National University of La Plata

About: National University of La Plata is a education organization based out in La Plata, Argentina. It is known for research contribution in the topics: Population & Stars. The organization has 12993 authors who have published 30013 publications receiving 495118 citations. The organization is also known as: UNLP & Universidad Nacional de La Plata.

Cationic exchange in nanosized Zn Fe 2 O 4 spinel revealed by experimental and simulated near-edge absorption structure

Abstract: The nonequilibrium cation site occupancy in nanosized zinc ferrites $(\ensuremath{\sim}6--13\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ with different degree of inversion ($\ensuremath{\sim}0.2$ to 0.4) was investigated using Fe and Zn $K$-edge x-ray absorption near edge spectroscopy (XANES) and extended x-ray absorption fine structure, and magnetic measurements. The very good agreement between experimental and ab initio calculations on the Zn $K$-edge XANES region clearly shows the large ${\mathrm{Zn}}^{2+}(\mathrm{A})\ensuremath{\rightarrow}{\mathrm{Zn}}^{2+}[\mathrm{B}]$ transference that takes place in addition to the well-identified ${\mathrm{Fe}}^{3+}[\mathrm{B}]\ensuremath{\rightarrow}{\mathrm{Fe}}^{3+}(\mathrm{A})$ one, without altering the long-range structural order. XANES spectra features as a function of the spinel inversion were shown to depend on the configuration of the ligand shells surrounding the absorbing atom. This XANES approach provides a direct way to sense cationic inversion in these Zn-containing spinel ferrites. We also demonstrated that a mechanical crystallization takes place on nanocrystalline spinel that causes an increase of both grain and magnetic sizes and, simultaneously, generates a significant augment of the inversion.

A New Chronology for Middle Eocene-Early Miocene South American Land Mammal Ages

Abstract: Cenozoic South American Land Mam- mal Ages (SALMAs) have historically been correlated to the geologic time scale using 40 Ar/ 39 Ar dating and magnetostratigraphy. At Gran Barranca (68.7°W, 45.7°S)—one of South America's key areas for constraining SALMAs—existing radioisotopic ages have uncertainties of up to 4 m.y. To better con- strain the ages of mammalian assemblages, we employed high-precision (±<40 k.y.) U-Pb dating using single zircon crystals. We dated nine tuffs from the Sarmiento Formation containing middle Eocene-early Miocene faunas (Barrancan, Mustersan, Tinguiriri- can, Deseadan, Colhuehuapian, and "Pin- turan"). The new dates span from 39.861 ± 0.037 Ma to 19.041 ± 0.027 Ma. The La Can- cha Tuff, occurring within the Tinguirirican faunal level yielded an age of 33.581 ± 0.015 Ma, confi rming that the Vera Member con- tains the only fossiliferous geologic section encompassing the Eocene-Oligocene transi- tion in the Southern Hemisphere. The pre- Deseadan fauna, La Cantera, is ≤30.77 Ma, the age of the Colhuehuapian is expanded to 21.1-20.1 Ma, and the Pinturan may be as old as ca. 19 Ma. The new U-Pb dates confi rm that at- mospheric temperatures and vegetation remained constant across the Eocene- Oligocene transition in Patagonia and that hypsodonty occurred in South American un- gulates much earlier than on any other conti- nent. Additionally, refi nement of the SALMA boundaries will eventually provide the con- text necessary to compare faunal transitions across continents, although currently too much data are missing to allow such compar- isons. Finally, the new ages provide a high- resolution age model from which hypotheses about rates of environmental and evolution- ary change at Gran Barranca can be tested.

Search for pair and single production of new heavy quarks that decay to a Z boson and a third-generation quark in pp collisions at √s=8 TeV with the ATLAS detector

Abstract: A search is presented for the production of new heavy quarks that decay to a Z boson and a third-generation Standard Model quark. In the case of a new charge +2/3 quark (T ), the decay targeted is T → Zt, while the decay targeted for a new charge −1/3 quark (B) is B → Zb. The search is performed with a dataset corresponding to 20.3 fb−1 of pp collisions at s√=8 TeV recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider. Selected events contain a high transverse momentum Z boson candidate reconstructed from a pair of oppositely charged same-flavor leptons (electrons or muons), and are analyzed in two channels defined by the absence or presence of a third lepton. Hadronic jets, in particular those with properties consistent with the decay of a b-hadron, are also required to be present in selected events. Different requirements are made on the jet activity in the event in order to enhance the sensitivity to either heavy quark pair production mediated by the strong interaction, or single production mediated by the electroweak interaction. No significant excess of events above the Standard Model expectation is observed, and lower limits are derived on the mass of vector-like T and B quarks under various branching ratio hypotheses, as well as upper limits on the magnitude of electroweak coupling parameters.