Precise control of band filling in Na _xCoO ₂

Electronic properties of the sodium cobaltate Na _xCoO ₂ are systematically studied through a precise control of band filling. Resistivity, magnetic susceptibility and specific heat measurements are carried out on a series of high-quality polycrystalline samples prepared at 200 °C with Na content in a wide range of 0:35 ≤ x ≤ 0:70. It is found that dramatic changes in electronic properties take place at a critical Na concentration x* that lies between 0.58 and 0.59, which separates a Pauli paramagnetic and a Curie-Weiss metals. It is suggested that at x* the Fermi level touches the bottom of the a _1g band at the Γ point, leading to a crucial change in the density of states across x* and the emergence of a small electron pocket around the Γ point for x > x*.