“Pudding Mold” Band Drives Large Thermopower in Na x CoO 2

In the present study, we pin down the origin of the coexistence of the large thermopower and the large conductivity 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 a1g 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.