5,568 research outputs found
Gaussian Effective Potential and the Coleman's normal-ordering Prescription : the Functional Integral Formalism
For a class of system, the potential of whose Bosonic Hamiltonian has a
Fourier representation in the sense of tempered distributions, we calculate the
Gaussian effective potential within the framework of functional integral
formalism. We show that the Coleman's normal-ordering prescription can be
formally generalized to the functional integral formalism.Comment: 6 pages, revtex; With derivation details and an example added. To
appear in J. Phys.
Simulating changes in shape of thermionic cathodes during operation of high-pressure arc discharges
A numerical model of current transfer to thermionic cathodes of high-pressure arc discharges
is developed with account of deviations from local thermodynamic equilibrium occurring
near the cathode surface, in particular, of the near-cathode space-charge sheath, melting of the
cathode, and motion of the molten metal under the effect of the plasma pressure, the Lorentz
force, gravity, and surface tension. Modelling results are reported for a tungsten cathode of
an atmospheric-pressure argon arc and the computed changes in the shape of the cathode
closely resemble those observed in the experiment. The modelling has shown that the time
scale of change of the cathode shape during arc operation is very sensitive to the temperature
attained by the cathode. The fact that the computed time scales conform to those observed
in the experiment indicate that the model of non-equilibrium near-cathode layers in high pressure arc discharges, employed in this work, predicts the cathode temperature for a given
arc current with adequate accuracy. In contrast, modelling based on the assumption of local
thermodynamic equilibrium in the whole arc plasma computation domain up to the cathode
surface could hardly produce a similar agreement.info:eu-repo/semantics/publishedVersio
Cryogenic setup for trapped ion quantum computing
We report on the design of a cryogenic setup for trapped ion quantum
computing containing a segmented surface electrode trap. The heat shield of our
cryostat is designed to attenuate alternating magnetic field noise, resulting
in 120~dB reduction of 50~Hz noise along the magnetic field axis. We combine
this efficient magnetic shielding with high optical access required for single
ion addressing as well as for efficient state detection by placing two lenses
each with numerical aperture 0.23 inside the inner heat shield. The cryostat
design incorporates vibration isolation to avoid decoherence of optical qubits
due to the motion of the cryostat. We measure vibrations of the cryostat of
less than 20~nm over 2~s. In addition to the cryogenic apparatus, we
describe the setup required for an operation with
Ca and Sr ions.
The instability of the laser manipulating the optical qubits in
Ca is characterized yielding a minimum of its
Allan deviation of 2.410 at 0.33~s. To evaluate the
performance of the apparatus, we trapped Ca
ions, obtaining a heating rate of 2.14(16)~phonons/s and a Gaussian decay of
the Ramsey contrast with a 1/e-time of 18.2(8)~ms
Accuracy of low-dose computed tomography coronary angiography using prospective electrocardiogram-triggering: first clinical experience
AIMS: To evaluate the accuracy of low-dose computed tomography coronary angiography (CTCA) using prospective ECG-triggering for the assessment of coronary artery disease (CAD). METHODS AND RESULTS: A total of 30 patients (19 males, 11 females, mean age 58.8 +/- 9.9 years) underwent low-dose CTCA and invasive coronary angiography (CA) [median 2 days (0, 41)]. Before CT scanning, intravenous beta-blocker was administered in 18 of 30 patients as heart rate (HR) was >65 b.p.m., achieving a mean HR of 55.7 +/- 7.9 b.p.m. CAD was defined as coronary artery narrowing > or =50%, using CA as standard of reference. The estimated mean effective radiation dose was 2.1 +/- 0.7 mSv (range: 1.0-3.3), yielding 96.0% (383/399) of evaluable segments. On an intention-to-diagnose-base, all non-evaluative segments were included in the analysis. Vessels with a non-evaluative segment and no further finding were censored as false positive. Patient-based analysis revealed sensitivity, specificity, positive predictive value, and negative predictive value of 100, 83.3, 90.0, and 100%, respectively. The respective values per vessel were 100, 88.9, 85.7, and 100%, respectively. CONCLUSION: Prospective ECG-triggering allows low-dose CTCA and provides high diagnostic accuracy in the assessment of CAD in patients with stable sinus rhythm and a low heart rat
Restenosis after directional coronary atherectomy: Differences between primary atheromatoes and restenosls lesions and influence of subintimal tissue resection
AbstractRates of restenosis were evaluated in 70 patients (74 lesions) after successful directional coronary atherectomy. The extent of vascular tissue resection was correlated with restenosis rates for coronary (n = 59) and vein bypass graft (n = 15) lesions.After 6 months, the overall restenosis rate was 50% (37 of 74 lesions); it was 42% (15 of 36 lesions) when intima alone was resected, 50% (7 of 14 lesions) when media was resected and 63% (15 of 24 lesions) when adventitia was resected. Subintimal tissue resection increased the restenosis rate for vein grafts (43% with intimal resection versus 100% with subintimal resection, p = 0.01) but not for coronary arteries (50% versus 48%). There was no overall difference in restenosis rates after atherectomy between primary lesions and restenosis lesions that occurred after balloon angioplasty (46% versus 54%). Among postballoon angioplasty restenosis lesions, a higher rate of restenosis after atherectomy was found with subintimal than with intimal resection (78% versus 32%, p = 0.01).Tissues from patients undergoing a second atherectomy for restenosis after initial atherectomy (n = 8) demonstrated neointimal hyperplasia that appeared histotogically identical to restenotic tissue developing after balloon angioplasty (n = 37).These data suggest that the cellular response to directional coronary atherectomy is characterized by neointimal proliferation similar to that which may develop after balloon angioplasty. The extent of fibrous hyperplasia appears to be related to the depth of tissue resection in vein graft lesions and coronary artery restenosis lesions that occur after balloon angioplasty but not in primary atheromatous coronary artery lesions
The optical response of Ba_{1-x}K_xBiO_3: Evidence for an unusual coupling mechanism of superconductivity?
We have analysed optical reflectivity data for Ba_{1-x}K_xBiO_3 in the
far-infrared region using Migdal-Eliashberg theory and found it inconsistent
with standard electron-phonon coupling: Whereas the superconducting state data
could be explained using moderate coupling, \lambda=0.7, the normal state
properties indicate \lambda \le 0.2. We have found that such behaviour could be
understood using a simple model consisting of weak standard electron-phonon
coupling plus weak coupling to an unspecified high energy excitation near 0.4
eV. This model is found to be in general agreement with the reflectivity data,
except for the predicted superconducting gap size. The additional high energy
excitation suggests that the dominant coupling mechanism in Ba_{1-x}K_xBiO_3 is
not standard electron-phonon.Comment: 5 pages REVTex, 5 figures, 32 refs, accepted for publication in Phys.
Rev.
Molecular Hydrogen and Global Star Formation Relations in Galaxies
(ABRIDGED) We use hydrodynamical simulations of disk galaxies to study
relations between star formation and properties of the molecular interstellar
medium (ISM). We implement a model for the ISM that includes low-temperature
(T<10^4K) cooling, directly ties the star formation rate to the molecular gas
density, and accounts for the destruction of H2 by an interstellar radiation
field from young stars. We demonstrate that the ISM and star formation model
simultaneously produces a spatially-resolved molecular-gas surface density
Schmidt-Kennicutt relation of the form Sigma_SFR \propto Sigma_Hmol^n_mol with
n_mol~1.4 independent of galaxy mass, and a total gas surface density -- star
formation rate relation Sigma_SFR \propto Sigma_gas^n_tot with a power-law
index that steepens from n_tot~2 for large galaxies to n_tot>~4 for small dwarf
galaxies. We show that deviations from the disk-averaged Sigma_SFR \propto
Sigma_gas^1.4 correlation determined by Kennicutt (1998) owe primarily to
spatial trends in the molecular fraction f_H2 and may explain observed
deviations from the global Schmidt-Kennicutt relation.Comment: Version accepted by ApJ, high-res version available at
http://kicp.uchicago.edu/~brant/astro-ph/molecular_ism/rk2007.pd
Towards an understanding of isospin violation in pion-nucleon scattering
We investigate isospin breaking in low-energy pion-nucleon scattering in the
framework of chiral perturbation theory. This work extends the systematic
analysis of [1] to the energy range above threshold. Various relations, which
identically vanish in the limit of isospin symmetry, are used to quantify
isospin breaking effects. We study the energy dependence of the S- and P-wave
projections of these ratios and find dramatic effects in the S-waves of those
two relations which are given in terms of isoscalar quantities only. This
effect drops rather quickly with growing center-of-mass energy.Comment: 12 pp, REVTeX, 8 figs, FZJ-IKP(TH)-2000-2
Central Nucleon-Nucleon Potential and Chiral Scalar Form Factor
The central two-pion exchange NN potential at large distances is studied in
the framework of relativistic chiral symmetry and related directly to the
nucleon scalar form factor, which describes the mass density of its pion cloud.
This relationship is well supported by phenomenology and allows the dependence
of the asymptotic potential on the nucleon mass to be assessed. Results in the
heavy baryon limit are about 25% larger than those corresponding to the
empirical nucleon mass in the region of physical interest. This indicates that
it is very important to keep this mass finite in precise descriptions of the NN
system and supports the efficacy of the relativistic chiral framework. One also
estimates the contribution of subleading effects and presents a simple
discussions of the role of the quark condensate in this problem.Comment: 16 pages, 8 figure
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