Decoupling of the magnetic and insulating states in single-crystal (Sr_1-xA_x)₂IrO₄ (A=Ca or Ba)

Sr₂IrO₄ is the archetype J_eff=1/2 Mott insulator with a Néel temperature T_N = 240 K and an energy gap of 0.62 eV. Here we report results of our recent study of Ca or Ba doped Sr₂IrO_4. Substituting Ca²⁺ (up to 15%) or Ba²⁺ (up to 3%) ion for the Sr²⁺ ion provides no additional charge carriers, but alters the lattice parameters and reduces IrO₆ octahedra distortions in Sr₂IrO₄. This study reveals that Ca or Ba doping retains the Néel temperature, however, drastically reduces the electrical resistivity by more than five orders of magnitude or even precipitates a sharp insulator-to-metal transition at lower temperatures. In essence, the vanishing insulating state due to Ca or Ba doping is accompanied by an unchanged Néel temperature. This study provides strong evidence that there is an unusual correlation between the magnetic and insulating state in this spin-orbit tuned insulator.