637 research outputs found
Magnetization plateaux in the classical Shastry-Sutherland lattice
We investigated the classical Shastry-Sutherland lattice under an external
magnetic field in order to understand the recently discovered magnetization
plateaux in the rare-earth tetraborides compounds RB. A detailed study of
the role of thermal fluctuations was carried out by mean of classical spin
waves theory and Monte-Carlo simulations. Magnetization quasi-plateaux were
observed at 1/3 of the saturation magnetization at non zero temperature. We
showed that the existence of these quasi-plateaux is due to an entropic
selection of a particular collinear state. We also obtained a phase diagram
that shows the domains of existence of different spin configurations in the
magnetic field versus temperature plane.Comment: 4 pages, proceedings of HFM200
Influence of lattice distortions in classical spin systems
We investigate a simple model of a frustrated classical spin chain coupled to
adiabatic phonons under an external magnetic field. A thorough study of the
magnetization properties is carried out both numerically and analytically. We
show that already a moderate coupling with the lattice can stabilize a plateau
at 1/3 of the saturation and discuss the deformation of the underlying lattice
in this phase. We also study the transition to saturation where either a first
or second order transition can occur, depending on the couplings strength.Comment: Submitted to Phys. Rev.
Magnetization Process of the Classical Heisenberg Model on the Shastry-Sutherland Lattice
We investigate classical Heisenberg spins on the Shastry-Sutherland lattice
and under an external magnetic field. A detailed study is carried out both
analytically and numerically by means of classical Monte-Carlo simulations.
Magnetization pseudo-plateaux are observed around 1/3 of the saturation
magnetization for a range of values of the magnetic couplings. We show that the
existence of the pseudo-plateau is due to an entropic selection of a particular
collinear state. A phase diagram that shows the domains of existence of those
pseudo-plateaux in the plane is obtained.Comment: 9 pages, 11 figure
Magnetic-field effects on transport in carbon nanotube junctions
Here we address a theoretical study on the behaviour of electronic states of
heterojunctions and quantum dots based on carbon nanotubes under magnetic
fields. Emphasis is put on the analysis of the local density of states, the
conductance, and on the characteristic curves of current versus voltage. The
heterostructures are modeled by joining zigzag tubes through single
pentagon-heptagon pair defects, and described within a simple tight binding
calculation. The conductance is calculated using the Landauer formula in the
Green functions formalism. The used theoretical approach incorporates the
atomic details of the topological defects by performing an energy relaxation
via Monte Carlo calculation. The effect of a magnetic field on the conductance
gap of the system is investigated and compared to those of isolated constituent
tubes. It is found that the conductance gap of the studied CNHs exhibits
oscillations as a function of the magnetic flux. However, unlike the pristine
tubes case, they are not Aharonov-Bohm periodic oscillations
Negative differential resistance in molecular junctions: application to graphene ribbon junctions
Using self-consistent calculations based on Non-Equilibrium Green's Function
(NEGF) formalism, the origin of negative differential resistance (NDR) in
molecular junctions and quantum wires is investigated. Coupling of the molecule
to electrodes becomes asymmetric at high bias due to asymmetry between its
highest occupied molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) levels. This causes appearance of an asymmetric potential
profile due to a depletion of charge and reduction of screening near the source
electrode. With increasing bias, this sharp potential drop leads to an enhanced
localization of the HOMO and LUMO states in different parts of the system. The
reduction in overlap, caused by localization, results in a significant
reduction in the transmission coefficient and current with increasing bias. An
atomic chain connected to two Graphene ribbons was investigated to illustrate
these effects. For a chain substituting a molecule, an even-odd effect is also
observed in the NDR characteristics.Comment: 8 pages, 8 figure
Impact of Controlling the Site Distribution of Al Atoms on Catalytic Properties in Ferrierite-Type Zeolites
Zeolites with the ferrierite (FER) topology are synthesized using a combination of tetramethylammonium
(TMA) cations with differently sized cyclic amines (pyrrolidine (Pyr), hexamethyleneimine (HMI), and 1,4-
diazabicyclo[2.2.2]octane (DAB)). Using these organic structure-directing agents (SDAs), low Si/Al ratios
and concentrated synthesis mixtures favor the crystallization of FER materials. Increasing the size of the
cyclic amine or decreasing the aluminum content leads to the crystallization of other phases or the creation
of excessive amounts of connectivity defects. TMA cations play a decisive role in the synthesis of the FER
materials, and their presence allows the use of HMI to synthesize FER. Proton MAS NMR is used to quantify
the accessibility of pyridine to acid sites in these FER samples, where it is found that the FER + HMI + TMA
sample contains only 27% acid sites in the 8-MR channels, whereas FER + Pyr and FER + Pyr + TMA
contain 89% and 84%, respectively. The constraint index (CI) test and the carbonylation of dimethyl ether
(DME) with carbon monoxide are used as probe reactions to evaluate how changes in the aluminum distribution
in these FER samples affect their catalytic behavior. Results show that the use of Pyr as an SDA results in
the selective population of acid sites in the 8-MR channels, whereas the use of HMI generates FER zeolites
with an increased concentration of acid sites in the 10-MR channels
Surface plasmons at composite surfaces with diffusive charges
Metal surfaces with disorder or with nanostructure modifications are studied,
allowing for a localized charge layer (CL) in addition to continuous charges
(CC) in the bulk, both charges having a compressional or diffusive non-local
response. The notorious problem of "additional boundary conditions" is resolved
with the help of a Boltzmann equation that involves the scattering between the
two charge types. Depending on the strength of this scattering, the oscillating
charges can be dominantly CC or CL; the surface plasmon (SP) resonance acquires
then a relatively small linewidth, in agreement with a large set of data. With
a few parameters our model describes a large variety of SP dispersions
corresponding to observed data.Comment: 6 pages, 2 figure
On the Origin of the -4.4 eV Band in CdTe(100)"
We calculate the bulk- (infinite system), (100)-bulk-projected- and
(100)-Surface-projected Green's functions using the Surface Green's Function
Matching method (SGFM) and an empirical tight-binding hamiltonian with
tight-binding parameters (TBP) that describe well the bulk band structure of
CdTe. In particular, we analyze the band (B--4) arising at --4.4 eV from the
top of the valence band at according to the results of Niles and
H\"ochst and at -4.6 eV according to Gawlik {\it et al.} both obtained by
Angle-resolved photoelectron spectroscopy (ARPES). We give the first
theoretical description of this band.Comment: 17 pages, Rev-TEX, CIEA-Phys. 02/9
Neurohormonal activation induces intracellular iron deficiency and mitochondrial dysfunction in cardiac cells
Cèl·lula cardĂaca; Deficiència de ferro; ActivaciĂł neurohormonalCardiac cell; Iron deficiency; Neurohormonal activationCĂ©lula cardĂaca; Deficiencia de hierro; ActivaciĂłn neurohormonalBackground
Iron deficiency (ID) is common in patients with heart failure (HF) and is associated with poor outcomes, yet its role in the pathophysiology of HF is not well-defined. We sought to determine the consequences of HF neurohormonal activation in iron homeostasis and mitochondrial function in cardiac cells.
Methods
HF was induced in C57BL/6 mice by using isoproterenol osmotic pumps and embryonic rat heart-derived H9c2 cells were subsequently challenged with Angiotensin II and/or Norepinephrine. The expression of several genes and proteins related to intracellular iron metabolism were assessed by Real time-PCR and immunoblotting, respectively. The intracellular iron levels were also determined. Mitochondrial function was analyzed by studying the mitochondrial membrane potential, the accumulation of radical oxygen species (ROS) and the adenosine triphosphate (ATP) production.
Results
Hearts from isoproterenol-stimulated mice showed a decreased in both mRNA and protein levels of iron regulatory proteins, transferrin receptor 1, ferroportin 1 and hepcidin compared to control mice. Furthermore, mitoferrin 2 and mitochondrial ferritin were also downregulated in the hearts from HF mice. Similar data regarding these key iron regulatory molecules were found in the H9c2 cells challenged with neurohormonal stimuli. Accordingly, a depletion of intracellular iron levels was found in the stimulated cells compared to non-stimulated cells, as well as in the hearts from the isoproterenol-induced HF mice. Finally, neurohormonal activation impaired mitochondrial function as indicated by the accumulation of ROS, the impaired mitochondrial membrane potential and the decrease in the ATP levels in the cardiac cells.
Conclusions
HF characteristic neurohormonal activation induced changes in the regulation of key molecules involved in iron homeostasis, reduced intracellular iron levels and impaired mitochondrial function. The current results suggest that iron could be involved in the pathophysiology of HF.This work was funded by the following Grants: unrestricted grant from Vifor Pharma and Basic Research Competitive Grant in Cardiology from the Spanish Society of Cardiology 2015
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