13,788 research outputs found
Why is solar cycle 24 an inefficient producer of high-energy particle events?
The aim of the study is to investigate the reason for the low productivity of
high-energy SEPs in the present solar cycle. We employ scaling laws derived
from diffusive shock acceleration theory and simulation studies including
proton-generated upstream Alfv\'en waves to find out how the changes observed
in the long-term average properties of the erupting and ambient coronal and/or
solar wind plasma would affect the ability of shocks to accelerate particles to
the highest energies. Provided that self-generated turbulence dominates
particle transport around coronal shocks, it is found that the most crucial
factors controlling the diffusive shock acceleration process are the number
density of seed particles and the plasma density of the ambient medium.
Assuming that suprathermal populations provide a fraction of the particles
injected to shock acceleration in the corona, we show that the lack of most
energetic particle events as well as the lack of low charge-to-mass ratio ion
species in the present cycle can be understood as a result of the reduction of
average coronal plasma and suprathermal densities in the present cycle over the
previous one
Landau damping of Bogoliubov excitations in optical lattices at finite temperature
We study the damping of Bogoliubov excitations in an optical lattice at
finite temperatures. For simplicity, we consider a Bose-Hubbard tight-binding
model and limit our analysis to the lowest excitation band. We use the Popov
approximation to calculate the temperature dependence of the number of
condensate atoms in each lattice well. We calculate the Landau
damping of a Bogoliubov excitation in an optical lattice due to coupling to a
thermal cloud of excitations. While most of the paper concentrates on 1D
optical lattices, we also briefly present results for 2D and 3D lattices. For
energy conservation to be satisfied, we find that the excitations in the
collision process must exhibit anomalous dispersion ({\it i.e.} the excitation
energy must bend upward at low momentum), as also exhibited by phonons in
superfluid . This leads to the sudden disappearance of all damping
processes in -dimensional simple cubic optical lattice when , where is the on-site interaction, and is the hopping matrix
element. Beliaev damping in a 1D optical lattice is briefly discussed.Comment: 28 pages, 9 figure
Damping of Bogoliubov Excitations in Optical Lattices
Extending recent work to finite temperatures, we calculate the Landau damping
of a Bogoliubov excitation in an optical lattice, due to coupling to a thermal
cloud of such excitations. For simplicity, we consider a 1D Bose-Hubbard model
and restrict ourselves to the first energy band. For energy conservation to be
satisfied, the excitations in the collision processes must exhibit ``anomalous
dispersion'', analogous to phonons in superfluid . This leads to the
disappearance of all damping processes when , where is
the on-site interaction, is the hopping matrix element and
is the number of condensate atoms at a lattice site. This phenomenon also
occurs in 2D and 3D optical lattices. The disappearance of Beliaev damping
above a threshold wavevector is noted.Comment: 4pages, 5figures, submitted to Phys. Rev. Let
Direct Evidence for a Magnetic f-electron Mediated Cooper Pairing Mechanism of Heavy Fermion Superconductivity in CeCoIn5
To identify the microscopic mechanism of heavy-fermion Cooper pairing is an
unresolved challenge in quantum matter studies; it may also relate closely to
finding the pairing mechanism of high temperature superconductivity.
Magnetically mediated Cooper pairing has long been the conjectured basis of
heavy-fermion superconductivity but no direct verification of this hypothesis
was achievable. Here, we use a novel approach based on precision measurements
of the heavy-fermion band structure using quasiparticle interference (QPI)
imaging, to reveal quantitatively the momentum-space (k-space) structure of the
f-electron magnetic interactions of CeCoIn5. Then, by solving the
superconducting gap equations on the two heavy-fermion bands
with these magnetic interactions as mediators of the
Cooper pairing, we derive a series of quantitative predictions about the
superconductive state. The agreement found between these diverse predictions
and the measured characteristics of superconducting CeCoIn5, then provides
direct evidence that the heavy-fermion Cooper pairing is indeed mediated by the
f-electron magnetism.Comment: 19 pages, 4 figures, Supplementary Information: 31 pages, 5 figure
A method of estimating the yield of a missing plot in field experimental work
In cases of field experiments when one plot is missing, a method has been developed for furnishing an estimate of the yield of the missing plot, based on all the other values. The calculation is given for (a) a Randomised Block experiment, and (b) a Latin Square arrangement. In both cases the actual arithmetic is very simple.The steps in the procedure are:(1) Determine the desired value by an application of the equation (A) or (B) according as the experiment was arranged in Randomised Blocks or in a Latin Square.(2) Proceed as usual with the analysis of variance, using the estimated figure for the missing yield, and remembering to deduct one from the number of degrees of freedom ascribable to error
EPR and ferromagnetism in diluted magnetic semiconductor quantum wells
Motivated by recent measurements of electron paramagnetic resonance (EPR)
spectra in modulation-doped CdMnTe quantum wells, [F.J. Teran {\it et al.},
Phys. Rev. Lett. {\bf 91}, 077201 (2003)], we develop a theory of collective
spin excitations in quasi-two-dimensional diluted magnetic semiconductors
(DMSs). Our theory explains the anomalously large Knight shift found in these
experiments as a consequence of collective coupling between Mn-ion local
moments and itinerant-electron spins. We use this theory to discuss the physics
of ferromagnetism in (II,Mn)VI quantum wells, and to speculate on the
temperature at which it is likely to be observed in n-type modulation doped
systems.Comment: 4 pages, 1 figur
Electro-optic techniques for longitudinal electron bunch diagnostics
Electro-optic techniques are becoming increasingly important in ultrafast electron bunch longitudinal diagnostics and have been successfully implemented at various accelerator laboratories. The longitudinal bunch shape is directly obtained from a single-shot, non-intrusive measurement of the temporal electric field profile of the bunch. Further- more, the same electro-optic techniques can be used to measure the temporal profile of terahertz / far-infrared opti- cal pulses generated by a CTR screen, at a bending magnet (CSR), or by an FEL. This contribution summarizes the re- sults obtained at FELIX and FLASH
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