43,994 research outputs found
H-NMR spin-echo measurements of the static and dynamic spin properties in -(BETS)FeCl
H-NMR spin-echo measurements of the spin-echo decay with a
decay rate 1/ and the frequency shift under applied
magnetic field = 9 T along the a-axis over a temperature
range 2.0180 K are reported for a single crystal of the organic conductor
-(BETS)FeCl. It provides the spin dynamic and static
properties in the paramagnetic metal (PM) and antiferromagnetic insulator (AFI)
states as well as across the PMAFI phase transition. A large slow beat
structure in the spin-echo decay is observed with a typical beat frequency of
7 kHz and it varies across the spectrum. Its origin is attributed to
the HH dipole interactions rather than to the much larger
dipolar field contribution from the Fe electrons (spin = 5/2). A
simple phenomenological model provides an excellent fit to the data. The
dominant H-NMR frequency shift comes from the dipolar field from the 3d
Fe ions, and the Fe Fe exchange interactions ()
( includes the dd exchange interactions through the electrons)
have a substantial effect to the local field at the proton sites expecially at
low temperatures. A good fit is obtained with = - 1.7 K. The data of
the spin-echo decay rate 1/ indicates that there is a significant change
in the slow fluctuations of the local magnetic field at the H-sites on
traversing the PM to AFI phase. This evidence supports earlier reports that the
PMAFI phase transition in -(BETS)FeCl is driven
magnetically and first order.Comment: 9 pages, 10 figures, resubmitted to Phys. Rev. B in response to
comments of Editor and reviewers on March 23, 200
The Wilson Effective K\"ahler Potential For Supersymmetric Nonlinear Sigma Models
Renormalization group methods are used to determine the evolution of the low
energy Wilson effective action for supersymmetric nonlinear sigma models in
four dimensions. For the case of supersymmetric models, the
K\"ahler potential is determined exactly and is shown to exhibit a nontrivial
ultraviolet fixed point in addition to a trivial infrared fixed point. The
strong coupling behavior of the theory suggests the possible existence of
additional relevant operators or nonperturbative degrees of freedom.Comment: 9 pages, LaTeX, 1 eps figur
|V|: New insight into the circular polarization of radio pulsars
We present a study of single pulses from nine bright northern pulsars to
investigate the behaviour of circular polarisation, V. The observations were
conducted with the Effelsberg 100-m radio telescope at 1.41 GHz and 4.85 GHz
and the Westerbork radio telescope at 352 MHz. For the first time, we present
the average profile of the absolute circular polarisation |V| in the single
pulses. We demonstrate that the average profile of |V| is the distinguishing
feature between pulse components that exhibit low V in the single pulses and
components that exhibit high V of either handedness, despite both cases
resulting in a low mean. We also show that the |V| average profile remains
virtually constant with frequency, which is not generally the case for V,
leading us to the conclusion that |V| is a key quantity in the pulsar emission
problem.Comment: 5 pages, accepted for publication in MNRAS letter
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⁺
Rhodium(II)-catalyzed stereocontrolled synthesis of dihydrofuran-3-imines from 1-Tosyl-1,2,3-triazoles
Rhodium(II) acetate catalyzes the denitrogenative transformation of 5-substituted and 4,5-disubstituted 1-sulfonyl-1,2,3-triazoles with pendent allyl and propargyl ether motifs to oxonium ylides that undergo [2,3]-sigmatropic rearrangement to give substituted dihydrofuran-3-imines in high yield and diastereoselectivity
Measurement of vertical velocity using clear-air Doppler radars
A new clear air Doppler radar was constructed, called the Flatland radar, in very flat terrain near Champaign-Urbana, Illinois. The radar wavelength is 6.02 m. The radar has been measuring vertical velocity every 153 s with a range resolution of 750 m almost continuously since March 2, 1987. The variance of vertical velocity at Flatland is usually quite small, comparable to the variance at radars located near rough terrain during periods of small background wind. The absence of orographic effects over very flat terrain suggests that clear air Doppler radars can be used to study vertical velocities due to other processes, including synoptic scale motions and propagating gravity waves. For example, near rough terrain the shape of frequency spectra changes drastically as the background wind increases. But at Flatland the shape at periods shorter than a few hours changes only slowly, consistent with the changes predicted by Doppler shifting of gravity wave spectra. Thus it appears that the short period fluctuations of vertical velocity at Flatland are alsmost entirely due to the propagating gravity waves
Shape control of QDs studied by cross-sectional scanning tunneling microscopy
In this cross-sectional scanning tunneling microscopy study we investigated
various techniques to control the shape of self-assembled quantum dots (QDs)
and wetting layers (WLs). The result shows that application of an indium flush
during the growth of strained InGaAs/GaAs QD layers results in flattened QDs
and a reduced WL. The height of the QDs and WLs could be controlled by varying
the thickness of the first capping layer. Concerning the technique of antimony
capping we show that the surfactant properties of Sb result in the preservation
of the shape of strained InAs/InP QDs during overgrowth. This could be achieved
by both a growth interrupt under Sb flux and capping with a thin GaAsSb layer
prior to overgrowth of the uncapped QDs. The technique of droplet epitaxy was
investigated by a structural analysis of strain free GaAs/AlGaAs QDs. We show
that the QDs have a Gaussian shape, that the WL is less than 1 bilayer thick,
and that minor intermixing of Al with the QDs takes place.Comment: 7 pages, 10 figure
Phonon resonances in atomic currents through Bose-Fermi mixtures in optical lattices
We present an analysis of Bose-Fermi mixtures in optical lattices for the
case where the lattice potential of the fermions is tilted and the bosons (in
the superfluid phase) are described by Bogoliubov phonons. It is shown that the
Bogoliubov phonons enable hopping transitions between fermionic Wannier-Stark
states; these transitions are accompanied by energy dissipation into the
superfluid and result in a net atomic current along the lattice. We derive a
general expression for the drift velocity of the fermions and find that the
dependence of the atomic current on the lattice tilt exhibits negative
differential conductance and phonon resonances. Numerical simulations of the
full dynamics of the system based on the time-evolving block decimation
algorithm reveal that the phonon resonances should be observable under the
conditions of a realistic measuring procedure.Comment: 8 pages, 5 figure
On dynamical mass generation in three dimensional supersymmetric U(1) gauge field theory
We investigate and contrast the non-perturbative infra red structure of N=1
and N=2 supersymmetric non-compact U(1) gauge field theory in three space-time
dimensions with N matter flavours. We study the Dyson-Schwinger equations in a
general gauge using superfield formalism; this ensures that supersymmetry is
kept manifest, though leads to spurious infra red divergences which we have to
avoid carefully. In the N=1 case the superfield formalism allows us to choose a
vertex which satisfies the U(1) Ward identity exactly, and we find the expected
critical behaviour in the wavefunction renormalization and strong evidence for
the existence of a gauge independent dynamically generated mass, but with no
evidence for a critical flavour number. We study the N=2 model by dimensional
reduction from four dimensional N=1 electrodynamics, and we refine the old
gauge dependence argument that there is no dynamical mass generation. We
recognize that the refinement only holds after dimensional reduction.Comment: 32 pages RevTeX; 3 axodraw figures include
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