3,059 research outputs found
Central twisted transformation groups and group -algebras of central group extensions
We examine the structure of central twisted transformation group \cs-algebras
C_{0}(X)\rtimes_{\id,u}G, and apply our results to the group \cs-algebras of
central group extensions. Our methods require that we study Moore's cohomology
group H^{2}\bigl(G,C(X,\T)\bigr), and, in particular, we prove an inflation
result for pointwise trivial cocyles which may be of use elsewhere.Comment: 22 pages. To appear in the Indiana U. Math.
Disorder-induced freezing of dynamical spin fluctuations in underdoped cuprates
We study the dynamical spin susceptibility of a correlated d-wave
superconductor (dSC) in the presence of disorder, using an unrestricted
Hartree-Fock approach. This model provides a concrete realization of the notion
that disorder slows down spin fluctuations, which eventually "freeze out". The
evolution of disorder-induced spectral weight transfer agrees qualitatively
with experimental observations on underdoped cuprate superconductors. For
sufficiently large disorder concentrations, static spin density wave (SDW)
order is created when droplets of magnetism nucleated by impurities overlap. We
also study the disordered stripe state coexisting with a dSC and compare its
magnetic fluctuation spectrum to that of the disorder-generated SDW phase.Comment: 5 pages, 4 figure
Analysis of Rayleigh-Plesset dynamics for sonoluminescing bubbles
Recent work on single bubble sonoluminescence (SBSL) has shown that many
features of this phenomenon, especially the dependence of SBSL intensity and
stability on experimental parameters, can be explained within a hydrodynamic
approach. More specifically, many important properties can already be derived
from an analysis of bubble wall dynamics. This dynamics is conveniently
described by the Rayleigh-Plesset (RP) equation. In this work we derive
analytical approximations for RP dynamics and subsequent analytical laws for
parameter dependences. These results include (i) an expression for the onset
threshold of SL, (ii) an analytical explanation of the transition from
diffusively unstable to stable equilibria for the bubble ambient radius
(unstable and stable sonoluminescence), and (iii) a detailed understanding of
the resonance structure of the RP equation. It is found that the threshold for
SL emission is shifted to larger bubble radii and larger driving pressures if
surface tension is enlarged, whereas even a considerable change in liquid
viscosity leaves this threshold virtually unaltered. As an enhanced viscosity
stabilizes the bubbles against surface oscillations, we conclude that the ideal
liquid for violently collapsing, surface stable SL bubbles should have small
surface tension and large viscosity, although too large viscosity (>40 times
the viscosity of water) will again preclude collapses.Comment: 41 pages, 21 eps and ps figures; LaTeX stylefiles replaced because
the PostScript file produced at the archive had misplaced and misscaled
figure
Energy transfer within phycocyanin trimers of Mastigocladus laminosus studied by picosecond time-resolved transient absorption spectroscopy
The transient absorption recovery induced in phycocyanin trimers by picosecond pulses of
variable wavelength (570 — 620 nm) has been recorded and analyzed by applying a least-squares
multi-exponential fit procedure.
The results suggest that in native PC trimers the chromophores exhibit a microheterogeneity
with the effect that the derived apparent lifetimes are functions of excitation and probing wavelength.
It is suggested that, due to strong excitonic coupling between a-84 and ß-84
chromophores, the lifetime of the terminal acceptor state is reduced to about 900 ps; the apparent
energy transfer time from chromophore β-155 to a-84 and ß-84 chromophores varies between
20—50 ps depending on the actual chromophore-protein arrangement (microheterogeneity)
- …