1,756 research outputs found
Proton NMR measurements of the local magnetic field in the paramagnetic metal and antiferromagnetic insulator phases of -(BETS)FeCl
Measurements of the H-NMR spectrum of a small ( 4 g) single
crystal of the organic conductor -(BETS)FeCl are reported
with an applied magnetic field = 9 T parallel to the a-axis in
the -plane over a temperature range 2.0 180 K. They provide the
distribution of the static local magnetic field at the proton sites in the
paramagnetic metal (PM) and antiferromagnetic insulator (AFI) phases, along
with the changes that occur at the PMAFI phase transition. The spectra have
six main peaks that are significantly broadened and shifted at low . The
origin of these features is attributed to the large dipolar field from the 3d
Fe ion moments (spin = 5/2). Their amplitude and
dependence are modeled using a modified Brillouin function that includes a
mean field approximation for the total exchange interaction () between
one Fe ion and its two nearest neighbors. A good fit is obtained using
= 1.7 K. At temperatures below the PMAFI transition temperature
= 3.5 K, an extra peak appears on the high frequency side of the
spectrum and the details of the spectrum become smeared. Also, the rms
linewidth and the frequency shift of the spectral distribution are
discontinuous, consistent with the transition being first-order. These
measurements verify that the dominant local magnetic field contribution is from
the Fe ions and indicate that there is a significant change in the
static local magnetic field distribution at the proton sites on traversing the
PM to AFI phase transition.Comment: 11 pages, 7 figures. Revised version of cond-mat/0605044 resubmitted
to Phys. Rev. B in response to comments of Editor and reviewer
Ionic Liquid-Based Microemulsions in Catalysis
The design and properties of surface-active
ionic liquids that
are able to form stable microemulsions with heptane and water are
presented, and their promise as reaction media for thermomorphic palladium-catalyzed
cross-coupling reactions is demonstrated
The Error and Repair Catastrophes: A Two-Dimensional Phase Diagram in the Quasispecies Model
This paper develops a two gene, single fitness peak model for determining the
equilibrium distribution of genotypes in a unicellular population which is
capable of genetic damage repair. The first gene, denoted by ,
yields a viable organism with first order growth rate constant if it
is equal to some target ``master'' sequence . The second
gene, denoted by , yields an organism capable of genetic repair
if it is equal to some target ``master'' sequence . This
model is analytically solvable in the limit of infinite sequence length, and
gives an equilibrium distribution which depends on \mu \equiv L\eps , the
product of sequence length and per base pair replication error probability, and
\eps_r , the probability of repair failure per base pair. The equilibrium
distribution is shown to exist in one of three possible ``phases.'' In the
first phase, the population is localized about the viability and repairing
master sequences. As \eps_r exceeds the fraction of deleterious mutations,
the population undergoes a ``repair'' catastrophe, in which the equilibrium
distribution is still localized about the viability master sequence, but is
spread ergodically over the sequence subspace defined by the repair gene. Below
the repair catastrophe, the distribution undergoes the error catastrophe when exceeds \ln k/\eps_r , while above the repair catastrophe, the
distribution undergoes the error catastrophe when exceeds , where denotes the fraction of deleterious mutations.Comment: 14 pages, 3 figures. Submitted to Physical Review
Intrinsiccone adaptation modulates feedback efficiency from horizontal cells to cones
Processing of visual stimuli by the retina changes strongly during light/dark adaptation. These changes are due to both local photoreceptor-based processes and to changes in the retinal network. The feedback pathway from horizontal cells to cones is known to be one of the pathways that is modulated strongly during adaptation. Although this phenomenon is well described, the mechanism for this change is poorly characterized. The aim of this paper is to describe the mechanism for the increase in efficiency of the feedback synapse from horizontal cells to cones. We show that a train of flashes can increase the feedback response from the horizontal cells, as measured in the cones, up to threefold. This process has a time constant of ∼3 s and can be attributed to processes intrinsic to the cones. It does not require dopamine, is not the result of changes in the kinetics of the cone light response and is not due to changes in horizontal cells themselves. During a flash train, cones adapt to the mean light intensity, resulting in a slight (4 mV) depolarization of the cones. The time constant of this depolarization is ∼3 s. We will show that at this depolarized membrane potential, a light-induced change of the cone membrane potential induces a larger change in the calcium current than in the unadapted condition. Furthermore, we will show that negative feedback from horizontal cells to cones can modulate the calcium current more efficiently at this depolarized cone membrane potential. The change in horizontal cell response properties during the train of flashes can be fully attributed to these changes in the synaptic efficiency. Since feedback has major consequences for the dynamic, spatial, and spectral processing, the described mechanism might be very important to optimize the retina for ambient light conditions
Energy scan of the cross sections and evidence for decays into charged bottomonium-like states
Using data collected with the Belle detector at the KEKB asymmetric-energy
collider, we measure the energy dependence of the cross sections from thresholds up to GeV.
We find clear and peaks with little or no
continuum contribution. We study the resonant substructure of the
transitions and find evidence that they
proceed entirely via the intermediate isovector states and
. The relative fraction of these states is loosely constrained by
the current data: the hypothesis that only is produced is excluded
at the level of 3.3 standard deviations, while the hypothesis that only
is produced is not excluded at a significant level.Comment: 8 pages, 4 figures, submitted to Physical Review Letter
Search for a massive invisible particle in and decays
We present a search for a non-Standard-Model invisible particle in the
mass range in and
decays. The results are obtained from a data sample that corresponds to pairs,
collected at the resonance with the Belle detector at the KEKB
collider. One meson is fully reconstructed in a hadronic mode to
determine the momentum of the lepton of the signal decay in the rest frame of
the recoiling partner meson. We find no evidence of a signal and set upper
limits on the order of .Comment: 8 pages, 4 figures, 3 table
First Observation of Doubly Cabibbo-Suppressed Decay of a Charmed Baryon:
We report the first observation of the decay using a 980 data sample collected by the
Belle detector at the KEKB asymmetric-energy collider. This is the
first doubly Cabibbo-suppressed decay of a charmed baryon to be observed. We
measure the branching ratio of this decay with respect to its Cabibbo-favored
counterpart to be , where the uncertainties are
statistical and systematic, respectively.Comment: 6 pages, 3 figure
Versatile Coordination of Cyclopentadienyl-Arene Ligands and Its Role in Titanium-Catalyzed Ethylene Trimerization
Cationic titanium(IV) complexes with ansa-(η5-cyclopentadienyl,η6-arene) ligands were synthesized and characterized by X-ray crystallography. The strength of the metal-arene interaction in these systems was studied by variable-temperature NMR spectroscopy. Complexes with a C1 bridge between the cyclopentadienyl and arene moieties feature hemilabile coordination behavior of the ligand and consequently are active ethylene trimerization catalysts. Reaction of the titanium(IV) dimethyl cations with CO results in conversion to the analogous cationic titanium(II) dicarbonyl species. Metal-to-ligand backdonation in these formally low-valent complexes gives rise to a strongly bonded, partially reduced arene moiety. In contrast to the η6-arene coordination mode observed for titanium, the more electron-rich vanadium(V) cations [cyclopentadienyl-arene]V(NiPr2)(NC6H4-4-Me)+ feature η1-arene binding, as determined by a crystallographic study. The three different metal-arene coordination modes that we experimentally observed model intermediates in the cycle for titanium-catalyzed ethylene trimerization. The nature of the metal-arene interaction in these systems was studied by DFT calculations.
First model-independent Dalitz analysis of , decay
We report a measurement of the amplitude ratio of
and decays with a Dalitz analysis of decays, for the first time using a model-independent method.
We set an upper limit at the 68\% confidence level, using the full
data sample of pairs collected at the
resonance with the Belle detector at the KEKB collider. This result is
obtained from observables , , and , where , and
is the weak (strong) phase difference between and .Comment: arXiv admin note: substantial text overlap with arXiv:1502.0755
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