Germán R. Castro

“Charge Leakage” at LaMnO3/SrTiO3 Interfaces

We report on the charge transfer at the interface between a band (SrTiO3) and a Mott insulator (LaMnO3) in epitaxial superlattices. We have used combined atomic resolution electron microscopy and spectroscopy, synchrotron X ray reciprocal space maps and magneto transport measurements, to characterize the interface properties. The LaMnO3 layers are always started and terminated in (LaO) planes, giving an overall electron doping to the system. However, the direction of charge leakage is determined by the manganite to titanate thickness ratio in a way controlled by the different epitaxial strain patterns. This result may provide a clue to optimize oxide devices such as magnetic tunnel junctions and field effect transistors whose operation is determined by the interface properties.

Optical design of the general-purpose Spanish X-ray beamline for absorption and diffraction

This paper describes the optical design of the general-purpose Spanish beamline for absorption and diffraction (SpLine), which will be installed on a bending-magnet port of the ESRF. This beamline is planned to cover the needs of the Spanish synchroton radiation community with broad scientific fields of interest, covering physics, chemistry, materials science and biology. By using the advantages of a wide front end (9 mrad), the Spanish beamline will be split into two branches. Both branches will be equipped with focusing optics and experimental stations. Thus, each branch can be operated simultaneously and independently from each other. One branch (A) will have two experimental stations, one for high-resolution powder diffraction (HRPD) and the other for X-ray absorption spectroscopy (XAS) and X-ray standing waves (XSW). The other branch (B) will have facilities for macromolecular crystallography and for single-crystal diffraction analysis, including that of surfaces and interfaces, as well as an X-ray diffraction/scattering camera for non-crystalline specimens.

Structural and magnetic properties of ultrathin epitaxial La0.7Ca0.3MnO3 manganite films: Strain versus finite size effects

We present a structural and magnetic study of La0.7Ca0.3MnO3 epitaxial films (the thickness ranges from 2.4 to 80 nm) on SrTiO3. Their structure, as obtained by x-ray diffraction, is orthorhombic with a 45° rotated 2×2 square lattice, referred to the SrTiO3 substrate, in the film plane. The 2.4 nm film adopts a different structure as evidenced by the extinction of particular diffraction peaks and supported by the behavior of the size of the in-plane structural domains. While no important structural changes are detected for the other films as the layer thickness is reduced, Tc decreases drastically while the remanent magnetization is constant. Tc follows the functional dependence of a mean field approximation for the limitation of the divergence of the spin–spin correlations by the film thickness.

Does the rotational state of a molecule influence trapping in a precursor? An investigation of N2/W(100), CO/FeSi(100) and O2/Ni(111)

Abstract We have used seeded nozzle beams to obtain molecules of identical translational energy but quite different rotational energy to investigate the adsorption dynamics for precursor adsorption in the systems N 2 /W(100), CO/FeSi(100) and O 2 /Ni(111). The result of the investigation is unambiguous: there is no discernible influence of the rotational state on the sticking coefficient. This is true for an intrinsic as well as for an extrinsic precursor. This leads to the conclusion that rotational energy does not have to be accommodated immediately upon the first impact to facilitate trapping. Apparently the molecule can rotate more or less freely and the rotational energy can be accommodated subsequently to trapping.

Crystal Structures of Bacterial Peptidoglycan Amidase Ampd and an Unprecedented Activation Mechanism.

AmpD is a cytoplasmic peptidoglycan (PG) amidase involved in bacterial cell-wall recycling and in induction of β-lactamase, a key enzyme of β-lactam antibiotic resistance. AmpD belongs to the amidase_2 family that includes zinc-dependent amidases and the peptidoglycan-recognition proteins (PGRPs), highly conserved pattern-recognition molecules of the immune system. Crystal structures of Citrobacter freundii AmpD were solved in this study for the apoenzyme, for the holoenzyme at two different pH values, and for the complex with the reaction products, providing insights into the PG recognition and the catalytic process. These structures are significantly different compared with the previously reported NMR structure for the same protein. The NMR structure does not possess an accessible active site and shows the protein in what is proposed herein as an inactive “closed” conformation. The transition of the protein from this inactive conformation to the active “open” conformation, as seen in the x-ray structures, was studied by targeted molecular dynamics simulations, which revealed large conformational rearrangements (as much as 17 Å) in four specific regions representing one-third of the entire protein. It is proposed that the large conformational change that would take the inactive NMR structure to the active x-ray structure represents an unprecedented mechanism for activation of AmpD. Analysis is presented to argue that this activation mechanism might be representative of a regulatory process for other intracellular members of the bacterial amidase_2 family of enzymes.

Structure of the clean NiAl(1 1 0) surface and the Al2O3/NiAl(1 1 0) interface by measurements of crystal truncation rods

The clean NiAl(1 1 0) surface and the Al2O3/NiAl(1 1 0) interface have been investigated by synchrotron X-ray diffraction experiments. In the case of the oxide surface the analysis of the NiAl(1 1 0) crystal truncation rods (CTR) provide information about the interface between the Al2O3 film and the NiAl substrate. The analysis of the CTR-data shows clearly a rippled Ni-Al topmost surface with an amplitude value of RNi/Al = 0.16 ± 0.01 A for the clean surface and RNi/Al = 0.18 ± 0.02 A for the oxide covered surface. On the clean surface the Al sites are expanded by +3.8\% (outwards) and the Ni sites are contracted by -3.2\% (inwards) respect to the unrelaxed interlayer separation. For the oxide covered surface an increase of the expansion of the outermost Al atoms (+7.3\%) relative to their bulk positions has been found, while the Ni atoms remain (-0.9\%) at the bulk position. On both cases, an ideal surface stoichiometry (1:1) was obtained. However, some intermixing (chemical disorder) of one specimen in the sites of the other and vice versa was present (less than 4\%). This chemical disorder was not enhanced by the presence of the Al2O3 overlayer. Neither rippling nor oscillatory relaxation in deeper layers was detectable.

Properties of potassium adsorbed on Si(100)2×1

The properties of potassium adsorbed on a Si(100)2×1 surface were studied using work function change measurements, thermal desorption spectra of potassium, and coadsorbed xenon as well as ultraviolet photoemission spectroscopy measurements of clean and oxygen exposed K/Si surfaces at 300 and 55 K. The results strongly support the notion that potassium overlayers of ΘK≥0.5 exhibit metallic character.

Synthesis and Characterization of a Coupled Binuclear CuI/CuIII Complex

The synthesis through reaction of a C(alpha),C(ortho) dilithiated phosphazene with CuBr and structural characterization of the first example of a binuclear mixed valence [Cu(I)(N(2))/Cu(III)(C(4))] complex showing a metal-metal bond, as well as its applications in cyclopropanation and oxidation reactions, are described.

Formation of titanium monoxide (001) single-crystalline thin film induced by ion bombardment of titanium dioxide (110)

A plethora of technological applications justify why titanium dioxide is probably the most studied oxide, and an optimal exploitation of its properties quite frequently requires a controlled modification of the surface. Low-energy ion bombardment is one of the most extended techniques for this purpose and has been recently used in titanium oxides, among other applications, to favour resistive switching mechanisms or to form transparent conductive layers. Surfaces modified in this way are frequently described as reduced and defective, with a high density of oxygen vacancies. Here we show, at variance with this view, that high ion doses on rutile titanium dioxide (110) induce its transformation into a nanometric and single-crystalline titanium monoxide (001) thin film with rocksalt structure. The discovery of this ability may pave the way to new technical applications of ion bombardment not previously reported, which can be used to fabricate heterostructures and interfaces.

New Nanostructured Materials: Synthesis of Dodecanuclear NiII Complexes and Surface Deposition Studies

Microwave-assisted synthesis has been used to obtain the family of dodecanuclear Ni(II) complexes [Ni12(NO3)(MeO)12(MeC6H4CO2)9(MeOH)10(H2O)2][ClO4]2 (1), [Ni12(NO3)(MeO)12(BrC6H4CO2)9(MeOH)10(H2O)2][ClO4]2 (2), [Ni12(CO3)(MeO)12(MeC6H4CO2)9(MeOH)10(H2O)2]2[SO4] (3) and [Ni12(NO3)(MeO)12(MeC6H4CO2)9(MeOH)8(H2O)7][NO3]2 (4). They contain three {Ni4O4} cubane units which template around a central μ6 anion, either NO3(-) or CO3(2-). Their magnetic properties have been studied by superconducting quantum interference device (SQUID) magnetometry and high-field EPR measurements. The nanostructuration of the Ni12 species on mica surfaces is studied by AFM and grazing-incidence X-ray diffraction, which reveal the formation of polycrystalline thin layers.