Topological insulators and metal-insulator transition in the pyrochlore iridates

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 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.