106,011 research outputs found
Numerical design of streamlined tunnel walls for a two-dimensional transonic test
An analytical procedure is discussed for designing wall shapes for streamlined, nonporous, two-dimensional, transonic wind tunnels. It is based upon currently available 2-D inviscid transonic and boundary layer analysis computer programs. Predicted wall shapes are compared with experimental data obtained from the NASA Langley 6 by 19 inch Transonic Tunnel where the slotted walls were replaced by flexible nonporous walls. Comparisons are presented for the empty tunnel operating at a Mach number of 0.9 and for a supercritical test of an NACA 0012 airfoil at zero lift. Satisfactory agreement is obtained between the analytically and experimentally determined wall shapes
Atomic-phase interference devices based on ring-shaped Bose-Einstein condensates: Two ring case
We theoretically investigate the ground-state properties and quantum dynamics
of a pair of adjacent ring-shaped Bose-Einstein condensates that are coupled
via tunneling. This device, which is the analogue of a symmetric
superconducting quantum interference device, is the simplest version of what we
term an Atomic-Phase Interference Device (APHID). The two-ring APHID is shown
to be sensitive to rotation.Comment: 8 page
Band structure of Charge Ordered Doped Antiferromagnets
We study the distribution of electronic spectral weight in a doped
antiferromagnet with various types of charge order and compare to angle
resolved photoemission experiments on lightly doped LaSrCuO
(LSCO) and electron doped NdCeCuO. Calculations on
in-phase stripe and bubble phases for the electron doped system are both in
good agreement with experiment including in particular the existence of in-gap
spectral weight. In addition we find that for in-phase stripes, in contrast to
anti-phase stripes, the chemical potential is likely to move with doping. For
the hole doped system we find that ``staircase'' stripes which are globally
diagonal but locally vertical or horizontal can reproduce the photoemission
data whereas pure diagonal stripes cannot. We also calculate the magnetic
structure factors of such staircase stripes and find that as the stripe
separation is decreased with increased doping these evolve from diagonal to
vertical separated by a coexistence region. The results suggest that the
transition from horizontal to diagonal stripes seen in neutron scattering on
underdoped LSCO may be a crossover between a regime where the typical length of
straight stripe segments is longer than the inter-stripe spacing to one where
it is shorter and that locally the stripes are always aligned with the Cu-O
bonds.Comment: 13 pages, 16 figure
Superfluid Suppression in d-Wave Superconductors due to Disordered Magnetism
The influence of static magnetic correlations on the temperature-dependent
superfluid density \rho_s(T) is calculated for d-wave superconductors. In
self-consistent calculations, itinerant holes form incommensurate spin density
waves (SDW) which coexist with superconductivity. In the clean limit, the
density of states is gapped, and \rho_s(T << T_c) is exponentially activated.
In inhomogeneously-doped cases, the SDW are disordered and both the density of
states and \rho_s(T) obtain forms indistinguishable from those in dirty but
pure d-wave superconductors, in accordance with experiments. We conclude that
the observed collapse of \rho_s at x\approx 0.35 in underdoped YBCO may
plausibly be attributed to the coexistence of SDW and superconductivity.Comment: 6 pages, 5 figures. Expanded discussio
Vacuum polarization of scalar fields near Reissner-Nordstr\"{o}m black holes and the resonance behavior in field-mass dependence
We study vacuum polarization of quantized massive scalar fields in
equilibrium at black-hole temperature in Reissner-Nordstr\"{o}m background. By
means of the Euclidean space Green's function we analytically derive the
renormalized expression at the event horizon with the area
. It is confirmed that the polarization amplitude
is free from any divergence due to the infinite red-shift
effect. Our main purpose is to clarify the dependence of on
field mass in relation to the excitation mechanism. It is shown for
small-mass fields with how the excitation of
caused by finite black-hole temperature is suppressed as increases, and it
is verified for very massive fields with that
decreases in proportion to with the amplitude equal to the
DeWitt-Schwinger approximation. In particular, we find a resonance behavior
with a peak amplitude at in the field-mass dependence of
vacuum polarization around nearly extreme (low-temperature) black holes. The
difference between Scwarzschild and nearly extreme black holes is discussed in
terms of the mass spectrum of quantum fields dominant near the event horizon.Comment: 24 pages, 1 figure Accepted in PR
Size Gap for Zero Temperature Black Holes in Semiclassical Gravity
We show that a gap exists in the allowed sizes of all zero temperature static
spherically symmetric black holes in semiclassical gravity when only
conformally invariant fields are present. The result holds for both charged and
uncharged black holes. By size we mean the proper area of the event horizon.
The range of sizes that do not occur depends on the numbers and types of
quantized fields that are present. We also derive some general properties that
both zero and nonzero temperature black holes have in all classical and
semiclassical metric theories of gravity.Comment: 4 pages, ReVTeX, no figure
Spontaneous superconductivity and optical properties of high-Tc cuprates
We suggest that the high temperature superconductivity in cuprate compounds
may emerge due to interaction between copper-oxygen layers mediated by in-plane
plasmons. The strength of the interaction is determined by the c-axis geometry
and by the ab-plane optical properties. Without making reference to any
particular in-plane mechanism of superconductivity, we show that the interlayer
interaction favors spontaneous appearance of the superconductivity in the
layers. At a qualitative level the model describes correctly the dependence of
the transition temperature on the interlayer distance, and on the number of
adjacent layers in multilayered homologous compounds. Moreover, the model has a
potential to explain (i) a mismatch between the optimal doping levels for
critical temperature and superconducting density and (ii) a universal scaling
relation between the dc-conductivity, the superfluid density, and the
superconducting transition temperature.Comment: 4.4 pages, 2 figures; v2 matches the published version (clarifying
remarks and references are added
Quantum Cosmological Relational Model of Shape and Scale in 1-d
Relational particle models are useful toy models for quantum cosmology and
the problem of time in quantum general relativity. This paper shows how to
extend existing work on concrete examples of relational particle models in 1-d
to include a notion of scale. This is useful as regards forming a tight analogy
with quantum cosmology and the emergent semiclassical time and hidden time
approaches to the problem of time. This paper shows furthermore that the
correspondence between relational particle models and classical and quantum
cosmology can be strengthened using judicious choices of the mechanical
potential. This gives relational particle mechanics models with analogues of
spatial curvature, cosmological constant, dust and radiation terms. A number of
these models are then tractable at the quantum level. These models can be used
to study important issues 1) in canonical quantum gravity: the problem of time,
the semiclassical approach to it and timeless approaches to it (such as the
naive Schrodinger interpretation and records theory). 2) In quantum cosmology,
such as in the investigation of uniform states, robustness, and the qualitative
understanding of the origin of structure formation.Comment: References and some more motivation adde
Deterministic creation, pinning, and manipulation of quantized vortices in a Bose-Einstein condensate
We experimentally and numerically demonstrate deterministic creation and
manipulation of a pair of oppositely charged singly quantized vortices in a
highly oblate Bose-Einstein condensate (BEC). Two identical blue-detuned,
focused Gaussian laser beams that pierce the BEC serve as repulsive obstacles
for the superfluid atomic gas; by controlling the positions of the beams within
the plane of the BEC, superfluid flow is deterministically established around
each beam such that two vortices of opposite circulation are generated by the
motion of the beams, with each vortex pinned to the \emph{in situ} position of
a laser beam. We study the vortex creation process, and show that the vortices
can be moved about within the BEC by translating the positions of the laser
beams. This technique can serve as a building block in future experimental
techniques to create, on-demand, deterministic arrangements of few or many
vortices within a BEC for precise studies of vortex dynamics and vortex
interactions.Comment: 9 pages, 7 figure
Superposition Formulas for Darboux Integrable Exterior Differential Systems
In this paper we present a far-reaching generalization of E. Vessiot's
analysis of the Darboux integrable partial differential equations in one
dependent and two independent variables. Our approach provides new insights
into this classical method, uncovers the fundamental geometric invariants of
Darboux integrable systems, and provides for systematic, algorithmic
integration of such systems. This work is formulated within the general
framework of Pfaffian exterior differential systems and, as such, has
applications well beyond those currently found in the literature. In
particular, our integration method is applicable to systems of hyperbolic PDE
such as the Toda lattice equations, 2 dimensional wave maps and systems of
overdetermined PDE.Comment: 80 page report. Updated version with some new sections, and major
improvements to other
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