67 research outputs found
Competing magnetic interactions in spin-1/2 square lattice: hidden order in SrVO
With decreasing temperature SrVO undergoes two structural phase
transitions, tetragonal-to-orthorhombic-to-tetragonal, without long-range
magnetic order. Recent experiments suggest, that only at very low temperature
SrVO might enter some, yet unknown, phase with long-range magnetic
order, but without orthorhombic distortion. By combining relativistic density
functional theory with an extended spin-1/2 compass-Heisenberg model we find an
antiferromagnetic single-stripe ground state with highly competing exchange
interactions, involving a non negligible inter-layer coupling, which places the
system at the crossover between between the XY and Heisenberg picture. Most
strikingly, we find a strong two-site "spin-compass" exchange anisotropy which
is relieved by the orthorhombic distortion induced by the spin stripe order.
Based on these results we discuss the origin of the hidden order phase and the
possible formation of a spin-liquid at low temperatures
Strain-induced tuning of the electronic Coulomb interaction in 3d transition metal oxide perovskites
Epitaxial strain offers an effective route to tune the physical parameters in
transition metal oxides. So far, most studies have focused on the effects of
strain on the bandwidths and crystal field splitting, but recent experimental
and theoretical works have shown that also the effective Coulomb interaction
changes upon structural modifications. This effect is expected to be of
paramount importance in current material engineering studies based on
epitaxy-based material synthesization. Here, we perform constrained random
phase approximation calculations for prototypical oxides with a different
occupation of the d shell, LaTiO3 (d1), LaVO3 (d2), and LaCrO3 (d3), and
systematically study the evolution of the effective Coulomb interactions
(Hubbard U and Hund's J) when applying epitaxial strain. Surprisingly, we find
that the response upon strain is strongly dependent on the material. For
LaTiO3, the interaction parameters are determined by the degree of localization
of the orbitals, and grow with increasing tensile strain. In contrast, LaCrO3
shows the opposite trends: the interactions parameters shrink upon tensile
strain. This is caused by the enhanced screening due to the larger electron
filling. LaVO3 shows an intermediate behavior
Converged GW quasiparticle energies for transition metal oxide perovskites
The ab initio calculation of quasiparticle (QP) energies is a technically and
computationally challenging problem. In condensed matter physics the most
widely used approach to determine QP energies is the GW approximation. Although
the GW method has been widely applied to many typical semiconductors and
insulators, its application to more complex compounds such as transition metal
oxide perovskites has been comparatively rare, and its proper use is not well
established from a technical point of view. In this work, we have applied the
single-shot G0W0 method to a representative set of transition metal oxide
perovskites including 3d (SrTiO3, LaScO3, SrMnO3, LaTiO3, LaVO3, LaCrO3,
LaMnO3, and LaFeO3), 4d (SrZrO3, SrTcO3, and Ca2RuO4) and 5d (SrHfO3, KTaO3 and
NaOsO3) compounds with different electronic configurations, magnetic orderings,
structural characteristics and bandgaps ranging from 0.1 to 6.1 eV. We discuss
the proper procedure to obtain well converged QP energies and accurate bandgaps
within single-shot G0W0 by comparing the conventional approach based on an
incremental variation of a specific set of parameters (number of bands, energy
cutoff for the plane-wave expansion and number of k-points and the basis-set
extrapolation scheme [Phys. Rev. B 90, 075125 (2014)]. In addition, we have
inspected the difference between the adoption of norm-conserving and ultrasoft
potentials in GW calculations. A minimal statistical analysis indicates that
the correlation of the GW data with the DFT gap is more robust than the
correlation with the experimental gaps; moreover we identify the static
dielectric constant as alternative useful parameter for the approximation of GW
gap in high-throughput automatic procedures. Finally, we compute the QP band
structure and spectra within the random phase approximation and compare the
results with available experimental data.Comment: Physical Review Materials, accepte
Lifshitz transition driven by spin fluctuations and spin-orbit renormalization in NaOsO
In systems where electrons form both dispersive bands and small local spins,
we show that changes of the spin configuration can tune the bands through a
Lifshitz transition, resulting in a continuous metal-insulator transition
associated with a progressive change of the Fermi surface topology. In contrast
to a Mott-Hubbard and Slater pictures, this spin-driven Lifshitz transition
appears in systems with small electron-electron correlation and large
hybridization. We show that this situation is realized in 5 distorted
perovskites with an half-filled bands such as NaOsO, where the
strong hybridization reduces the local moment, and spin-orbit coupling
causes a large renormalization of the electronic mobility. This weakens the
role of electronic correlations and drives the system towards an itinerant
magnetic regime which enables spin-fluctuations
Relativistic +BSE study of the optical properties of Ruddlesden-Popper iridates
We study the optical properties of the Ruddlesden-Popper series of iridates
SrIrO (=1, 2 and ) by solving the
Bethe-Salpeter equation (BSE), where the quasiparticle (QP) energies and
screened interactions are obtained by the approximation including
spin-orbit coupling. The computed optical conductivity spectra show strong
excitonic effects and reproduce very well the experimentally observed
double-peak structure, in particular for the spin-orbital Mott insulators
SrIrO and SrIrO. However, does not account well for
the correlated metallic state of SrIrO owing to a much too small band
renormalization, and this affects the overall quality of the optical
conductivity. Our analysis describes well the progressive redshift of the main
optical peaks as a function of dimensionality (), which is correlated with
the gradual decrease of the electronic correlation (quantified by the
constrained random phase approximation) towards the metallic limit.
We have also assessed the quality of a computationally cheaper BSE approach
that is based on a model dielectric function and conducted on top of DFT+
one-electron energies. Unfortunately, this model BSE approach does not
accurately reproduce the outcome of the full +BSE method and leads to
larger deviations to the measured spectra.Comment: 13 pages, 8 figure
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