925 research outputs found
Spectral signatures of the BCS-BEC crossover in the excitonic insulator phase of the extended Falicov-Kimball model
We explore the spontaneous formation of an excitonic insulator state at the
semimetal-semiconductor transition of mixed-valence materials in the framework
of the spinless Falicov-Kimball model with direct - electron hopping.
Adapting the projector-based renormalization method, we obtain a set of
renormalization differential equations for the extended Falicov-Kimball model
parameters and finally derive analytical expressions for the order parameter,
as well as for the renormalized - and -electron dispersions, momentum
distributions, and wave-vector resolved single-particle spectral functions. Our
numerical results proved the valence transition picture, related to the
appearance of the excitonic insulator phase, in the case of overlapping and
bands. Thereby the photoemission spectra show significant differences
between the weak-to-intermediate and intermediate-to-strong Coulomb attraction
regimes, indicating a BCS-BEC transition of the excitonic condensate.Comment: final version, minor corrections in the text, references update
Universal properties of Fermi gases in arbitrary dimensions
We consider spin-1/2 Fermi gases in arbitrary, integer or non-integer spatial
dimensions, interacting via a Dirac delta potential. We first generalize the
method of Tan's distributions and implement short-range boundary conditions to
arbitrary dimension and we obtain a set of universal relations for the Fermi
gas. Three-dimensional scattering under very general conditions of transversal
confinement is described by an effectively reduced-dimensional scattering
length, which we show depends on the three-dimensional scattering length in a
universal way. Our formula for non-integer dimensions interpolates between the
known results in integer dimensions 1, 2 and 3. Without any need to solve the
associated multichannel scattering problem, we find that confinement-induced
resonances occur in all dimensions different from D=2, while
reduced-dimensional contacts, related to the tails of the momentum
distributions, are connected to the three-dimensional contact by a correction
factor of purely geometric origin.Comment: 6 pages, 0 figure
Cumulant approach to weakly doped antiferromagnets
We present a new approach to static and dynamical properties of holes and
spins in weakly doped antiferromagnets in two dimensions. The calculations are
based on a recently introduced cumulant approach to ground--state properties of
correlated electronic systems. The present method allows to evaluate hole and
spin--wave dispersion relations by considering hole or spin excitations of the
ground state. Usually, these dispersions are found from time--dependent
correlation functions. To demonstrate the ability of the approach we first
derive the dispersion relation for the lowest single hole excitation at
half--filling. However, the main purpose of this paper is to focus on the
mutual influence of mobile holes and spin waves in the weakly doped system. It
is shown that low-energy spin excitations strongly admix to the ground--state.
The coupling of spin waves and holes leads to a strong suppression of the
staggered magnetization which can not be explained by a simple rigid--band
picture for the hole quasiparticles. Also the experimentally observed doping
dependence of the spin--wave excitation energies can be understood within our
formalism.Comment: REVTEX, 25 pages, 7 figures (EPS), to be published in Phys. Rev.
Trimethylsilyl tag for probing protein-ligand interactions by NMR
Protein-ligand titrations can readily be monitored with a trimethylsilyl (TMS) tag. Owing to the intensity, narrow line shape and unique chemical shift of a TMS group, dissociation constants can be determined from straightforward 1D 1H-NMR spectra not only in the fast but also in the slow exchange limit. The tag is easily attached to cysteine residues and a sensitive reporter of ligand binding also at sites where it does not interfere with ligand binding or catalytic efficiency of the target protein. Its utility is demonstrated for the Zika virus NS2B-NS3 protease and the human prolyl isomerase FK506 binding protein.C.N. and G.O. thank the Alexander von Humboldt Foundation for a
Feodor Lynen Fellowship and the Australian Research Council for a
Laureate Fellowship, respectively. Financial project support by the
Australian Research Council, the Austrian Science Fund (FWF) (DK
Molecular Enzymology W901 to K.Z.) and by NAWI Graz is gratefully
acknowledged
Traditional wisdom confirmed by scientific research: Jatropha species from Mexico is non-toxic
The seeds of _Jatropha platyphylla_, available on the pacific coast from Sinaloa to Michoacán including the Nayarit and Jalisco states in Mexico, are consumed by local communities after roasting them in a manner similar to that used for peanuts. The kernels and kernel meal of _J. platyphylla_ were found to be free of phorbol esters but the kernel meal contained antinutritional factors: trypsin inhibitor, lectins and phytate at levels similar to those in _J. curcas_ kernel meal. However, trypsin inhibitor and lectins are heat labile so this explains why the local people can eat roasted seeds without ill effect. The fatty acid profile of _J. platyphylla_ oil was similar to that of _J. curcas_ and the kernels of both these species contained _ca_ 58% oil. The crude protein content of _J. platyphylla_ kernel meal was 75% against 60% in _J. curcas_, and the amino acid compositions were similar. All the essential amino acids, except lysine, were more abundant than, or comparable to those, in soybean meal or the FAO Reference protein.

	Tilapia and other fish species have been found to be particularly sensitive to Jatropha toxins. For this reason they are useful subjects for bioassays. The heated _J. platyphylla_ kernel meal was included in a standard diet (crude protein 36%) for Nile tilapia (_Oreochromis niloticus_) to replace 50% of the fish meal protein. The fish grew as well as those on a control diet in which none of the fish meal was replaced and those on a diet in which 50% of the fish meal protein was replaced by soybean meal. In all the three groups, blood biochemical parameters that serve as biomarkers for toxicity were within the normal ranges. This study confirmed the non-toxic nature of _J. platyphylla_. 

	The kernels and kernel meal of this species could help alleviate protein and energy deficiency in malnourished children in the lower mountain skirts of pacific coastal areas of Mexico. It could also be used to produce new cultivars with desirable traits using conventional inter species cross breeding and molecular breeding techniques. 

Hole motion in an arbitrary spin background: Beyond the minimal spin-polaron approximation
The motion of a single hole in an arbitrary magnetic background is
investigated for the 2D t-J model. The wavefunction of the hole is described
within a generalized string picture which leads to a modified concept of spin
polarons. We calculate the one-hole spectral function using a large string
basis for the limits of a Neel ordered and a completely disordered background.
In addition we use a simple approximation to interpolate between these cases.
For the antiferromagnetic background we reproduce the well-known quasiparticle
band. In the disordered case the shape of the spectral function is found to be
strongly momentum-dependent, the quasiparticle weight vanishes for all hole
momenta. Finally, we discuss the relevance of results for the lowest energy
eigenvalue and its dispersion obtained from calculations using a polaron of
minimal size as found in the literature.Comment: 13 pages, 8 figures, to appear in Phys. Rev.
Impact of dislocations and dangling bond defects on the electrical performance of crystalline silicon thin films
A wide variety of liquid and solid phase crystallized silicon films are
investigated in order to determine the performance limiting defect types in
crystalline silicon thin-film solar cells. Complementary characterization
methods, such as electron spin resonance, photoluminescence, and electron
microscopy, yield the densities of dangling bond defects and dislocations
which are correlated with the electronic material quality in terms of solar
cell open circuit voltage. The results indicate that the strongly differing
performance of small-grained solid and large-grain liquid phase crystallized
silicon can be explained by intra-grain defects like dislocations rather than
grain boundary dangling bonds. A numerical model is developed containing both
defect types, dislocations and dangling bonds, describing the experimental
results
Neuroinflammatory TNFα Impairs Memory via Astrocyte Signaling.
The occurrence of cognitive disturbances upon CNS inflammation or infection has been correlated with increased levels of the cytokine tumor necrosis factor-α (TNFα). To date, however, no specific mechanism via which this cytokine could alter cognitive circuits has been demonstrated. Here, we show that local increase of TNFα in the hippocampal dentate gyrus activates astrocyte TNF receptor type 1 (TNFR1), which in turn triggers an astrocyte-neuron signaling cascade that results in persistent functional modification of hippocampal excitatory synapses. Astrocytic TNFR1 signaling is necessary for the hippocampal synaptic alteration and contextual learning-memory impairment observed in experimental autoimmune encephalitis (EAE), an animal model of multiple sclerosis (MS). This process may contribute to the pathogenesis of cognitive disturbances in MS, as well as in other CNS conditions accompanied by inflammatory states or infections
Fluxes in M-theory on 7-manifolds and G structures
We consider warp compactifications of M-theory on 7-manifolds in the presence
of 4-form fluxes and investigate the constraints imposed by supersymmetry. As
long as the 7-manifold supports only one Killing spinor we infer from the
Killing spinor equations that non-trivial 4-form fluxes will necessarily curve
the external 4-dimensional space. On the other hand, if the 7-manifold has at
least two Killing spinors, there is a non-trivial Killing vector yielding a
reduction of the 7-manifold to a 6-manifold and we confirm that 4-form fluxes
can be incorporated if one includes non-trivial SU(3) structures.Comment: 13 pages, Latex; minor changes & add reference
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