Thermoelectric properties of the misfit cobaltate Ca3_3Co4_4O9_9

The layered misfit cobaltate Ca$_3$Co$_4$O$_9$, also known as Ca$_2$CoO$_3$[CoO$_2$]$_{1.62}$, is a promising p-type thermoelectric oxide. Employing density functional theory, we study its electronic structure and determine, on the basis of Boltzmann theory within the constant-relaxation-time approximation, the thermoelectric transport coefficients. The dependence on strain and temperature is determined. In particular, we find that the $xx$-component of the thermopower is strongly enhanced, while the $yy$-component is strongly reduced, when applying 2% tensile strain. A similar anisotropy is also found in the power factor. The temperature dependence of the conductivity in the $a$-$b$ plane is found to be rather weak above 200 K, which clearly indicates that the experimentally observed transport properties are dominated by inhomogeneities arising during sample growth, i.e., are not intrinsic.Comment: 12 pages (preprint style), 5 figure

Tailoring the intrinsic doping at YBa2Cu3O7-metal-contacts

Charge redistribution at interfaces between high-Tc superconductors and metals is problematic for technological applications due to local variations in the electronic structure. In particular, the interface affects the charge density in the superconducting CuO2-planes. For obtaining quantitative insight into effects of interface doping, we address YBCO-metal-contacts by means of first principle supercell calculations within density functional theory. On the one hand, we find that the CuO2-planes are intrinsically electron-overdoped, i.e. hole-underdoped. On the other hand, very strong effects on the near-contact electronic states are also caused by electronegative impurities incorporated into the metal. Doping of such impurities consequently paves the way for a controlled re-extracting of charge from intrinsically doped interfaces and, therefore, for a tailoring of the interface properties

VO2: A Novel View from Band Theory

New calculations for vanadium dioxide, one of the most controversely discussed materials for decades, reveal that band theory as based on density functional theory is well capable of correctly describing the electronic and magnetic properties of the metallic as well as both the insulating M1 and M2 phases. Considerable progress in the understanding of the physics of VO2 is achieved by the use of the recently developed hybrid functionals, which include part of the electron-electron interaction exactly and thereby improve on the weaknesses of semilocal exchange functionals as provided by the local density and generalized gradient approximations. Much better agreement with photoemission data as compared to previous calculations is found and a consistent description of the rutile-type early transition-metal dioxides is achieved.Comment: 5 pages, 4 figure