  In this study an extended low energy phase diagram for NaxCoO2 is
experimentally established with emphasis on the high x range. It is based on
systematic heat capacity studies on both polycrystalline and single crystalline
samples and on uSR measurements. Main features are the existence of mass
enhancement, spin fluctuations without long-range order, and magnetic order
with associated Fermi surface gapping. The latter is seen in the electronic
density of states (DOS) and suppression of nuclear specific heat. While there
is agreement between the band structure and the low energy DOS in the low x
range, in the high x range (x > 0.6) the thermodynamically determined DOS is
approximately three times that deduced from the angle-resolved photoemission
spectroscopy (ARPES)-measured band dispersion or local-density approximation
(LDA) calculations.
