Infrared Hall conductivity of Na0.7 Co O2

We report the infrared Hall conductivity σxy (ω) of Na0.7 Co O2 thin films at ω=1100 cm-1 determined from Faraday rotation θF measurements. σxy (ω) exhibits two types of hole conduction, Drude and incoherent carriers. The coherent Drude carriers appear at low temperatures and show a large renormalized mass and Fermi-liquid-like behavior of Hall scattering rate, γH ∼a T2. The spectral weight is suppressed with increasing temperature and disappears at T=120 K. The incoherent carrier response is centered at mid-IR frequency and shifts to lower energy with increasing T. The infrared Hall constant is positive and almost independent of temperature, in sharp contrast with the dc Hall constant and theoretical predictions.