22,632 research outputs found
Identifying and Indexing Icosahedral Quasicrystals from Powder Diffraction Patterns
We present a scheme to identify quasicrystals based on powder diffraction
data and to provide a standardized indexing. We apply our scheme to a large
catalog of powder diffraction patterns, including natural minerals, to look for
new quasicrystals. Based on our tests, we have found promising candidates
worthy of further exploration.Comment: 4 pages, 1 figur
Quantum Versus Mean Field Behavior of Normal Modes of a Bose-Einstein Condensate in a Magnetic Trap
Quantum evolution of a collective mode of a Bose-Einstein condensate
containing a finite number N of particles shows the phenomena of collapses and
revivals. The characteristic collapse time depends on the scattering length,
the initial amplitude of the mode and N. The corresponding time values have
been derived analytically under certain approximation and numerically for the
parabolic atomic trap. The revival of the mode at time of several seconds, as a
direct evidence of the effect, can occur, if the normal component is
significantly suppressed.
We also discuss alternative means to verify the proposed mechanism.Comment: minor corrections are introduced into the tex
Fitting the Means to the Ends: One Schoolâs Experience with Quantitative and Qualitative Methods in Curriculum Evaluation During Curriculum Change
Curriculum evaluation plays an important role in substantive curriculum change. The experience of the University of Texas Medical Branch (UTMB) with evaluation processes developed for the new Integrated Medical Curriculum (IMC) illustrates how evaluation methods may be chosen to match the goals of the curriculum evaluation process. Quantitative data such as ratings of courses or scores on external exams are useful for comparing courses or assessing whether standards have been met. Qualitative data such as studentsâ comments about aspects of courses are useful for eliciting explanations of observed phenomena and describing relationships between curriculum features and outcomes. The curriculum evaluation process designed for the IMC used both types of evaluation methods in a complementary fashion. Quantitative and qualitative methods have been used for formative evaluation of the new IMC courses. They are now being incorporated into processes to judge the IMC against its goals and objectives
The mechanical response of semiflexible networks to localized perturbations
Previous research on semiflexible polymers including cytoskeletal networks in
cells has suggested the existence of distinct regimes of elastic response, in
which the strain field is either uniform (affine) or non-uniform (non-affine)
under external stress. Associated with these regimes, it has been further
suggested that a new fundamental length scale emerges, which characterizes the
scale for the crossover from non-affine to affine deformations. Here, we extend
these studies by probing the response to localized forces and force dipoles. We
show that the previously identified nonaffinity length [D.A. Head et al. PRE
68, 061907 (2003).] controls the mesoscopic response to point forces and the
crossover to continuum elastic behavior at large distances.Comment: 16 pages, 18 figures; substantial changes to text and figures to
clarify the crossover to continuum elasticity and the role of finite-size
effect
Density-Polarization Functional Theory of the response of a periodic insulating solid to an electric field.
The response of an infinite, periodic, insulating, solid to an
infinitesimally small electric field is investigated in the framework of
Density Functional Theory. We find that the applied perturbing potential is not
a unique functional of the periodic density change~: it depends also on the
change in the macroscopic {\em polarization}. Moreover, the dependence of the
exchange-correlation energy on polarization induces an exchange-correlation
electric field. These effects are exhibited for a model semiconductor. We also
show that the scissor-operator technique is an approximate way of bypassing
this polarization dependence.Comment: 11 pages, 1 Fig
A nonlocal eigenvalue problem and the stability of spikes for reaction-diffusion systems with fractional reaction rates
We consider a nonlocal eigenvalue problem which arises in the study of stability of spike solutions for reaction-diffusion systems with
fractional reaction rates such as the Sel'kov model, the
Gray-Scott system, the hypercycle Eigen and Schuster, angiogenesis, and the generalized Gierer-Meinhardt
system.
We give some sufficient and explicit conditions for stability
by studying the corresponding nonlocal eigenvalue problem in a new
range of parameters
Exact renormalization-group analysis of first order phase transitions in clock models
We analyze the exact behavior of the renormalization group flow in
one-dimensional clock-models which undergo first order phase transitions by the
presence of complex interactions. The flow, defined by decimation, is shown to
be single-valued and continuous throughout its domain of definition, which
contains the transition points. This fact is in disagreement with a recently
proposed scenario for first order phase transitions claiming the existence of
discontinuities of the renormalization group. The results are in partial
agreement with the standard scenario. However in the vicinity of some fixed
points of the critical surface the renormalized measure does not correspond to
a renormalized Hamiltonian for some choices of renormalization blocks. These
pathologies although similar to Griffiths-Pearce pathologies have a different
physical origin: the complex character of the interactions. We elucidate the
dynamical reason for such a pathological behavior: entire regions of coupling
constants blow up under the renormalization group transformation. The flows
provide non-perturbative patterns for the renormalization group behavior of
electric conductivities in the quantum Hall effect.Comment: 13 pages + 3 ps figures not included, TeX, DFTUZ 91.3
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