ICT2014 Abstracts

The interplay between the spin-orbit coupling and non-trivial band topology gives rise to a new quantum state of matter, having an inverted bulk band gap with conducting edge or surface states, known as topological insulators. These surface states have nearly linear energy dispersion and are topologically protected by the time reversal symmetry.  Most of the known topological insulators are excellent thermoelectric materials having very high figure of merit. Thus, we investigate the bulk and surface electronic properties of GeBi₂Te₄ (GBT124), using the first-principles density functional theory calculations, with projector augmented wave basis, using the VASP (Vienna Ab- initio Simulation Package) code. The GGA approximation is used for the exchange correlation potential and the SOC is included self consistently. Since many thermoelectric properties of other similar systems in literature are based on the Wien2K package with Boltzmann theory and the constant scattering time approximation (CSTA), so we have used Wien2k package with BoltzTrap code for calculating the thermoelectric properties. Our bulk and surface electronic structure results show that GBT124 is a topological insulator, with Z₂ =1;(111), having a single Dirac cone surface state at the Γ - point. The non-trivial surface state has a large in-plane polarization with spin-momentum locked spin-texture, where spin is locked perpendicular to momentum. An out-of-plane spin-component starts appearing as we move away from the Dirac point, due to the dominant effect from the  cubic Dressalhaus spin-orbit coupling term. The calculated thermopower of GBT124 is in the order of few hundred μV/K and is different for electron and hole carriers. This difference in the thermopower is attributed to different atomic type bands near the Fermi level. The calculated thermopower for both the hole and electron carriers extends over a high temperature region and thus making GBT124 a suitable candidate for high temperature thermoelectric applications.