528 research outputs found
Magnetic Moment Formation in Quantum Point Contacts
We study the formation of local magnetic moments in quantum point contacts.
Using a Hubbard-like model to describe point contacts formed in a two
dimensional system, we calculate the magnetic moment using the unrestricted
Hartree approximation. We analyze different type of potentials to define the
point contact, for a simple square potential we calculate a phase diagram in
the parameter space (Coulomb repulsion - gate voltage). We also present an
analytical calculation of the susceptibility to give explicit conditions for
the occurrence of a local moment, we present a simple scaling argument to
analyze how the stability of the magnetic moment depends on the point contact
dimensions.Comment: 7 pages, 2 figure
STM conductance of Kondo impurities on open and structured surfaces
We study the scanning tunneling microscopy response for magnetic atoms on
open and structured surfaces using Wilson's renormalization group. We observe
Fano resonances associated with Kondo resonances and interference effects. For
a magnetic atom in a quantum corral coupled to the confined surface states, and
experimentally relevant parameters, we observe a large confinement induced
effect not present in the experiments. These results suggest that the Kondo
screening is dominated by the bulk electrons rather than the surface ones.Comment: 6 pages, 6 figure
Magnetic Structure of Hydrogen Induced Defects on Graphene
Using density functional theory (DFT), Hartree-Fock, exact diagonalization,
and numerical renormalization group methods we study the electronic structure
of diluted hydrogen atoms chemisorbed on graphene. A comparison between DFT and
Hartree-Fock calculations allows us to identify the main characteristics of the
magnetic structure of the defect. We use this information to formulate an
Anderson-Hubbard model that captures the main physical ingredients of the
system, while still allowing a rigorous treatment of the electronic
correlations. We find that the large hydrogen-carbon hybridization puts the
structure of the defect half-way between the one corresponding to an adatom
weakly coupled to pristine graphene and a carbon vacancy. The impurity's
magnetic moment leaks into the graphene layer where the electronic correlations
on the C atoms play an important role in stabilizing the magnetic solution.
Finally, we discuss the implications for the Kondo effect.Comment: 10 pages, 10 fig
Protein kinase C isoenzymes in human neuroblasts involvement of PKCε in cell differentiation
AbstractAlthough neuronal cells are a major target of phorbol ester action, the activity of the various protein kinase C (PKC) isoenzymes have not been studied in detail in human neuroblasts. Differentiation of the LAN-5 human neuroblastoma cell line by interferon-γ (IFN-γ) is accompanied by a twofold increase in PKC activity. Since PKC is a multigene family, we investigated which isoforms were expressed in control and differentiated cells, and which of these isoenzymes is involved in neuronal differentiation. We found that: (1) PKC activity is higher in differentiated than in undifferentiated cells; (2) RT-PCR analysis showed the expression of mRNA for PKCα, -γ, -δ -ε and-ζ and the absence of mRNA for β in untreated LAN-5 cells; (3) Western blot evaluation with PKC isoform-specific antibodies showed the same pattern of PKC expression in non-differentiated cells; (4) Expression of PKCε mRNA was significantly enhanced by IFN-γ-induced differentiation, while the other isoforms were not affected; (5) Differentiation of LAN-5 cells with IFN-γ or retinoic acid induced overexpression of the PKCε protein, while inhibition of cell proliferation by fetal calf serum starvation was without effect. These findings suggest that expression of PKCε isoform is tightly coupled with neuronal differentiation and may play a role in the maintenance of the differentiated state
Quantum transport through a deformable molecular transistor
The linear transport properties of a model molecular transistor with
electron-electron and electron-phonon interactions were investigated
analytically and numerically. The model takes into account phonon modulation of
the electronic energy levels and of the tunnelling barrier between the molecule
and the electrodes. When both effects are present they lead to asymmetries in
the dependence of the conductance on gate voltage. The Kondo effect is observed
in the presence of electron-phonon interactions. There are important
qualitative differences between the cases of weak and strong coupling. In the
first case the standard Kondo effect driven by spin fluctuations occurs. In the
second case, it is driven by charge fluctuations. The Fermi-liquid relation
between the spectral density of the molecule and its charge is altered by
electron-phonon interactions. Remarkably, the relation between the
zero-temperature conductance and the charge remains unchanged. Therefore, there
is perfect transmission in all regimes whenever the average number of electrons
in the molecule is an odd integer.Comment: 9 pages, 6 figure
Rapid increase of resistance to erythromycin and clindamycin in Streptococcus pyogenes in Italy, 1993-1995. The Italian Surveillance Group for Antimicrobial Resistance.
A survey of antibiotic resistance in Streptococcus pyogenes in Italy showed a sharp increase in erythromycin resistance. In 1993, the incidence of erythromycin-resistant strains was on average 5.1%, with marked variations by geographic area. Two years later, the incidence of these strains had registered a 1.5- to roughly 20-fold increase, with a mean value of 25.9%, exceeding 40% in three centers out of 13 and 30% in another four. For all the strains studied, normal levels of susceptibility to penicillin were reported
Effective electron-electron and electron-phonon interactions in the Hubbard-Holstein model
We investigate the interplay between the electron-electron and the
electron-phonon interaction in the Hubbard-Holstein model. We implement the
flow-equation method to investigate within this model the effect of correlation
on the electron-phonon effective coupling and, conversely, the effect of
phonons in the effective electron-electron interaction. Using this technique we
obtain analytical momentum-dependent expressions for the effective couplings
and we study their behavior for different physical regimes. In agreement with
other works on this subject, we find that the electron-electron attraction
mediated by phonons in the presence of Hubbard repulsion is peaked at low
transferred momenta. The role of the characteristic energies involved is also
analyzed.Comment: 23 pages, 5 figure
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