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  • J. Gazquez, M. Stengel, R. Mishra, M. Scigaj, M. Varela, M.A. Roldan, J. Fontcuberta, F. Sánchez, G. Herranz. “Competition between Polar and Nonpolar Lattice Distortions in Oxide Quantum Wells: New Critical Thickness at Polar Interfaces”. Physical Review Letters, 119, 106102 (2017). Here we addressed a long-standing question in the fundamental physics of perovskite ferroics: what is the critical thickness for a spontaneous lattice distortion to occur in a thin film? We explored whether nanoscale quantum confinement could be a general tool to switch, in an arbitrary perovskite film, the antiferrodistortive distortions (AFD) on and off at will, being AFD tilts the most common distortion of the Oxygen octahedra in perovskites. By combining scanning transmission electron microscopy and first-principles based models, we find a crossover between a bulk-like LaAlO3 structure to a strongly polar state with no AFD tilts at a thickness of approximately three-unit cells. The phenomena here described are completely general and applicable to a broad range of physical systems.

  • J. Gazquez, R. Guzman, R. Mishra, E. artolome, J. Salafranca, C. Magen, M. Varela, M. Coll, A. Palau, S.M. Valvidares, P. Gargiani, E. Pellegrin, J. Herrero-Martin, S. Pennycook, S.T. Pantelides, T. Puig and X. Obradors. “Emerging diluted ferromagnetism in high-Tc superconductors driven by point defect clusters”. Advanced Science, 1500295 (2016)”.  Here it is reported, for the first time, the existence of a dilute ferromagnetic system in the superconducting state of YBa2Cu3O7−δ (YBCO) cuprates below Tc. This work connects a dilute magnetic behavior, a macroscopic phenomenon, probed using XMCD measurements with atomic-scale structure and chemistry obtained using STEM imaging and spectroscopy, both of which are tied up through DFT calculations.

  • A. Llordés, G. Garcia, J. Gazquez, & D. J. Milliron. “Tunable near-infrared and visible-light transmittance in nanocrystal-in-glass composites”. Nature 500, 323–326 (2013). Here it is reported a new electrochromic material, made from nanocrystals of indium tin oxide embedded in a glassy matrix of niobium oxide. The resulting composite material combines two distinct functionalities—one providing control over visible light and the other, control over near infra-red. My task comprised the microstructural study of these nanocomposites, a key point for the performance of the material.

  • A. Carretero-Genevrier, M. Gich, L. Picas, J. Gazquez, G. L. Drisko, C. Boissiere, D. Grosso, J. Rodriguez-Carvajal, C. “Soft-Chemistry–Based Routes to Epitaxial a-Quartz Thin Films with Tunable Textures”. Science 340 (2013). This is the only reported growth of epitaxial quartz on silicon. The films are fabricated by thermal treatment of amorphous silica films prepared by chemical solution deposition. My task comprised the microstructural study of these Quartz films. In several other publications we have reported the specifics of the growth process.

  • C. Cantoni, J. Gazquez, F. Miletto Granozio, M. P. Oxley, M. Varela, A. R. Lupini, S. J. Pennycook, C. Aruta, U. Scotti di Uccio, P. Perna, D. Maccariello. "Electron transfer and ionic displacements at the origin of the 2D electron gas at the LAO/STO interface: Direct measurements with atomic-column spatial resolution”. Advanced Materials 24, 3952-3957 (2012). This work shows a direct interatomic distance measurement, which reveals clear distortions of the perovskite sublattices near the interface and within the LAO film, similarly to what had been theoretically predicted.

  • J. Salafranca*, J.Gazquez*, N. Pérez, A. Labarta, S.T. Pantelides, S.J. Pennycook, X. Batlle, M. Varela, “Surfactant Organic Molecules Restore Magnetism in Metal-Oxide Nanoparticle Surfaces”. Nano Letters 12, 2499-2503 (2012). In this publication it is reported, for the first time, a real-space magnetization map, using electron magnetic circular dichroism (EMCD) performed in nanodiffraction mode in the STEM, of nanoparticles one nanometer spatial resolution. Combining theory and experiments, it was found that the organic molecules attached to the nanoparticles, though not magnetically active, prevent the formation of a magnetically-inactive or dead layer at the surface.

  • J. Gazquez, W. Luo, S.T. Pantelides, M.A. Torija,M.Sharma, C. Leighton, M.P. Oxley, S.J. Pennycook and M. Varela. “Atomic-resolution imaging of spin-state superlattices in perovskite cobaltite films”, Nano Letters 11, 973-976 (2011). This is a result seminal in the field of magnetic materials microscopy: no technique had previously been able to produce maps of the magnetic spin state in real space atoms with atomic resolution in bulk materials. Unlike the bulk material, where no Co spin-state ordering is found, La0.5Sr0.5CoO3 (LSCO) thin films present a strain-induced domain structure due to oxygen vacancy ordering, inside of which nanometer sized domains show high-spin Co ions in the planes containing O vacancies, and low-spin Co ions in the stoichiometric planes.

  • A. Llordes, A. Palu, J.Gazquez, M. Coll, R. Vlad, A. Pomar, J. Arbiol, R. Guzman, S. Ye, V. Rouco, F. Sandiumenge, S. Ricart, T. Puig, M. Varela, D. Chateigner, J. Vanacken, J.Gutiérrez, V. Moshchalkov, G. Deutscher, C. Magen, and X. Obradors. “Nanoscale strain-induced pair suppression as a vortex-pinning mechanism in high-temperature superconductors”. Nature Materials 11 329-336 (2012). In this work it was suggested a new vortex-pinning mechanism based on the bond-contraction pairing model, where pair formation is quenched under tensile strain, forming new and effective core-pinning regions. Strain maps of high-resolution STEM images allowed me to quantify the lattice distortion built-up around local defects and confirming the presence of highly strained three-dimensional regions within the YBCO, which draws a new contribution to the vortex-pinning landscape of high temperature superconductors. These results have also been included in a Spanish patent.

  • M.A. Torija, M. Sharma, J. Gazquez, M. Varela, C. He, J. Schmitt, J.A. Borchers, M. Laver, S. El-Khatib and C. Leighton. “Chemically-driven nanoscopic magnetic phase separation at the SrTiO3(001)/La1 La0.5Sr0.5CoO3 interface”. Advanced Materials 23, 2711-2715 (2011). La0.5Sr0.5CoO3 (LSCO) thin films present deterioration in magnetism and conductivity in the near-interface region at compositions that are electronically homogeneous in bulk. In this publication, STEM/EELS demonstrated that magneto-electric phase separation occurs due to nanoscale hole doping inhomogeneity near the interface, due to depth-wise variations in Sr and O. We showed that spatially resolved measurements of quantities such as hole doping are possible.

  • J.Gazquez, F. Sandiumenge, M.Coll, A. Pomar, N. Mestres, T. Puig, X. Obradors, Y. Kihn, M.J. Casanove, C. Ballesteros “Precursor evolution and mechanism of YBa2Cu3Ox films by TFA metal-organic decomposition”. Chemistry of Materials 18, 6211 (2006). The superconducting properties of YBCO thin films strongly depend on their microstructure, which is intimately tied to their particular fabrication process and will determine their applicability and performance as coated conductors. In this publication it was reported, for the first time, the microstructural analysis, by means of X-Ray and TEM, of the conversion process of Y, Ba, and Cu TFA precursors to epitaxial YBCO films. This work allowed our group to properly tune the growth process and enhance the physical properties of the resulting YBCO thin films. This and other results on YBCO thin films during my PhD also appeared in a book’s chapter and in two Spanish patents.

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