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Search Results: 1 - 10 of 225 matches for " Vassilios Kapaklis "
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Nanolithographic templates using diblock copolymer films on chemically heterogeneous substrates
Vassilios Kapaklis,Spyridon Grammatikopoulos,Roman Sordan,Alessio Miranda,Floriano Traversi,Hans von K?nel,Dimitrios Trachylis,Panagiotis Poulopoulos,Constantin Politis
Physics , 2010, DOI: 10.1166/jnn.2010.2583
Abstract: The orientation of the lamellae formed by the phase separation of symmetric diblock copolymer thin films is strongly affected by the wetting properties of the polymer blocks with respect to the substrate. On bare silicon wafers the lamellae of polystyrene-b-polymethylmethacrylate thin films tend to order parallel to the wafer surface, with the polymethylmethacrylate block preferentially wetting silicon. We have developed a methodology for inducing the arrangement of lamellae perpendicular to the substrate by using chemically modified substrates. This is done by chemisorbing a self-assembled monolayer of thiol-terminated alkane chains on thin gold films deposited on silicon wafers. We also show that it is possible to spatially control the perpendicular orientation of the lamellae at sub-micron length scales by using simple chemical patterns and etch them, in order to produce nanolithographic templates. This method may be of great technological interest for the preparation of well-defined templates using block copolymer thin films.
Surface plasmons and magneto-optic activity in hexagonal Ni anti-dot arrays
Evangelos Th. Papaioannou,Vassilios Kapaklis,Emil Melander,Bj?rgvin Hj?rvarsson,Spiridon D. Pappas,Piotr Patoka,Michael Giersig,Paul Fumagalli,Antonio Garcia-Martin,Georgios Ctistis
Physics , 2011, DOI: 10.1364/OE.19.023867
Abstract: The influence of surface plasmons on the magneto-optic activity in a two-dimensional hexagonal array is addressed. The experiments were performed using hexagonal array of circular holes in a ferromagnetic Ni film. Well pronounced troughs are observed in the optical reflectivity, resulting from the presence of surface plasmons. The surface plasmons are found to strongly enhance the magneto-optic response (Kerr rotation), as compared to a continuous film of the same composition. The influence of the hexagonal symmetry of the pattern on the coupling between the plasmonic excitations is demonstrated, using optical diffraction measurements and theoretical calculations of the magneto-optic and of the angular dependence of the optical activity.
A new look on the two-dimensional Ising model: thermal artificial spins
Unnar B. Arnalds,Jonathan Chico,Henry Stopfel,Vassilios Kapaklis,Oliver B?renbold,Marc A. Verschuuren,Ulrike Wolff,Volker Neu,Anders Bergman,Bj?rgvin Hj?rvarsson
Physics , 2015,
Abstract: We present a direct experimental investigation of the thermal ordering in an artificial analogue of an asymmetric two dimensional Ising system composed of a rectangular array of nano-fabricated magnetostatically interacting islands. During fabrication and below a critical thickness of the magnetic material the islands are thermally fluctuating and thus the system is able to explore its phase space. Above the critical thickness the islands freeze-in resulting in an arrested thermalized state for the array. Determining the magnetic state of the array we demonstrate a genuine artificial two-dimensional Ising system which can be analyzed in the context of nearest neighbour interactions.
Light localisation and magneto-optic enhancement in Ni anti-dot arrays
Markus Rollinger,Philip Thielen,Emil Melander,Erik ?stman,Vassilios Kapaklis,Bj?rn Obry Mirko Cinchetti,A. Garcia-Martin,Martin Aeschlimann,Evangelos Th. Papaioannou
Physics , 2015,
Abstract: The excitation of surface plasmons in magnetic nano-structures can strongly influence their magneto-optical properties. Here, we use photoemission electron microscopy to map the spatial distribution of the electric near-field on a nano-patterned magnetic surface that supports plasmon polaritons. By using different photon energies and polarization states of the incident light we reveal that the electric near-field is either concentrated in spots forming a hexagonal lattice with the same symmetry as the Ni nano-pattern or in stripes oriented along the $\Gamma$-K direction of the lattice and perpendicular to the polarization direction. We show that the polarization-dependent near-field enhancement on the patterned surface is directly correlated to both the excitation of surface plasmon polaritons on the patterned surface as well as the enhancement of the polar magneto-optical Kerr effect.
Melting artificial spin ice
Vassilios Kapaklis,Unnar B. Arnalds,Adam Harman-Clarke,Evangelos Th. Papaioannou,Masoud Karimipour,Panagiotis Korelis,Andrea Taroni,Peter C. W. Holdsworth,Steven T. Bramwell,Bj?rgvin Hj?rvarsson
Physics , 2011, DOI: 10.1088/1367-2630/14/3/035009
Abstract: Artificial spin ice arrays of micromagnetic islands are a means of engineering additional energy scales and frustration into magnetic materials. Despite much progress in elucidating the properties of such arrays, the `spins' in the systems studied so far have no thermal dynamics as the kinetic constraints are too high. Here we address this problem by using a material with an ordering temperature near room temperature. By measuring the temperature dependent magnetization in different principal directions, and comparing with simulations of idealized statistical mechanical models, we confirm a dynamical `pre-melting' of the artificial spin ice structure at a temperature well below the intrinsic ordering temperature of the island material. We thus create a spin ice array that has real thermal dynamics of the artificial spins over an extended temperature range.
Influence of the magnetic field on the plasmonic properties of transparent Ni anti-dot arrays
Emil Melander,Erik ?stman,Janine Keller,Jan Schmidt,Evangelos Th. Papaioannou,Vassilios Kapaklis,Unnar B. Arnalds,B. Caballero,A. García-Martín,J. C. Cuevas,Bj?rgvin Hj?rvarsson
Physics , 2012, DOI: 10.1063/1.4742931
Abstract: Extraordinary optical transmission is observed due to the excitation of surface plasmon polaritons (SPPs) in 2-Dimensional hexagonal anti-dot patterns of pure Ni thin films, grown on sapphire substrates. A strong enhancement of the polar Kerr rotation is recorded at the surface plasmon related transmission maximum. Angular resolved reflectivity measurements under an applied field, reveal an enhancement and a shift of the normalized reflectivity difference upon reversal of the magnetic saturation (transverse magneto-optical Kerr effect-TMOKE). The change of the TMOKE signal clearly shows the magnetic field modulation of the dispersion relation of SPPs launched in a 2D patterned ferromagnetic Ni film.
The descriptive set-theoretic complexity of the set of points of continuity of a multi-valued function (Extended Abstract)
Vassilios Gregoriades
Electronic Proceedings in Theoretical Computer Science , 2010, DOI: 10.4204/eptcs.24.13
Abstract: In this article we treat a notion of continuity for a multi-valued function F and we compute the descriptive set-theoretic complexity of the set of all x for which F is continuous at x. We give conditions under which the latter set is either a G_delta set or the countable union of G_delta sets. Also we provide a counterexample which shows that the latter result is optimum under the same conditions. Moreover we prove that those conditions are necessary in order to obtain that the set of points of continuity of F is Borel i.e., we show that if we drop some of the previous conditions then there is a multi-valued function F whose graph is a Borel set and the set of points of continuity of F is not a Borel set. Finally we give some analogue results regarding a stronger notion of continuity for a multi-valued function. This article is motivated by a question of M. Ziegler in "Real Computation with Least Discrete Advice: A Complexity Theory of Nonuniform Computability with Applications to Linear Algebra", (submitted).
New Light on Early Christianity in Nubia: The Martyrium of Saint Athanasios of Clysma
Vassilios CHRISTIDES
Byzantina Symmeikta , 1994,
Abstract: No abstract
Atomism versus Holism in Science and Philosophy
Vassilios Karakostas
Physics , 2009,
Abstract: The pros and cons of various forms of atomism and holism that are applicable both in physical science and today's philosophy of nature are evaluated. To this end, Lewis' thesis of Humean supervenience is presented as an important case study of an atomistic doctrine in philosophical thought. According to the thesis of Humean supervenience, the world is fragmented into local matters of particular fact and everything else supervenes upon them in conjunction with the spatiotemporal relations among them. It is explicitly shown that Lewis' ontological doctrine of Humean supervenience incorporates at its foundation the so-called separability principle of classical physics. In view of the systematic violation of the latter within quantum mechanics, it is argued that contemporary physical science posits non-supervenient relations over and above the spatiotemporal ones. It is demonstrated that the relation of quantum entanglement constitutes the prototypical example of a holistic, irreducible physical relation that does not supervene upon a spatiotemporal arrangement of Humean qualities, undermining, thereby, the thesis of Humean supervenience. It is concluded, in this respect, that the assumption of ontological reductionism, as expressed in Lewis' Humean doctrine, cannot be regarded as a reliable code of the nature of the physical world and its contents. It is proposed instead that - due to the undeniable existence of generic non-supervenient relations - a metaphysic of relations of a moderate kind ought to be acknowledged as an indispensable part of our understanding of the natural world at a fundamental level.
Humean Supervenience in the Light of Contemporary Science
Vassilios Karakostas
Physics , 2009,
Abstract: It is shown that Lewis' ontological doctrine of Humean supervenience incorporates at its foundation the so-called separability principle of classical physics. In view of the systematic violation of the latter within quantum mechanics, the claim that contemporary physical science may posit non-supervenient relations beyond the spatiotemporal ones is reinforced on a foundational basis concerning constraints on the state-representation of physical systems. Depending on the mode of assignment of states to physical systems, unit state vectors versus statistical density operators, we distinguish between strongly and weakly non-Humean, non-supervenient relations. It is demonstrated that in either case the relations of quantum entanglement constitute prototypical examples of irreducible physical relations that do not supervene upon a spatiotemporal arrangement of Humean qualities, weakening, thereby, the thesis of Humean supervenience. It is examined, in this respect, the status of Lewis' recombination principle, whereas his conception of lawhood is critically investigated. It is concluded that the assumption of ontological reductionism, as expressed in Lewis' Humean doctrine, cannot be regarded as a reliable code of the nature of the physical world and its contents. It is proposed instead that, due to the undeniable existence of non-supervenient relations, a metaphysic of relations of a moderate kind ought to be acknowledged as an indispensable part of our understanding of the natural world at a fundamental level.
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