Strongly correlated properties of the thermoelectric cobalt oxide Ca3Co4O9

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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 Tmin≈ 63 K, a strongly correlated Fermi liquid up to T*≈ 140 K, with ρ=ρ₀+AT² and A≈ 3.63 10⁻² μΩcm/K², followed by an incoherent metal with kFl≤ 1 and a high temperature insulator above T**≈ 510 K. 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 μΩcm.K²/(mJ²mol⁻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/a 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.