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^1,2. Recent discoveries of pseudogap^3,4,5, magnetic multipolar ordered⁶ and possible d-wave superconducting phases^7,8 in doped Sr₂IrO₄ have reinforced this analogy among the single layer variants. However, unlike the bilayer cuprates⁹, no electronic instabilities have been reported in the doped bilayer iridate Sr₃Ir₂O₇. Here we show that Sr₃Ir₂O₇ realizes a weak Mott state with no cuprate analogue⁹ 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 Sr₃Ir₂O₇ at temperatures (T_DW) close to 200 K via the coherent oscillations of its collective modes, which is reminiscent of that observed in cuprates^10,11. The absence of any signatures of a new spatial periodicity below T_DW from diffraction¹², scanning tunnelling¹² and photoemission^13,14 based probes suggests an unconventional and possibly short-ranged nature of this density wave order.

层状钙钛矿铱酸盐实现了罕见的Mott绝缘子类别，该绝缘子预计将是铜酸盐高温超导体^1,2的母态的强自旋轨道耦合类似物。在掺杂的Sr ₂ IrO ₄中伪间隙^3,4,5，磁性多极有序⁶和可能的d波超导相^7,8的最新发现在单层变体中加强了这种类比。然而，与双层铜酸盐^9不同，在掺杂的双层铱酸盐Sr ₃ Ir ₂ O _7中没有电子不稳定的报道。在这里我们显示Sr ₃Ir ₂ O ₇通过使用超快的时间分辨光反射率来揭示其绝缘间隙与反铁磁性之间的紧密联系，从而实现了无铜酸盐类似物⁹的弱Mott态。然而，我们通过其集体模态的相干振荡，在接近200 K的温度（T _DW）下，检测到金属电子掺杂的Sr ₃ Ir ₂ O ₇中微妙的电荷密度波状费米表面不稳定性，这让人想起了铜币^10,11。衍射¹²在T _DW以下没有新的空间周期性的任何特征，基于扫描隧道¹²和光发射^13,14的探针表明，这种密度波阶具有非常规且可能是短距离的性质。