Low-temperature thermopower and phonon-drag effect in the quasi-one-dimensional cobaltate Ba3Co2O6(CO3)0.7

The quasi-one-dimensional cobaltate Ba3Co2O6(CO3)0.7 exhibits a high thermoelectric performance characterized by the large thermopower and high electrical conductivity above room temperature, while the low-temperature transport properties have been less investigated so far. Here, we systematically measure the thermopower of single-crystalline Ba3Co2O6(CO3)0.7 samples at low temperatures. The temperature dependence of the thermopower near room temperature is similar to that of other thermoelectric cobaltates, such as layered cobaltates, but exhibits a sample-dependent prominent peak structure near T = 40 K. We evaluate the sample dependence of the carrier density and the mobility and find that the saturation effect of phonon drag is essential for the observed peak structure in the thermopower. The enhanced thermopower utilizing a phonon-drag effect in the present material may be crucial for low-temperature thermoelectrics.