Investigation of topological phases and thermoelectricity in half Heusler compounds: an Ab initio study

Abstract

Discovering new materials is a key aspect of semiconductor physics for sustaining the continuous progress and improvements in current electronic devices. The discovery of the non-trivial topological phase was a deviation from the conventional classification of materials, i.e., metals, insulators, and semiconductors. The discovery of the topological phase has become a center of attention in the condensed matter community due to riveting underlying physics and opening vast opportunities for their applications in next-generation spintronic devices. The topological phase in the materials is realized by time-reversal protected surface states, a distinctive quantum state of matter for the transport of spins. In recent years, immense research ploughed into discovering many such new materials at the horizon of physics and material science. More recently, the search has been extended to ternary compounds, particularly noncentrosymmetric half Heusler compounds. In the thesis, we have carried the detailed investigations of topological phase and thermoelectric properties in the half Heusler compounds. In the first problem, we report theoretical investigations of topological and thermoelectric properties of non-centrosymmetric half Heusler compounds XPtS (X = Sr, Ba) using first-principles calculations. In addition, we also investigated the effect of static strain (up to 10%) on their topological and thermoelectric properties. Our detailed investigations show that the XPtS compounds are topological insulators and continue as topological insulators up to a strain of 10%. However, the bandgap becomes a maximum of 0.213 eV under a strain of 3% for SrPtS and 0.164 eV at a strain of 5% for BaPtS. Thermoelectric investigations show that the Figure of merit (a measure of thermoelectric performance) ZT becomes maximum (0.222) at room temperature for BaPtS under a strain of 1% (Chapter 3). In the second problem, we investigated the topological and thermoelectric properties of the non-centrosymmetric compound LiAuTe, which forms into a dynamically stable FCC structure of space group F¯43m. While HSE calculations reveal the compound as a topological semi-metal with a band inversion at Γ point, a high value of m∗e /m∗ h as per its band structure calculations indicates its possible thermoelectric potential. From further investigations of thermoelectricity, the Seebeck coefficient and power factor are -136 μV/K and 2.1 x 1011 W.m−1.K−2.s−1, respectively, which are comparable to that of well known thermoelectric materials like HgTe, SnTe, etc. (Chapter 4). In the third problem, we report detailed investigations of topological phases in non-centrosymmetric half Heusler compound LiAuBi up to a pressure of 30.0 GPa. It is found that the compound forms into a dynamically stable face-centered cubic (FCC) lattice structure of space group F¯43m (216) at ambient pressure. The compound is topologically non-trivial at ambient pressure but undergoes a quantum phase transition to trivial topological phase at 23.4 GPa. However, the detailed investigations show a structural phase transition from FCC lattice (space group 216) to a honeycomb lattice (space group 194) at 13.0 GPa, which is also associated with a non-trivial to the trivial topological phase transition. Further investigations show that the compound also carries appreciable thermoelectric properties at ambient pressure (Chapter 5). The detailed theoretical investigations provide new materials as possible candidates for the non-trivial topological family and also a theoretical platform for experiments and their potential applications in spintronics and thermoelectricity.