Strongly correlated properties of the thermoelectric cobalt oxide Ca₃Co₄O₉

We have performed both in-plane resistivity, Hall effect, and specific heat measurements on the thermoelectric cobalt oxide Ca₃Co₄O₉. Four distinct transport regimes are found as a function of temperature, corresponding to a low temperature insulating one up to T_min≈63 K, a strongly correlated Fermi liquid up to T^*≈140 K, with ρ=ρ₀+AT² and A≈3.63×10⁻² μΩ cm∕K², followed by an incoherent metal with k_Fl⩽1 and a high temperature insulator above T^**≈510 K. The specific heat Sommerfeld coefficient γ=93 mJ∕(mol K²) confirms a rather large value of the electronic effective mass and fulfils the Kadowaki-Woods ratio A∕γ²≈0.45×10⁻⁵ μΩ cm K²∕(mJ² mol⁻²). Resistivity measurements under pressure reveal a decrease of the Fermi liquid transport coefficient A with an increase of T^* as a function of pressure while the product A(T^*)²∕b₂ remains constant and of order h∕e². Both thermodynamic and transport properties suggest a strong renormalization of the quasiparticles coherence scale of order T^* that seems to govern also thermopower.