Charge ordering at the interface in (LaMnO$_3$)$_{2n}$/(SrMnO$_3$)$_n$ superlattices as the origin of their insulating state

We have performed ab initio calculations within the LDA+U method in the multilayered system (LaMnO$_3$)$_{2n}$ / (SrMnO$_3$)$_n$. Our results suggest a charge-ordered state that alternates Mn$^{3+}$ and Mn$^{4+}$ cations in a checkerboard in-plane pattern is developed at the interfacial layer, leading to a gap opening. Such an interfacial charge-ordered situation would be the energetically favored reconstruction between LaMnO$_3$ and SrMnO$_3$. This helps understanding the insulating behavior observed experimentally in these multilayers at intermediate values of $n$, whose origin is known to be due to some interfacial mechanism.

Strain effects to optimize the thermoelectric properties of hole-doped La$_2$NiO$_{4+delta}$ via ab initio calculations

Thermoelectric properties of the system La$_2$NiO$_{4+delta}$ have been recently discussed [Phys. Rev. B 86, 165114 (2012)] via ab initio calculations. An optimum hole-doping value was obtained with reasonable thermopower and thermoelectric figure of merit being calculated. Here, a large increase in the thermoelectric performance through lattice strain and the corresponding atomic relaxations is predicted. This increase would be experimentally attainable via growth in thin films of the material on top of different substrates. A small tensile strain would produce large thermoelectric figures of merit at high temperatures, $zT$ $sim$ 1 in the range of oxygen excess $delta$ $sim$ 0.05 - 0.10 and in-plane lattice parameter in the range 3.95 - 4.05 AA. In that relatively wide range of parameters, thermopower values close to 200 $mu$V/K are obtained. The best performance of this compound is expected to occur in the high temperature limit.

Enhanced thermoelectric response of hole-doped La$_2$NiO$_{4+delta}$ by ab initio calculations

Thermoelectric properties of the system La$_2$NiO$_{4+delta}$ have been studied ab initio. Large Seebeck coefficient values are predicted for the parent compound, and to some extent remain in the hole-doped metallic phase, accompanied of an increase in the conductivity. This system, due to its layered structure would be a suitable candidate for an improvement of its thermoelectric figure of merit by nanostructurization in thin films, that has already been shown to increase the electrical conductivity ($sigma$). Our calculations show that in the region around La$_2$NiO$_{4.05}$ the system has a large thermopower at high temperatures and also a substantially increased $sigma$. Films grown with this low-doping concentration will show an optimal relationship between thermopower and $sigma$. This result is obtained for various exchange-correlation schemes (correlated, uncorrelated and parameter-free) that we use to analyze the electronic structure of the hole-doped compound.