276 research outputs found
Graded Orbital Occupation near Interfaces in a La2NiO4 - La2CuO4 Superlattice
X-ray absorption spectroscopy and resonant soft x-ray reflectivity show a
non-uniform distribution of oxygen holes in a La2NiO4 - La2CuO4 (LNO-LCO)
superlattice, with excess holes concentrated in the LNO layers. Weak
ferromagnetism with Tc = 160 K suggests a coordinated tilting of NiO6
octahedra, similar to that of bulk LNO. Ni d3z2-r2 orbitals within the LNO
layers have a spatially variable occupation. This variation of the Ni valence
near LNO-LCO interfaces is observed with resonant soft x-ray reflectivity at
the Ni L edge, at a reflection suppressed by the symmetry of the structure, and
is possible through graded doping with holes, due to oxygen interstitials taken
up preferentially by inner LNO layers. Since the density of oxygen atoms in the
structure can be smoothly varied with standard procedures, this orbital
occupation, robust up to at least 280 K, is tunable.Comment: 11 pages, 8 figure
Observation of Macroscopic Structural Fluctuations in bcc Solid 4He
We report neutron diffraction studies of low density bcc and hcp solid 4He.
In the bcc phase, we observed a continuous dynamical behaviour involving
macroscopic structural changes of the solid. The dynamical behaviour takes
place in a cell full of solid, and therefore represents a solidsolid
transformation. The structural changes are consistent with a gradual rotation
of macroscopic grains separated by low angle grain boundaries. We suggest that
these changes are triggered by random momentary vibrations of the experimental
system. An analysis of Laue diffraction patterns indicates that in some cases
these structural changes, once initiated by a momentary impulse, seem to
proceed at a constant rate over times approaching an hour. The energy
associated with these macroscopic changes appears to be on the order of kT.
Under similar conditions (temperature and pressure), these effects were absent
in the hcp phase.Comment: 14 pages, 6 figure, accepted for PR
Algorithms for Reconstructing DDoS Attack Graphs using Probabilistic Packet Marking
DoS and DDoS attacks are widely used and pose a constant threat. Here we
explore Probability Packet Marking (PPM), one of the important methods for
reconstructing the attack-graph and detect the attackers. We present two
algorithms. Differently from others, their stopping time is not fixed a priori.
It rather depends on the actual distance of the attacker from the victim. Our
first algorithm returns the graph at the earliest feasible time, and turns out
to guarantee high success probability. The second algorithm enables attaining
any predetermined success probability at the expense of a longer runtime. We
study the performance of the two algorithms theoretically, and compare them to
other algorithms by simulation. Finally, we consider the order in which the
marks corresponding to the various edges of the attack graph are obtained by
the victim. We show that, although edges closer to the victim tend to be
discovered earlier in the process than farther edges, the differences are much
smaller than previously thought.Comment: 30 pages, 4 figures, 4 table
New excitations in bcc He - an inelastic neutron scattering study
We report neutron scattering measurements on bcc solid % He. We studied
the phonon branches and the recently discovered ''optic-like'' branch along the
main crystalline directions. In addition, we discovered another, dispersionless
"optic-like'' branch at an energy around 1 meV (~11K). The properties of
the two "optic-like" branches seem different. Since one expects only 3 acoustic
phonon branches in a monoatomic cubic crystal, these new branches must
represent different type of excitations. One possible interpretation involves
localized excitations unique to a quantum solid.Comment: 4 pages, 3 figures, accepted by PRB, Rapid Communication
Cosmic Swarms: A search for Supermassive Black Holes in the LISA data stream with a Hybrid Evolutionary Algorithm
We describe a hybrid evolutionary algorithm that can simultaneously search
for multiple supermassive black hole binary (SMBHB) inspirals in LISA data. The
algorithm mixes evolutionary computation, Metropolis-Hastings methods and
Nested Sampling. The inspiral of SMBHBs presents an interesting problem for
gravitational wave data analysis since, due to the LISA response function, the
sources have a bi-modal sky solution. We show here that it is possible not only
to detect multiple SMBHBs in the data stream, but also to investigate
simultaneously all the various modes of the global solution. In all cases, the
algorithm returns parameter determinations within (as estimated from
the Fisher Matrix) of the true answer, for both the actual and antipodal sky
solutions.Comment: submitted to Classical & Quantum Gravity. 19 pages, 4 figure
BCC vs. HCP - The Effect of Crystal Symmetry on the High Temperature Mobility of Solid He
We report results of torsional oscillator (TO) experiments on solid He at
temperatures above 1K. We have previously found that single crystals, once
disordered, show some mobility (decoupled mass) even at these rather high
temperatures. The decoupled mass fraction with single crystals is typically 20-
30%. In the present work we performed similar measurements on polycrystalline
solid samples. The decoupled mass with polycrystals is much smaller, 1%,
similar to what is observed by other groups. In particular, we compared the
properties of samples grown with the TO's rotation axis at different
orientations with respect to gravity. We found that the decoupled mass fraction
of bcc samples is independent of the angle between the rotation axis and
gravity. In contrast, hcp samples showed a significant difference in the
fraction of decoupled mass as the angle between the rotation axis and gravity
was varied between zero and 85 degrees. Dislocation dynamics in the solid
offers one possible explanation of this anisotropy.Comment: 10 pages, 5 figures, to appear in Journal of Low Temperature Physics
- special issue on Supersolidit
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