Effects of electronic correlations and disorder on the thermopower of NaxCoO2

Abstract

For the thermoelectric properties of NaxCoO2, we analyze the effect of local Coulomb interaction and (disordered) potential differences for Co-sites with adjacent Na-ion or vacancy. The disorder potential alone increases the resistivity and reduces the thermopower, while the Coulomb interaction alone leads only to minor changes compared to the one-particle picture of the local density approximation. Only combined, these two terms give rise to a substantial increase of the thermopower: the number of (quasi-)electrons around the Fermi level is much more suppressed than that of the (quasi-)holes. Hence, there is a particle-hole imbalance acting in the same direction as a similar imbalance for the group velocities. Together, this interplay results in a large positive thermopower. Introducing a thermoelectric spectral density, we located the energies and momenta regions most relevant for the thermopower and changes thereof.

Enhancement of the effective disorder potential and thermopower in NaxCoO2through electron-phonon coupling

“…16 and 18 in Ref. 8) the conductivity will be reduced by the electron-phonon scattering. At the same time, less disorder also means a reduced disorder scattering and hence a higher conductivity.…”

“…As already introduced in Ref. 8, sites with an adjacent Na atom (i ∈ Na) are not altered, while a local disorder potential is added to the "vacancy" sites (i ∈ Vac).…”

“…4 It has been shown that several conditions are important for getting the high thermopower: a "pudding mold"-like shape of the band structure, 5 strong correlation, and also disorder. [6][7][8][9] Phonons have also been argued to have a positive effect on the thermopower: 10 "nano phonon engineering" as in heterostructures, thin films, cage structures, and nanowires can indeed overcome the obstacle that a high electrical conductivity is usually connected with a high thermal conductivity, which is inversely proportional to the figure of merit. However, in materials characterized by the presence of several concurrent physical mechanisms, not only the individual effects but also the interplay and the possible synergies between them have to be carefully analyzed.…”

“…The effect of two inequivalent sites on the electronic correlations was then analyzed by Marianetti and Kotliar 6,14 by means of LDA plus dynamical mean-field theory (DMFT) 22 with the inclusion of the disorder at the level of the coherent potential approximation (CPA). Further LDA + DMFT studies which focused on reproducing the thermopower of Na x CoO 2 7,8 analyzed the mechanism behind the influence of disorder on electronic correlations: because of the disorder the lattice sites have locally different occupations and, since a fraction of them moves towards half filling, correlation effects get effectively enhanced. This increase in correlation leads to stronger electron-hole asymmetry in the LDA + DMFT spectra with respect to the LDA and this favors a large thermopower.…”

Ab initio study of thermoelectric properties of doped SnO2 superlattices

“…Among them, we can mention layered cobalt oxides, perovskite-structured compounds, transparent conductive oxides (TCOs), among others. Various p-and n-type oxide semiconductors, such as Ca 3 Co 4 O 9 , Na x CoO 2 , LaCoO 3 , SrTiO 3 , CaMnO 3 , ZnO, TiO 2 and SnO 2 have been studied [6][7][8][9][10][11][12][13][14][15][16][17][18][19]. Despite the efforts, a suitable value for ZT has not yet been achieved.…”