Pudding mold band drives large thermopower in NaxCoO2

In the present study, we pin down the origin of the coexistence of the large thermopower and the low resistivity in NaxCoO2. It is revealed that not just the density of states (DOS), the effective mass, nor the band width, but the peculiar shape of the a,, band referred to as the pudding mold type, which consists of a dispersive portion and a somewhat flat portion, is playing an important role in this phenomenon. The present study provides a new guiding principle for designing good thermoelectric materials.