242 research outputs found
Estrutura atômica e eletrônica em niquelatos (TRNiO3) através de espectroscopia de níveis profundos
Orientador: Helio Cesar Nogueira TolentinoTese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb WataghinResumo: Neste trabalho de tese estudamos a estrutura atômica e eletrônica de sistemas de perovskitas de Ni com terras-raras (TRNiO3) utilizando a espectroscopia de absorção de níveis profundos. A fonte de luz síncrotron foi explorada na região de raios X moles para o estudo das bordas LIII e LII do Ni, e na região de raios X duros para o estudo da borda K do Ni e LIII dos terras raras. A propriedade mais notável nesses sistemas é a ocorrência de uma transição metal-isolante ao variarmos a temperatura da amostra. Essa tem-peratura de transição depende do íon terra-rara, e aumenta ao reduzirmos o tamanho do mesmo. Esses sistemas possuem uma estrutura perovskita distorcida, onde os octaedros NiO6 giram para preencher o espaço em torno do íon terra-rara. Quanto menor o íon, maior a distorção. Observamos assim uma estreita correlação entre a distorção da rede e a transição metal-isolante.Para sistemas com TR variando desde Pr ao Gd foi encontrada uma estrutura cristalográfica de simetria ortorômbica, onde o Ni ocupa um sítio simétrico.Baseado nisto, um primeiro modelo proposto sugeria que a transição seria devido ao estreitamento da banda formada pelos orbitais hibridizados Ni-3d e O-2p, quando o ângulo Ni-O-Ni diminui. No entanto, modificações na temperatura de transição metal-isolante pela substituição isotópica do O mostraram que o acoplamento elétron-fônon deveria ser importante. De fato, para sistemas com íons TR menores (de Ho a Lu) foi encontrada uma distorção monoclínica na fase isolante desses sistemas. Em tal estrutura o Ni ocupa sítios distintos com distâncias Ni-O diferentes. Um ordenamento de cargas nesses sítios distintos explica a fase isolante, bem como o ordenamento antiferromagnético observado para alguns dos compostos. Nossos resultados mostram que essas duas distâncias Ni-O coexistem para todos os sistemas, independentemente da estrutura cristalográfica de longo alcance, e em ambas fases eletrônicas. O sítio maior, de fraca hibridização, é o responsável pela localização eletrônica e coexiste com uma matriz condutora de forte hibridização. A transição metal isolante é explicada pelas modificações na proporção entre esses dois sítios. Dentro deste contexto, um forte acoplamento dos elétrons de condução com a rede é esperado, bem como a supressão da fase isolante sob pressãoAbstract: In this work, we studied the atomic and electronic structure of Ni perovskite systems (TRNiO3, TR=rare earth) using core level absorption spectroscopy. The synchrotron light source was exploited in the soft X-ray range to study Ni LIII and LII edges and in the hard X-ray range to study Ni K edge and rare earths LIII edges. The most remarkable property in these systems is a metal to insulator transition with temperature. This transition temperature depends on the rare-earth ion, increasing its value as the rare earth size is re-duced. These systems have a distorted perovskite structure, where the Ni O6octahedra rotate to fill the empty space left around the rare-earth ion. The smaller the ion, the larger the distortion. This indicates a straight correlation between the net distortion and the metal-insulator transition. For systems with TR varying from Pr to Gd it was found a crystallographic structure with orthorhombic symmetry, where Ni occupies a very symmetric site. Based on these results, it was proposed a model suggesting that the bandwidth would decrease due to a smaller hybridization between Ni3d and O2p bands caused by a decrease at the Ni-O-Ni angle. However, modifications in the transition temperature by the O isotope substitution, showed that the electron-phonon coupling plays an important role. Indeed, for systems with smaller TR ions (from Ho to Lu) it was found a monoclinic distortion in the insulating phase. In such structure Ni occupies two different sites with different Ni-O distances. A charge ordering associated to these different sites explains the insulating phase, as well as the antiferromagnetic ordering observed for some systems. Our results show that these two Ni-O distances coexist in all systems, inde-pendent of its long range crystallographic structure, and in both electronic phases. The site with longer Ni-O distance, which is weakly hybridized, is the responsible for the electronic localization and it is immersed in a con-ducting matrix with stronger hybridization. The metal-insulator transition is explained by the modifications in the proportion between these two Ni sites. In this context, a strong electron-phonon coupling is expected, as well as the suppression of the insulating phase under pressureDoutoradoFísicaDoutor em Ciência
An electron hole doping and soft x-ray spectroscopy study on La1-xSrxFe0.75Ni0.25O3-{\delta}
The conductivity of the electron hole and polaron conductor
La1-xSrxFe0.75Ni0.25O3-{\delta}, a potential cathode material for intermediate
temperature solid oxide fuel cells, was studied for 0 <x < 1 and for
temperatures 300 K <T < 1250 K. In LaSrFe-oxide, an ABO3 type perovskite,
A-site substitu-tion of the trivalent La3+ by the divalent Sr2+ causes
oxidation of Fe3+ towards Fe4+, which forms conducting electron holes. Here we
have in addition a B-site substitution by Ni. The compound for x = 0.5 is
identified as the one with the highest conductivity ({\sigma} ~ 678 S/cm) and
lowest activation energy for polaron conductivity (Ep = 39 meV). The evolution
of the electronic structure was monitored by soft x-ray Fe and oxygen K-edge
spectroscopy. Homogeneous trend for the oxida-tion state of the Fe was
observed. The variation of the ambient temperature conductivity and activation
energy with relative Sr content (x) shows a correlation with the ratio of
(eg/eg+t2g) in Fe L3 edge up to x=0.5. The hole doping process is reflected by
an almost linear trend by the variation of the pre-peaks of the oxygen K-edge
soft x-ray absorption spectra
Environment of Er in a-Si:H: co-sputtering versus ion implantation
We report a comparative Extended X-Ray Fine Structure (EXAFS) study of Er in a-Si:H prepared by Er implantation in a-Si:H and by co-sputtering undergoing the same cumulative annealing processes. It was found that the Er environment in as-implanted samples is formed by Si atoms, which are replaced by oxygen under annealing. In the co-sputtered samples, the initial low coordination oxygen environment evolves under thermal treatment to an Er2O3 -like neighborhood.756759Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq
Evidence for Coexistence of Bulk Superconductivity and Itinerant Antiferromagnetism in the Heavy Fermion System CeCo(InCd)
In the generic phase diagram of heavy fermion systems, tuning an external
parameter such as hydrostatic or chemical pressure modifies the superconducting
transition temperature. The superconducting phase forms a dome in the
temperature-tuning parameter phase diagram, which is associated with a maximum
of the superconducting pairing interaction. Proximity to antiferromagnetism
suggests a relation between the disappearance of antiferromagnetic order and
superconductivity. We combine muon spin rotation, neutron scattering, and x-ray
absorption spectroscopy techniques to gain access to the magnetic and
electronic structure of CeCo(InCd) at different time scales.
Different magnetic structures are obtained that indicate a magnetic order of
itinerant character, coexisting with bulk superconductivity. The suppression of
the antiferromagnetic order appears to be driven by a modification of the
bandwidth/carrier concentration, implying that the electronic structure and
consequently the interplay of superconductivity and magnetism is strongly
affected by hydrostatic and chemical pressure.Comment: Article + Supplementary information 33 pages, 13 figure
Spin-orbit induced mixed-spin ground state in NiO perovskites probed by XAS: new insight into the metal to insulator transition
We report on a Ni L edges x-ray absorption spectroscopy (XAS) study
in NiO perovskites. These compounds exhibit a metal to insulator ()
transition as temperature decreases. The L edge presents a clear
splitting in the insulating state, associated to a less hybridized ground
state. Using charge transfer multiplet calculations, we establish the
importance of the crystal field and 3d spin-orbit coupling to create a
mixed-spin ground state. We explain the transition in NiO
perovskites in terms of modifications in the Ni crystal field splitting
that induces a spin transition from an essentially low-spin (LS) to a
mixed-spin state.Comment: 4 pages, 4 figures, accepted as PRB - Rapid Comm. Dez. 200
Origin of interface magnetism in BiMnO3/SrTiO3 and LaAlO3/SrTiO3 heterostructures
Possible ferromagnetism induced in otherwise non-magnetic materials has been
motivating intense research in complex oxide heterostructures. Here we show
that a confined magnetism is realized at the interface between SrTiO3 and two
insulating polar oxides, BiMnO3 and LaAlO3. By using polarization dependent
x-ray absorption spectroscopy, we find that in both cases the magnetic order is
stabilized by a negative exchange interaction between the electrons transferred
to the interface and local magnetic moments. These local magnetic moments are
associated to Ti3+ ions at the interface itself for LaAlO3/SrTiO3 and to Mn3+
ions in the overlayer for BiMnO3/SrTiO3. In LaAlO3/SrTiO3 the induced magnetic
moments are quenched by annealing in oxygen, suggesting a decisive role of
oxygen vacancies in the stabilization of interfacial magnetism.Comment: 5 pages, 4 figure
Short-range charge-order in NiO perovskites (=Pr,Nd,Eu) probed by X-ray absorption spectroscopy
The short-range organization around Ni atoms in orthorhombic NiO
(=Pr,Nd,Eu) perovskites has been studied over a wide temperature range by Ni
K-edge x-ray absorption spectroscopy. Our results demonstrate that two
different Ni sites, with different average Ni-O bond lengths, coexist in those
orthorhombic compounds and that important modifications in the Ni nearest
neighbors environment take place across the metal-insulator transition. We
report evidences for the existence of short-range charge-order in the
insulating state, as found in the monoclinic compounds. Moreover, our results
suggest that the two different Ni sites coexists even in the metallic state.
The coexistence of two different Ni sites, independently on the ion,
provides a common ground to describe these compounds and shed new light in the
understanding of the phonon-assisted conduction mechanism and unusual
antiferromagnetism present in all NiO compounds.Comment: 4 pages, 3 figures, accepted PRB - Brief Report Dec.200
The impact of the near-surface region on the interpretation of x-ray absorption spectroscopy
Transition metal oxides (TMOs) exhibit a broad spectrum of electronic,
magnetic, and optical properties, making them intriguing materials for various
technological applications. Soft x-ray absorption spectroscopy (XAS) is widely
used to study TMOs, shedding light on their chemical state, electronic
structure, orbital polarization, element-specific magnetism, and more.
Different XAS acquisition modes feature different information depth regimes in
the sample. Here, we employ two XAS acquisition modes, having surface-sensitive
versus bulk probing depths, on the prototypical TMO SrVO3. We illustrate and
elucidate a strong apparent discrepancy between the different modes,
emphasizing the impact of the near-surface region on the interpretation of XAS
data. These findings highlight the importance of the acquisition mode selection
in XAS analysis. Moreover, the results highlight the role of the near-surface
region not only in the characterization of TMOs, but also in the design of
future nanoscale oxide electronics
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