Phase diagram of the sodium‐rich NaxCoO2 cobaltates

Specific heat of layered Na x CoO 2 single crystals (x=0.76-0.87) down to 100 mK and the dc-susceptibility down to 2 K in magnetic field applied parallel to the layers has been measured. The specific heat data of single crystals of exceptional high quality with 0.71 ≤ x ≤ 0.747 reported in L. Balicas et al. [Phys. Rev. Lett., 100, 126405 (2008)] with the kind permission of the authors have been also analyzed. Non-Fermi liquid (NFL) features observed in Na concentration dependent temperature region have been analyzed and discussed in terms of disorder induced NFL behavior with quantum Griffiths phases in the background. The phase diagram is proposed, the results indicate possible quantum critical point below x = 0.7 and closeness to metal-insulator transition at doping above x = 0.87.