Spontaneous formation of patterns is a common occurrence in nature. From the Turing patterns during chemical reactions to the wavy stripes of sand dunes, one can find a plethora of examples with almost periodic shapes and forms. In addition to their aesthetically pleasant regularity, such patterns usually appear at length scales surprisingly larger than their building blocks. In this study we illustrate the formation of Pd adlayer patterns on W(110) induced by the elastic interaction between adlayer steps.

T.O. Menteş, A. Locatelli, L. Aballe, and E. Bauer;
Phys. Rev. Lett. 101, 085701 (2008).
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Magnetic tunnel junctions (MTJ) are probably one of the most studied devices in the so called spintronics research field. Their active part is composed of two ferromagnetic electrodes separated by a thin insulating layer (0,5 to 3 nm) that a tunnel current exist between the two electrodes. The tunnel conduction depends on the density of states in each electrodes, and therefore for a ferromagnetic material on the spin. This leads to the most attractive property of MTJs: their electrical resistance varies strongly with the magnetic configuration of the electrodes. Combining high resolution magnetic imaging at Elettra and micromagnetic simulations, we have demonstrated the strong influence of a dipolar magnetic coupling on the magnetization reversal of MTJs.

M. Hehn, D. Lacour, F. Montaigne, J. Briones, R. Belkhou, S. El Moussaoui, F. Maccherozzi, and N. Rougemaille;
Appl. Phys. Lett. 92, 072501 (2008).
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The synthesis of semiconductor nanostructures and their organization into functional macroassemblies remain a fundamental challenge. In the Ge/Si model heteroepitaxial system, metal patterns on Si allow for remarkable control over the morphology and organization of Ge islands. For the case of a Si surface that is decorated with a square pattern of Au dots, Ge islands grow into a highly ordered, square array. Central to the ordering process is the existence of a barrier to Ge diffusion and an island-free region, or denuded zone, surrounding each Au site. Our in-situ spectromicroscopy studies at the Nanospectroscopy beamline of the Elettra synchrotron have uncovered the origin of this remarkable ordering process.

J.T. Robinson, F. Ratto, O. Moutanabbir, S. Heun, A. Locatelli, T.O. Menteş, L. Aballe, and O. D. Dubon;
Nano lett. 7, 2655-2659 (2007).
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Well-ordered semiconductor nanostructures offer a great technological potential for the application in electronic and opto-electronic devices. Especially, nanoscale three-dimensional (3D) islands, often referred to as quantum dots, are of high interest. These islands generally do not exhibit an ordered arrangement and, thus, show a broad size distribution. In order to obtain regularly arranged nanoscale three-dimensional (3D) islands of more uniform size, different approaches based on self-organization are possible. Methods using surface strain modulation by SiGe buffer layers, unfortunately, reduce the choice of the composition of nanoislands and surrounding matrix, and hence, affect the flexibility for bandgap engineering. Here, we present a novel technique, employing a chemical surface modulation induced by partial passivation of the substrate with gallium.

Th. Schmidt, J.I. Flege, S. Gangopadhyay, T. Clausen, A. Locatelli, S. Heun, and J. Falta;
Phys. Rev. Lett., 98, 066104 (2007).
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The ferromagnetic hexagonal α-phase of MnAs has a phase transition to the non-ferromagnetic orthorhombic β-phase at 318 K. The nature of magnetism in this phase has been the subject of many experimental and theoretical studies. The most recent theoretical work predicts either an antiferromagnetic or paramagnetic state. We have addressed this problem also from the experimental side, using epitaxial MnAs layers on GaAs. MnAs layers on GaAs are highly strained, which broadens the phase transition over a range of about 30 K. In this range α- and β-phase coexist. This allows studying both phases by combining XMLD- and XMCD-PEEM to determine the magnetic state of the β-phase and its interrelation with the ferromagnetic α-phase.

E. Bauer, R. Belkhou, S. Cherifi, A. Locatelli, A. Pavlovska, and N. Rougemaille;
J. Vac. Sci. Technol. B 25, 1470 (2007).
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Spinodal decomposition of homogeneous mixtures provides an attractive route to the fabrication of ordered micro- and nano-structures. This method exploits the minimization of the system total free energy to obtain phase separation of its elemental components, and is accompanied by the development of regular periodic patterns. In the case of a reactive mixture, a chemical reaction can provide a means to control the structure wavelength and size by effectively freezing the phase separation. Surprisingly, these concepts hold for very different systems, e.g. polymer mixtures and adsorbates on transition metal catalysts during surface chemical reactions.

A. Locatelli, T.O. Menteş, L. Aballe, A.S. Mikhailov, M. Kiskinova;
J. Phys. Chem. B. (Letter) 110, 19108-19111 (2006).
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The ability to grow one-dimensional Au on the thoroughly studied TiO₂ is highly desirable, as this system can significantly contribute understanding the high catalytic activity of finely dispersed Au on oxide supports in a variety of reactions. Here we report an experimental and theoretical study of Au on TiO₂, indicating a novel pathway to creation of one-dimensional structures by Au deposition on oxygen vacancy rows. Photon stimulated desorption (PSD) was used to produce ordered vacancies by removing the most weakly bound oxygen atoms (i.e. the bridge-bonded atoms). Temperature was kept sufficiently low so that no significant Ti and O exchange between surface and volume occurs, but repulsive interactions between vacancies could lead to long range ordering resulting in a (1x2) structure.

A. Locatelli, T. Pabisiak, A. Pavlovska, T.O. Menteş, L. Aballe, A. Kiejna and E. Bauer;
J. Phys.: Condens. Matter 19, 082202 (2007).
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T.O. Menteş, A. Locatelli, L. Aballe, A. Pavlovska, E. Bauer, T. Pabisiak and A. Kiejna;
Phys. Rev. B 76, 155413 (2007).
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The chemical reactivity of surfaces is a crucial property of many technological materials. It determines the chemisorption of molecules and atoms, the first reaction step playing a decisive role in their interaction with gases. A distinct feature of commonly used catalysts, protective coatings and gas sensors is that they are composed of nano-particles or thin films on metal, semiconductor or oxide supports, i.e. materials with strongly reduced dimensions in one or more directions. These materials typically present a chemical activity strongly modified with respect to the bulk parent. Our study aims at a quantitative understanding of the effect of electron confinement on surface reactivity in ultrathin metal films.

L. Aballe, A. Barinov, A. Locatelli, S. Heun, and M. Kiskinova;
Phys. Rev. Lett. 93, 196103 (2004).
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Heterogeneous catalytic reactions occurring at solid surfaces involve complex chemical interactions which lead to changes in the surface structure and to mass transport phenomena. Such systems often show oscillatory behavior and spatio-temporal pattern formation and have become a field of very active research. The development of laterally inhomogeneous interfaces usually affects the adspecies, even if they are not involved in the reaction, and may determine massive surface reconstruction/de-reconstruction processes.

A. Locatelli, C. Sbraccia, S. Heun, S. Baroni, M. Kiskinova;
J. Am. Chem. Soc. 127, 2351-2357 (2005).
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A. Locatelli, S. Heun, M. Kiskinova;
Surf. Sci. 566-568, 1130-1136 (2004).
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