239 research outputs found
Structural determination and electronic properties of 4d perovskite SrPdO3
The structure and ground state electronic structure of the recently
synthesized SrPdO perovskite [A. Galal {\em et al.}, J. Power Sources, {\bf
195}, 3806 (2010)] have been studied by means of screened hybrid functional and
the GW approximation with the inclusion of electron-hole interaction within the
test-charge/test-charge scheme. By conducting a structural search based on
lattice dynamics and group theoretical method we identify the orthorhombic
phase with space group as the most stable crystal structure. The
phase transition from the ideal cubic perovskite structure to the one
is explained in terms of the simultaneous stabilization of the
antiferrodistortive phonon modes and . Our results indicate that
SrPdO exhibits an insulating ground state, substantiated by a GW gap of
about 1.1 eV. Spin polarized calculations suggests that SrPdO adopts a low
spin state
(), and
is expected to exhibit spin excitations and spin state crossovers at finite
temperature, analogous to the case of 3 isoelectronic LaCoO. This would
provide a new playground for the study of spin state transitions in 4 oxides
and new opportunity to design multifunctional materials based on 4
building block
Ab initio prediction of the high-pressure phase diagram of BaBiO3
BaBiO3 is a well-known example of a 3D charge density wave (CDW) compound, in which the CDW behavior is induced by charge disproportionation at the Bi site. At ambient pressure, this compound is a charge-ordered insulator, but little is known about its high-pressure behavior. In this work, we study from first principles the high-pressure phase diagram of BaBiO3
using phonon mode analysis and evolutionary crystal structure prediction. We show that charge disproportionation is very robust in this compound and persists up to 100 GPa. This causes the system to remain insulating up to the highest pressure we studied
study of (=Ba, Sr, Ca) under high pressure
Using crystal structure prediction we study the
high-pressure phase diagram of bismuthates (=Ba, Sr, Ca)
in a pressure range up to 100GPa. All compounds show a transition from the
low-pressure perovskite structure to highly distorted, low-symmetry phases at
high pressures (PD transition), and remain charge disproportionated and
insulating up to the highest pressure studied. The PD transition at high
pressures in bismuthates can be understood as a combined effect of steric
arguments and of the strong tendency of bismuth to charge-disproportionation.
In fact, distorted structures permit to achieve a very efficient atomic
packing, and at the same time, to have Bi-O bonds of different lengths. The
shift of the PD transition to higher pressures with increasing cation size
within the series can be explained in terms of chemical
pressure
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
Combined first-principles and model Hamiltonian study of the perovskite series RMnO3 (R = La, Pr, Nd, Sm, Eu and Gd)
We merge advanced ab initio schemes (standard density functional theory,
hybrid functionals and the GW approximation) with model Hamiltonian approaches
(tight-binding and Heisenberg Hamiltonian) to study the evolution of the
electronic, magnetic and dielectric properties of the manganite family RMnO3 (R
= La, Pr, Nd, Sm, Eu and Gd). The link between first principles and
tight-binding is established by downfolding the physically relevant subset of
3d bands with e_g character by means of maximally localized Wannier functions
(MLWFs) using the VASP2WANNIER90 interface. The MLWFs are then used to
construct a tight-binding Hamiltonian. The dispersion of the TB e_g bands at
all levels are found to match closely the MLWFs. We provide a complete set of
TB parameters which can serve as guidance for the interpretation of future
studies based on many-body Hamiltonian approaches. In particular, we find that
the Hund's rule coupling strength, the Jahn-Teller coupling strength, and the
Hubbard interaction parameter U remain nearly constant for all the members of
the RMnO3 series, whereas the nearest neighbor hopping amplitudes show a
monotonic attenuation as expected from the trend of the tolerance factor.
Magnetic exchange interactions, computed by mapping a large set of hybrid
functional total energies onto an Heisenberg Hamiltonian, clarify the origin of
the A-type magnetic ordering observed in the early rare-earth manganite series
as arising from a net negative out-of-plane interaction energy. The obtained
exchange parameters are used to estimate the Neel temperature by means of Monte
Carlo simulations. The resulting data capture well the monotonic decrease of
the ordering temperature down the R series, in agreement with experiments.Comment: 13 pages, 9 figures, 3 table
Interplay between adsorbates and polarons: CO on rutile TiO(110)
Polaron formation plays a major role in determining the structural,
electrical and chemical properties of ionic crystals. Using a combination of
first principles calculations and scanning tunneling microscpoy/atomic force
microscopy (STM/AFM), we analyze the interaction of polarons with CO molecules
adsorbed on the rutile TiO(110) surface. Adsorbed CO shows attractive
coupling with polarons in the surface layer, and repulsive interaction with
polarons in the subsurface layer. As a result, CO adsorption depends on the
reduction state of the sample. For slightly reduced surfaces, many adsorption
configurations with comparable adsorption energies exist and polarons reside in
the subsurface layer. At strongly reduced surfaces, two adsorption
configurations dominante: either inside an oxygen vacancy, or at surface
Ti sites, coupled with a surface polaron
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