Carrier Mass Enhancement in La_0.5Sr_0.5CoO_3

We report measurements of optical conductivity and specific heat on La_0.5Sr_0.5CoO_3, a highly-conducting transition-metal oxide which exhibits magnetoresistance. These measurements show a mass enhancement for electronic carriers of roughly an order of magnitude over the electron bare mass. The optical conductivity shows a non-Drude behavior with a maximum at zero frequency and falling off by 1000 cm^-1 to a nearly frequency-independent value roughly half of the dc value. Analyzed in terms of the generalized Drude model, the data suggest a frequency-dependent scattering rate and mass m with the mass enhancement at zero frequency of m/m₀ = 10--15. Specific heat measurements below 15 K show a linear term γ T about 50 times greater than that calculated assuming a simple free-electron system at the same density with the bare mass m_0. The disagreement between the two estimates of the mass may be a consequence of contributions from disordered magnetic spins, which will increase the value of γ.