125 research outputs found
Dynamical Mean Field Theory of Nickelate Superlattices
Dynamical mean field methods are used to calculate the phase diagram,
many-body density of states, relative orbital occupancy and Fermi surface shape
for a realistic model of -based superlattices. The model is derived
from density functional band calculations and includes oxygen orbitals. The
combination of the on-site Hunds interaction and charge-transfer between the
transition metal and the oxygen orbitals is found to reduce the orbital
polarization far below the levels predicted either by band structure
calculations or by many-body analyses of Hubbard-type models which do not
explicitly include the oxygen orbitals. The findings indicate that
heterostructuring is unlikely to produce one band model physics and demonstrate
the fundamental inadequacy of modeling the physics of late transition metal
oxides with Hubbard-like models.Comment: Values of orbitals polarizations reported in Fig. 2 corrected. We
thank E. Benckiser and M. Wu for pointing out the error
Chemical control of orbital polarization in artificially structured transition-metal oxides: La2NiXO6 (X=B, Al, Ga, In) from first principles
The application of modern layer-by-layer growth techniques to
transition-metal oxide materials raises the possibility of creating new classes
of materials with rationally designed correlated electron properties. An
important step toward this goal is the demonstration that electronic structure
can be controlled by atomic composition. In compounds with partially occupied
transition-metal d shells, one important aspect of the electronic structure is
the relative occupancy of different d orbitals. Previous work has established
that strain and quantum confinement can be used to influence orbital occupancy.
In this paper we demonstrate a different modality for orbital control in
transition-metal oxide heterostructures, using density-functional band
calculations supplemented by a tight-binding analysis to show that the choice
of nontransition-metal counterion X in transition-metal oxide heterostructures
composed of alternating LaNiO3 and LaXO3 units strongly affects orbital
occupancy, changing the magnitude and in some cases the sign of the orbital
polarization
Failure mechanisms of graphene under tension
Recent experiments established pure graphene as the strongest material known
to mankind, further invigorating the question of how graphene fails. Using
density functional theory, we reveal the mechanisms of mechanical failure of
pure graphene under a generic state of tension. One failure mechanism is a
novel soft-mode phonon instability of the -mode, whereby the graphene
sheet undergoes a phase transition and is driven towards isolated benzene rings
resulting in a reduction of strength. The other is the usual elastic
instability corresponding to a maximum in the stress-strain curve. Our results
indicate that finite wave vector soft modes can be the key factor in limiting
the strength of monolayer materials
Covalency, double-counting and the metal-insulator phase diagram in transition metal oxides
Dynamical mean field theory calculations are used to show that for late
transition-metal-oxides a critical variable for the Mott/charge-transfer
transition is the number of d-electrons, which is determined by charge transfer
from oxygen ions. Insulating behavior is found only for a narrow range of
d-occupancy, irrespective of the size of the intra-d Coulomb repulsion. The
result is useful in interpreting 'density functional +U' and 'density
functional plus dynamical mean field' methods in which additional correlations
are applied to a specific set of orbitals and an important role is played by
the 'double counting correction' which dictates the occupancy of these
correlated orbitals. General considerations are presented and are illustrated
by calculations for two representative transition metal oxide systems: layered
perovskite Cu-based "high-Tc" materials, an orbitally non-degenerate
electronically quasi-two dimensional systems, and pseudocubic rare earch
nickelates, an orbitally degenerate electronically three dimensional system.
Density functional calculations yield d-occupancies very far from the Mott
metal-insulator phase boundary in the nickelate materials, but closer to it in
the cuprates, indicating the sensitivity of theoretical models of the cuprates
to the choice of double counting correction and corroborating the critical role
of lattice distortions in attaining the experimentally observed insulating
phase in the nickelates.Comment: 10+ pages, 5 figure
How to produce cheap and easy custom-made sterilizable filtering facepiece 2/3 masks for healthcare providers during pandemic COVID-19 emergency
On January 8, 2020, a novel coronavirus was officially announced as the causative pathogen of coronavirus disease (COVID-19) by the Chinese Center for Disease Control and Prevention.On February 26, COVID-19 has been recognized in 34 countries, with a total of 80,239 laboratory-confirmed patients and 2700 deaths.Protecting healthcare workers from infectious hazards is paramount to ensuring their safety in delivering health care.In addition, being able to protect healthcare workers, constituting the front-line response against high-threat respiratory pathogens, such as severe acute respiratory syndrome coronavirus 2, is important for reducing secondary transmission in healthcare-associated outbreaks.Authors present a simple, reliable, and cheap protocol to produce a custom-made sterilizable filtering facepiece 2/3 masks for healthcare providers during pandemic COVID-19 emergency
First-principles prediction of redox potentials in transition-metal compounds with LDA+U
First-principles calculations within the Local Density Approximation (LDA) or
Generalized Gradient Approximation (GGA), though very successful, are known to
underestimate redox potentials, such as those at which lithium intercalates in
transition metal compounds. We argue that this inaccuracy is related to the
lack of cancellation of electron self-interaction errors in LDA/GGA and can be
improved by using the DFT+ method with a self-consistent evaluation of the
parameter. We show that, using this approach, the experimental lithium
intercalation voltages of a number of transition metal compounds, including the
olivine LiMPO (M=Mn, Fe Co, Ni), layered LiMO (Co,
Ni) and spinel-like LiMO (M=Mn, Co), can be reproduced
accurately.Comment: 19 pages, 6 figures, Phys. Rev. B 70, 235121 (2004
S=1/2 chains and spin-Peierls transition in TiOCl
We study TiOCl as an example of an S=1/2 layered Mott insulator. From our
analysis of new susceptibility data, combined with LDA and LDA+U band structure
calculations, we conclude that orbital ordering produces quasi-one-dimensional
spin chains and that TiOCl is a new example of Heisenberg-chains which undergo
a spin-Peierls transition. The energy scale is an order of magnitude larger
than that of previously known examples. The effects of non-magnetic Sc
impurities are explained using a model of broken finite chains.Comment: 5 pages, 5 figures (color); details on crystal growth added; to be
published in Phys. Rev.
Phase Separation in LiFePO Induced by Correlation Effects
We report on a significant failure of LDA and GGA to reproduce the phase
stability and thermodynamics of mixed-valence LiFePO compounds.
Experimentally, LiFePO compositions () are known to be
unstable and phase separate into Li FePO and FePO. However,
first-principles calculations with LDA/GGA yield energetically favorable
intermediate compounds an d hence no phase separation. This qualitative failure
of LDA/GGA seems to have its origin in the LDA/GGA self-interaction which de
localizes charge over the mixed-valence Fe ions, and is corrected by explicitly
considering correlation effects in this material. This is demonstrated with
LDA+U calculations which correctly predict phase separation in LiFePO
for eV. T he origin of the destabilization of intermediate
compounds is identified as electron localization and charge ordering at
different iron sites. Introduction of correlation also yields more accurate
electrochemical reaction energies between FePO/LiFePO and
Li/Li electrodes.Comment: 12 pages, 5 figures, Phys. Rev. B 201101R, 200
Hundredfold Enhancement of Light Emission via Defect Control in Monolayer Transition-Metal Dichalcogenides
Two dimensional (2D) transition-metal dichalcogenide (TMD) based
semiconductors have generated intense recent interest due to their novel
optical and electronic properties, and potential for applications. In this
work, we characterize the atomic and electronic nature of intrinsic point
defects found in single crystals of these materials synthesized by two
different methods - chemical vapor transport and self-flux growth. Using a
combination of scanning tunneling microscopy (STM) and scanning transmission
electron microscopy (STEM), we show that the two major intrinsic defects in
these materials are metal vacancies and chalcogen antisites. We show that by
control of the synthetic conditions, we can reduce the defect concentration
from above to below . Because these point
defects act as centers for non-radiative recombination of excitons, this
improvement in material quality leads to a hundred-fold increase in the
radiative recombination efficiency
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