Toroidal moments as indicator for magneto-electric coupling: the case of BiFeO_3 versus FeTiO_3

In this paper we present an analysis of the magnetic toroidal moment and its relation to the various structural modes in R3c-distorted perovskites with magnetic cations on either the perovskite A or B site. We evaluate the toroidal moment in the limit of localized magnetic moments and show that the full magnetic symmetry can be taken into account by considering small induced magnetic moments on the oxygen sites. Our results give a transparent picture of the possible coupling between magnetization, electric polarization, and toroidal moment, thereby highlighting the different roles played by the various structural distortions in multiferroic BiFeO_3 and in the recently discussed isostructural material FeTiO_3, which has been predicted to exhibit electric field-induced magnetization switching.Comment: 6 pages, 2 figure

Labour markets in a Post-Keynesian growth model: the effects of endogenous productivity growth and working time reduction

We study endogenous employment and distribution dynamics in a Post-Keynesian model of Kalecki-Steindl tradition. Productivity adjustments stabilize employment and the labour share in the long run: technological change allows firms to replenish the reserve army of workers in struggle over income shares and thereby keep wage demands in check. We discuss stability conditions and the equilibrium dynamics. This allows us to study how legal working time and its reduction affect the equilibrium. We find that a demand shock is likely to lower the profit share and increase the employment rate. A supply shock in contrast tends to have detrimental effects on employment and income distribution. Labour market institutions and a working time reduction have no long-term effect on growth, distribution and inflation in the model. The effects on the level of capital stock and output however are positive in a wage-led demand regime. Furthermore, an erosion of labour market institutions dampens inflation temporarily. The model provides possible explanations as to the causes of several current economic phenomena such as secular stagnation, digitalisation, and the break-down of the Philips curve.Series: Ecological Economic Paper

First-Principles-Based Strain and Temperature Dependent Ferroic Phase Diagram of SrMnO3_3

Perovskite structure SrMnO$_3$ is a rare example of a multiferroic material where strain-tuning and/or cation substitution could lead to coinciding magnetic and ferroelectric ordering temperatures, which would then promise strong magnetoelectric coupling effects. Here, we establish the temperature and strain dependent ferroic phase diagram of SrMnO$_3$ using first-principles-based effective Hamiltonians. All parameters of these Hamiltonians are calculated using density functional theory, i.e., no fitting to experimental data is required. Temperature dependent properties are then obtained from Monte Carlo and molecular dynamics simulations. We observe a sequence of several magnetic transitions under increasing tensile strain, with a moderate variation of the corresponding critical temperatures. In contrast, the ferroelectric Curie temperature increases strongly after its onset around 2.5\,\% strain, and indeed crosses the magnetic transition temperature just above 3\,\% strain. Our results indicate pronounced magnetoelectric coupling, manifested in dramatic changes of the magnetic ordering temperatures and different magnetic ground states as function of the ferroelectric distortion. In addition, coexisting ferroelectric and ferromagnetic order is obtained for strains above 4\,\%. Our calculated phase diagram suggests the possibility to control the magnetic properties of SrMnO$_3$ through an applied electric field, significantly altering the magnetic transition temperatures, or even inducing transitions between different magnetic states.Comment: 13 pages, 8 figure

Interplay between breathing mode distortion and magnetic order in rare-earth nickelates RRNiO3_3 within DFT+UU

We present a systematic density functional theory (DFT) plus Hubbard $U$ study of structural trends and the stability of different magnetically ordered states across the rare-earth nickelate series, $R$NiO$_3$, with $R$ from Lu to La. In particular, we investigate how the magnetic order, the change of the rare-earth ion, and the Hubbard interaction $U$ are affecting the bond-length disproportionation between the nickel sites. Our results show that structural parameters can be obtained that are in very good agreement with present experimental data, and that DFT+$U$ is in principle able to capture the most important structural trends across the nickelate series. However, the amplitude of the bond-length disproportionation depends very strongly on the specific value used for the Hubbard $U$ parameter and also on the type of magnetic order imposed in the calculation. Regarding the relative stability of different magnetic orderings, a realistic antiferromagnetic order, consistent with the experimental observations, is favored for small $U$ values, and becomes more and more favorable compared to the ferromagnetic state towards the end of the series (i.e., towards $R$=Pr). Nevertheless, it seems that the stability of the ferromagnetic state is generally overestimated within the DFT+$U$ calculations. Our work provides a profound starting point for more detailed experimental investigations, and also for future studies using more advanced computational techniques such as, e.g., DFT combined with dynamical mean-field theory.Comment: 13 pages, 11 figures, 1 tabl

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.Comment: 11 pages, 11 figures, 2 table