15 research outputs found
Oxide Heterostructures from a Realistic Many-Body Perspective
Oxide heterostructures are a new class of materials by design, that open the
possibility for engineering challenging electronic properties, in particular
correlation effects beyond an effective single-particle description. This short
review tries to highlight some of the demanding aspects and questions,
motivated by the goal to describe the encountered physics from first
principles. The state-of-the-art methodology to approach realistic many-body
effects in strongly correlated oxides, the combination of density functional
theory with dynamical mean-field theory, will be briefly introduced. Discussed
examples deal with prominent Mott-band- and band-band-insulating type of oxide
heterostructures, where different electronic characteristics may be stabilized
within a single architectured oxide material.Comment: 19 pages, 9 figure
What is the valence of a correlated solid? The double life of delta-plutonium
Plutonium displays phase transitions with enormous volume differences among
its phases and both its Pauli like magnetic susceptibility and resistivity are
an order of magnitude larger than those of simple metals. Curium is also highly
resistive but its susceptibility is Curie-like at high temperatures and orders
antiferromagnetically at low temperatures. The anomalous properties of the late
actinides stem from the competition between the itinerancy and localization of
its f electrons, which makes the late actinides elemental strongly correlated
materials. A central problem in this field is to understand the mechanism by
which these materials resolve these conflicting tendencies. In this letter we
identify the electronic mechanisms responsible for the anomalous behaviour of
late actinides. We revisit the concept of valence using theoretical approach
that treats magnetism, Kondo screening, atomic multiplet effects, spin orbit
coupling and crystal field splitting on the same footing. Plutonium is found to
be in a rare mixed valent state, namely its ground state is a superposition of
two distinct valencies. Curium settles in a single valence magnetically ordered
state at low temperatures. The f7 atomic configuration of Curium is contrasted
with the multiple configuration manifolds present in Plutonium ground state
which we characterize by a valence histogram. The balance between the Kondo
screening and magnetism is determined by the competition between spin orbit
coupling and the strength of atomic multiplets which is in turn regulated by
the degree of itinerancy. The approach presented here, highlights the
electronic origin of the bonding anomalies in plutonium and can be applied to
predict generalized valences and the presence or absence of magnetism in other
compounds starting from first principles.Comment: 2 figures, 1 tabl
In situ atomic-scale observation of oxygen-driven core-shell formation in Pt3Co nanoparticles
The catalytic performance of core-shell platinum alloy nanoparticles is typically superior to that of pure platinum nanoparticles for the oxygen reduction reaction in fuel cell cathodes. Thorough understanding of core-shell formation is critical for atomic-scale design and control of the platinum shell, which is known to be the structural feature responsible for the enhancement. Here we reveal details of a counter-intuitive core-shell formation process in platinum-cobalt nanoparticles at elevated temperature under oxygen at atmospheric pressure, by using advanced in situ electron microscopy. Initial segregation of a thin platinum, rather than cobalt oxide, surface layer occurs concurrently with ordering of the intermetallic core, followed by the layer-by-layer growth of a platinum shell via Ostwald ripening during the oxygen annealing treatment. Calculations based on density functional theory demonstrate that this process follows an energetically favourable path. These findings are expected to be useful for the future design of structured platinum alloy nanocatalysts.Core-shell platinum alloy nanoparticles are promising catalysts for oxygen reduction, however a deeper understanding of core-shell formation is still required. Here the authors report oxygen-driven formation of core-shell Pt3Co nanoparticles, seen at the atomic scale with in situ electron microscopy at ambient pressure
The most incompressible metal osmium at static pressures above 750 gigapascals
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