1,017 research outputs found
Arginase from kiwifruit: properties and seasonal variation
The in vitro activity of arginase (EC 3.5.3.1) was investigated in youngest-mature leaves and roots (1-3 mm diameter) of kiwifruit vines (Actinidia deliciosa var. deliciosa) during an annual growth cycle, and enzyme from root material partially purified. No seasonal trend in the specific activity of arginase was observed in roots. Measurements in leaves, however, rose gradually during early growth and plateaued c. 17 weeks after budbreak. Changes in arginase activity were not correlated with changes in the concentration of arginine (substrate) or glutamine (likely end-product of arginine catabolism) in either tissue during the growth cycle. Purification was by (NH4)2SO4 precipitation and DEAE-cellulose chromatography. The kinetic properties of the enzyme, purified 60-fold over that in crude extracts, indicated a pH optimum of 8.8, and a Km (L-arginine) of 7.85 mM. Partially-purified enzyme was deactivated by dialysis against EDTA, and reactivated in the presence of Mn²⁺, Co²⁺, and Ni²⁺
The free surface of superfluid 4He at zero temperature
The structure and energetics of the free surface of superfluid He are
studied using the diffusion Monte Carlo method. Extending a previous
calculation by Vall\'es and Schmidt, which used the Green's function Monte
Carlo method, we study the surface of liquid He within a slab geometry
using a larger number of particles in the slab and an updated interatomic
potential. The surface tension is accurately estimated from the energy of slabs
of increasing surface density and its value is close to one of the two existing
experimental values. Results for the density profiles allow for the calculation
of the surface width which shows an overall agreement with recent experimental
data. The dependence on the transverse direction to the surface of other
properties such as the two-body radial distribution function, structure factor,
and one-body density matrix is also studied. The condensate fraction, extracted
from the asymptotic behavior of the one-body density matrix, shows an
unambiguous enhancement when approaching the surface.Comment: RevTex, 11 pages, accepted in Phys. Rev.
Li Induced Spin and Charge Excitations in a Spin Ladder
A lithium dopant in a cuprate spin ladder acts as a vacant (non-magnetic)
site accompanied by an extra hole bound by a Coulomb potential. We find that,
although the undoped ladder spin gap is not essentially altered by Li doping, a
dopant-magnon bound state appears within the gap. Contrary to the case of
Zn-doped ladders, we predict that, in the Li-doped ladder, the spin liquid
character is very robust against antiferromagnetism. We also predict the
spatial dependence of the density of states in the vicinity of the dopant which
could be measured by local spectroscopic probes.Comment: 4 pages, Submitted to Physical Review Letter
Numerical renormalization-group study of spin correlations in one-dimensional random spin chains
We calculate the ground-state two-spin correlation functions of spin-1/2
quantum Heisenberg chains with random exchange couplings using the real-space
renormalization group scheme. We extend the conventional scheme to take account
of the contribution of local higher multiplet excitations in each decimation
step. This extended scheme can provide highly accurate numerical data for large
systems. The random average of staggered spin correlations of the chains with
random antiferromagnetic (AF) couplings shows algebraic decay like ,
which verifies the Fisher's analytic results. For chains with random
ferromagnetic (FM) and AF couplings, the random average of generalized
staggered correlations is found to decay more slowly than a power-law, in the
form close to . The difference between the distribution functions of
the spin correlations of the random AF chains and of the random FM-AF chains is
also discussed.Comment: 14 pages including 8 figures, REVTeX, submitted to Physical Review
Bond-Dilution-Induced Quantum Phase Transitions in Heisenberg Antiferromagnets
Bond-dilution effects on the ground state of the square-lattice
antiferromagnetic Heisenberg model, consisting of coupled bond-alternating
chains, are investigated by means of the quantum Monte Carlo simulation. It is
found that, when the ground state of the non-diluted system is a non-magnetic
state with a finite spin gap, a sufficiently weak bond dilution induces a
disordered state with a mid gap in the original spin gap, and under a further
stronger bond dilution an antiferromagnetic long-range order emerges. While the
site-dilution-induced long-range order is induced by an infinitesimal
concentration of dilution, there exists a finite critical concentration in the
case of bond dilution. We argue that this essential difference is due to the
occurrence of two types of effective interactions between induced magnetic
moments in the case of bond dilution, and that the antiferromagnetic
long-range-ordered phase does not appear until the magnitudes of the two
interactions become comparable.Comment: 7 pages, 13 figure
Structural and dynamical properties of superfluid helium: a density functional approach
We present a novel density functional for liquid 4He, properly accounting for
the static response function and the phonon-roton dispersion in the uniform
liquid. The functional is used to study both structural and dynamical
properties of superfluid helium in various geometries. The equilibrium
properties of the free surface, droplets and films at zero temperature are
calculated. Our predictions agree closely to the results of ab initio Monte
Carlo calculations, when available. The introduction of a phenomenological
velocity dependent interaction, which accounts for backflow effects, is
discussed. The spectrum of the elementary excitations of the free surface and
films is studied.Comment: 37 pages, REVTeX 3.0, figures on request at [email protected]
A π-Extended Donor-Acceptor-Donor Triphenylene Twin linked via a Pyrazine-bridge
Beta-amino triphenylenes can be accessed via palladium catalyzed amination of the corresponding triflate using benzophe-none imine. Transformation of amine 6 to benzoyl amide 18 is also straightforward and its wide mesophase range demon-strates that the new linkage supports columnar liquid crystal formation. Amine 6 also undergoes clean aerobic oxidation to give a new twinned structure linked through an electron-poor pyrazine ring. The new discotic liquid crystal motif contains donor and acceptor fragments, and is more oval in shape rather than disk-like. It forms a wide range columnar mesophase. Absorption spectra are strong and broad; emission is also broad and occurs with a Stokes shift of ca. 0.7 eV, indicative of charge-transfer characte
Enhancement of Antiferromagnetic Correlations Induced by Nonmagnetic Impurities: Origin and Predictions for NMR Experiments
Spin models that have been proposed to describe dimerized chains, ladders,
two dimensional antiferromagnets, and other compounds are here studied when
some spins are replaced by spinless vacancies, such as it occurs by
doping. A small percentage of vacancies rapidly destroys the spin gap, and
their presence induces enhanced antiferromagnetic correlations near those
vacancies. The study is performed with computational techniques which includes
Lanczos, world-line Monte Carlo, and the Density Matrix Renormalization Group
methods. Since the phenomenon of enhanced antiferromagnetism is found to occur
in several models and cluster geometries, a common simple explanation for its
presence may exist. It is argued that the resonating-valence-bond character of
the spin correlations at short distances of a large variety of models is
responsible for the presence of robust staggered spin correlations near
vacancies and lattice edges. The phenomenon takes place regardless of the long
distance properties of the ground state, and it is caused by a ``pruning'' of
the available spin singlets in the vicinity of the vacancies. The effect
produces a broadening of the low temperature NMR signal for the compounds
analyzed here. This broadening should be experimentally observable in the
structurally dimerized chain systems
, , , and
, in ladder materials such as , in the
spin-Peierls systems and , and in several others since it
is a universal effect common to a wide variety of models and compounds.Comment: 18 pages revtex with 26 figures include
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