11,285 research outputs found
Far-field radiation of aft turbofan noise
Approximate expressions were developed for the noise radiation from the aft duct. The results of approximate aft radiation equation compare favorably to more exact Wiener-Hopf radiation results. Refraction as well as convective effects in the multiple flow streams is considered. The peak in the radiation pattern, which occurs nearly at engine sideline, is composed of modes with relatively large cut-off ratios. This implies that aft fan radiation will be inherently more difficult to suppress that the fan inlet noise. The theoretical multimodal radiation pattern is compared to experimental data for the first two harmonics of blade passage frequency for three full scale fans at two speeds. The agreement between theory and experiment is quite good
Disproportionation and electronic phase separation in parent manganite LaMnO_3
Nominally pure undoped parent manganite LaMnO_3 exhibits a puzzling behavior
inconsistent with a simple picture of an A-type antiferromagnetic insulator
(A-AFI) with a cooperative Jahn-Teller ordering. We do assign its anomalous
properties to charge transfer instabilities and competition between insulating
A-AFI phase and metallic-like dynamically disproportionated phase formally
separated by a first-order phase transition at T_{disp}=T_{JT}\approx 750 K.
The unconventional high-temperature phase is addressed to be a specific
electron-hole Bose liquid (EHBL) rather than a simple "chemically"
disproportionated R(Mn^{2+}Mn^{4+})O_3 phase. New phase does nucleate as a
result of the charge transfer (CT) instability and evolves from the
self-trapped CT excitons, or specific EH-dimers, which seem to be a precursor
of both insulating and metallic-like ferromagnetic phases observed in
manganites. We arrive at highly frustrated system of triplet (e_g^2)^3A_{2g}
bosons moving in a lattice formed by hole Mn^{4+} centers. Starting with
different experimental data we have reproduced a typical temperature dependence
of the volume fraction of high-temperature mixed-valent EHBL phase. We argue
that a slight nonisovalent substitution, photo-irradiation, external pressure
or magnetic field gives rise to an electronic phase separation with a
nucleation or an overgrowth of EH-droplets. Such a scenario provides a
comprehensive explanation of numerous puzzling properties observed in parent
and nonisovalently doped manganite LaMnO_3 including an intriguing
manifestation of superconducting fluctuations.Comment: 20 pages, 8 figure
Pairing state in multicomponent superconductors
We use the microscopic weak coupling theory to predict the pairing state in
superconductors of cubic, hexagonal, or tetragonal symmetry, where the order
parameter is multicomponent, i.e., transforms according to either a
2-dimensional or a 3-dimensional representation of the crystal point group. We
show that the superconducting phase usually breaks the time-reversal symmetry
for singlet multicomponent superconductors. The superconducting order parameter
for triplet superconductors in most cases turns out to be non-magnetic.Comment: 7 page
Enhanced Coherence of Antinodal Quasiparticles in a Dirty d-wave Superconductor
Recent ARPES experiments show a narrow quasiparticle peak at the gap edge
along the antinodal [1,0]-direction for the overdoped cuprate superconductors.
We show that within weak coupling BCS theory for a d-wave superconductor the
s-wave single-impurity scattering cross section vanishes for energies of the
gap edge. This coherence effect occurs through multiple scattering off the
impurity. For small impurity concentrations the spectral function has a
pronounced increase of the (scattering) lifetime for antinodal quasiparticles
but shows a very broad peak in the nodal direction, in qualitative agreement
with experiment and in strong contrast to the behavior observed in underdoped
cuprates.Comment: 4 pages, 3 figures, submitte
Residual mean first-passage time for jump processes: theory and applications to L\'evy flights and fractional Brownian motion
We derive a functional equation for the mean first-passage time (MFPT) of a
generic self-similar Markovian continuous process to a target in a
one-dimensional domain and obtain its exact solution. We show that the obtained
expression of the MFPT for continuous processes is actually different from the
large system size limit of the MFPT for discrete jump processes allowing
leapovers. In the case considered here, the asymptotic MFPT admits
non-vanishing corrections, which we call residual MFPT. The case of L/'evy
flights with diverging variance of jump lengths is investigated in detail, in
particular, with respect to the associated leapover behaviour. We also show
numerically that our results apply with good accuracy to fractional Brownian
motion, despite its non-Markovian nature.Comment: 13 pages, 8 figure
Temperature dependent spin susceptibility in a two-dimensional metal
We consider a two-dimensional electron system with Coulomb interaction
between particles at a finite temperature T. We show that the dynamic Kohn
anomaly in the response function at 2K_F leads to a linear-in-T correction to
the spin susceptibility, same as in systems with short-range interaction.
We show that the singularity of the Coulomb interaction at q=0 does not
invalidate the expansion in powers of r_s, but makes the expansion
non-analytic. We argue that the linear temperature dependence is consistent
with the general structure of Landau theory and can be viewed as originating
from the non-analytic component of the Landau function near the Fermi surface.Comment: 4 pages, no figure
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