Structural determination and electronic properties of 4d perovskite SrPdO3

The structure and ground state electronic structure of the recently synthesized SrPdO$_3$ 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 $P_{nma}$ space group as the most stable crystal structure. The phase transition from the ideal cubic perovskite structure to the $P_{nma}$ one is explained in terms of the simultaneous stabilization of the antiferrodistortive phonon modes $R_4^+$ and $M_3^+$. Our results indicate that SrPdO$_3$ exhibits an insulating ground state, substantiated by a GW$_0$ gap of about 1.1 eV. Spin polarized calculations suggests that SrPdO$_3$ adopts a low spin state ($t_{2g}^{\uparrow\downarrow\uparrow\downarrow\uparrow\downarrow}e_g^0$), and is expected to exhibit spin excitations and spin state crossovers at finite temperature, analogous to the case of 3$d$ isoelectronic LaCoO$_3$. This would provide a new playground for the study of spin state transitions in 4$d$ oxides and new opportunity to design multifunctional materials based on 4$d$ $P_{nma}$ 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

Ab-initio\textit{Ab-initio} study of ABiO3\textit{A}BiO_3 (AA=Ba, Sr, Ca) under high pressure

Using $\textit{ab-initio}$ crystal structure prediction we study the high-pressure phase diagram of $\textit{A}BiO_3$ bismuthates ($A$=Ba, Sr, Ca) in a pressure range up to 100$~$GPa. 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 $\textit{A}BiO_3$ series can be explained in terms of chemical pressure

Competing magnetic interactions in spin-1/2 square lattice: hidden order in Sr2_2VO4_4

With decreasing temperature Sr$_2$VO$_4$ undergoes two structural phase transitions, tetragonal-to-orthorhombic-to-tetragonal, without long-range magnetic order. Recent experiments suggest, that only at very low temperature Sr$_{2}$VO$_{4}$ 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 TiO2_2(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$_2$(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$_{5c}$ sites, coupled with a surface polaron