Lattice Distortions Around a Tl+ Impurity in NaI:Tl+ and CsI:Tl+ Scintillators. An Ab Initio Study Involving Large Active Clusters

Ab initio Perturbed Ion cluster-in-the-lattice calculations of the impurity centers NaI:Tl+ and CsI:Tl+ are pressented. We study several active clusters of increasing complexity and show that the lattice relaxation around the Tl+ impurity implies the concerted movement of several shells of neighbors. The results also reveal the importance of considering a set of ions that can respond to the geometrical displacements of the inner shells by adapting selfconsistently their wave functions. Comparison with other calculations involving comparatively small active clusters serves to assert the significance of our conclusions. Contact with experiment is made by calculating absorption energies. These are in excellent agreement with the experimental data for the most realistic active clusters considered.Comment: 7 pages plus 6 postscript figures, LaTeX. Submmited to Phys, Rev.

Artificial chemical and magnetic structure at the domain walls of an epitaxial oxide

Progress in nanotechnology requires new approaches to materials synthesis that make it possible to control material functionality down to the smallest scales. An objective of materials research is to achieve enhanced control over the physical properties of materials such as ferromagnets1, ferroelectrics2 and superconductors3. In this context, complex oxides and inorganic perovskites are attractive because slight adjustments of their atomic structures can produce large physical responses and result in multiple functionalities4,5. In addition, these materials often contain ferroelastic domains6. The intrinsic symmetry breaking that takes place at the domain walls can induce properties absent from the domains themselves7, such as magnetic or ferroelectric order and other functionalities, as well as coupling between them. Moreover, large domain wall densities create intense strain gradients, which can also affect the material’s properties8,9. Here we show that, owing to large local stresses, domain walls can promote the formation of unusual phases. In this sense, the domain walls can function as nanoscale chemical reactors. We synthesize a two-dimensional ferromagnetic phase at the domain walls of the orthorhombic perovskite terbium manganite (TbMnO3), which was grown in thin layers under epitaxial strain on strontium titanate (SrTiO3) substrates. This phase is yet to be created by standard chemical routes. The density of the two-dimensional sheets can be tuned by changing the film thickness or the substrate lattice parameter (that is, the epitaxial strain), and the distance between sheets can be made as small as 5 nanometres in ultrathin films10, such that the new phase at domain walls represents up to 25 per cent of the film volume. The general concept of using domain walls of epitaxial oxides to promote the formation of unusual phases may be applicable to other materials systems, thus giving access to new classes of nanoscale materials for applications in nanoelectronics and spintronics.Fil: Farokhipoor, S.. University of Groningen; Países BajosFil: Magén, C.. Universidad de Zaragoza; EspañaFil: Venkatesan, S.. Technische Universitat Munchen; Alemania. Universite de Tours; Francia. Gerencia de Investigación y Aplicaciones; Argentina. Instituto de Nanociencias y Nanotecnología; ArgentinaFil: Íñiguez, J.. Consejo Superior de Investigaciones Científicas; EspañaFil: Daumont, C.J.M.. University of Groningen; Países Bajos. Universite de Tours; Francia. Gerencia de Investigación y Aplicaciones; Argentina. Instituto de Nanociencias y Nanotecnología; ArgentinaFil: Rubi, Diego. University of Groningen; Países Bajos. Universite de Tours; Francia. Gerencia de Investigación y Aplicaciones; Argentina. Instituto de Nanociencias y Nanotecnología; ArgentinaFil: Snoeck, E.. Universidad de Zaragoza; EspañaFil: Mostovoy, M.. University of Groningen; Países BajosFil: De Graaf, C.. University of Groningen; Países Bajos. Universitat Rovira I Virgili; España. Institució Catalana de Recerca i Estudis Avançats. Barcelona; EspañaFil: Müller, A.. Technische Universitat Munchen; Alemania. Universite de Tours; Francia. Gerencia de Investigación y Aplicaciones; Argentina. Instituto de Nanociencias y Nanotecnología; ArgentinaFil: Döblinger, M.. Technische Universitat Munchen; AlemaniaFil: Scheu, C.. Technische Universitat Munchen; Alemania. Universite de Tours; Francia. Gerencia de Investigación y Aplicaciones; Argentina. Instituto de Nanociencias y Nanotecnología; ArgentinaFil: Noheda, B.. University of Groningen; Países Bajo