3,280 research outputs found
Double Exchange in a Magnetically Frustrated System
This work examines the magnetic order and spin dynamics of a double-exchange
model with competing ferromagnetic and antiferromagnetic Heisenberg
interactions between the local moments. The Heisenberg interactions are
periodically arranged in a Villain configuration in two dimensions with
nearest-neighbor, ferromagnetic coupling and antiferromagnetic coupling
. This model is solved at zero temperature by performing a
expansion in the rotated reference frame of each local moment.
When exceeds a critical value, the ground state is a magnetically
frustrated, canted antiferromagnet. With increasing hopping energy or
magnetic field , the local moments become aligned and the ferromagnetic
phase is stabilized above critical values of or . In the canted phase, a
charge-density wave forms because the electrons prefer to sit on lines of sites
that are coupled ferromagnetically. Due to a change in the topology of the
Fermi surface from closed to open, phase separation occurs in a narrow range of
parameters in the canted phase. In zero field, the long-wavelength spin waves
are isotropic in the region of phase separation. Whereas the average spin-wave
stiffness in the canted phase increases with or , it exhibits a more
complicated dependence on field. This work strongly suggests that the jump in
the spin-wave stiffness observed in PrCaMnO with at a field of 3 T is caused by the delocalization of the electrons rather
than by the alignment of the antiferromagnetic regions.Comment: 28 pages, 12 figure
Particle acceleration, magnetic field generation, and emission in relativistic pair jets
Shock acceleration is a ubiquitous phenomenon in astrophysical plasmas.
Plasma waves and their associated instabilities (e.g., Buneman, Weibel and
other two-stream instabilities) created in collisionless shocks are responsible
for particle (electron, positron, and ion) acceleration. Using a 3-D
relativistic electromagnetic particle (REMP) code, we have investigated
particle acceleration associated with a relativistic jet front propagating into
an ambient plasma. We find that the growth times of Weibel instability are
proportional to the Lorentz factors of jets. Simulations show that the Weibel
instability created in the collisionless shock front accelerates jet and
ambient particles both perpendicular and parallel to the jet propagation
direction.Comment: 4 pages, 2 figures, submitted to Il nuovo cimento (4th Workshop
Gamma-Ray Bursts in the Afterglow Era, Rome, 18-22 October 2004
Long-term source monitoring with BATSE
The uncollimated Burst and Transient Source Experiment (BATSE) large area detectors (LADs) are well suited to nearly continuous monitoring of the stronger hard x-ray sources, and time series analysis for pulsars. An overview of the analysis techniques presently being applied to the data are discussed, including representative observations of the Crab Nebula, Crab pulsar, and summaries of the sources detected to data. Results of a search for variability in the Crab Pulsar pulse profile are presented
Occultation analysis of BATSE data: Operational aspects
The Burst and Transient Source Experiment (BATSE) large area detectors are being used to monitor hard x-ray/gamma ray sources on a daily basis for evidence of transient behavior. Flux measurements are performed using a simple earth occultation technique. Daily searches are also being performed to detect occultation steps of sources which are not being routinely monitored. Topics concerning the operational aspects of the occultation measurements are presented. Preliminary spectral results are also presented for several of the brighter sources
Particle acceleration in electron-ion jets
Weibel instability created in collisionless shocks is responsible for
particle (electron, positron, and ion) acceleration. Using a 3-D relativistic
electromagnetic particle (REMP) code, we have investigated particle
acceleration associated with a relativistic electron-ion jet fronts propagating
into an ambient plasma without initial magnetic fields with a longer simulation
system in order to investigate nonlinear stage of the Weibel instability and
its acceleration mechanism. The current channels generated by the Weibel
instability induce the radial electric fields. The z component of the Poynting
vector (E x B) become positive in the large region along the jet propagation
direction. This leads to the acceleration of jet electrons along the jet. In
particular the E x B drift with the large scale current channel generated by
the ion Weibel instability accelerate electrons effectively in both parallel
and perpendicular directions.Comment: 2 pages, 1 figure, Proceedings for Astrophysical Sources of High
Energy Particles and Radiation, AIP proceeding Series, eds . T. Bulik, G.
Madejski and B. Ruda
New Magnetic Excitations in the Spin-Density-Wave of Chromium
Low-energy magnetic excitations of chromium have been reinvestigated with a
single-Q crystal using neutron scattering technique. In the transverse
spin-density-wave phase a new type of well-defined magnetic excitation is found
around (0,0,1) with a weak dispersion perpendicular to the wavevector of the
incommensurate structure. The magnetic excitation has an energy gap of E ~ 4
meV and at (0,0,1) exactly corresponds to the Fincher mode previously studied
only along the incommensurate wavevector.Comment: 4 pages, 4 figure
Radiation from relativistic jets
Nonthermal radiation observed from astrophysical systems containing
relativistic jets and shocks, e.g., gamma-ray bursts (GRBs), active galactic
nuclei (AGNs), and Galactic microquasar systems usually have power-law emission
spectra. Recent PIC simulations of relativistic electron-ion
(electron-positron) jets injected into a stationary medium show that particle
acceleration occurs within the downstream jet. In the presence of relativistic
jets, instabilities such as the Buneman instability, other two-streaming
instability, and the Weibel (filamentation) instability create collisionless
shocks, which are responsible for particle (electron, positron, and ion)
acceleration. The simulation results show that the Weibel instability is
responsible for generating and amplifying highly nonuniform, small-scale
magnetic fields. These magnetic fields contribute to the electron's transverse
deflection behind the jet head. The ``jitter'' radiation from deflected
electrons in small-scale magnetic fields has different properties than
synchrotron radiation which is calculated in a uniform magnetic field. This
jitter radiation, a case of diffusive synchrotron radiation, may be important
to understand the complex time evolution and/or spectral structure in gamma-ray
bursts, relativistic jets, and supernova remnants.Comment: 8 pages,3 figures, accepted for the Proceedings of Science of the
Workshop on Blazar Variability across the Electromagnetic Spectrum, April 22
to 25, 200
Mean Field Theory of Josephson Junction Arrays with Charge Frustration
Using the path integral approach, we provide an explicit derivation of the
equation for the phase boundary for quantum Josephson junction arrays with
offset charges and non-diagonal capacitance matrix. For the model with nearest
neighbor capacitance matrix and uniform offset charge , we determine,
in the low critical temperature expansion, the most relevant contributions to
the equation for the phase boundary. We explicitly construct the charge
distributions on the lattice corresponding to the lowest energies. We find a
reentrant behavior even with a short ranged interaction. A merit of the path
integral approach is that it allows to provide an elegant derivation of the
Ginzburg-Landau free energy for a general model with charge frustration and
non-diagonal capacitance matrix. The partition function factorizes as a product
of a topological term, depending only on a set of integers, and a
non-topological one, which is explicitly evaluated.Comment: LaTex, 24 pages, 8 figure
Spin and charge excitations in incommensurate spin density waves
Collective excitations both for spin- and charge-channels are investigated in
incommensurate spin density wave (or stripe) states on two-dimensional Hubbard
model. By random phase approximation, the dynamical susceptibility
\chi(q,\omega) is calculated for full range of (q,\omega) with including all
higher harmonics components. An intricate landscape of the spectra in
\chi(q,\omega) is obtained. We discuss the anisotropy of the dispersion cones
for spin wave excitations, and for the phason excitation related to the motion
of the stripe line. Inelastic neutron experiments on Cr and its alloys and
stripe states of underdoped cuprates are proposed
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