Enhanced thermoelectric properties of the topological phase of monolayer HfC

Thermoelectric properties of a topological insulator, monolayer HfC, are calculated using first-principles calculation, which accounts for the two contributions from edge and bulk states. By applying strain up to 8% along the a axis, the monolayer HfC shows the topological phase while it is in a non-topological state without strain. The figure of merit, ZT, for the topological phase becomes two-ordered magnitude larger (ZT ∼ 2) because of larger electric conductivity than that of the non-topological phase due to edge current. The total Seebeck coefficient S, and ZT have maximum values when the chemical potential is located at the highest energy of the edge state. The peak of ZT comes from the fact that the product of divergent S and quickly-decreasing electric conductivity above the highest energy of the edge state. We further optimize S and ZT by changing the sample size and temperature.