Roman Kovacik

Mechanism of ferroelectric instabilities in non-d0perovskites: LaCrO3versus CaMnO3

The incompatibility of partial d occupation on the perovskite B-site with the standard charge transfer mechanism for ferroelectricity has been a central paradigm in multiferroics research. Nevertheless, it was recently shown by density functional theory calculations that CaMnO_3 exhibits a polar instability that even dominates over the octahedral tilting for slightly enlarged unit cell volume. Here, we present similar calculations for LaCrO_3, which has the same d^3 B-site electron configuration as CaMnO_3. We find that LaCrO_3 exhibits a very similar, albeit much weaker, polar instability as CaMnO_3. In addition, while the Born effective charge (BEC) of the Mn^{4+} cation in CaMnO_3 is highly anomalous, the BEC of Cr^{3+} in LaCrO_3 is only slightly enhanced. By decomposing the BECs into contributions of individual Wannier functions we show that the ferroelectric instabilities in both systems can be understood in terms of charge transfer between TM d and O p states, analogously to the standard d^0 perovskite ferroelectrics.

Rubidium superoxide: A p-electron Mott insulator

Rubidium superoxide, RbO_2, is a rare example of a solid with partially-filled electronic p states, which allows to study the interplay of spin and orbital order and other effects of strong electronic correlations in a material that is quite different from the conventional d or f electron systems. Here we show, using a combination of density functional theory (DFT) and dynamical mean-field theory, that at room temperature RbO_2 is indeed a paramagnetic Mott insulator. We construct the metal-insulator phase diagram as a function of temperature and Hubbard interaction parameters U and J. Due to the strong particle-hole asymmetry of the RbO_2 band-structure, we find strong differences compared to a simple semi-elliptical density of states, which is often used to study the multiband Hubbard model. In agreement with our previous DFT study, we also find indications for complex spin and orbital order at low temperatures.

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 general tight-binding Hamiltonian written as a sum of the kinetic term, the Hunds rule coupling, the JT coupling, and the electron-electron interaction. The dispersion of the tight-binding (TB) eg 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 Hunds 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 series from R = La to Gd, in agreement with experiments. This trend correlates well with the modulation of structural properties, in particular with the progressive reduction of the Mn-O-Mn bond angle which is associated with the quenching of the volume and the decrease of the tolerance factor due to the shrinkage of the ionic radii of R going from La to Gd.

Effect of Hubbard U on the construction of low-energy Hamiltonians for LaMnO(3) via maximally localized Wannier functions

We use maximally localized Wannier functions to construct tight-binding (TB) parameterizations for the e_g bands of LaMnO_3 based on first principles electronic structure calculations. We compare two different ways to represent the relevant bands around the Fermi level: i) a d-p model that includes atomic-like orbitals corresponding to both Mn(d) and O(p) states in the TB basis, and ii) an effective e_g model that includes only two e_g-like Wannier functions per Mn site. We first establish the effect of the Jahn-Teller distortion within the d-p model, and then compare the TB representations for both models obtained from GGA+U calculations with different values of the Hubbard parameter U. We find that in the case of the d-p model the TB parameters are rather independent on the specific value of U, if compared with the mean-field approximation of an appropriate multi-band Hubbard Hamiltonian. In contrast, the U dependence of the TB parameters for the effective e_g model cannot easily be related to a corresponding mean-field Hubbard model, and therefore these parameters depend critically on the specific value of U, and more generally on the specific exchange-correlation functional, used in the electronic structure calculation.

Spin transport and spin-caloric effects in (Cr,Zn)Te half-metallic nanostructures: Effect of spin disorder at elevated temperatures from first principles

An important contribution to the thermoelectric and spin-caloric transport properties in magnetic materials at elevated temperatures is the formation of a spin-disordered state due to local moment fluctuations. This effect has not been largely investigated so far. We focus on various magnetic nanostructures of CrTe in the form of thin layers or nanowires embedded in ZnTe matrix, motivated by the miniaturization of spintronics devices and by recent suggestions that magnetic nanostructures can lead to extraordinary thermoelectric effects due to quantum confinement. The electronic structure of the studied systems is calculated within the multiple scattering screened Korringa-Kohn-Rostoker Green function (KKR-GF) framework. The Monte Carlo method is used to simulate the magnetization in the temperature induced spin disorder. The transport properties are evaluated from the transmission probability obtained using the Baranger-Stone approach within the KKR-GF framework. We find qualitative and quantitative changes in the thermoelectric and spin-caloric coefficients when spin disorder is included in the calculation. Furthermore, we show that substitutional impurities in CrTe nanowires could considerably enhance the Seebeck coefficient and the thermoelectric figure of merit.

Spin-caloric transport properties of cobalt nanostructures: Spin disorder effects from first principles

The fundamental aspects of spin-dependent transport processes and their interplay with temperature gradients, as given by the spin Seebeck coefficient, are still largely unexplored and a multitude of contributing factors must be considered. We used density functional theory together with a Monte-Carlo-based statistical method to simulate simple nanostructures, such as Co nanowires and films embedded in a Cu host or in vacuum, and investigated the influence of spin-disorder scattering on electron transport at elevated temperatures. While we show that the spin-dependent scattering of electrons due to temperature induced disorder of the local magnetic moments contributes significantly to the resistance, thermoelectric and spin-caloric transport coefficients, we also conclude that the actual magnitude of these effects cannot be predicted, quantitatively or qualitatively, without such detailed calculations.