3,983 research outputs found
FÖRSTER TRANSFER CALCULATIONS BASED ON CRYSTAL STRUCTURE DATA FROM Agmenellum quadruplicatum C-PHYCOCYANIN
Excitation energy transfer in C-phycocyanin is modeled using the Forster inductive resonance mechanism. Detailed calculations are carried out using coordinates and orientations of the chromophores derived from X-ray crystallographic studies of C-phycocyanin from two different species (Schirmer et al, J. Mol. Biol. 184, 257–277 (1985) and ibid., 188, 651-677 (1986)). Spectral overlap integrals are estimated from absorption and fluorescence spectra of C-phycocyanin of Mastigocladus laminosus and its separated subunits. Calculations are carried out for the β-subunit, αβ-monomer, (αβ)3-trimer and (αβ)0-hexamer species with the following chromophore assignments: β155 = 's’(sensitizer), β84 =‘f (fluorescer) and α84 =‘m’(intermediate):]:. The calculations show that excitation transfer relaxation occurs to 3=98% within 200 ps in nearly every case; however, the rates increase as much as 10-fold for the higher aggregates. Comparison with experimental data on fluorescence decay and depolarization kinetics from the literature shows qualitative agreement with these calculations. We conclude that Forster transfer is sufficient to account for all of the observed fluorescence properties of C-phycocyanin in aggregation states up to the hexamer and in the absence of linker polypeptides
Interplane magnetic coupling effects in the multilattice compound Y_2Ba_4Cu_7O_{15}
We investigate the interplane magnetic coupling of the multilattice compound
Y_2Ba_4Cu_7O_{15} by means of a bilayer Hubbard model with inequivalent planes.
We evaluate the spin response, effective interaction and the intra- and
interplane spin-spin relaxation times within the fluctuation exchange
approximation. We show that strong in-plane antiferromagnetic fluctuations are
responsible for a magnetic coupling between the planes, which in turns leads to
a tendency of the fluctuation in the two planes to equalize.
This equalization effect grows whit increasing in-plane antiferromagnetic
fluctuations, i. e., with decreasing temperature and decreasing doping, while
it is completely absent when the in-layer correlation length becomes of the
order of one lattice spacing. Our results provide a good qualitative
description of NMR and NQR experiments in Y_2Ba_4Cu_7O_{15}.Comment: Final version, to appear. in Phys. Rev. B (Rapid Communications),
sched. Jan. 9
Effects of Electronic Correlations on the Thermoelectric Power of the Cuprates
We show that important anomalous features of the normal-state thermoelectric
power S of high-Tc materials can be understood as being caused by doping
dependent short-range antiferromagnetic correlations. The theory is based on
the fluctuation-exchange approximation applied to Hubbard model in the
framework of the Kubo formalism. Firstly, the characteristic maximum of S as
function of temperature can be explained by the anomalous momentum dependence
of the single-particle scattering rate. Secondly, we discuss the role of the
actual Fermi surface shape for the occurrence of a sign change of S as a
function of temperature and doping.Comment: 4 pages, with eps figure
Spectral properties of entanglement witnesses
Entanglement witnesses are observables which when measured, detect
entanglement in a measured composed system. It is shown what kind of relations
between eigenvectors of an observable should be fulfilled, to allow an
observable to be an entanglement witness. Some restrictions on the signature of
entaglement witnesses, based on an algebraic-geometrical theorem will be given.
The set of entanglement witnesses is linearly isomorphic to the set of maps
between matrix algebras which are positive, but not completely positive. A
translation of the results to the language of positive maps is also given. The
properties of entanglement witnesses and positive maps express as special cases
of general theorems for -Schmidt witnesses and -positive maps. The
results are therefore presented in a general framework.Comment: published version, some proofs are more detailed, mistakes remove
THERMAL DENATURATION OF MONOMERIC AND TRIMERIC PHYCOCYANINS STUDIED BY STATIC AND SPECTROSCOPY POLARIZED TIME-RESOLVED FLUORESCENCE
C-Phycocyanin (PC) and allophycocyanin (APC). as well as the a-subunit of PC. have been
isolated from the blue-green alga (cyanobacterium). Spirulina platensis. The effects of partial thermal
denaturation of PC and of its state of aggregation have been studied by ps time-resolved, polarized
fluorescence spectroscopy. All measurements have been performed under low photon fluxes (< 10’ ’
photonsipulse x cm’) to minimize singlet-singlet annihilation processes. A complex decay is obtained
under most conditions, which can be fitted satisfactorily with a bi-exponential (7’ = 70400 ps. T? =
1000-3000 ps) for both the isotropic and the polarized part, but with different intensities and time
constants for the two decay curves. The data are interpreted in the frameworkof the model first developed
by Teak and Dale (Biochern. J. 116, 161 (1970)], which divides the spectroscopically different
chromophores in (predominantly) sensitizing (s) and fluorescing U, ones. If one assumes temperature
dependent losses in the energy transfer from the s to the f and between f chromophores. both the
biexponential nature of the isotropic fluorescence decay and the polarization data can be rationalized. In
the isotropic emission (corresponding to the population of excited states) the short lifetime is related to the
s-,f transfer. the longer one to the “free“ decay of the final acceptor(s) (= f). The polarized part is
dominated by an extremely short decay time. which is related to s+f transfer, as well as to resonance
transfer between the f-chromophores
Coherent vs incoherent pairing in 2D systems near magnetic instability
We study the superconductivity in 2D fermionic systems near antiferromagnetic
instability, assuming that the pairing is mediated by spin fluctuations. This
pairing involves fully incoherent fermions and diffusive spin excitations. We
show that the competition between fermionic incoherence and strong pairing
interaction yields the pairing instability temperature which
increases and saturates as the magnetic correlation length . We
argue that in this quantum-critical regime the pairing problem is qualitatively
different from the BCS one.Comment: 7 pages, 2 figure
Theory for Dynamical Short Range Order and Fermi Surface Volume in Strongly Correlated Systems
Using the fluctuation exchange approximation of the one band Hubbard model,
we discuss the origin of the changing Fermi surface volume in underdoped
cuprate systems due to the transfer of occupied states from the Fermi surface
to its shadow, resulting from the strong dynamical antiferromagnetic short
range correlations. The momentum and temperature dependence of the quasi
particle scattering rate shows unusual deviations from the conventional Fermi
liquid like behavior. Their consequences for the changing Fermi surface volume
are discussed. Here, we investigate in detail which scattering processes
might be responsible for a violation of the Luttinger theorem. Finally, we
discuss the formation of hole pockets near half filling.Comment: 5 pages, Revtex, 4 postscript figure
A matrix product solution for a nonequilibrium steady state of an XX chain
A one dimensional XX spin chain of finite length coupled to reservoirs at
both ends is solved exactly in terms of a matrix product state ansatz. An
explicit representation of matrices of fixed dimension 4 independent of the
chain length is found. Expectations of all observables are evaluated, showing
that all connected correlations, apart from nearest neighbor z-z, are zero.Comment: 11 page
Possible high superconductivity mediated by antiferromagnetic spin fluctuations in systems with Fermi surface pockets
We propose that if there are two small pocket-like Fermi surfaces, and the
spin susceptibility is pronounced around a wave vector {\bf Q} that bridges the
two pockets, the spin-singlet superconductivity mediated by spin fluctuations
may have a high transition temperature. Using the fluctuation exchange
approximation, this idea is confirmed for the Hubbard on a lattice with
alternating hopping integrals, for which is estimated to be almost an
order of magnitude larger than those for systems with a large connected Fermi
surface.Comment: 5 pages, uses RevTe
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