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Search Results: 1 - 10 of 461912 matches for " A. Lappas "
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Magnetic Anisotropy of SrCu$_2$(BO$_3$)$_2$ System as Revealed by X-Band ESR
A. Zorko,D. Ar?on,H. Kageyama,A. Lappas
Physics , 2003,
Abstract: X-band ESR measurements on a single crystal of the highly frustrated SrCu$_2$(BO$_3$)$_2$ system are shown to provide an essential inspection of the magnetic anisotropy present in this compound. The very broad absorption lines seem to be consistent with the largest anisotropy term, namely, the antisymmetric Dzyaloshinsky-Moriya (DM) interaction allowed by symmetry. However, the previously well-accepted model of only interdimer interaction is generalized with additional intradimer DM terms. Moreover, spin-phonon coupling is recognized as the cause on the linewidth broadening with increasing temperature.
Anti-Inflammatory Properties of Sirtuin 6 in Human Umbilical Vein Endothelial Cells
Martha Lappas
Mediators of Inflammation , 2012, DOI: 10.1155/2012/597514
Abstract: A prominent feature of inflammatory diseases is endothelial dysfunction. Factors associated with endothelial dysfunction include proinflammatory cytokines, adhesion molecules, and matrix degrading enzymes. At the transcriptional level, they are regulated by the histone deacetylase sirtuin (SIRT) 1 via its actions on the proinflammatory transcription factor nuclear factor- B (NF- B). The role of SIRT6, also a histone deacetylase, in regulating inflammation in endothelial cells is not known. The aim of this study was to determine the effect of SIRT6 knockdown on inflammatory markers in human umbilical vein endothelial cells (HUVECs) in the presence of lipopolysaccharide (LPS). LPS decreased expression of SIRT6 in HUVECs. Knockdown of SIRT6 increased the expression of proinflammatory cytokines (IL-1 , IL-6, IL-8), COX-prostaglandin system, ECM remodelling enzymes (MMP-2, MMP-9 and PAI-1), the adhesion molecule ICAM-1, and proangiogenic growth factors VEGF and FGF-2; cell migration; cell adhesion to leukocytes. Loss of SIRT6 increased the expression of NF- B, whereas overexpression of SIRT6 was associated with decreased NF- B transcriptional activity. Taken together, these results demonstrate that the loss of SIRT6 in endothelial cells is associated with upregulation of genes involved in inflammation, vascular remodelling, and angiogenesis. SIRT6 may be a potential pharmacological target for inflammatory vascular diseases. 1. Introduction Endothelial dysfunction is associated with vasoconstriction, a proinflammatory state, and prothrombotic properties. A number of diseases, including most forms of cardiovascular disease, diabetes, cancer, rheumatoid arthritis, and aging, are associated with endothelial dysfunction [1–3]. Endothelial dysfunction is also a characteristic of bacterial sepsis [4, 5]. To reduce or prevent the high incidence of morbidity and mortality associated with vascular complications, it is essential to understand further the mechanism by which inflammation regulates endothelial function. Systemic inflammation causes an upregulation of a wide number of factors that lead to severe injury of vascular endothelial cells [6–10]. These include growth factors (basic fibroblast growth factor (bFGF or FGF-2), vascular endothelial growth factor (VEGF)); proinflammatory cytokines (IL-1 , IL-6) and chemokines (IL-8); extracellular matrix (ECM) degrading enzymes (plasminogen activator inhibitor-1 (PAI-1), matrix metalloproteinases (MMPs)); cell adhesion molecules (intercellular adhesion molecule (ICAM-1), vascular cell adhesion protein (VCAM)-1,
On Flat Connections Induced Over Covering Maps
Dionyssios Lappas
Mathematics , 2003,
Abstract: Flat connections induced over covering maps are studied and the trivial ones among them are described. In the sequel, we deal with the resulting holonomy bundles.
Magnetic inhomogeneity on a triangular lattice: the magnetic-exchange versus the elastic energy and the role of disorder
A. Zorko,J. Kokalj,M. Komelj,O. Adamopoulos,H. Luetkens,D. Ar?on,A. Lappas
Physics , 2015, DOI: 10.1038/srep09272
Abstract: Inhomogeneity in the ground state is an intriguing, emergent phenomenon in magnetism. Recently, it has been observed in the magnetostructural channel of the geometrically frustrated $\alpha$-NaMnO$_2$, for the first time in the absence of active charge degrees of freedom. Here we report an in-depth numerical and local-probe experimental study of the isostructural sister compound CuMnO$_2$ that emphasizes and provides an explanation for the crucial differences between the two systems. The experimentally verified, much more homogeneous, ground state of the stoichiometric CuMnO$_2$ is attributed to the reduced magnetoelastic competition between the counteracting magnetic-exchange and elastic-energy contributions. The comparison of the two systems additionally highlights the role of disorder and allows an understanding of the puzzling phenomenon of phase separation in uniform antiferromagnets.
Electric and Magnetic Properties of Sputter Deposited BiFeO3 Films
N. Siadou,I. Panagiotopoulos,N. Kourkoumelis,T. Bakas,K. Brintakis,A. Lappas
Advances in Materials Science and Engineering , 2013, DOI: 10.1155/2013/857465
Abstract: Polycrystalline BiFeO3 films have been magnetron sputter deposited at room temperature and subsequently heat-treated ex situ at temperatures between 400 and 700°C. The deposition was done in pure Ar atmosphere, as the use of oxygen-argon mixture was found to lead to nonstoichiometric films due to resputtering effects. At a target-to-substrate distance the BiFeO3 structure can be obtained in larger range process gas pressures (2–7?mTorr) but the films do not show a specific texture. At codeposition from BiFeO3 and Bi2O3 has been used. Films sputtered at low rate tend to grow with the (001) texture of the pseudo-cubic BiFeO3 structure. As the film structure does not depend on epitaxy similar results are obtained on different substrates. A result of the volatility of Bi, Bi rich oxide phases occur after heat treatment at high temperatures. A Bi2SiO5 impurity phase forms on the substrate side, and does not affect the properties of the main phase. Despite the deposition on amorphous silicon oxide substrate weak ferromagnetism phenomena and displaced loops have been observed at low temperatures showing that their origin is not strain. Ba, La, Ca, and Sr doping suppress the formation of impurity phases and leakage currents. 1. Introduction Recently there is a revival of the interest in magnetoelectric materials for novel multifunctional devices [1] and spintronic [2] applications. BiFeO3 (BFO) is both ferroelectric ( ?K) and antiferromagnetic ( ?K) at room temperature, and thus it is very promising for such applications [3]. Pulsed laser deposition [4–6] and sputtering [7–10] are standard methods to deposit BFO films. Magnetron sputtering is a very reproducible and easily controlled deposition technique that can be used to prepare heterostructures combining different types of layers. However, when magnetrons are used to prepare binary oxide materials, severe resputtering of the film during deposition can alter its composition [11]. In order to find the conditions which lead to formation of the crystalline BFO phase sputtering at different substrate-to-target distances and process gas pressures have been tested. Here, we present a study of magnetron sputtered deposition of polycrystalline BiFeO3 films in view of their use as exchange-biasing layers. 2. Experimental Polycrystalline BFO and iron oxide films have been magnetron sputter deposited at room temperature using a MANTIS deposition system from a commercial BiFeO3 ( and ), Bi2O3 ( ), and Fe3O4 ( ) targets of Kurt J. Lesker. The substrates are placed opposite to the sputtering targets (on axis). Pure (5N)
Multiferroicity and hydrogen-bond ordering in (C2H5NH3)2CuCl4 featuring dominant ferromagnetic interactions
B. Kundys,A. Lappas,M. Viret,V. Kapustianyk,V. Rudyk,S. Semak,Ch. Simon,I. Bakaimi
Physics , 2010, DOI: 10.1103/PhysRevB.81.224434
Abstract: We demonstrate that ethylammonium copper chloride, (C2H5NH3)2CuCl4, a member of the hybrid perovskite family is an electrically polar and magnetic compound with dielectric anomaly around the Curie point (247 K). We have found large spontaneous electric polarization below this point accompanied with a color change in the sample. The system is also ferroelectric, with large remnant polarization (37{\mu}C/cm2) that is comparable to classical ferroelectric compounds. The results are ascribed to hydrogen-bond ordering of the organic chains. The coexistence of ferroelectricity and dominant ferromagnetic interactions allows to relate the sample to a rare group of magnetic multiferroic compounds. In such hybrid perovskites the underlying hydrogen bonding of easily tunable organic building blocks in combination with the 3d transition-metal layers offers an emerging pathway to engineer multifuctional multiferroics.
Embodied Mathematics and the Origins of Geometry
Dionyssios Lappas,Panayotis Spyrou
Mathematics , 2003,
Abstract: In this paper, we propose that 'embodied mathematics' should be studied not only by reduction to the present individual bodily experience but in an historical context as well, as far as the origins of mathematics are concerned. Some early mathematical results are the Theorems of Geometry and arose as attempts to objectively render the main perceptual categories such as verticality, horizontality, similarity (or its varieties). Inasmuch as these are of a qualitative nature, it was required that they be expressed in a quantitative way in order to be objectified. The first form of this objectification occurred in the case of 'archetypal results', namely the Pythagorean triads and the internal ratio of the legs in the right triangles. In the next stage, a 'scientific' treatment would come from a shift of objectification and descriptions inside an abstract theory, which would constitute the first logicomathematical knowledge. In this theory, the 'archetypal results' were incorporated, generalized and acquired their unquestionable, supertemporal validity. The study presents a particular epistemological analysis of some of the main terms used in the beginnings of Geometrical Thought and Euclid's Elements, utilizing the theoretical apparatus of the theory of 'embodied mathematics'. It also traces models of objectification for the 'archetypal results' and indicates their diffusion in later mathematical developments.
Assembly-mediated Interplay of Dipolar Interactions and Surface Spin Disorder in Colloidal Maghemite Nanoclusters
A. Kostopoulou,K. Brintakis,M. Vasilakaki,K. N. Trohidou,A. P. Douvalis,A. Lascialfari,L. Manna,A. Lappas
Physics , 2013, DOI: 10.1039/c3nr06103e
Abstract: Controlled assembly of single-crystal, colloidal maghemite nanoparticles is facilitated via a high-temperature polyol-based pathway. Structural characterization shows that size-tunable nanoclusters of 50 and 86 nm diameters (D), with high dispersibility in aqueous media, are composed of $\sim$ 13 nm (d) crystallographically oriented nanoparticles. The interaction effects are examined against the increasing volume fraction, $\phi$, of the inorganic magnetic phase that goes from individual colloidal nanoparticles ($\phi$= 0.47) to clusters ($\phi$= 0.72). The frozen-liquid dispersions of the latter exhibit weak ferrimagnetic behavior at 300 K. Comparative Mossbauer spectroscopic studies imply that intra-cluster interactions come into play. A new insight emerges from the clusters temperature-dependent ac susceptibility that displays two maxima in $\chi$''(T), with strong frequency dispersion. Scaling-law analysis, together with the observed memory effects suggest that a superspin glass state settles-in at T$_{B}$ $\sim$ 160-200 K, while at lower-temperatures, surface spin-glass freezing is established at T$_{f}$ $\sim$40- 70 K. In such nanoparticle-assembled systems, with increased $\phi$, Monte Carlo simulations corroborate the role of the inter-particle dipolar interactions and that of the constituent nanoparticles surface spin disorder in the emerging spin-glass dynamics.
Colloidal Assemblies of Oriented Maghemite Nanocrystals and their NMR Relaxometric Properties
A. Kostopoulou,S. K. P. Velu,K. Thangavel,F. Orsini,K. Brintakis,S. Psycharakis,A. Ranella,L. Bordonali,A. Lappas,A. Lascialfari
Physics , 2014,
Abstract: Elevated-temperature polyol-based colloidal-chemistry approach allows for the development of size-tunable (50 and 86 nm) assemblies of maghemite iso-oriented nanocrystals, with enhanced magnetization. 1H-Nuclear Magnetic Resonance (NMR) relaxometric experiments show that the ferrimagnetic cluster-like colloidal entities exhibit a remarkable enhancement (4 to 5 times) in the transverse relaxivity, if compared to that of the superparamagnetic contrast agent Endorem, over an extended frequency range (1-60 MHz). The marked increase of the transverse relaxivity r2 at a clinical magnetic field strength (1.41 T), which is 405.1 and 508.3 mM-1 s-1 for small and large assemblies respectively, allows to relate the observed response to the raised intra-aggregate magnetic material volume fraction. Furthermore, cell tests with murine fibroblast culture medium confirmed the cell viability in presence of the clusters. We discuss the NMR dispersion profiles on the basis of relaxivity models to highlight the magneto-structural characteristics of the materials for improved T2-weighted magnetic resonance images.
One-dimensional magnetic fluctuations in the spin-2 triangular lattice α-NaMnO2
C. Stock,L. C. Chapon,O. Adamopoulos,A. Lappas,M. Giot,J. W. Taylor,M. A. Green,C. M. Brown,P. G. Radaelli
Physics , 2009, DOI: 10.1103/PhysRevLett.103.077202
Abstract: The S=2 anisotropic triangular lattice alpha-NaMnO2 is studied by neutron inelastic scattering. Antiferromagnetic order occurs at T ~ 45 K with opening of a spin gap. The spectral weight of the magnetic dynamics above the gap (Delta ~ 7.5 meV) has been analysed by the single-mode approximation. Excellent agreement with the experiment is achieved when a dominant exchange interaction (|J|/k_B ~ 73 K), along the monoclinic b-axis and a sizeable easy-axis magnetic anisotropy (|D|/k_B ~ 3 K) are considered. Despite earlier suggestions for two-dimensional spin interactions, the dynamics illustrate strongly coupled antiferromagnetic S=2 chains and cancellation of the interchain exchange due to the lattice topology. alpha-NaMnO2 therefore represents a model system where the geometric frustration is resolved through the lowering of the dimensionality of the spin interactions.
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