Magnetic field dependent specific heat and enhanced Wilson ratio in strongly correlated layered cobalt oxide

We have investigated the low-temperature specific-heat properties as a function of magnetic field in the strongly correlated layered cobalt oxide BiBa 0.66 K 0.36 O 2 CoO 2. These measurements reveal two kinds of magnetic field dependent contributions in qualitative agreement with the presence of a previously inferred magnetic quantum critical point QCP. First, the coefficient of the low-temperature T 3 behavior of the specific heat turns out to sizably decrease near a magnetic field consistent with the critical value reported in a recent paper. In addition, a moderate but significant enhancement of the Sommerfeld coefficient is found in the vicinity of the QCP suggesting a slight increase in the electronic effective mass. This result contrasts with the divergent behavior of the previously reported Pauli susceptibility. Thus, a strongly enhanced Wilson ratio is deduced, suggesting efficient ferromagnetic fluctuations in the Fermi-liquid regime which could explain the unusual magnetic field dependent specific heat. As a strong check, the high magnetic field Wilson ratio asymptotically recovers the universal limit of the local Fermi liquid against ferromagnetism. Transition-metal oxides have demonstrated over the last decades how the strong correlations could lead to unantici-pated electronic properties. Outstanding examples 1 are super-conducting cuprates, manganites with their colossal negative magnetoresistance, 2 vanadates displaying the Mott metal-insulator transition, 3 and the layered cobalt oxides which exhibit an unexpected large thermopower at room temperature. 4 Most of these oxides share in common that they are doped Mott insulator, i.e., their metallicity originates from the introduction of charge carriers by doping; otherwise , the strong Coulomb repulsion would localize electrons to form a Mott insulating state. 1,5 Belonging to this class of materials the layered cobalt oxides have revealed, besides their enhanced room-temperature thermopower, 6 a very rich phase diagram as well as striking properties 7-9 including large negative magnetoresistance in some compounds 10 or giant electron-electron scattering in Na 0.7 CoO 2. 11 Interestingly , the latter observation has already led to conjecture a possible influence of a magnetic QCP in the aforementioned compound. Density-functional calculations have also predicted at the local spin-density approximation level weak itinerant ferromagnetic state competing with weak itinerant antiferromagnetic st