Thermoelectric Effect in a Topological Dirac Semimetal under Strain and Confinement

The topological Dirac semimetal Cd₃As₂ is a potential thermoelectric material due to its high electron mobility and low lattice thermal conductivity. However, in its bulk form, Cd₃As₂ has no band gap, which is detrimental for the thermopower. One way to open a band gap in the bulk is through quantum confinement and strain in thin films. In this project, we investigate and contrast the thermoelectric properties of epitaxial Cd₃As₂ films with two different thicknesses (950 nm and 95 nm). Resistivity and quantum oscillation measurements established the semimetal nature of the thick sample, while the thin sample showed semiconducting features, suggesting the presence of a bulk gap, likely due to epitaxial strain and a weak confinement effect. The thermoelectric properties of these samples were qualitatively different. The thick sample exhibited a semi-metallic behavior, where its Seebeck coefficient can be fitted by a semiclassical model, while a sign change of the Seebeck coefficient was observed in the thin sample at low temperatures. In addition, the Nernst coefficient of the thick sample showed a dispersive (Drude-like) peak versus the magnetic field and an anomalous Nernst effect appeared below 50 K, in agreement with previous bulk measurements. In contrast, the thin sample showed a monotonically increasing Nernst coefficient versus magnetic field and an absence of anomalous Nernst effect down to 2 K. Our results open possibilities for engineering the thermoelectric properties of topological semimetals through strain and quantum confinement effect.