537 research outputs found
Nonvanishing cohomology and classes of Gorenstein rings
We give counterexamples to the following conjecture of Auslander: given a
finitely generated module over an Artin algebra , there exists a
positive integer such that for all finitely generated -modules
, if \Ext_{\Lambda}^i(M,N)=0 for all , then
\Ext_{\Lambda}^i(M,N)=0 for all . Some of our examples moreover
yield homologically defined classes of commutative local rings strictly between
the class of local complete intersections and the class of local Gorenstein
rings.Comment: 16 page
Electron momentum distribution of a single mobile hole in the t-J model
We investigate the electron momentum distribution function (EMDF) for the
two-dimensional t-J model. The results are based on the self-consistent Born
approximation (SCBA) for the self-energy and the wave function. In the Ising
limit of the model we give the results in a closed form, in the Heisenberg
limit the results are obtained numerically. An anomalous momentum dependence of
EMDF is found and the anomaly is in the lowest order in number of magnons
expressed analitycally. We interpret the anomaly as a fingerprint of an
emerging large Fermi surface coexisting with hole pockets.Comment: M2S - submitted to Physica
Kinetic and ion pairing contributions in the dielectric spectra of electrolyte aqueous solutions
Understanding dielectric spectra can reveal important information about the
dynamics of solvents and solutes from the dipolar relaxation times down to
electronic ones. In the late 1970s, Hubbard and Onsager predicted that adding
salt ions to a polar solution would result in a reduced dielectric permittivity
that arises from the unexpected tendency of solvent dipoles to align opposite
to the applied field. So far, this effect has escaped an experimental
verification, mainly because of the concomitant appearance of dielectric
saturation from which the Hubbard-Onsager decrement cannot be easily separated.
Here we develop a novel non-equilibrium molecular dynamics simulation approach
to determine this decrement accurately for the first time. Using a
thermodynamic consistent all-atom force field we show that for an aqueous
solution containing sodium chloride around 4.8 Mol/l, this effect accounts for
12\% of the total dielectric permittivity. The dielectric decrement can be
strikingly different if a less accurate force field for the ions is used. Using
the widespread GROMOS parameters, we observe in fact an {\it increment} of the
dielectric permittivity rather than a decrement. We can show that this
increment is caused by ion pairing, introduced by a too low dispersion force,
and clarify the microscopic connection between long-living ion pairs and the
appearance of specific features in the dielectric spectrum of the solution
Entanglement of two delocalised electrons
Several convenient formulae for the entanglement of two indistinguishable
delocalised spin-1/2 particles are introduced. This generalizes the standard
formula for concurrence, valid only in the limit of localised or
distinguishable particles. Several illustrative examples are given.Comment: 4 page
Synthesis, Conformational Studies and Enantioselective Homogeneous Catalytic Hydrogenation with CRC-PHOS, and Some Congeners
The lactone of (1S,3R)-1-hydroxy-1-diphenylphosphino metyl-
2,2,3-trimethylcyclopentan-3-carboxylic acid (8, CRC-PHOS),
and (1R,3R)-bis(diphenylphosphinoxymethyl)-2,2,3-trimethylcyclopentane
(16), were prepared starting from ( + )-camphanic and (-)-
isocamphoric acid, respectively. Their complex salts [Rh(norbornadiene)
lactone of (1S,3R)-1-hydroxy-1-diphenylphosphinomethyl-
2,2,3-trimethylcyclopentan-3-carboxylic acid] perchlorate (27), and
[Rh(norbornadiene (1R,3R)-1,2,2-trimethyl-1,3-bis ( diphenylphosphinoxymethyl)
cyclopentane)] perchlorate (28) were isolated and their
catalytic and enantioselective ability tested on some model prochiral
carboxylic acids. The asymetric bias did not exceed 35°/o e. e. in
either case. Attepmts at preparation of the diphosphine congener
of 16, i.e. 21, as well as isolation of the phosphinite congener of
8, i. e. 22, failed. NMR LIS study of the conformation in solution
of 8, and model compounds 6 and 9 revealed that 6 and 8 possess
in their most stable conformations a dihedral angle 1P of 165°,
(Figure 4.) while for 9 two stable conformations with 1P 200° and
350° are found. These results indicate that bidentate binding of
metal to heteroatom X (0, P) in the side chain, and to the tetrahedral
oxygen within lactone group is scarcely possible
Electron Spin Resonance of SrCu2(BO3)2 at High Magnetic Field
We calculate the electron spin resonance (ESR) spectra of the
quasi-two-dimensional dimer spin liquid SrCu2(BO3)2 as a function of magnetic
field B. Using the standard Lanczos method, we solve a Shastry-Sutherland
Hamiltonian with additional Dzyaloshinsky-Moriya (DM) terms which are crucial
to explain different qualitative aspects of the ESR spectra. In particular, a
nearest-neighbor DM interaction with a non-zero D_z component is required to
explain the low frequency ESR lines for B || c. This suggests that crystal
symmetry is lowered at low temperatures due to a structural phase transition.Comment: 4 pages, 4 b&w figure
Layer-by-layer formation of oligoelectrolyte multilayers: a combined experimental and computational study
For the first time, the combination of experimental preparation and results
of fully atomistic simulations of an oligoelectrolyte multilayer (OEM) made of
poly(diallyl dimethyl ammonium chloride)/poly(styrene sulfonate sodium salt)
(PDADMAC/PSS) is presented. The layer-by-layer growth was carried out by
dipping silica substrates in oligoelectrolyte solutions and was modeled by
means of atomistic molecular dynamics simulations with a protocol that mimics
the experimental procedure up to the assembly of four layers. Measurements of
OEM thickness, surface roughness and amount of adsorbed oligoelectrolyte chains
obtained from both approaches are compared. A good agreement between simulated
and experimental results was found, with some deviations due to intrinsic
limitations of both methods. However, the combination of information extracted
from simulations to support the analysis of experimental data can overcome such
restrictions and improve the interpretation of experimental results. On the
other hand, processes dominated by slower kinetics, like the destabilization of
adsorbed layers upon equilibration with the surrounding environment, are out of
reach for the simulation modeling approach, but they can be investigated by
monitoring in situ the oligoelectrolyte adsorption during the assembly process.
This demonstrates how the synergistic use of simulation and experiments
improves the knowledge of OEM properties down to the molecular scale
FePO(4)NPs Are an Efficient Nutritional Source for Plants: Combination of Nano-Material Properties and Metabolic Responses to Nutritional Deficiencies
Phosphorous and iron are a macro- and micronutrient, respectively, whose low bioavailability can negatively affect crop productivity. There is ample evidence that the use of conventional P and Fe fertilizers has several environmental and economical disadvantages, but even though great expectations surround nanotechnology and its applications in the field of plant nutrition, little is known about the mechanisms underlying the uptake and use of these sub-micron particles (nanoparticles, NPs) by crop species. This work shows that cucumber and maize plants both use the nutrients borne by FePO(4)NPs more efficiently than those supplied as bulk. However, morpho-physiological parameters and nutrient content analyses reveal that while cucumber plants (aStrategy Ispecies with regard to Fe acquisition) mainly use these NPs as a source of P, maize (aStrategy IIspecies) uses them preferentially for Fe. TEM analyses of cucumber root specimens revealed no cell internalization of the NPs. On the other hand, electron-dense nanometric structures were evident in proximity of the root epidermal cell walls of the NP-treated plants, which after ESEM/EDAX analyses can be reasonably identified as iron-oxyhydroxide. It appears that the nutritional interaction between roots and NPs is strongly influenced by species-specific metabolic responses
A Chiral Phosphine (CRC-PHOS) Derived from ( + )-Champhanic Acid. A New Ligand for Homogeneous Asymmetric Hydrogenation
The lactone of 1S,3S-1-hydroxy-1-diphenylphosphinomethyl-
2,2,3-tri-methyl-cyclopentan-3-carboxylic acid (4, CRC-PHOS) was
prepared from ( + )-camphanic acid methylester 1, and its complex
salt [Rh(CRC-PHOS)2NBD] · CI04 • THF (5) was isolated. The latter
exhibited rather a high rate, but low enantioselectivity in the first
attempt at asymmetric hydrogenation
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