Large thermopower in the antiferromagnetic semiconductor BaMn$_2$Bi$_2$

We report electrical and thermal transport properties of Mn-based material BaMn$_2$Bi$_2$ with ThCr$_2$Si$_2$ structure. The resistivity of the antiferromagnetic BaMn$_2$Bi$_2$ shows a metal-semiconductor transition at $\sim 80$ K with decreasing temperature. Correspondingly, the thermopower $S$ shows a peak at the same temperature, approaching ~150 $\mu$V/K. With increasing temperature $S$ decreases to about 125 $\mu$V/K at the room temperature. The magnetic field enhances the peak value to 210 $\mu$V/K. The Hall resistivity reveals an abrupt change of the carrier density close to the metal-semiconductor transition temperature.