Topological insulators and metal-insulator transition in the pyrochlore iridates

TL;DR

This paper investigates the conditions under which topological insulators can exist in pyrochlore iridates, emphasizing the roles of crystal field effects, lattice distortions, and external pressure in stabilizing or destabilizing topological phases.

Contribution

It reveals how trigonal crystal fields and lattice distortions influence topological phases, proposing external pressure as a method to induce topological insulators in pyrochlore iridates.

Findings

Trigonal crystal field destabilizes topological insulator, leading to metallic state.

Lattice distortions can restore topological insulator phase.

External pressure along [111] can induce topological insulating state.

Abstract

The possible existence of topological insulators in cubic pyrochlore iridates A$_{2}$Ir$_{2}$O$_{7}$ (A = Y or rare-earth elements) is investigated by taking into account the strong spin-orbit coupling and trigonal crystal field effect. It is found that the trigonal crystal field effect, which is always present in real systems, may destabilize the topological insulator proposed for the ideal cubic crystal field, leading to a metallic ground state. Thus the trigonal crystal field is an important control parameter for the metal-insulator changeover. We propose that this could be one of the reasons why distinct low temperature ground states may arise for the pyrochlore iridates with different A-site ions. On the other hand, examining the electron-lattice coupling, we find that softening of the $\textbf{q}$=0 modes corresponding to trigonal or tetragonal distortions of the Ir pyrochlore lattice leads to the resurrection of the strong topological insulator. Thus, in principle, a finite temperature transition to a low-temperature topological insulator can occur via structural changes. We also suggest that the application of the external pressure along [111] or its equivalent directions would be the most efficient way of generating strong topological insulators in pyrochlore iridates.