Origin of the large thermoelectric power in oxygen-variable RBaCo2O5+x (R=Gd,Nd)

Thermoelectric properties of GdBaCo2O5+x and NdBaCo2O5+x single crystals have been studied upon continuous doping of CoO2 planes with either electrons or holes. The thermoelectric response and the resistivity behavior reveal a hopping character of the transport in both compounds, providing the basis for understanding the recently found remarkable divergence of the Seebeck coefficient at x=0.5. The doping dependence of the thermoelectric power evinces that the configurational entropy of charge carriers, enhanced by their spin and orbital degeneracy, plays a key role in the origin of the large thermoelectric response in these correlated oxides.