13,670 research outputs found
Jastrow-Correlated Wavefunctions for Flat-Band Lattices
The electronic band structure of many compounds, e.g., carbon-based
structures, can exhibit essentially no dispersion. Models of electrons in
flat-band lattices define non-perturbative strongly correlated problems by
default. We construct a set of Jastrow-correlated ansatz wavefunctions to
capture the low energy physics of interacting particles in flat bands. We test
the ansatz in a simple Coulomb model of spinless electrons in a honeycomb
ribbon. We find that the wavefunction accurately captures the ground state in a
transition from a crystal to a uniform quantum liquid.Comment: 5 pages, 4 figures, update context, references and publication
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Comparative Study of BCS-BEC Crossover Theories above : the Nature of the Pseudogap in Ultra-Cold Atomic Fermi Gases
This paper presents a comparison of two finite-temperature BCS-Bose Einstein
condensation (BEC) crossover theories above the transition temperature:
Nozieres Schmitt-Rink (NSR) theory and finite -extended BCS-Leggett theory.
The comparison is cast in the form of numerical studies of the behavior of the
fermionic spectral function both theoretically and as constrained by
(primarily) radio frequency (RF) experiments. Both theories include pair
fluctuations and exhibit pseudogap effects, although the nature of this
pseudogap is very different. The pseudogap in finite -extended BCS-Leggett
theory is found to follow a BCS-like dispersion which, in turn, is associated
with a broadened BCS-like self energy, rather more similar to what is observed
in high temperature superconductors (albeit, for a d-wave case). The fermionic
quasi-particle dispersion is different in NSR theory and the damping is
considerably larger. We argue that the two theories are appropriate in
different temperature regimes with the BCS-Leggett approach more suitable
nearer to condensation. There should, in effect, be little difference at higher
as the pseudogap becomes weaker and where the simplifying approximations
used in the BCS-Leggett approach break down. On the basis of
momentum-integrated radio frequency studies of unpolarized gases, it would be
difficult to distinguish which theory is the better. A full comparison for
polarized gases is not possible since there is claimed to be inconsistencies in
the NSR approach (not found in the BCS-Leggett scheme). Future experiments
along the lines of momentum resolved experiments look to be very promising in
distinguishing the two theories.Comment: 16 pages, 11 figure
Numerical Simulation of the Generation of Axisymmetric Mode Jet Screech Tones
An imperfectly expanded supersonic jet, invariably, radiates both broadband noise and discrete frequency sound called screech tones. Screech tones are known to be generated by a feedback loop driven by the large scale instability waves of the jet flow. Inside the jet plume is a quasi-periodic shock cell structure. The interaction of the instability waves and the shock cell structure, as the former propagates through the latter, is responsible for the generation of the tones. Presently, there are formulas that can predict the tone frequency fairly accurately. However, there is no known way to predict the screech tone intensity. In this work, the screech phenomenon of an axisymmetric jet at low supersonic Mach number is reproduced by numerical simulation. The computed mean velocity profiles and the shock cell pressure distribution of the jet are found to be in good agreement with experimental measurements. The same is true with the simulated screech frequency. Calculated screech tone intensity and directivity at selected jet Mach number are reported in this paper. The present results demonstrate that numerical simulation using computational aeroacoustics methods offers not only a reliable way to determine the screech tone intensity and directivity but also an opportunity to study the physics and detailed mechanisms of the phenomenon by an entirely new approach
Dust emission from a parsec-scale structure in the Seyfert 1 nucleus of NGC 4151
We report mid-IR interferometric measurements with \sim 10 mas resolution,
which resolve the warm (T = 285 +25 / -50 K) thermal emission at the center of
NGC 4151. Using pairs of VLT 8.2 m telescopes with MIDI and by comparing the
data to a Gaussian model, we determined the diameter of the dust emission
region, albeit only along one position angle, to be 2.0 +/- 0.4 pc (FWHM). This
is the first size and temperature estimate for the nuclear warm dust
distribution in a Seyfert 1 galaxy. The parameters found are comparable to
those in Seyfert 2 galaxies, thus providing direct support for the unified
model. Using simple analytic temperature distributions, we find that the
mid-infrared emission is probably not the smooth continuation of the hot
nuclear source that is marginally resolved with K band interferometry. We also
detected weak excess emission around 10.5 micron in our shorter baseline
observation, possibly indicating that silicate emission is extended to the
parsec scale.Comment: 5 pages, 4 figures, accepted for publication in The Astrophysical
Journal Letter
Conditioning of BPM pickup signals for operations of the Duke storage ring with a wide range of single-bunch current
The Duke storage ring is a dedicated driver for the storage ring based
oscillator free-electron lasers (FELs), and the High Intensity Gamma-ray Source
(HIGS). It is operated with a beam current ranging from about 1 mA to 100 mA
per bunch for various operations and accelerator physics studies. High
performance operations of the FEL and gamma-ray source require a stable
electron beam orbit, which has been realized by the global orbit feedback
system. As a critical part of the orbit feedback system, the electron beam
position monitors (BPMs) are required to be able to precisely measure the
electron beam orbit in a wide range of the single-bunch current. However, the
high peak voltage of the BPM pickups associated with high single-bunch current
degrades the performance of the BPM electronics, and can potentially damage the
BPM electronics. A signal conditioning method using low pass filters is
developed to reduce the peak voltage to protect the BPM electronics, and to
make the BPMs capable of working with a wide range of single-bunch current.
Simulations and electron beam based tests are performed. The results show that
the Duke storage ring BPM system is capable of providing precise orbit
measurements to ensure highly stable FEL and HIGS operations
Screech Tones of Supersonic Jets from Bevelled Rectangular Nozzles
It is known experimentally that an imperfectly expanded rectangular jet from a thin-lip convergent nozzle emits only a single dominant screech tone. The frequency of the screech tone decreases continuously with increase in jet Mach number. However, for a supersonic jet issued from a bevelled nozzle or a convergent-divergent nozzle with straight side walls, the shock cell structure and the screech frequency pattern are fairly complicated and have not been predicted before. In this paper, it is shown that the shock cell structures of these jets can be decomposed into waveguide modes of the jet flow. The screech frequencies are related to the higher-order waveguide modes following the weakest-link screech tone theory. The measured screech frequencies are found to compare well with the predicted screech frequency curves
Bioactive ceramic-reinforced composites for bone augmentation
Biomaterials have been used to repair the human body for millennia, but it is only since the 1970s that man-made composites have been used. Hydroxyapatite (HA)-reinforced polyethylene (PE) is the first of the âsecond-generationâ biomaterials that have been developed to be bioactive rather than bioinert. The mechanical properties have been characterized using quasi-static, fatigue, creep and fracture toughness testing, and these studies have allowed optimization of the production method. The in vitro and in vivo biological properties have been investigated with a range of filler content and have shown that the presence of sufficient bioactive filler leads to a bioactive composite. Finally, the material has been applied clinically, initially in the orbital floor and later in the middle ear. From this initial combination of HA in PE other bioactive ceramic polymer composites have been developed
Resonance NLS Solitons as Black Holes in Madelung Fluid
A new resonance version of NLS equation is found and embedded to the
reaction-diffusion system, equivalent to the anti-de Sitter valued Heisenberg
model, realizing a particular gauge fixing condition of the Jackiw-Teitelboim
gravity. The space-time points where dispersion change the sign correspond to
the event horizon, and the soliton solutions to the AdS black holes. The
soliton with velocity bounded above describes evolution on the hyperboloid with
nontrivial winding number and create under collisions the resonance states with
a specific life time.Comment: Plain Tex, 12 pages, 6 figure
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