137 research outputs found
Charge ordering in quarter-filled ladder systems coupled to the lattice
We investigate charge ordering in the presence of electron-phonon coupling
for quarter-filled ladder systems by using Exact Diagonalization. As an example
we consider NaV2O5 using model parameters obtained from first-principles
band-structure calculations. The relevant Holstein coupling to the lattice
considerably reduces the critical value of the nearest-neighbor Coulomb
repulsion at which formation of the zig-zag charge-ordered state occurs, which
is then accompanied by a static lattice distortion. Energy and length of a
kink-like excitation on the background of the distorted lattice are calculated.
Spin and charge spectra on ladders with and without static distortion are
obtained, and the charge gap and the effective spin-spin exchange parameter J
are extracted. J agrees well with experimental results. Analysis of the
dynamical Holstein model, restricted to a small number of phonons, shows that
low frequency lattice vibrations increase the charge order, accompanied by
dynamically produced zig-zag lattice distortions.Comment: 11 pages, 17 figures, revised version as to appear in Phys. Rev.
Optical properties, electron-phonon coupling, and Raman scattering of vanadium ladder compounds
The electronic structure of two V-based ladder compounds, the quarter-filled
NaVO in the symmetric phase and the iso-structural half-filled
CaVO is investigated by ab initio calculations. Based on the
bandstructure we determine the dielectric tensor of these
systems in a wide energy range. The frequencies and eigenvectors of the fully
symmetric A phonon modes and the corresponding electron-phonon and
spin-phonon coupling parameters are also calculated from first-principles. We
determine the Raman scattering intensities of the A phonon modes as a
function of polarization and frequency of the exciting light.
All results, i.e. shape and magnitude of the dielectric function, phonon
frequencies and Raman intensities show very good agreement with available
experimental data.Comment: 11 pages, 10 figure
Controlling frustrated magnetism on the kagome lattice by uniaxial-strain tuning
It is predicted that strongly interacting spins on a frustrated lattice may
lead to a quantum disordered ground state or even form a quantum spin liquid
with exotic low-energy excitations. However, a thorough tuning of the
frustration strength, separating its effects from those of disorder and other
factors, is pending. Here we break the symmetry of a kagome-lattice compound in
a controlled manner by applying uniaxial stress. The transition
temperature of is linearly enhanced with strain,
upon in-plane compression of order
, providing clear evidence for a release of frustration and its pivotal
role for magnetic order. Our comprehensive H NMR results suggest a
state under unstrained conditions and
further reveal an incomplete antiferromagnetic transition with fluctuating
moments in this strongly frustrated system.Comment: 7 pages total, 4 pages main text, 5 figure
Modeling of Diffusion and Incorporation of Interstitial Oxygen Ions at the TiN/SiO2 Interface
Silica-based resistive random access memory devices have become an active research area due to complementary metal-oxide-semiconductor compatibility and recent dramatic increases in their performance and endurance. In spite of both experimental and theoretical insights gained into the electroforming process, many atomistic aspects of the set and reset operation of these devices are still poorly understood. Recently a mechanism of electroforming process based on the formation of neutral oxygen vacancies (VO0) and interstitial O ions (Oi2-) facilitated by electron injection into the oxide has been proposed. In this work, we extend the description of the bulk (Oi2-) migration to the interface of amorphous SiO2 with the polycrystaline TiN electrode, using density functional theory simulations. The results demonstrate a strong kinetic and thermodynamic drive for the movement of Oi2- to the interface, with dramatically reduced incorporation energies and migration barriers close to the interface. The arrival of Oi2- at the interface is accompanied by preferential oxidation of undercoordinated Ti sites at the interface, forming a Ti-O layer. We investigate how O ions incorporate into a perfect and defective ∑5(012)[100] grain boundary (GB) in TiN oriented perpendicular to the interface. Our simulations demonstrate the preferential incorporation of Oi at defects within the TiN GB and their fast diffusion along a passivated grain boundary. They explain how, as a result of electroforming, the system undergoes very significant structural changes with the oxide being significantly reduced, interface being oxidized, and part of the oxygen leaving the system
MDR1 causes resistance to the antitumour drug miltefosine
Miltefosine (hexadecylphosphocholine) is used for topical treatment of breast cancers. It has been shown previously that a high percentage of breast carcinomas express MDR1 or MRP. We investigated the sensitivity of MDR1 -expressing cells to treatment with miltefosine. We show that cells overexpressing MDR1 (NCI/ADR-RES, KB-8-5, KB-C1, CCRF/VCR1000, CCRF/ADR5000) were less sensitive to miltefosine treatment when compared to the sensitive parental cell lines. HeLa cells transfected with MDR1 exhibited resistance to the compound, indicating that expression of this gene is sufficient to reduce the sensitivity to miltefosine. The resistance of MDR1 -expressing cells to miltefosine was less pronounced than that to adriamycin or vinblastine. Expression of MDR2 did not correlate with the resistance to miltefosine. As shown by a fluorescence quenching assay using MIANS-labelled P-glycoprotein (PGP), miltefosine bound to PGP with a K d of approximately 7 μM and inhibited PGP-ATPase activity with an IC 50 of approximately 35 μM. Verapamil was not able to reverse the resistance to miltefosine. Concentrations of miltefosine up to approximately 60 μM stimulated, whereas higher concentrations inhibited the transport of [3H]-colchicine with an IC 50 of approximately 297 μM. Binding studies indicated that miltefosine seems to interact with the transmembrane domain and not the cytosolic nucleotide-binding domain of PGP. These data indicate that expression of MDR1 may reduce the response to miltefosine in patients and that this compound interacts with PGP in a manner different from a number of other substrates. © 2001 Cancer Research Campaign www.bjcancer.co
Insect chemical ecology: chemically mediated interactions and novel applications in agriculture
Forum PaperInsect chemical ecology (ICE) evolved as a discipline concerned with plant–insect interactions, and also with a strong
focus on intraspecific pheromone-mediated communication. Progress in this field has rendered a more complete picture of
how insects exploit chemical information in their surroundings in order to survive and navigate their world successfully.
Simultaneously, this progress has prompted new research questions about the evolution of insect chemosensation and related
ecological adaptations, molecular mechanisms that mediate commonly observed behaviors, and the consequences of chemically
mediated interactions in different ecosystems. Themed meetings, workshops, and summer schools are ideal platforms
for discussing scientific advancements as well as identifying gaps and challenges within the discipline. From the 11th to the
22nd of June 2018, the 11th annual PhD course in ICE was held at the Swedish University of Agricultural Sciences (SLU)
Alnarp, Sweden. The course was made up of 35 student participants from 22 nationalities (Fig. 1a) as well as 32 lecturers.
Lectures and laboratory demonstrations were supported by literature seminars, and four broad research areas were covered:
(1) multitrophic interactions and plant defenses, (2) chemical communication focusing on odor sensing, processing, and
behavior, (3) disease vectors, and (4) applied aspects of basic ICE research in agriculture. This particular article contains a
summary and brief synthesis of these main emergent themes and discussions from the ICE 2018 course. In addition, we also
provide suggestions on teaching the next generation of ICE scientists, especially during unprecedented global situationsinfo:eu-repo/semantics/publishedVersio
Effects of pressure on the local atomic structure of CaWO4 and YLiF4: Mechanism of the scheelite-to-wolframite and scheelite-to-fergusonite transitions
The pressure response of the scheelite phase of CaWO4 (YLiF4) and the
occurrence of the pressure induced scheelite-to-wolframite (M-fergusonite)
transition are reviewed and discussed. It is shown that the change of the axial
parameters under compression is related with the different pressure dependence
of the W-O (Li-F) and Ca-O (Y-F) interatomic bonds. Phase transition mechanisms
for both compounds are proposed. Furthermore, a systematic study of the phase
transition in 16 different scheelite ABX4 compounds indicates that the
transition pressure increases as the packing ratio of the anionic BX4 units
around the A cations increases.Comment: 38 pages, 10 figures (Figure 5 corrected), accepted for publication
in Journal of Solid State Chemistr
A Dual Receptor Crosstalk Model of G-Protein-Coupled Signal Transduction
Macrophage cells that are stimulated by two different ligands that bind to G-protein-coupled receptors (GPCRs) usually respond as if the stimulus effects are additive, but for a minority of ligand combinations the response is synergistic. The G-protein-coupled receptor system integrates signaling cues from the environment to actuate cell morphology, gene expression, ion homeostasis, and other physiological states. We analyze the effects of the two signaling molecules complement factors 5a (C5a) and uridine diphosphate (UDP) on the intracellular second messenger calcium to elucidate the principles that govern the processing of multiple signals by GPCRs. We have developed a formal hypothesis, in the form of a kinetic model, for the mechanism of action of this GPCR signal transduction system using data obtained from RAW264.7 macrophage cells. Bayesian statistical methods are employed to represent uncertainty in both data and model parameters and formally tie the model to experimental data. When the model is also used as a tool in the design of experiments, it predicts a synergistic region in the calcium peak height dose response that results when cells are simultaneously stimulated by C5a and UDP. An analysis of the model reveals a potential mechanism for crosstalk between the Gαi-coupled C5a receptor and the Gαq-coupled UDP receptor signaling systems that results in synergistic calcium release
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