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Register a new userEnhanced thermoelectric response of hole-doped La$_2$NiO$_{4+delta}$ by ab initio calculations
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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.
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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.
We show how an accurate first-principles treatment of the antiferromagnetic (AFM) ground state of La$_2$CuO$_4$ can be obtained without invoking any free parameters such as the Hubbard $U$. The magnitude and orientation of our theoretically predicted magnetic moment of $0.495 mu_{B}$ on Cu-sites along the (100) direction are in excellent accord with experimental results. The computed values of the band gap (1.00 eV) and the exchange-coupling (-138 meV) match the corresponding experimental values. We identify interesting band splittings below the Fermi energy, including an appreciable Hunds splitting of 1.25 eV. The magnetic form factor obtained from neutron scattering experiments is also well described by our calculations. Our study opens up a new pathway for first-principles investigations of electronic and atomic structures and phase diagrams of cuprates and other complex materials.
In view of the continuous theoretical efforts aimed at an accurate microscopic description of the strongly correlated transition metal oxides and related materials, we show that with continuum quantum Monte Carlo (QMC) calculations it is possible to obtain the value of the spin superexchange coupling constant of a copper oxide in a quantitatively excellent agreement with experiment. The variational nature of the QMC total energy allows us to identify the best trial wave function out of the available pool of wave functions, which makes the approach essentially free from adjustable parameters and thus truly ab initio. The present results on magnetic interactions suggest that QMC is capable of accurately describing ground state properties of strongly correlated materials.
We present the TRIQS/DFTTools package, an application based on the TRIQS library that connects this toolbox to realistic materials calculations based on density functional theory (DFT). In particular, TRIQS/DFTTools together with TRIQS allows an efficient implementation of DFT plus dynamical mean-field theory (DMFT) calculations. It supplies tools and methods to construct Wannier functions and to perform the DMFT self-consistency cycle in this basis set. Post-processing tools, such as band-structure plotting or the calculation of transport properties are also implemented. The package comes with a fully charge self-consistent interface to the Wien2k band structure code, as well as a generic interface that allows to use TRIQS/DFTTools together with a large variety of DFT codes. It is distributed under the GNU General Public License (GPLv3).
We investigate charge distribution in the recently discovered high-$T_c$ superconductors, layered nickelates. With increasing value of charge-transfer energy we observe the expected crossover from the cuprate to the local triplet regime upon hole doping. We find that the $d-p$ Coulomb interaction $U_{dp}$ plays a role and makes Zhang-Rice singlets less favorable, while the amplitude of local triplets is enhanced. By investigating the effective two-band model with orbitals of $x^2-y^2$ and $s$ symmetries we show that antiferromagnetic interactions dominate for electron doping. The screened interactions for the $s$ band suggest the importance of rare-earth atoms in superconducting nickelates.
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