Metal-insulator Transition and Topological Phases of Pyrochlore Iridates
The 4d and 5d transition metal oxides are interesting because these materials incorporate both strong spin-orbit coupling and strong correlations, and consequently display distinct physical properties and the tantalizing possibility of novel topological phases. A prominent family in this class, the rare earth pyrochlore iridates, shows a metal-insulator transition and non-collinear complex magnetic ordering in the insulating state. We carry out magnetic band structure calculations using the GGA+U method, which reproduce the systematic trend that stronger order and larger gaps occur with decreasing rare earth radius.
A corresponding paramagnetic band calculation shows that Pr2Ir2O7 is a strong candidate for a nodal quadratic band touching state, in which the doubly degenerate conduction and valence bands touch at the zone center, right at the Fermi level. This suggests that Pr2Ir2O7 is very sensitive to perturbations, such as time reversal symmetry or cubic symmetry breaking terms, giving rise to the possibility of many novel phases. Indeed, we demonstrate using first-principles calculations that uniaxial strain applied along the (111) direction opens a band gap and converts the material to a strong topological insulator.