics covered include direct electric field acceleration in the black hole's
magnetosphere, ultra-high energy cosmic ray production, Blandford-Znajek
mechanism, centrifugal acceleration and magnetic reconnection, along with the
relevant efficiency constraints imposed by interactions with matter, radiation
and fields. By way of application, a detailed discussion of well-known sources
(Sgr A*; Cen A; M87; NGC1399) is presented.Comment: invited review for International Journal of Modern Physics D, 49
pages, 15 figures; minor typos corrected to match published versio
Integrability and Quantum Chaos in Spin Glass Shards
We study spin glass clusters ("shards") in a random transverse magnetic
field, and determine the regime where quantum chaos and random matrix level
statistics emerge from the integrable limits of weak and strong field.
Relations with quantum phase transition are also discussed.Comment: revtex, 4 pages, 5 figure
Computational Complexity of Determining the Barriers to Interface Motion in Random Systems
The low-temperature driven or thermally activated motion of several condensed
matter systems is often modeled by the dynamics of interfaces (co-dimension-1
elastic manifolds) subject to a random potential. Two characteristic
quantitative features of the energy landscape of such a many-degree-of-freedom
system are the ground-state energy and the magnitude of the energy barriers
between given configurations. While the numerical determination of the former
can be accomplished in time polynomial in the system size, it is shown here
that the problem of determining the latter quantity is NP-complete. Exact
computation of barriers is therefore (almost certainly) much more difficult
than determining the exact ground states of interfaces.Comment: 8 pages, figures included, to appear in Phys. Rev.
Fano Interference in Microwave Resonator Measurements
Resonator measurements are a simple but powerful tool to characterize a
material's microwave response. The losses of a resonant mode are quantified by
its internal quality factor , which can be extracted from the
scattering coefficient in a microwave reflection or transmission measurement.
Here we show that a systematic error on arises from Fano
interference of the signal with a background path. Limited knowledge of the
interfering paths in a given setup translates into a range of uncertainty for
, which increases with the coupling coefficient. We
experimentally illustrate the relevance of Fano interference in typical
microwave resonator measurements and the associated pitfalls encountered in
extracting . On the other hand, we also show how to characterize
and utilize the Fano interference to eliminate the systematic error
Local magnetic structure due to inhomogeneity of interaction in S=1/2 antiferromagnetic chain
We study the magnetic properties of antiferromagnetic Heisenberg
chains with inhomogeneity of interaction. Using a quantum Monte Carlo method
and an exact diagonalization method, we study bond-impurity effect in the
uniform chain and also in the bond-alternating chain. Here `bond
impurity' means a bond with strength different from those in the bulk or a
defect in the alternating order. Local magnetic structures induced by bond
impurities are investigated both in the ground state and at finite
temperatures, calculating the local magnetization, the local susceptibility and
the local field susceptibility. We also investigate the force acting between
bond impurities and find the force generally attractive.Comment: 15pages, 34figure
Critical Exponents of the pure and random-field Ising models
We show that current estimates of the critical exponents of the
three-dimensional random-field Ising model are in agreement with the exponents
of the pure Ising system in dimension 3 - theta where theta is the exponent
that governs the hyperscaling violation in the random case.Comment: 9 pages, 4 encapsulated Postscript figures, REVTeX 3.
New Relativistic Particle-In-Cell Simulation Studies of Prompt and Early Afterglows from GRBs
Nonthermal radiation observed from astrophysical systems containing
relativistic jets and shocks, e.g., gamma-ray bursts (GRBs), active galactic
nuclei (AGNs), and microquasars commonly exhibit power-law emission spectra.
Recent PIC simulations of relativistic electron-ion (or electron-positron) jets
injected into a stationary medium show that particle acceleration occurs within
the downstream jet. In collisionless, relativistic shocks, particle (electron,
positron, and ion) acceleration is due to plasma waves and their associated
instabilities (e.g., the Weibel (filamentation) instability) created in the
shock region. The simulations show that the Weibel instability is responsible
for generating and amplifying highly non-uniform, small-scale magnetic fields.
These fields contribute to the electron's transverse deflection behind the jet
head. The resulting "jitter" radiation from deflected electrons has different
properties compared to synchrotron radiation, which assumes a uniform magnetic
field. Jitter radiation may be important for understanding the complex time
evolution and/or spectra in gamma-ray bursts, relativistic jets in general, and
supernova remnants.Comment: : 4 pages, 1 figure and 1 table, typos are corrected, submitted for
the Proceedings of The 4th Heidelberg International Symposium on High Energy
Gamma-Ray Astronomy, July 7-11, 2008, in Heidelberg, German
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