759 research outputs found
Hysteresis and bi-stability by an interplay of calcium oscillations and action potential firing
Many cell types exhibit oscillatory activity, such as repetitive action
potential firing due to the Hodgkin-Huxley dynamics of ion channels in the cell
membrane or reveal intracellular inositol triphosphate (IP) mediated
calcium oscillations (CaOs) by calcium-induced calcium release channels
(IP-receptor) in the membrane of the endoplasmic reticulum (ER). The
dynamics of the excitable membrane and that of the IP-mediated CaOs have
been the subject of many studies. However, the interaction between the
excitable cell membrane and IP-mediated CaOs, which are coupled by
cytosolic calcium which affects the dynamics of both, has not been studied.
This study for the first time applied stability analysis to investigate the
dynamic behavior of a model, which includes both an excitable membrane and an
intracellular IP-mediated calcium oscillator. Taking the IP
concentration as a control parameter, the model exhibits a novel rich spectrum
of stable and unstable states with hysteresis. The four stable states of the
model correspond in detail to previously reported growth-state dependent states
of the membrane potential of normal rat kidney fibroblasts in cell culture. The
hysteresis is most pronounced for experimentally observed parameter values of
the model, suggesting a functional importance of hysteresis. This study shows
that the four growth-dependent cell states may not reflect the behavior of
cells that have differentiated into different cell types with different
properties, but simply reflect four different states of a single cell type,
that is characterized by a single model.Comment: 29 pages, 6 figure
Topological Phase Diagram of a Two-Subband Electron System
We present a phase diagram for a two-dimensional electron system with two
populated subbands. Using a gated GaAs/AlGaAs single quantum well, we have
mapped out the phases of various quantum Hall states in the density-magnetic
filed plane. The experimental phase diagram shows a very different topology
from the conventional Landau fan diagram. We find regions of negative
differential Hall resistance which are interpreted as preliminary evidence of
the long sought reentrant quantum Hall transitions. We discuss the origins of
the anomalous topology and the negative differential Hall resistance in terms
of the Landau level and subband mixing.Comment: 4 pages, 4 figure
Investigating the current knowledge and needs concerning a follow-up for long-term cardiovascular risks in Dutch women with a preeclampsia history:a qualitative study
Background There is increasing evidence that a history of preeclampsia is an important risk factor for future cardiovascular events. Awareness of this risk could provide opportunities for identification of women at risk, with opportunities for prevention and / or early intervention. A standardized follow-up has not yet been implemented in the north of the Netherlands. The objective of this qualitative study was to explore the opinions and wishes among women and physicians about the follow-up for women with a history of preeclampsia. Methods Semi-structured interviews with 15 women and 14 physicians (5 obstetricians, 4 general practitioners, 3 vascular medicine specialists and 2 cardiologists) were performed and addressed topics about knowledge on CVR, current - and future follow-up. Women were approached through the HELLP foundation and their physicians. Physicians were approached by email. The interviews were recorded, typed and coded using ATLAS.ti software. A theoretical-driven thematic analysis was performed. Results Women had some knowledge about the association between preeclampsia and the increased CVR, but missed information from their health care providers. Specialists were aware of the association, but the information and advice they provided to their patients was minimal and inconsistent according to themselves. Whereas some general practitioners regarded their own knowledge as limited. There was a clear desire among women for a more extensive follow-up with specific attention to both emotional and physical consequences of preeclampsia. Physicians indicated that they preferred to see a follow up program concerning the CVR at the general practitioner as part of the already existent cardiovascular risk management (CVRM) program. Conclusion Women and medical specialists consider it important to improve aftercare for women after a pregnancy complicated by preeclampsia. Introducing these women into the CVRM program at the general practitioner is regarded as a preferred first step. Further research is warranted to establish an evidence-based guideline for the follow-up of these women
Double Degeneracy and Jahn-Teller Effects in CMR Perovskites
Jahn-Teller (JT) electron-phonon coupling effects in the colossal
magnetoresistance perovskite compounds are investigated.
Electron-electron correlations between two degenerate Mn orbitals are
studied in the Gutzwiller approximation. The static JT distortion and
antiadiabatic polaron effects are studied in a modified Lang-Firsov
approximation. We find that (i) the electron or hole character of the charge
carrier depends on the static JT distortion, and (ii) due to the two-component
nature of the JT coupling, fluctuations in the JT distortion direction
contribute to the charge transport in similar fashion as the local spins.Comment: 11 RevTeX pages. 3 Figures available upon request. submitted to Phys.
rev. B (Rapid Communications
Charge Localization in Disordered Colossal-Magnetoresistance Manganites
The metallic or insulating nature of the paramagnetic phase of the
colossal-magnetoresistance manganites is investigated via a double exchange
Hamiltonian with diagonal disorder. Mobility edge trajectory is determined with
the transfer matrix method. Density of states calculations indicate that random
hopping alone is not sufficient to induce Anderson localization at the Fermi
level with 20-30% doping. We argue that the metal-insulator transtion is likely
due to the formation of localized polarons from nonuniform extended states as
the effective band width is reduced by random hoppings and electron-electron
interactions.Comment: 4 pages, RevTex. 4 Figures include
Double Exchange Alone Does Not Explain the Resistivity of
The system with has
traditionally been modelled with a ``double exchange'' Hamiltonian, in which it
is assumed that the only relevant physics is the tendency of carrier hopping to
line up neighboring spins. We present a solution of the double exchange model,
show it is incompatible with many aspects of the resistivity data, and propose
that a strong electron-phonon interaction arising from a Jahn-Teller splitting
of the outer Mn d-level plays a crucial role.Comment: Figure available via concentional mail. Contact
[email protected]
Monte Carlo Simulations for the Magnetic Phase Diagram of the Double Exchange Hamiltonian
We have used Monte Carlo simulation techniques to obtain the magnetic phase
diagram of the double exchange Hamiltonian. We have found that the Berry's
phase of the hopping amplitude has a negligible effect in the value of the
magnetic critical temperature. To avoid finite size problems in our simulations
we have also developed an approximated expression for the double exchange
energy. This allows us to obtain the critical temperature for the ferromagnetic
to paramagnetic transition more accurately. In our calculations we do not
observe any strange behavior in the kinetic energy, chemical potential or
electron density of states near the magnetic critical temperature. Therefore,
we conclude that other effects, not included in the double exchange
Hamiltonian, are needed to understand the metal-insulator transition which
occurs in the manganites.Comment: 6 pages Revtex, 8 PS figure
Ultrafast photoinduced reflectivity transients in
The temperature dependence of ultrafast photoinduced reflectivity transients
is reported in NdSrMnO thin film. The photoinduced
reflectivity shows a complex response with very different temperature
dependences on different timescales. The response on the sub-ps timescale
appears to be only weakly sensitive to the 270K-metal-insulator phase
transition. Below K the sub-ps response displays a two component
behavior indicating inhomogeneity of the film resulting from the substrate
induced strain. On the other hand, the slower response on the 10-100 ps
timescale is sensitive only to the metal-insulator phase transition and is in
agreement with some previously published results. The difference in the
temperature dependences of the responses on nanosecond and s timescales
indicates that thermal equilibrium between the different degrees of fredom is
established relatively slowly - on a nanosecond timescale
Impact of Charge Ordering on Magnetic Correlations in Perovskite (Bi,Ca)MnO_3
Single crystalline (Bi,Ca)MnO3 (74< %Ca <82) were studied with neutron
scattering, electron diffraction and bulk magnetic measurement. We discovered
dynamic ferromagnetic spin correlations at high temperatures, which are
replaced by antiferromagnetic spin fluctuations at a concomitant charge
ordering and structural transition. Our results indicate that thermal-activated
hopping of the Jahn-Teller active e_g electrons in these insulating materials,
nevertheless, induce ferromagnetic interaction through double-exchange
mechanism. It is the ordering of these charges competing with the
double-exchange ferromagnetic metallic state.Comment: 11 pages, 3 figures, Revte
Electron Correlation and Jahn-Teller Interaction in Manganese Oxides
The interplay between the electron repulsion and the Jahn-Teller
electron-phonon interation is studied with a large model for the
ferromagnetic state of the manganese oxides. These two interactions collaborate
to induce the local isospin (orbital) moments and reduce the bandwidth .
Especially the retardation effect of the Jahn-Teller phonon with the frequency
is effective to reduce , but the strong -dependence occurs
even when the Coulombic interaction is dominating () as long as
. The phonon spectrum consists of two components, i.e., the
temperature independent sharp peak at and that corresponding to the Kondo peak. These results
compared with the experiments suggest that in the metallic
manganese oxides.Comment: REVTE
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