51 research outputs found
Origin of the orbital and spin orderings in rare-earth titanates
Rare-earth titanates RTiO are Mott insulators displaying a rich physical
behavior, featuring most notably orbital and spin orders in their ground state.
The origin of their ferromagnetic to antiferromagnetic transition as a function
of the size of the rare-earth however remains debated. Here we show on the
basis of symmetry analysis and first-principles calculations that although
rare-earth titanates are nominally Jahn-Teller active, the Jahn-Teller
distortion is negligible and irrelevant for the description of the ground state
properties. At the same time, we demonstrate that the combination of two
antipolar motions produces an effective Jahn-Teller-like motion which is the
key of the varying spin-orbital orders appearing in titanates. Thus, titanates
are prototypical examples illustrating how a subtle interplay between several
lattice distortions commonly appearing in perovskites can produce orbital
orderings and insulating phases irrespective of proper Jahn-Teller motions.Comment: Accepted in Physical Review
Tailoring the interfacial magnetic anisotropy in multiferroic field-effect devices
Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).-- et al.Ferroelectric field-effect devices based on perovskite oxide materials offer a new possibility to exploit emergent interfacial effects such as the electrostatic modification of the transport and magnetic properties of strongly correlated materials and to prove the magneto-electric coupling at the interface between the two different ferroic materials. Here we report on the reversible modulation of the interfacial magnetic and magnetotransport properties of La0.825Sr0.175MnO3 thin films induced by switching the ferroelectric polarization of a top PbZr0.2Ti0.8O3 layer. Anisotropic magnetoresistance (AMR) measurements were performed applying a magnetic field H in a plane perpendicular to the current density. By rotating H from the out-of-plane towards the in-plane direction, upon the ferroelectric polarization switching, a modulation of the normalized AMR amplitude was achieved. The dynamical electrostatic coupling at the interface of the two oxides is responsible for a reconstruction of the Mn3deg orbitals which in turn affects the surface magnetic anisotropy of the magneto-electric system. The present work might have a broader impact, including in the field of multiferroic tunnel junctions, due to a better understanding of the coupling at the interface of the two ferroic oxides where the influence of the polarization on the magnetic degree of freedom is accomplished.D.P. thanks the European Community’s Seventh Framework Programme (FP7/2007-2013) for financial support under Grant Agreement No. NMP3-LA-2010-246102. I.F. acknowledges Beatriu de Pinós postdoctoral scholarship (2011 BP-A 00220) from the
Catalan Agency for Management of University and Research Grants (AGAUR-Generalitat de Catalunya).Peer Reviewe
Arsenic-fluoride co-contamination in groundwater: Background and anomalies in a volcanic-sedimentary aquifer in central Italy
Abstract In a volcanic-sedimentary aquifer in central Italy, we investigate the co-existence of arsenic and fluoride in groundwater, aiming at identifying the most probable processes deductible at regional/groundwater body scale leading to the observed co-contamination in groundwater. Further, the areas at risk for human health where high concentrations can produce a significant risk to human health have been investigated. The study area is located in Latium (Central Italy) where silica-undersaturated alkali-potassic formations of Plio-Pleistocene age largely outcrop above marine and continental sand and clay deposits (Neogene) and continental alluvial deposits (Lower Pleistocene–Middle Pleistocene). Geochemical data from groundwater at 322 wells and 76 springs have been analyzed through statistical methods including clustering/PCA and geostatistical analysis. The results show exceedances of the drinking water standards for F (1.5 mg/L) and As (10.0 μg/L) in 29% and 55% of the sampled groundwater, respectively. Multivariate statistics suggest a widespread process of water-rock interaction with the K-alkaline volcanic formations releasing As, F, K, Si, V, Rb and PO4 to the groundwater. As and F show a good correlation (Pearson's r = 0.61, Spearman's rs = 0.59) and define a separate PCA component, confirming that their background in groundwater might be governed by a common process. Kriging interpolations have been used to study the spatial distribution of the two parameters, identifying areas with the highest concentrations and highest probability of exceeding the standards for human consumption. Moreover, by resampling the As-F data with the jackknife technique it was possible to identify the variations of their correlation index in the study area, due to specific As or F anomalies. While in the peripheral areas of the volcanic districts, dominated by sedimentary deposits, the As-F correlation index does not present important fluctuations, Indicator Kriging shows specific As or F correlation anomalies within the volcanic groundwater bodies and along the Tyrrhenian coastline. These anomalies seem to correspond to the zones with the highest thermal flux and/or are located near important structural lineaments. Fluoride correlation anomalies close to mining sites (fluorite) have also been observed. We hypothesize that, unlike the regional co-contamination, these local anomalies are related to the upwelling of geothermal fluids along fracture/fault systems that mix with cold groundwater, or to the interaction with mineral deposits particularly enriched of these elements
Proposal for an integrated method of natural background levels assessment in groundwater
Natural Background Level (NBL), geochemical approach, statistical methods
Crossover of conduction mechanism in Sr2IrO4 epitaxial thin films
High quality epitaxial Sr2IrO4 thin films with various thicknesses (9-300 nm)
have been grown on SrTiO3 (001) substrates, and their electric transport
properties have been investigated. All samples showed the expected insulating
behavior with a strong resistivity dependence on film thickness, that can be as
large as three orders of magnitude at low temperature. A close examination of
the transport data revealed interesting crossover behaviors for the conduction
mechanism upon variation of thickness and temperature. While Mott variable
range hopping (VRH) dominated the transport for films thinner than 85 nm, high
temperature thermal activation behavior was observed for films with large
thickness, which was followed by a crossover from Mott to Efros-Shklovskii (ES)
VRH in the low temperature range. This low temperature crossover from Mott to
ES VRH indicates the presence of a Coulomb gap (~3 meV). Our results
demonstrate the competing and tunable conduction in Sr2IrO4 thin films, which
in turn would be helpful for understanding the insulating nature related to
strong spin-orbit-coupling of the 5d iridates
A proposal for groundwater sampling guidelines: application to a case study in Southern Latium
Groundwater monitoring , body status, best sampling techniques, quaternary alluvial, pyroclastic deposit
Charge distribution across capped and uncapped infinite-layer neodymium nickelate thin films
Charge ordering (CO) phenomena have been widely debated in
strongly-correlated electron systems mainly regarding their role in
high-temperature superconductivity. Here, we elucidate the structural and
charge distribution in NdNiO thin films prepared with and without capping
layers, and characterized by the absence and presence of CO. Our
microstructural and spectroscopic analysis was done by scanning transmission
electron microscopy-electron energy loss spectroscopy (STEM-EELS) and hard
x-ray photoemission spectroscopy (HAXPES). Capped samples show Ni, with
an out-of-plane (o-o-p) lattice parameter of around 3.30 angstroms indicating
good stabilization of the infinite-layer structure. Bulk-sensitive HAXPES on
Ni-2p shows weak satellite feature indicating large charge-transfer energy. The
uncapped samples evidence an increase of the o-o-p parameter up to 3.65
angstroms on the thin-film top, and spectroscopies show signatures of higher
valence in this region (towards Ni). Here, 4D-STEM demonstrates (3,0,3)
oriented stripes which emerge from partially occupied apical oxygen. Those
stripes form quasi-2D coherent domains viewed as rods in the reciprocal space
with r.l.u. extension located at Q = () r.l.u. and Q = () r.l.u. The stripes associated with oxygen re-intercalation
concomitant with hole doping suggests a possible link to the previously
reported CO in infinite-layer nickelate thin films
Coherent Fe-rich nano-scale perovskite oxide phase in epitaxial Sr2FeMoO6 films grown on cubic and scandate substrates
We report the growth of high-quality epitaxial Sr2FeMoO6 (SFMO) thin films on various unconventional oxide substrates, such as TbScO3, DyScO3, and Sr2Al0.3Ga0.7TaO6 (SAGT) as well as on the most commonly used one, SrTiO3 (STO), by pulsed laser deposition. The films were found to contain a foreign nano-scale phase coherently embedded inside the SFMO film matrix. Through energy dispersive X-ray spectroscopy and scanning transmission electron microscopy, we identified the foreign phase to be Sr2−xFe1+yMo1−yO6, an off-stoichiometric derivative of the SFMO compound with Fe rich content (y ≈ 0.6) and a fairly identical crystal structure to SFMO. The films on STO and SAGT exhibited very good magnetic properties with high Curie temperature values. All the samples have fairly good conducting behavior albeit the presence of a foreign phase. Despite the relatively large number of items of the foreign phase, there is no significant deterioration in the properties of the SFMO films. We discuss in detail how magneto-transport properties are affected by the foreign phase.
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Physiological Profiling and Functional Diversity of Groundwater Microbial Communities in a Municipal Solid Waste Landfill Area
The disposal of municipal solid wastes in landfills represents a major threat for aquifer environments at the global scale. The aim of this study was to explore how groundwater geochemical characteristics can influence the microbial community functioning and the potential degradation patterns of selected organic substrates in response to different levels of landfill-induced alterations. Groundwaters collected from a landfill area were monitored by assessing major physical-chemical parameters and the microbiological contamination levels (total coliforms and fecal indicators—Colilert-18). The aquatic microbial community was further characterized by flow cytometry and Biolog EcoPlatesTM assay. Three groundwater conditions (i.e., pristine, mixed, and altered) were identified according to their distinct geochemical profiles. The altered groundwaters showed relatively higher values of organic matter concentration and total cell counts, along with the presence of fecal indicator bacteria, in comparison to samples from pristine and mixed conditions. The kinetic profiles of the Biolog substrate degradation showed that the microbial community thriving in altered conditions was relatively more efficient in metabolizing a larger number of organic substrates, including those with complex molecular structures. We concluded that the assessment of physiological profiling and functional diversity at the microbial community level could represent a supportive tool to understand the potential consequences of the organic contamination of impacted aquifers, thus complementing the current strategies for groundwater management
Interface-mediated ferroelectric patterning and Mn valency in nano-structured PbTiO <sub>3</sub> /La <sub>0.7</sub> Sr <sub>0.3</sub> MnO <sub>3</sub>
International audienceWe employed a multitechnique approach using piezo-force response microscopy and photoemission microscopy to investigate a self-organizing polarization domain pattern in PbTiO3/La0.7Sr0.3MnO3 (PTO/LSMO) nanostructures. The polarization is correlated with the nanostructure morphology as well as with the thickness and Mn valence of the LSMO template layer. On the LSMO dots, the PTO is upwards polarized, whereas outside the nanodots, the polarization appears both strain and interface roughness dependent. The results suggest that the electronic structure and strain of the PTO/LSMO interface contribute to determining the internal bias of the ferroelectric layer
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