A charge density wave-like instability in a doped spin-orbit assisted weak Mott insulator

Abstract

Layered perovskite iridates realize a rare class of Mott insulators that are predicted to be strongly spin–orbit coupled analogues of the parent state of cuprate high-temperature superconductors. Recent discoveries of pseudogap, magnetic multipolar ordered6 and possible d-wave superconducting phases in doped Sr_2IrO_4 have reinforced this analogy among the single layer variants. However, unlike the bilayer cuprates, no electronic instabilities have been reported in the doped bilayer iridate Sr_3Ir_2O_7. Here we show that Sr_3Ir_2O_7 realizes a weak Mott state with no cuprate analogue by using ultrafast time-resolved optical reflectivity to uncover an intimate connection between its insulating gap and antiferromagnetism. However, we detect a subtle charge density wave-like Fermi surface instability in metallic electron doped Sr_3Ir_2O_7 at temperatures (T_(DW)) close to 200 K via the coherent oscillations of its collective modes, which is reminiscent of that observed in cuprates. The absence of any signatures of a new spatial periodicity below T_(DW) from diffraction, scanning tunnelling and photoemission based probes suggests an unconventional and possibly short-ranged nature of this density wave order.