27,136 research outputs found
Monte Carlo evaluation of path integrals for the nuclear shell model
We present in detail a formulation of the shell model as a path integral and
Monte Carlo techniques for its evaluation. The formulation, which linearizes
the two-body interaction by an auxiliary field, is quite general, both in the
form of the effective `one-body' Hamiltonian and in the choice of ensemble. In
particular, we derive formulas for the use of general (beyond monopole) pairing
operators, as well as a novel extraction of the canonical (fixed-particle
number) ensemble via an activity expansion. We discuss the advantages and
disadvantages of the various formulations and ensembles and give several
illustrative examples. We also discuss and illustrate calculation of the
imaginary-time response function and the extraction, by maximum entropy
methods, of the corresponding strength function. Finally, we discuss the
"sign-problem" generic to fermion Monte Carlo calculations, and prove that a
wide class of interactions are free of this limitation.Comment: 38 pages, RevTeX v3.0, figures available upon request; Caltech
Preprint #MAP-15
Generating loop graphs via Hopf algebra in quantum field theory
We use the Hopf algebra structure of the time-ordered algebra of field
operators to generate all connected weighted Feynman graphs in a recursive and
efficient manner. The algebraic representation of the graphs is such that they
can be evaluated directly as contributions to the connected n-point functions.
The recursion proceeds by loop order and vertex number.Comment: 22 pages, LaTeX + AMS + eepic; new section with alternative recursion
formula added, further minor changes and correction
Influence of Anomalous Dispersion on Optical Characteristics of Quantum Wells
Frequency dependencies of optical characteristics (reflection, transmission
and absorption of light) of a quantum well are investigated in a vicinity of
interband resonant transitions in a case of two closely located excited energy
levels. A wide quantum well in a quantizing magnetic field directed normally to
the quantum-well plane, and monochromatic stimulating light are considered.
Distinctions between refraction coefficients of barriers and quantum well, and
a spatial dispersion of the light wave are taken into account. It is shown that
at large radiative lifetimes of excited states in comparison with nonradiative
lifetimes, the frequency dependence of the light reflection coefficient in the
vicinity of resonant interband transitions is defined basically by a curve,
similar to the curve of the anomalous dispersion of the refraction coefficient.
The contribution of this curve weakens at alignment of radiative and
nonradiative times, it is practically imperceptible at opposite ratio of
lifetimes . It is shown also that the frequency dependencies similar to the
anomalous dispersion do not arise in transmission and absorption coefficients.Comment: 10 pages, 6 figure
Precision CW laser automatic tracking system investigated
Precision laser tracker capable of tracking a low acceleration target to an accuracy of about 20 microradians rms is being constructed and tested. This laser tracking has the advantage of discriminating against other optical sources and the capability of simultaneously measuring range
Behavioural clusters and predictors of performance during recovery from stroke
We examined the patterns and variability of recovery post-stroke in multiple behavioral domains. A large cohort of first time stroke patients with heterogeneous lesions was studied prospectively and longitudinally at 1-2 weeks, 3 months and one year post-injury with structural MRI to measure lesion anatomy and in-depth neuropsychological assessment. Impairment was described at all timepoints by a few clusters of correlated deficits. The time course and magnitude of recovery was similar across domains, with change scores largely proportional to the initial deficit and most recovery occurring within the first three months. Damage to specific white matter tracts produced poorer recovery over several domains: attention and superior longitudinal fasciculus II/III, language and posterior arcuate fasciculus, motor and corticospinal tract. Finally, after accounting for the severity of the initial deficit, language and visual memory recovery/outcome was worse with lower education, while the occurrence of multiple deficits negatively impacted attention recovery
Multiwavelength Observations of Massive Stellar Cluster Candidates in the Galaxy
The Galaxy appears to be richer in young, massive stellar clusters than
previously known, due to advances in infrared surveys which have uncovered
deeply embedded regions of star formation. Young, massive clusters can
significantly impact the surrounding interstellar medium (ISM) and hence radio
observations can also be an important tracer of their activity. Several hundred
cluster candidates are now known by examining survey data. Here we report on
multiwavelength observations of six of these candidates in the Galaxy. We
carried out 4.9 and 8.5 GHz VLA observations of the radio emission associated
with these clusters to obtain the physical characteristics of the surrounding
gas, including the Lyman continuum photon flux and ionized gas mass. Spitzer
Infrared Array Camera observations were also made of these regions, and provide
details on the stellar population as well as the dust continuum and polycyclic
aromatic hydrocarbon emission. When compared to the known young, massive
clusters in the Galaxy, the six cluster candidates have less powerful Lyman
ionizing fluxes and ionize less of the H II mass in the surrounding ISM.
Therefore, these cluster candidates appear to be more consistent with
intermediate-mass clusters (10^3-10^4 Msun).Comment: 39 pages, 20 figures. Accepted in the Astronomical Journal; to be
published Fall 201
Effect of the Spatial Dispersion on the Shape of a Light Pulse in a Quantum Well
Reflectance, transmittance and absorbance of a symmetric light pulse, the
carrying frequency of which is close to the frequency of interband transitions
in a quantum well, are calculated. Energy levels of the quantum well are
assumed discrete, and two closely located excited levels are taken into
account. A wide quantum well (the width of which is comparable to the length of
the light wave, corresponding to the pulse carrying frequency) is considered,
and the dependance of the interband matrix element of the momentum operator on
the light wave vector is taken into account. Refractive indices of barriers and
quantum well are assumed equal each other. The problem is solved for an
arbitrary ratio of radiative and nonradiative lifetimes of electronic
excitations. It is shown that the spatial dispersion essentially affects the
shapes of reflected and transmitted pulses. The largest changes occur when the
radiative broadening is close to the difference of frequencies of interband
transitions taken into account.Comment: 7 pages, 5 figure
Elastic Light Scattering by Semiconductor Quantum Dots
Elastic light scattering by low-dimensional semiconductor objects is
investigated theoretically. The differential cross section of resonant light
scattering on excitons in quantum dots is calculated. The polarization and
angular distribution of scattered light do not depend on the quantum-dot form,
sizes and potential configuration if light wave lengths exceed considerably the
quantum-dot size. In this case the magnitude of the total light scattering
cross section does not depend on quantum-dot sizes. The resonant total light
scattering cross section is about a square of light wave length if the exciton
radiative broadening exceeds the nonradiative broadening. Radiative broadenings
are calculated
Profile alterations of a symmetrical light pulse coming through a quantum well
The theory of a response of a two-energy-level system, irradiated by
symmetrical light pulses, has been developed.(Suchlike electronic system
approximates under the definite conditions a single ideal quantum well (QW) in
a strong magnetic field {\bf H}, directed perpendicularly to the QW's plane, or
in magnetic field absence.) The general formulae for the time-dependence of
non-dimensional reflection {\cal R}(t), absorption {\cal A}(t) and transmission
{\cal T}(t) of a symmetrical light pulse have been obtained. It has been shown
that the singularities of three types exist on the dependencies {\cal R}(t),
{\cal A}(t), {\cal T}(t). The oscillating time dependence of {\cal R}(t), {\cal
A}(t), {\cal T}(t) on the detuning frequency \Delta\omega=\omega_l-\omega_0
takes place. The oscillations are more easily observable when
\Delta\omega\simeq\gamma_l. The positions of the total absorption, reflection
and transparency singularities are examined when the frequency \omega_l is
detuned.Comment: 9 pages, 13 figures with caption
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