Enhanced Seebeck coefficient by a filling-induced Lifshitz transition in KxRhO2

We have systematically measured the transport properties in the layered rhodium oxide ${\mathrm{K}}_x{\mathrm{RhO}}₂$ single crystals ($0.5\ensuremath{\lesssim}x\ensuremath{\lesssim}0.67$), which is isostructural to the thermoelectric oxide ${\mathrm{Na}}_x{\mathrm{CoO}}₂$. We find that below $x=0.64$ the Seebeck coefficient is anomalously enhanced at low temperatures with increasing $x$, while it is proportional to the temperature as a conventional metal above $x=0.65$, suggesting the existence of a critical content ${x}^*\ensuremath{\simeq}0.65$. For the origin of this anomalous behavior, we discuss a filling-induced Lifshitz transition, which is characterized by a sudden topological change in the cylindrical hole Fermi surfaces at the critical content ${x}^*$.