8 research outputs found
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The effect of core-shell engineering on the energy product of magnetic nanometals.
Solution-based growth of magnetic FePt-FeCo (core-shell) nanoparticles with a controllable shell thickness has been demonstrated. The transition from spin canting to exchange coupling of FePt-FeCo core-shell nanostructures leads to a 28% increase in the coercivity (12.8 KOe) and a two-fold enhancement in the energy product (9.11 MGOe)
Electronic structure of negative charge transfer CaFeO3 across the metal-insulator transition
We investigated the metal-insulator transition for epitaxial thin films of
the perovskite CaFeO3, a material with a significant oxygen ligand hole
contribution to its electronic structure. We find that biaxial tensile and
compressive strain suppress the metal-insulator transition temperature. By
combining hard X-ray photoelectron spectroscopy, soft X-ray absorption
spectroscopy, and density functional calculations, we resolve the
element-specific changes to the electronic structure across the metal-insulator
transition. We demonstrate that the Fe electron valence undergoes no observable
change between the metallic and insulating states, whereas the O electronic
configuration undergoes significant changes. This strongly supports the
bond-disproportionation model of the metal-insulator transition for CaFeO3 and
highlights the importance of ligand holes in its electronic structure. By
sensitively measuring the ligand hole density, however, we find that it
increases by ~5-10% in the insulating state, which we ascribe to a further
localization of electron charge on the Fe sites. These results provide detailed
insight into the metal-insulator transition of negative charge transfer
compounds and should prove instructive for understanding metal-insulator
transitions in other late transition metal compounds such as the nickelates.Comment: Minor typographic changes mad
Recommended from our members
The effect of core-shell engineering on the energy product of magnetic nanometals.
Solution-based growth of magnetic FePt-FeCo (core-shell) nanoparticles with a controllable shell thickness has been demonstrated. The transition from spin canting to exchange coupling of FePt-FeCo core-shell nanostructures leads to a 28% increase in the coercivity (12.8 KOe) and a two-fold enhancement in the energy product (9.11 MGOe)
Strain-Engineered Oxygen Vacancies in CaMnO₃ Thin Films
We demonstrate a novel pathway to control and stabilize oxygen vacancies in complex transition-metal oxide thin films. Using atomic layer-by-layer pulsed laser deposition (PLD) from two separate targets, we synthesize high-quality single-crystalline CaMnO3 films with systematically varying oxygen vacancy defect formation energies as controlled by coherent tensile strain. The systematic increase of the oxygen vacancy content in CaMnO3 as a function of applied in-plane strain is observed and confirmed experimentally using high-resolution soft X-ray absorption spectroscopy (XAS) in conjunction with bulk-sensitive hard X-ray photoemission spectroscopy (HAXPES). The relevant defect states in the densities of states are identified and the vacancy content in the films quantified using the combination of first-principles theory and core–hole multiplet calculations with holistic fitting. Our findings open up a promising avenue for designing and controlling new ionically active properties and functionalities of com..
Inter-Layer Coupling Induced Valence Band Edge Shift in Mono- to Few-Layer MoS2
Recent progress in the synthesis of monolayer MoS2, a two-dimensional direct band-gap semiconductor, is paving new pathways toward atomically thin electronics. Despite the large amount of literature, fundamental gaps remain in understanding electronic properties at the nanoscale. Here, we report a study of highly crystalline islands of MoS2 grown via a refined chemical vapor deposition synthesis technique. Using high resolution scanning tunneling microscopy and spectroscopy (STM/STS), photoemission electron microscopy/spectroscopy (PEEM) and μ-ARPES we investigate the electronic properties of MoS2 as a function of the number of layers at the nanoscale and show in-depth how the band gap is affected by a shift of the valence band edge as a function of the layer number. Green’s function based electronic structure calculations were carried out in order to shed light on the mechanism underlying the observed bandgap reduction with increasing thickness, and the role of the interfacial Sulphur atoms is clarified. Our study, which gives new insight into the variation of electronic properties of MoS2 films with thickness bears directly on junction properties of MoS2, and thus impacts electronics application of MoS2.publishedVersionPeer reviewe
Electronic Structure of a Graphene-like Artificial Crystal of NdNiO3
Artificial complex-oxide heterostructures containing ultrathin buried layers grown along the pseudocubic [111] direction have been predicted to host a plethora of exotic quantum states arising from the graphene-like lattice geometry and the interplay between strong electronic correlations and band topology. To date, however, electronic-structural investigations of such atomic layers remain an immense challenge due to the shortcomings of conventional surface-sensitive probes with typical information depths of a few angstroms. Here, we use a combination of bulk-sensitive soft X-ray angle-resolved photoelectron spectroscopy (SX-ARPES), hard X-ray photoelectron spectroscopy (HAXPES), and state-of-the-art first-principles calculations to demonstrate a direct and robust method for extracting momentum-resolved and angle-integrated valence-band electronic structure of an ultrathin buckled graphene-like layer of NdNiO3 confined between two 4-unit cell-thick layers of insulating LaAlO3. The momentum-resolved dispersion of the buried Ni d states near the Fermi level obtained via SX-ARPES is in excellent agreement with the first-principles calculations and establishes the realization of an antiferro-orbital order in this artificial lattice. The HAXPES measurements reveal the presence of a valence-band bandgap of 265 meV. Our findings open a promising avenue for designing and investigating quantum states of matter with exotic order and topology in a few buried layers
Strain-Engineered Oxygen Vacancies in CaMnO<sub>3</sub> Thin Films
We
demonstrate a novel pathway to control and stabilize oxygen vacancies
in complex transition-metal oxide thin films. Using atomic layer-by-layer
pulsed laser deposition (PLD) from two separate targets, we synthesize
high-quality single-crystalline CaMnO<sub>3</sub> films with systematically
varying oxygen vacancy defect formation energies as controlled by
coherent tensile strain. The systematic increase of the oxygen vacancy
content in CaMnO<sub>3</sub> as a function of applied in-plane strain
is observed and confirmed experimentally using high-resolution soft
X-ray absorption spectroscopy (XAS) in conjunction with bulk-sensitive
hard X-ray photoemission spectroscopy (HAXPES). The relevant defect
states in the densities of states are identified and the vacancy content
in the films quantified using the combination of first-principles
theory and core–hole multiplet calculations with holistic fitting.
Our findings open up a promising avenue for designing and controlling
new ionically active properties and functionalities of complex transition-metal
oxides via strain-induced oxygen-vacancy formation and ordering