49,293 research outputs found
THEORETICAL STUDIES OF BILIPROTEIN CHROMOPHORES AND RELATED BILE PIGMENTS BY MOLECULAR ORBITAL AND RAMACHANDRAN TYPE CALCULATIONS
Ramachandran calculations have been used to gain insight into steric hindrance in bile
pigments related to biliprotein chromophores. The high optical activity of denatured phycocyanin, as
compared to phycoerythrin, has been related to the asymmetric substitution at ring A, which shifts the
equilibrium towards the P-helical form of the chromophore. Geometric effects on the electronic structures
and transitions have then been studied by molecular orbital calculations for several conjugation
systems including the chromophores of phycocyanin. phytochrome P,, cations, cation radicals and
tautomeric forms. For these different chromophores some general trends can be deduced. For instance,
for a given change in the gross shape (e.g. either unfolding of the molecule from a cyclic-helical to a fully
extended geometry, or upon out-of-plane twists of the pyrrole ring A) of the molecules under study, the
predicted absorption spectra all change in a simikar way. Nonetheless, there are characteristic distinctions
between the different n-systems, both in the transition energies and the charge distribution, which
can be related to their known differences in spectroscopic properties and their reactivity
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Two novel nonlinear companding schemes with iterative receiver to reduce PAPR in multi-carrier modulation systems
Companding transform is an efficient and simple method to reduce the Peak-to-Average Power Ratio (PAPR) for Multi-Carrier Modulation (MCM) systems. But if the MCM signal is only simply operated by inverse companding transform at the receiver, the resultant spectrum may exhibit severe in-band and out-of-band radiation of the distortion components, and considerable peak regrowth by excessive channel noises etc. In order to prevent these problems from occurring, in this paper, two novel nonlinear companding schemes with a iterative receiver are proposed to reduce the PAPR. By transforming the amplitude or power of the original MCM signals into uniform distributed signals, the novel schemes can effectively reduce PAPR for different modulation formats and sub-carrier sizes. Despite moderate complexity increasing at the receiver, but it is especially suitable to be combined with iterative channel estimation. Computer simulation results show that the proposed schemes can offer good system performances without any bandwidth expansion
Light pseudoscalar eta and H->eta eta decay in the simplest little Higgs mode
The SU(3) simplest little Higgs model in its original framework without the
so-called mu term inevitably involves a massless pseudoscalar boson eta, which
is problematic for b-physics and cosmological axion limit. With the mu term
introduced by hand, the eta boson acquires mass m_eta ~ mu, which can be
lighter than half the Higgs boson mass in a large portion of the parameter
space. In addition, the introduced mu term generates sizable coupling of
H-eta-eta. The Higgs boson can dominantly decay into a pair of eta's especially
when mH below the WW threshold. Another new decay channel of H->Z+eta can be
dominant or compatible with H -> WW for mH above the Z+eta threshold. We show
that the LEP bound on the Higgs boson mass is loosened to some extent due to
this new H->eta eta decay channel as well as the reduced coupling of H-Z-Z. The
Higgs boson mass bound falls to about 110 GeV for f=3-4 TeV. Since the eta
boson decays mainly into a bb pair, H-> eta eta -> 4b and H-> Z eta -> Z bb
open up other interesting search channels in the pursuit of the Higgs boson in
the future experiments. We discuss on these issues.Comment: major modification considering the simplest little Higgs model with
the mu ter
Connections of activated hopping processes with the breakdown of the Stokes-Einstein relation and with aspects of dynamical heterogeneities
We develop a new extended version of the mode-coupling theory (MCT) for glass
transition, which incorporates activated hopping processes via the dynamical
theory originally formulated to describe diffusion-jump processes in crystals.
The dynamical-theory approach adapted here to glass-forming liquids treats
hopping as arising from vibrational fluctuations in quasi-arrested state where
particles are trapped inside their cages, and the hopping rate is formulated in
terms of the Debye-Waller factors characterizing the structure of the
quasi-arrested state. The resulting expression for the hopping rate takes an
activated form, and the barrier height for the hopping is ``self-generated'' in
the sense that it is present only in those states where the dynamics exhibits a
well defined plateau. It is discussed how such a hopping rate can be
incorporated into MCT so that the sharp nonergodic transition predicted by the
idealized version of the theory is replaced by a rapid but smooth crossover. We
then show that the developed theory accounts for the breakdown of the
Stokes-Einstein relation observed in a variety of fragile glass formers. It is
also demonstrated that characteristic features of dynamical heterogeneities
revealed by recent computer simulations are reproduced by the theory. More
specifically, a substantial increase of the non-Gaussian parameter, double-peak
structure in the probability distribution of particle displacements, and the
presence of a growing dynamic length scale are predicted by the extended MCT
developed here, which the idealized version of the theory failed to reproduce.
These results of the theory are demonstrated for a model of the Lennard-Jones
system, and are compared with related computer-simulation results and
experimental data.Comment: 13 pages, 5 figure
Tunable Quantum Fluctuation-Controlled Coherent Spin Dynamics
Temporal evolution of a macroscopic condensate of ultra cold atoms is usually
driven by mean field potentials, either due to scattering between atoms or due
to coupling to external fields; and coherent quantum dynamics have been
observed in various cold-atom experiments. In this article, we report results
of studies of a class of quantum spin dynamics which are purely driven by zero
point quantum fluctuations of spin collective coordinates. Unlike the usual
mean-field coherent dynamics, quantum fluctuation-controlled spin dynamics or
QFCSD studied here are very sensitive to variation of quantum fluctuations and
can be tuned by four to five order of magnitude using optical lattices. They
have unique dependence on optical lattice potential depths and quadratic Zeeman
fields. QFCSD can be potentially used to calibrate quantum fluctuations and
investigate correlated fluctuations and various universal scaling properties
near quantum critical points.Comment: 14 pages, 12 figures included; including detailed discussions on
thermal effects, trapping potentials and spin exchange losses. (To appear in
PRA
Microstrip superconducting quantum interference device amplifiers with submicron Josephson junctions: enhanced gain at gigahertz frequencies
We present measurements of an amplifier based on a dc superconducting quantum
interference device (SQUID) with submicron Al-AlOx-Al Josephson junctions. The
small junction size reduces their self-capacitance and allows for the use of
relatively large resistive shunts while maintaining nonhysteretic operation.
This leads to an enhancement of the SQUID transfer function compared to SQUIDs
with micron-scale junctions. The device layout is modified from that of a
conventional SQUID to allow for coupling signals into the amplifier with a
substantial mutual inductance for a relatively short microstrip coil.
Measurements at 310 mK exhibit gain of 32 dB at 1.55 GHz.Comment: Version with high resolution figures at:
http://physics.syr.edu/~bplourde/bltp-publications.ht
The observation of a positive magnetoresistance and close correlation among lattice, spin and charge around TC in antipervoskite SnCMn3
The temperature dependences of magnetization, electrical transport, and
thermal transport properties of antiperovskite compound SnCMn3 have been
investigated systematically. A positive magnetoresistance (~11%) is observed
around the ferrimagnetic-paramagnetic transition (TC ~ 280 K) in the field of
50 kOe, which can be attributed to the field-induced magnetic phase transition.
The abnormalities of resistivity, Seebeck coefficient, normal Hall effect and
thermal conductivity near TC are suggested to be associated with an abrupt
reconstruction of electronic structure. Further, our results indicate an
essential interaction among lattice, spin and charge degrees of freedom around
TC. Such an interaction among various degrees of freedom associated with sudden
phase transition is suggested to be characteristic of Mn-based antiperovskite
compounds.Comment: 13 pages, 5 figure
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