Topological phase transitions in Tl<sub>2</sub>Ta<sub>2</sub>O<sub>7 </sub> under strain regulation

Topological electronic materials exhibit many novel physical properties, such as low dissipation transport and high carrier mobility. These extraordinary properties originate from their non-trivial topological electronic structures in momentum space. In recent years, topological phase transitions based on topological electronic materials have gradually become one of the hot topics in condensed matter physics. Using first-principles calculations, we explore the topological phase transitions driven by in-plane strain in ternary pyrochlore oxide Tl₂Ta₂O₇. Firstly, we analyze the atomic-orbital-resolved band structure and find that the O (p_<i>x</i>+p_<i>y</i>) and p_<i>z</i> orbitals of the system near the Fermi level have band inversion, indicating the emergence of topological phase transitions in the system. Then the tight-binding models are constructed to calculate the <i>Z</i>₂ topological invariants, which can determine the topologically non-trivial feature of the system. Finally, topological properties such as surface states and a three-dimensional Dirac cone are studied. It is found that Tl₂Ta₂O₇ without strain is a semimetal with a quadratic band touching point at Fermi level, while the in-plane strain can drive the topological phase transition via breaking crystalline symmetries. When the system is under the –1% in-plane compression strain and without considering the spin orbit coupling (SOC), the application of strain results in two triply degenerate nodal points formed in the –<i>Z</i> to <i>Γ</i> direction and <i>Γ</i> to <i>Z</i> direction, respectively. When the SOC is included, there are two fourfold degenerate Dirac points on the –<i>Z</i> to <i>Γ</i> path and <i>Γ</i> to <i>Z</i> path<i>,</i> respectively. Thus, the –1% in-plane compression strain makes the system transit from the quadratic contact point semimetal to a Dirac semimetal. When 1% in-plane expansion strain is applied and the SOC is neglected, there exists one band intersection along <i>Y→</i><i>Γ</i>. When the SOC is taken into consideration, the gap is opened. Therefore, the 1% in-plane expansion strain drives Tl₂Ta₂O₇ into a strong topological insulator. In addition, the system is also expected to have strong correlation effect and superconductivity due to the possible flat band. This work can guide the study of topological phase transitions in three-dimensional materials and provide a good material platform for the design of low-dissipation electronic devices.