1,812 research outputs found
Cellular Dynamical Mean Field Theory of the Periodic Anderson Model
We develop a cluster dynamical mean field theory of the periodic Anderson
model in three dimensions, taking a cluster of two sites as a basic reference
frame. The mean field theory displays the basic features of the Doniach phase
diagram: a paramagnetic Fermi liquid state, an antiferromagnetic state and a
transition between them.
In contrast with spin density wave theories, the transition is accompanied by
a large increase of the effective mass everywhere on the Fermi surface and a
substantial change of the Fermi surface shape across the transition. To
understand the nature and the origin of the phases near the transition, we
investigate the paramagnetic solution underlying the antiferromagnetic state,
and identify the transition as a point where the electrons decouple from
the conduction electrons undergoing an orbitally selective Mott transition.
This point turns out to be intimately related to the two impurity Kondo model
quantum critical point. In this regime, non local correlations become important
and result in significant changes in the photoemission spectra and the de
Haas-van Alphen frequencies. The transition involves considerable spectral
weight transfer from the Fermi level to its immediate vicinity, rather than to
the Hubbard bands as in single site DMFT.Comment: 7 pages, 7 figure
T=0 heavy fermion quantum critical point as an orbital selective Mott transition
We describe the T=0 quantum phase transition in heavy fermion systems as an
orbital selective Mott transition (OSMT) using a cluster extension of dynamical
mean field theory. This transition is characterized by the emergence of a new
intermediate energy scale corresponding to the opening of a pseudogap and the
vanishing of the low-energy hybridization between light and heavy electrons. We
identify the fingerprint of Mott physics in heavy electron systems with the
appearance of surfaces in momentum space where the self-energy diverges and we
derive experimental consequences of this scenario for photoemission,
compressibility, optical conductivity, susceptibility and specific heat.Comment: 4 pages, 3 figures. Published versio
Non-Fermi liquid behavior in multi-orbital Anderson impurity models and possible relevance for strongly correlated lattice models
Spectral properties of a two-orbital Anderson impurity model across a non-Fermi liquid fixed point
We study by NRG the spectral properties of a two-orbital Anderson impurity
model in the presence of an exchange splitting which follows either regular or
inverted Hund's rules. The phase diagram contains a non-Fermi liquid fixed
point separating a screened phase, where conventional Kondo effect occurs, from
an unscreened one, where the exchange-splitting takes care of quenching the
impurity degrees of freedom. On the Kondo screened side close to this fixed
point the impurity density of states shows a narrow Kondo-peak on top of a
broader resonance. This narrow peak transforms in the unscreened phase into a
narrow pseudo-gap inside the broad resonance. Right at the fixed point only the
latter survives. The fixed point is therefore identified by a jump of the
density of states at the chemical potential. We also show that particle-hole
perturbations which simply shift the orbital energies do not wash out the fixed
point, unlike those perturbations which hybridize the two orbitals.
Consequently the density-of-state jump at the chemical potential remains finite
even away from particle-hole symmetry, and the pseudo-gap stays pinned at the
chemical potential, although it is partially filled in. We also discuss the
relevance of these results for lattice models which map onto this Anderson
impurity model in the limit of large lattice-coordination. Upon approaching the
Mott metal-insulator transition, these lattice models necessarily enter a
region with a local criticality which reflects the impurity non-Fermi liquid
fixed point. However, unlike the impurity, the lattice can get rid of the
single-impurity fixed-point instability by spontaneously developing
bulk-coherent symmetry-broken phases, which we identify for different lattice
models.Comment: 43 pages, 11 figures. Minor corrections in the Appendi
Thermodynamics of the three-dimensional Hubbard model: Implications for cooling cold atomic gases in optical lattices
We present a comprehensive study of the thermodynamic properties of the
three-dimensional fermionic Hubbard model, with application to cold fermionic
atoms subject to an optical lattice and a trapping potential. Our study is
focused on the temperature range of current experimental interest. We employ
two theoretical methods - dynamical mean-field theory and high-temperature
series - and perform comparative benchmarks to delimitate their respective
range of validity. Special attention is devoted to understand the implications
that thermodynamic properties of this system have on cooling. Considering the
distribution function of local occupancies in the inhomogeneous lattice, we
show that, under adiabatic evolution, the variation of any observable (e.g.,
temperature) can be conveniently disentangled into two distinct contributions.
The first contribution is due to the redistribution of atoms in the trap during
the evolution, while the second one comes from the intrinsic change of the
observable. Finally, we provide a simplified picture of the cooling procedure
recently proposed in J.-S. Bernier et al., Phys. Rev. A 79, 061601 (2009) by
applying this method to an idealized model.Comment: 17 pages, 27 figures, version published in PR
Inhibitors of lactate dehydrogenase (hLDH5) from Polygala flavescens subsp. flavescens
The human isoform 5 of lactate dehydrogenase (hLDH5) is an enzyme up-regulated in tumor tissues since cancer cells depend mainly on anaerobic respiration and their glycolytic rate is up to 200 times higher than that of the normal tissue. hLDH5 inhibition should cause cancer cell death by starvation, without interfering with healthy cells that normally use oxidative phosphorylation for ATP generation. Inhibition of LDH is so considered as a promising target in cancer treatment, since it is possible to cause a starving of cancerous cells by reducing glycolysis or by inhibiting the conversion of glucose to lactate. In the course of our research program on the hLDH5 inhibitory activity of natural products [1], a chemical study of P. flavescens subsp. flavescens was carried out. Polygala L. genus (Polygalaceae) is well known to contain phenolic oligosaccharides, xanthones, lignans, and triterpenic saponins and it’s largely used in the traditional medicine [2]. Ten new compounds were isolated from the n-BuOH residue of the aerial parts through Sephadex and RP-HPLC separations, including four flavonol glycosides, two oligosaccharides, one α-ionone, and three triterpenoidic saponins, together with two known oligosaccharides and two flavonol glycosides. All structures were elucidated on the basis of their spectroscopic and spectrometric data. The isolates were assayed for their inhibitory activity against hLDH5 and 3,6'-di-O-sinapoylsucrose showed an IC50 value of 90.4 µM. Modeling studies were carried out to suggest the putative interaction mode of this compound in the enzyme active site. This analysis highlighted that 3,6'-di-O-sinapoylsucrose shows a high number of H-bonds and interacts with enzyme regions rarely explored by the known hLDH5 inhibitors
Wireless Sensing for the Respiratory Activity of Human Beings: Measurements and Wide-band Numerical Analysis
An electromagnetic sensing system for the measurement of the respiratory activity is presented. The aims are to demonstrate the feasibility of the proposed approach and in particular to evaluate the effect on the measured signal of the distance between the subject and the sensing apparatus. Moreover, an electromagnetic model of the system, including the monitored subject, is proposed as a tool to solve the problem of selecting working parameters for system design. The sensing system is based on the measurement of the phase variation of the reflection coefficient caused by the respiratory activity. The phase signal compared with the thorax displacement measured by a reference instrument shows a high correlation () for different subject postures (sitting, standing, and lying) and a reduction of the signal amplitude with the distance dB/cm is reported. The numerical simulations performed on a wide-band highlight the frequencies where the method exhibits the highest sensitivity to thorax movements. The sensitivity can be further improved by reducing the antenna beamwidth. Despite the signal amplitude reduction, the proposed system makes it possible to correctly operate at distances up to 2.5 m
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