78 research outputs found
Electronic correlations in vanadium chalcogenides: BaVSe3 versus BaVS3
Albeit structurally and electronically very similar, at low temperature the
quasi-one-dimensional vanadium sulfide BaVS3 shows a metal-to-insulator
transition via the appearance of a charge-density-wave state, while BaVSe3
apparently remains metallic down to zero temperature. This different behavior
upon cooling is studied by means of density functional theory and its
combination with the dynamical mean-field theory and the rotationally-invariant
slave-boson method. We reveal several subtle differences between these
chalcogenides that provide indications for the deviant behavior of BaVSe3 at
low temperature. In this regard, a smaller Hubbard U in line with an increased
relevance of the Hund's exchange J plays a vital role.Comment: 16 pages, 11 figures, published versio
Nature of the Mott transition in Ca2RuO4
We study the origin of the temperature-induced Mott transition in Ca2RuO4. As
a method we use the local-density approximation+dynamical mean-field theory. We
show the following. (i) The Mott transition is driven by the change in
structure from long to short c-axis layered perovskite (L-Pbca to S-Pbca); it
occurs together with orbital order, which follows, rather than produces, the
structural transition. (ii) In the metallic L-Pbca phase the orbital
polarization is ~0. (iii) In the insulating S-Pbca phase the lower energy
orbital, ~xy, is full. (iv) The spin-flip and pair-hopping Coulomb terms reduce
the effective masses in the metallic phase. Our results indicate that a similar
scenario applies to Ca_{2-x}Sr_xRuO_4 (x<0.2). In the metallic x< 0.5
structures electrons are progressively transferred to the xz/yz bands with
increasing x, however we find no orbital-selective Mott transition down to ~300
K.Comment: 4 pages, 3 figures; published versio
Interplay of charge-transfer and Mott-Hubbard physics approached by an efficient combination of self-interaction correction and dynamical mean-field theory
Late transition-metal oxides with small charge-transfer energy raise
issues for state-of-the-art correlated electronic structure schemes such as the
combination of density functional theory (DFT) with dynamical mean-field theory
(DMFT). The accentuated role of the oxygen valence orbitals in these compounds
asks for an enhanced description of ligand-based correlations. Utilizing the
rocksalt-like NiO as an example, we present an advancement of charge
self-consistent DFT+DMFT by including self-interaction correction (SIC) applied
to oxygen. This introduces explicit onsite O correlations as well as an
improved treatment of intersite correlations. Due to the efficient SIC
incorporation in a pseudopotential form, the DFT+sicDMFT framework is an
advanced but still versatile method to address the interplay of charge-transfer
and Mott-Hubbard physics. We revisit the spectral features of stoichiometric
NiO and reveal the qualitative sufficiency of local DMFT self-energies in
describing spectral peak structures usually associated with explicit nonlocal
processes. For LiNiO, prominent in-gap states are verified by the
present theoretical study.Comment: 8 pages, 6 figure
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
Plane-wave based electronic structure calculations for correlated materials using dynamical mean-field theory and projected local orbitals
The description of realistic strongly correlated systems has recently
advanced through the combination of density functional theory in the local
density approximation (LDA) and dynamical mean field theory (DMFT). This
LDA+DMFT method is able to treat both strongly correlated insulators and
metals. Several interfaces between LDA and DMFT have been used, such as (N-th
order) Linear Muffin Tin Orbitals or Maximally localized Wannier Functions.
Such schemes are however either complex in use or additional simplifications
are often performed (i.e., the atomic sphere approximation). We present an
alternative implementation of LDA+DMFT, which keeps the precision of the
Wannier implementation, but which is lighter. It relies on the projection of
localized orbitals onto a restricted set of Kohn-Sham states to define the
correlated subspace. The method is implemented within the Projector Augmented
Wave (PAW) and within the Mixed Basis Pseudopotential (MBPP) frameworks. This
opens the way to electronic structure calculations within LDA+DMFT for more
complex structures with the precision of an all-electron method. We present an
application to two correlated systems, namely SrVO3 and beta-NiS (a
charge-transfer material), including ligand states in the basis-set. The
results are compared to calculations done with Maximally Localized Wannier
functions, and the physical features appearing in the orbitally resolved
spectral functions are discussed.Comment: 15 pages, 17 figure
Magnetism, spin texture and in-gap states: Atomic specialization at the surface of oxygen-deficient SrTiO
Motivated by recent spin- and angular-resolved photoemission (SARPES)
measurements performed on the two-dimensional electronic states confined near
the (001) surface of SrTiO in the presence of oxygen vacancies, we explore
their spin structure by means of ab initio density functional theory (DFT)
calculations of slabs. Relativistic nonmagnetic DFT calculations display
Rashba-like spin winding with a splitting of a few meV and when surface
magnetism on the Ti ions is in- cluded, bands become spin-split with an energy
difference ~100 meV at the point, consistent with SARPES findings.
While magnetism tends to suppress the effects of the relativistic Rashba
interaction, signatures of it are still clearly visible in terms of complex
spin textures. Furthermore, we observe an atomic specialization phenomenon,
namely, two types of electronic contributions: one is from Ti atoms neighboring
the oxygen vacancies that acquire rather large magnetic moments and mostly
create in-gap states; another comes from the partly polarized t
itinerant electrons of Ti atoms lying further away from the oxygen vacancy,
which form the two-dimensional electron system and are responsible for the
Rashba spin winding and the spin splitting at the Fermi surface.Comment: 6 pages, 4 figures, for Suppl. Mat. please contact first autho
LDA+Gutzwiller Method for Correlated Electron Systems
Combining the density functional theory (DFT) and the Gutzwiller variational
approach, a LDA+Gutzwiller method is developed to treat the correlated electron
systems from {\it ab-initio}. All variational parameters are self-consistently
determined from total energy minimization. The method is computationally
cheaper, yet the quasi-particle spectrum is well described through kinetic
energy renormalization. It can be applied equally to the systems from weakly
correlated metals to strongly correlated insulators. The calculated results for
SrVO, Fe, Ni and NiO, show dramatic improvement over LDA and LDA+U.Comment: 4 pages, 3 figures, 1 tabl
Electronic phase separation at LaAlO3/SrTiO3 interfaces tunable by oxygen deficiency
Electronic phase separation is crucial for the fascinating macroscopic
properties of the LaAlO3/SrTiO3 (LAO/STO) paradigm oxide interface, including
the coexistence of superconductivity and ferromagnetism. We investigate this
phenomenon using angle-resolved photoelectron spectroscopy (ARPES) in the
soft-X-ray energy range, where the enhanced probing depth combined with
resonant photoexcitation allow access to fundamental electronic structure
characteristics (momentum-resolved spectral function, dispersions and ordering
of energy bands, Fermi surface) of buried interfaces. Our experiment uses X-ray
irradiation of the LAO/STO interface to tune its oxygen deficiency, building up
a dichotomic system where mobile weakly correlated Ti t2g-electrons co-exist
with localized strongly correlated Ti eg-ones. The ARPES spectra dynamics under
X-ray irradiation shows a gradual intensity increase under constant Luttinger
count of the Fermi surface. This fact identifies electronic phase separation
(EPS) where the mobile electrons accumulate in conducting puddles with fixed
electronic structure embedded in an insulating host phase, and allows us to
estimate the lateral fraction of these puddles. We discuss the physics of EPS
invoking a theoretical picture of oxygen-vacancy clustering, promoted by the
magnetism of the localized Ti eg-electrons, and repelling of the mobile
t2g-electrons from these clusters. Our results on the irradiation-tuned EPS
elucidate the intrinsic one taking place at the stoichiometric LAO/STO
interfaces.Comment: In review with Phys. Rev. Material
Unconventional Hund Metal in a Weak Itinerant Ferromagnet
The physics of weak itinerant ferromagnets is challenging due to their small magnetic moments and the ambiguous role of local interactions governing their electronic properties, many of which violate Fermi-liquid theory. While magnetic fluctuations play an important role in the materials’ unusual electronic states, the nature of these fluctuations and the paradigms through which they arise remain debated. Here we use inelastic neutron scattering to study magnetic fluctuations in the canonical weak itinerant ferromagnet MnSi. Data reveal that short-wavelength magnons continue to propagate until a mode crossing predicted for strongly interacting quasiparticles is reached, and the local susceptibility peaks at a coherence energy predicted for a correlated Hund metal by first-principles many-body theory. Scattering between electrons and orbital and spin fluctuations in MnSi can be understood at the local level to generate its non-Fermi liquid character. These results provide crucial insight into the role of interorbital Hund’s exchange within the broader class of enigmatic multiband itinerant, weak ferromagnets
Hubbard band or oxygen vacancy states in the correlated electron metal SrVO?
We study the effect of oxygen vacancies on the electronic structure of the
model strongly correlated metal SrVO. By means of angle-resolved
photoemission (ARPES) synchrotron experiments, we investigate the systematic
effect of the UV dose on the measured spectra. We observe the onset of a
spurious dose-dependent prominent peak at an energy range were the lower
Hubbard band has been previously reported in this compound, raising questions
on its previous interpretation. By a careful analysis of the dose dependent
effects we succeed in disentangling the contributions coming from the oxygen
vacancy states and from the lower Hubbard band. We obtain the intrinsic ARPES
spectrum for the zero-vacancy limit, where a clear signal of a lower Hubbard
band remains. We support our study by means of state-of-the-art ab initio
calculations that include correlation effects and the presence of oxygen
vacancies. Our results underscore the relevance of potential spurious states
affecting ARPES experiments in correlated metals, which are associated to the
ubiquitous oxygen vacancies as extensively reported in the context of a
two-dimensional electron gas (2DEG) at the surface of insulating
transition metal oxides.Comment: Manuscript + Supplemental Material, 12 pages, 9 figure
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