Doping-induced topological transition and enhancement of thermopower in the Dirac-semimetal system Cd${}_{3-x}$Zn${}_x$As${}_2$

Cd${}_3$As${}_2$ is one of the prototypical topological Dirac semimetals. Here, we manipulate the band inversion responsible for the emergence of Dirac nodes by alloying Cd${}_3$As${}_2$ with topologically trivial Zn${}_3$As${}_2$. We observe the expected topological phase transition around a Zn concentration of $x\sim 1$whilethecarrierdensitymonotonicallydecreasesas$x$ is increased. For larger $x$, the thermoelectric figure of merit exhibits comparably large values exceeding 0.3 at room temperature, due to the combined effects of a strong enhancement of the thermopower, an only moderate increase of the resistivity, and a suppression of the thermal conductivity. Complementary quantum-oscillation data and optical-conductivity measurements allow to infer that the enhanced thermoelectric performance is due to a flattening of the band structure in the higher-$x$ region in Cd${}_{3-x}$Zn${}_x$As${}_2$.