17,718 research outputs found
Performance deterioration: An airline perspective
Certain features of engine design and installation are clearly demonstrating important advantages in engine performance retention. Bearing arrangement no doubt has an important role in performance retention. Four bearings seem to be insufficient while six are probably more than are required. A well designed five-bearing system would seem to be an optimum configuration. Bearings with over-hung components such as fans, should be designed to minimize such over-hang to limit associated wobble, which in turn leads to shroud rub, or to allow closer running clearances. Almost any effort to improve stiffness and generally reduce flexing of the engine structure will pay off significantly in performance retention, even at a weight penalty. For this reason cowl load sharing to provide additional rigidity at a low cost in additional weight and complexity is favored
Competing Ordered Phases in URu2Si2: Hydrostatic Pressure and Re-substitution
A persistent kink in the pressure dependence of the \hidden order" (HO)
transition temperature of URu2-xRexSi2 is observed at a critical pressure Pc=15
kbar for 0 < x < 0.08. In URu2Si2, the kink at Pc is accompanied by the
destruction of superconductivity; a change in the magnitude of a spin
excitation gap, determined from electrical resistivity measurements; and a
complete gapping of a portion of the Fermi surface (FS), inferred from a change
in scattering and the competition between the HO state and superconductivity
for FS fraction
Pinning of stripes by local structural distortions in cuprate high-Tc superconductors
We study the spin-density wave (stripe) instability in lattices with mixed
low-temperature orthorhombic (LTO) and low-temperature tetragonal (LTT) crystal
symmetry. Within an explicit mean-field model it is shown how local LTT regions
act as pinning centers for static stripe formation. We calculate the
modulations in the local density of states near these local stripe regions and
find that mainly the coherence peaks and the van Hove singularity (VHS) are
spatially modulated. Lastly, we use the real-space approach to simulate recent
tunneling data in the overdoped regime where the VHS has been detected by
utilizing local normal state regions.Comment: Conference proceedings for Stripes1
Percolation Effects in Very High Energy Cosmic Rays
Most QCD models of high energy collisions predict that the inelasticity
is an increasing function of the energy. We argue that, due to percolation of
strings, this behaviour will change and, at GeV, the
inelasticity will start to decrease with the energy. This has straightforward
consequences in high energy cosmic ray physics: 1) the relative depth of the
shower maximum grows faster with energy above the knee; 2) the energy
measurements of ground array experiments at GZK energies could be
overestimated.Comment: Correction of equation (19) and figures 3 and 4. 4 pages, 4 figure
Variational study of the antiferromagnetic insulating phase of V2O3 based on Nth order Muffin-Tin-Orbitals
Motivated by recent results of th order muffin-tin orbital (NMTO)
implementation of the density functional theory (DFT), we re-examine
low-temperature ground-state properties of the anti-ferromagnetic insulating
phase of vanadium sesquioxide VO. The hopping matrix elements obtained
by the NMTO-downfolding procedure differ significantly from those previously
obtained in electronic structure calculations and imply that the in-plane
hopping integrals are as important as the out-of-plane ones. We use the NMTO
hopping matrix elements as input and perform a variational study of the ground
state. We show that the formation of stable molecules throughout the crystal is
not favorable in this case and that the experimentally observed magnetic
structure can still be obtained in the atomic variational regime. However the
resulting ground state (two electrons occupying the degenerate
doublet) is in contrast with many well established experimental observations.
We discuss the implications of this finding in the light of the non-local
electronic correlations certainly present in this compound.Comment: 7 pages, 2 figure
Cold Quark Matter
We perform an O(alpha_s^2) perturbative calculation of the equation of state
of cold but dense QCD matter with two massless and one massive quark flavor,
finding that perturbation theory converges reasonably well for quark chemical
potentials above 1 GeV. Using a running coupling constant and strange quark
mass, and allowing for further non-perturbative effects, our results point to a
narrow range where absolutely stable strange quark matter may exist. Absent
stable strange quark matter, our findings suggest that quark matter in compact
star cores becomes confined to hadrons only slightly above the density of
atomic nuclei. Finally, we show that equations of state including quark matter
lead to hybrid star masses up to M~2M_solar, in agreement with current
observations. For strange stars, we find maximal masses of M~2.75M_solar and
conclude that confirmed observations of compact stars with M>2M_solar would
strongly favor the existence of stable strange quark matter.Comment: 51 pages, 11 figures, v2: minor modifications and additional
reference
Coupled Magnetic Excitations in Single Crystal PrBa2Cu3O6.2
The dispersion of the low-energy magnetic excitations of the Pr sublattice in
PrBa2Cu3O6.2 is determined by inelastic neutron scattering measurements on a
single crystal. The dispersion, which shows the effect of interactions with the
Cu spin-waves, is well described by a model of the coupled Cu-Pr magnetic
system. This enables values for the principal exchange constants to be
determined, which suggest that both Pr-Pr and Cu-Pr interactions are important
in producing the anomalously high ordering temperature of the Pr sublattice.
Measurements of the Cu optic spin wave mode show that the inter-layer Cu-Cu
exchange is significantly lower than in YBa2Cu3O6.2.Comment: To be published Phys. Rev. Let
Anomalous diffusion and generalized Sparre-Andersen scaling
We are discussing long-time, scaling limit for the anomalous diffusion
composed of the subordinated L\'evy-Wiener process. The limiting anomalous
diffusion is in general non-Markov, even in the regime, where ensemble averages
of a mean-square displacement or quantiles representing the group spread of the
distribution follow the scaling characteristic for an ordinary stochastic
diffusion. To discriminate between truly memory-less process and the non-Markov
one, we are analyzing deviation of the survival probability from the (standard)
Sparre-Andersen scaling.Comment: 5 pages, 3 figure
Mott scattering of polarized electrons in a strong laser field
We present analytical and numerical results of the relativistic calculation
of the transition matrix element and differential cross section for
Mott scattering of initially polarized Dirac particles (electrons) in the
presence of strong laser field with linear polarization. We use exact
Dirac-Volkov wave functions to describe the dressed electrons and the collision
process is treated in the first Born approximation. The influence of the laser
field on the degree of polarization of the scattered electron is reported.Comment: 12 pages, 11 figures, Revte
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