Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userStrain effects to optimize the thermoelectric properties of hole-doped La$_2$NiO$_{4+delta}$ via ab initio calculations
123
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
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.
rate research
Read More
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.
Kondo insulator FeSb$_2$ with large Seebeck coefficient would have potential in thermoelectric applications in cryogenic temperature range if it had not been for large thermal conductivity $kappa$. Here we studied the influence of different chemical substitutions at Fe and Sb site on thermal conductivity and thermoelectric effect in high quality single crystals. At $5%$ of Te doping at Sb site thermal conductivity is suppressed from $sim 250$ W/Km in undoped sample to about 8 W/Km. However, Cr and Co doping at Fe site suppresses thermal conductivity more slowly than Te doping, and even at 20$%$ Cr/Co doping the thermal conductivity remains $sim 30$ W/Km. The analysis of different contributions to phonon scattering indicates that the giant suppression of $kappa$ with Te is due to the enhanced point defect scattering originating from the strain field fluctuations. In contrast, Te-doping has small influence on the correlation effects and then for small Te substitution the large magnitude of the Seebeck coefficient is still preserved, leading to the enhanced thermoelectric figure of merit ($ZTsim 0.05$ at $sim 100$ K) in Fe(Sb$_{0.9}$Te$_{0.1}$)$_2$.
The electronic structure and magnetic properties of CrSb$_2$ have been investigated by ab-initio calculations with an emphasis on the role of the magnetic structure for the ground state. The influence of correlation effects has been investigated by performing fixed spin moment (FSM) calculations showing their important role for the electronic and magnetic properties. The details of the electronic structure of CrSb$_2$ are analyzed by a comparison with those of FeSb$_2$. The results obtained contribute in particular to the understanding of the temperature dependence of transport and magnetic behavior observed experimentally.
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.
Doping Bi$_2$Se$_3$ by magnetic ions represents an interesting problem since it may break the time reversal symmetry needed to maintain the topological insulator character. Mn dopants in Bi$_2$Se$_3$ represent one of the most studied examples here. However, there is a lot of open questions regarding their magnetic ordering. In the experimental literature different Curie temperatures or no ferromagnetic order at all are reported for comparable Mn concentrations. This suggests that magnetic ordering phenomena are complex and highly susceptible to different growth parameters, which are known to affect material defect concentrations. So far theory focused on Mn dopants in one possible position, and neglected relaxation effects as well as native defects. We have used ab initio methods to calculate the Bi$_2$Se$_3$ electronic structure influenced by magnetic Mn dopants, and exchange interactions between them. We have considered two possible Mn positions, the substitutional and interstitial one, and also native defects. We have found a sizable relaxation of atoms around Mn, which affects significantly magnetic interactions. Surprisingly, very strong interactions correspond to a specific position of Mn atoms separated by van der Waals gap. Based on the calculated data we performed spin dynamics simulations to examine systematically the resulting magnetic order for various defect contents. We have found under which conditions the experimentally measured Curie temperatures ${T_{rm{C}}}$ can be reproduced, noticing that interstitial Mn atoms appear to be important here. Our theory predicts the change of ${T_{rm{C}}}$ with a shift of Fermi level, which opens the way to tune the system magnetic properties by selective doping.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
