786 research outputs found
Understanding/unravelling carotenoid excited singlet states.
Carotenoids are essential light-harvesting pigments in natural photosynthesis. They absorb in the blue–green region of the solar spectrum and transfer the absorbed energy to (bacterio-)chlorophylls, and thus expand the wavelength range of light that is able to drive photosynthesis. This process is an example of singlet–singlet excitation energy transfer, and carotenoids serve to enhance the overall efficiency of photosynthetic light reactions. The photochemistry and photophysics of carotenoids have often been interpreted by referring to those of simple polyene molecules that do not possess any functional groups. However, this may not always be wise because carotenoids usually have a number of functional groups that induce the variety of photochemical behaviours in them. These differences can also make the interpretation of the singlet excited states of carotenoids very complicated. In this article, we review the properties of the singlet excited states of carotenoids with the aim of producing as coherent a picture as possible of what is currently known and what needs to be learned
Ordering in Two-Dimensional Ising Models with Competing Interactions
We study the 2D Ising model on a square lattice with additional non-equal
diagonal next-nearest neighbor interactions. The cases of classical and quantum
(transverse) models are considered. Possible phases and their locations in the
space of three Ising couplings are analyzed. In particular, incommensurate
phases occurring only at non-equal diagonal couplings, are predicted. We also
analyze a spin-pseudospin model comprised of the quantum Ising model coupled to
XY spin chains in a particular region of interactions, corresponding to the
Ising sector's super-antiferromagnetic (SAF) ground state. The spin-SAF
transition in the coupled Ising-XY model into a phase with co-existent SAF
Ising (pseudospin) long-range order and a spin gap is considered. Along with
destruction of the quantum critical point of the Ising sector, the phase digram
of the Ising-XY model can also demonstrate a re-entrance of the spin-SAF phase.
A detailed study of the latter is presented. The mechanism of the re-entrance,
due to interplay of interactions in the coupled model, and the conditions of
its appearance are established. Applications of the spin-SAF theory for the
transition in the quarter-filled ladder compound NaV2O5 are discussed.Comment: Minor revisions and refs. added; published version of the invited
paper in a special issue of "Low Temp. Physics
Dynamics of lattice spins as a model of arrhythmia
We consider evolution of initial disturbances in spatially extended systems
with autonomous rhythmic activity, such as the heart. We consider the case when
the activity is stable with respect to very smooth (changing little across the
medium) disturbances and construct lattice models for description of
not-so-smooth disturbances, in particular, topological defects; these models
are modifications of the diffusive XY model. We find that when the activity on
each lattice site is very rigid in maintaining its form, the topological
defects - vortices or spirals - nucleate a transition to a disordered,
turbulent state.Comment: 17 pages, revtex, 3 figure
Calculation of the Coherent Synchrotron Radiation Impedance from a Wiggler
Most studies of Coherent Synchrotron Radiation (CSR) have only considered the
radiation from independent dipole magnets. However, in the damping rings of
future linear colliders, a large fraction of the radiation power will be
emitted in damping wigglers. In this paper, the longitudinal wakefield and
impedance due to CSR in a wiggler are derived in the limit of a large wiggler
parameter . After an appropriate scaling, the results can be expressed in
terms of universal functions, which are independent of . Analytical
asymptotic results are obtained for the wakefield in the limit of large and
small distances, and for the impedance in the limit of small and high
frequencies.Comment: 10 pages, 8 figure
Analysis of a three-component model phase diagram by Catastrophe Theory: Potentials with two Order Parameters
In this work we classify the singularities obtained from the Gibbs potential
of a lattice gas model with three components, two order parameters and five
control parameters applying the general theorems provided by Catastrophe
Theory. In particular, we clearly establish the existence of Landau potentials
in two variables or, in other words, corank 2 canonical forms that are
associated to the hyperbolic umbilic, D_{+4}, its dual the elliptic umbilic,
D_{-4}, and the parabolic umbilic, D_5, catastrophes. The transversality of the
potential with two order parameters is explicitely shown for each case. Thus we
complete the Catastrophe Theory analysis of the three-component lattice model,
initiated in a previous paper.Comment: 17 pages, 3 EPS figures, Latex file, continuation of Phys. Rev. B57,
13527 (1998) (cond-mat/9707015), submitted to Phys. Rev.
Analysis of a three-component model phase diagram by Catastrophe Theory
We analyze the thermodynamical potential of a lattice gas model with three
components and five parameters using the methods of Catastrophe Theory. We find
the highest singularity, which has codimension five, and establish its
transversality. Hence the corresponding seven-degree Landau potential, the
canonical form Wigwam or , constitutes the adequate starting point to
study the overall phase diagram of this model.Comment: 16 pages, Latex file, submitted to Phys. Rev.
Functional characterization of various algal carotenoid ketolases reveals that ketolating zeaxanthin efficiently is essential for high production of astaxanthin in transgenic Arabidopsis
Extending the carotenoid pathway to astaxanthin in plants is of scientific and industrial interest. However, expression of a microbial β-carotene ketolase (BKT) that catalyses the formation of ketocarotenoids in transgenic plants typically results in low levels of astaxanthin. The low efficiency of BKTs in ketolating zeaxanthin to astaxanthin is proposed to be the major limitation for astaxanthin accumulation in engineered plants. To verify this hypothesis, several algal BKTs were functionally characterized using an Escherichia coli system and three BKTs were identified, with high (up to 85%), moderate (∼38%), and low (∼1%) conversion rate from zeaxanthin to astaxanthin from Chlamydomonas reinhardtii (CrBKT), Chlorella zofingiensis (CzBKT), and Haematococcus pluvialis (HpBKT3), respectively. Transgenic Arabidopsis thaliana expressing the CrBKT developed orange leaves which accumulated astaxanthin up to 2 mg g−1 dry weight with a 1.8-fold increase in total carotenoids. In contrast, the expression of CzBKT resulted in much lower astaxanthin content (0.24 mg g−1 dry weight), whereas HpBKT3 was unable to mediate synthesis of astaxanthin in A. thaliana. The none-native astaxanthin was found mostly in a free form integrated into the light-harvesting complexes of photosystem II in young leaves but in esterified forms in senescent leaves. The alteration of carotenoids did not affect chlorophyll content, plant growth, or development significantly. The astaxanthin-producing plants were more tolerant to high light as shown by reduced lipid peroxidation. This study advances a decisive step towards the utilization of plants for the production of high-value astaxanthin
Exact correlation functions of Bethe lattice spin models in external fields
We develop a transfer matrix method to compute exactly the spin-spin
correlation functions of Bethe lattice spin models in the external magnetic
field h and for any temperature T. We first compute the correlation function
for the most general spin - S Ising model, which contains all possible
single-ion and nearest-neighbor pair interactions. This general spin - S Ising
model includes the spin-1/2 simple Ising model and the Blume-Emery-Griffiths
(BEG) model as special cases. From the spin-spin correlation functions, we
obtain functions of correlation length for the simple Ising model and BEG
model, which show interesting scaling and divergent behavior as T approaches
the critical temperature. Our method to compute exact spin-spin correlation
functions may be applied to other Ising-type models on Bethe and Bethe-like
lattices.Comment: 19 page
- …