Pudding-mold type band as the origin of large thermopower in transition metal oxides

Date & Time
9 May 2008 (Fri), 10:30 am - 12:00 am
8F large seminar room, Sengen site
Professor Ryotaro Arita(有田亮太郎准教授)
Department of Applied Physics, University of Tokyo(東京大学大学院工学系研究科物理工学専攻)
Pudding-mold type band as the origin of large thermopower in transition metal oxides
Conversion of temperature differences directly into electricity is one of the most fascinating phenomena in condensed matter physics. Designing and searching for good thermoelectric materials indeed have a long history of extensive studies due to the scientific interest and potential technological importance.
Hitherto, it has been believed that the coexistence of low resistivity and large thermopower is difficult to realize, so that the main target thermoelectric materials have been various insulators or semiconductors. More recently, the focus of interest shifted however to thermoelectrical metals, where one is searching for exceptionally large power factors. NaxCoO2 and LiRh2O4 are in fact good examples of such interesting thermoelectric metals.
Here we propose that a peculiar shape of conduction bands plays an important role to realize the coexistence of large conductivity and thermopower.
We first examine this idea for NaxCoO2 and LiRh2O4 by the Boltzmann equation approach based on the local density approximation (LDA). We then step forward to the combination of LDA and the dynamical mean field theory (LDA+DMFT) for LiRh2O4, a 3D frustrated paramagnetic metal for which DMFT is expected to be a good approximation. We find that the experimental values of the large thermopower are successfully reproduced. We also simulate how the power factor can be much further enhanced through doping.
Dr. Igor Solovyev