Short-range charge-order in RRNiO3_{3} perovskites (RR=Pr,Nd,Eu) probed by X-ray absorption spectroscopy

The short-range organization around Ni atoms in orthorhombic $R$NiO$_{3}$ ($R$=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 $R$ 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 $R$NiO$_{3}$ compounds.Comment: 4 pages, 3 figures, accepted PRB - Brief Report Dec.200

Local tetragonal distortion in La_{0.7}Sr_{0.3}MnO_3 strained thin films probed by x-ray absorption spectroscopy

We report on an angular resolved X-ray Absorption Spectroscopy study of $La_{0.7}Sr_{0.3}MnO_{3}$ thin films epitaxially grown by pulsed laser deposition on slightly mismatched substrates which induce tensile or compressive strains. XANES spectra give evidence of tetragonal distortion within the $MnO_{6}$ octahedra, with opposite directions for tensile and compressive strains. Quantitative analysis has been done and a model of tetragonal distortion reflecting the strain has been established. EXAFS data collected in plane for tensile substrate confirm the change in the $Mn-O$ average bond distance and the increase of $Mn-Mn$ length matching with the enlargement of the cell parameter. From these results we conclude that there is no significant change in the $Mn-O-Mn$ angle. Our observations conflict with the scenarios which this angle is the main driving parameter in the sensitivity of manganite films properties to external strains and suggest that the distortion within the octahedra plays a key role in the modification of the transport and magnetic properties.Comment: 8 pages, 6 figure

Phonon activity and intermediate glassy phase of YVO₃

We show that in YVO₃ additional hard phonons gradually become zone center infrared active below ∼210 K, verifying that a lattice phase transition takes place at about that temperature. Their gradual increment in intensity between ∼210 and ∼77 K is associated with a "glassy" behavior found in the temperature-dependent V K edge pseudoradial distribution. This translates into an increase in the Debye-Waller factors ascribed to the appearance of V local structural disorder below ∼150 K. Conflicts between various ordering mechanisms in YVO₃ bring up similarities of the intermediate phase to known results in dielectric incommensurate systems, suggesting the formation of commensurate domains below 116 K, the onset temperature of G-type antiferromagnetism. We propose that ∼210 and ∼77 K be understood as the temperatures where the commensurate-incommensurate and incommensurate-commensurate "lock-in" phase transitions take place. We found support for this interpretation in the inverted λ shapes of the measured heat capacity and in the overall temperature dependence of the hard phonons.Facultad de Ciencias ExactasCentro de Química Inorgánic

Robust perpendicular exchange coupling in an ultrathin CoO/PtFe double layer: strain and spin orientation

Sem informaçãoWe report on the exchange coupling and magnetic properties of a strained ultrathin CoO/PtFe double layer with perpendicular magnetic anisotropy. The cobalt oxide growth by reactive molecular beam epitaxy on a Pt-terminated PtFe/Pt(001) surface gives rise to a hexagonal surface and a monoclinic distorted CoO 3 nm film at room temperature. This distorted ultrathin CoO layer couples with the PtFe(001) layer establishing a robust perpendicular exchange bias shift. Soft x-ray absorption spectroscopy provides a full description of the spin orientations in the CoO/PtFe double layer. The exchange bias shift is preserved up to the Néel antiferromagnetic ordering temperature of TN = 293 K. This unique example of selfsame value for blocking and ordering temperatures, yet identical to the bulk ordering temperature, is likely related to the original strain-induced distortion and strengthened interaction between the two well-ordered spin layers.We report on the exchange coupling and magnetic properties of a strained ultrathin CoO/PtFe double layer with perpendicular magnetic anisotropy. The cobalt oxide growth by reactive molecular beam epitaxy on a Pt-terminated PtFe/Pt(001) surface gives rise to a hexagonal surface and a monoclinic distorted CoO 3 nm film at room temperature. This distorted ultrathin CoO layer couples with the PtFe(001) layer establishing a robust perpendicular exchange bias shift. Soft x-ray absorption spectroscopy provides a full description of the spin orientations in the CoO/PtFe double layer. The exchange bias shift is preserved up to the Néel antiferromagnetic ordering temperature of TN = 293 K. This unique example of selfsame value for blocking and ordering temperatures, yet identical to the bulk ordering temperature, is likely related to the original strain-induced distortion and strengthened interaction between the two well-ordered spin layers.881415Sem informaçãoSem informaçãoSem informaçã

Short-range charge order in RNiO₃ perovskites (R = Pr, Nd, Eu, Y) probed by x-ray-absorption spectroscopy

The short-range organization around Ni atoms in orthorhombic RNiO₃ (R = 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 R 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 RNiO₃ compounds.Facultad de Ciencias ExactasCentro de Química Inorgánic

Composition, Size, and Surface Functionalization dependent Optical Properties of Lead Bromide Perovskite Nanocrystals

The photoluminescence (PL), color purity, and stability of lead halide perovskite nanocrystals depend critically on the surface passivation. We present a study on the temperature dependent PL and PL decay dynamics of lead bromide perovskite nanocrystals characterized by different types of A cations, surface ligands, and nanocrystal sizes. Throughout, we observe a single emission peak from cryogenic to ambient temperature. The PL decay dynamics are dominated by the surface passivation, and a post-synthesis ligand exchange with a quaternary ammonium bromide (QAB) results in a more stable passivation over a larger temperature range. The PL intensity is highest from 50K-250K, which indicates that the ligand binding competes with the thermal energy at ambient temperature. Despite the favorable PL dynamics of nanocrystals passivated with QAB ligands (monoexponential PL decay over a large temperature range, increased PL intensity and stability), the surface passivation still needs improvement toward increased emission intensity in nanocrystal films.Comment: 19 pages, 5 figure

Temperature and high-pressure dependent x-ray absorption of SmNiO3 at the Ni K and Sm L3 edges

Wereport on x-ray absorption near-edge structure (XANES) and extended x-ray absorption fine; structure (EXAFS) measurements of SmNiO3 from 20 Kto 600 Kand up to 38 GPa at the NiKand Sm; L3 edges. A multiple component pre-NiKedge tail is understood, originating from 1 s transitions to; 3d-4p states while a post-edge shoulder increases distinctively smoothly, at about the insulator to; metal phase transition (TIM), due to the reduction of electron–phonon interactions as the Ni 3d andO; 2p band overlap triggers the metallic phase. This effect is concomitant with pressure-induced Ni-O-Ni; angle increments toward more symmetric Ni3+ octahedra of the rhombohedral R¯3c space group.; Room temperature pressure-dependent Ni white line peak energies have an abrupt∼3.10±0.04 GPa; valence discontinuity from non-equivalent Ni3+δ + Ni3–δ charge disproportionate net unresolved; absorber turning at∼TIM into Ni3+ of the orthorhombic Pbnm metal oxide phase. At 20 Kthe overall; white line response, still distinctive at TIM∼8.1±0.6 GPa is much smoother due to localization.; Octahedral bond contraction up to 38 GPa and at 300 Kand 20 Kshow breaks in its monotonic; increase at the different structural changes. The SmL3 edge does not show distinctive behaviors either; at 300 Kor 20 Kup to about 35 GPa but the perovskite Smcage, coordinated to eight oxygen atoms,; undergoes strong uneven bond contractions at intermediate pressures where we found the coexistence; of octahedral and rhombohedral superexchange angle distortions.Wefound that the white line; pressure-dependent anomaly may be used as an accurate alternative for delineating pressure–; temperature phase diagrams.Centro de Química Inorgánic

Phonon activity and intermediate glassy phase of YVO₃

We show that in YVO₃ additional hard phonons gradually become zone center infrared active below ∼210 K, verifying that a lattice phase transition takes place at about that temperature. Their gradual increment in intensity between ∼210 and ∼77 K is associated with a "glassy" behavior found in the temperature-dependent V K edge pseudoradial distribution. This translates into an increase in the Debye-Waller factors ascribed to the appearance of V local structural disorder below ∼150 K. Conflicts between various ordering mechanisms in YVO₃ bring up similarities of the intermediate phase to known results in dielectric incommensurate systems, suggesting the formation of commensurate domains below 116 K, the onset temperature of G-type antiferromagnetism. We propose that ∼210 and ∼77 K be understood as the temperatures where the commensurate-incommensurate and incommensurate-commensurate "lock-in" phase transitions take place. We found support for this interpretation in the inverted λ shapes of the measured heat capacity and in the overall temperature dependence of the hard phonons.Facultad de Ciencias ExactasCentro de Química Inorgánic

Short-range charge order in RNiO₃ perovskites (R = Pr, Nd, Eu, Y) probed by x-ray-absorption spectroscopy

The short-range organization around Ni atoms in orthorhombic RNiO₃ (R = 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 R 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 RNiO₃ compounds.Facultad de Ciencias ExactasCentro de Química Inorgánic

Stability of the Ni sites across the pressure-induced metallization in YNiO3

The local environment of nickel atoms in Y NiO3 across the pressure- induced insulator to metal (IM) transition was studied using X-ray absorption spectroscopy (XAS) supported by ab initio calculations. The monotonic contraction of the NiO6 units under applied pressure observed up to 13 GPa, stops in a limited pressure domain around 14 GPa, before resuming above 16 GPa. In this narrow pressure range, crystallographic modifications basically occur in the medium/long range, not in the NiO6 octahedron, whereas the evolution of the near-edge XAS features can be associated to metallization. Ab initio calculations show that these features are related to medium range order, provided that the Ni-O-Ni angle enables a proper overlap of the Ni eg and O 2p orbitals. Metallization is then not directly related to modifications in the average local geometry of the NiO6 units but more likely to an inter-octahedra rearrangement. These outcomes provides evidences of the bandwidth driven nature of the IM transition.Comment: 6 pages with figure