Polarized Raman spectroscopy study of metallic (Sr^sub 1−x^La^sub x^)^sub 3^Ir^sub 2^O^sub 7^ : A consistent picture of disorder-interrupted unidirectional charge order

We have used rotational anisotropic polarized Raman spectroscopy to study the symmetries, the temperature, and the doping dependence of the charge ordered state in metallic (Sr1−x Lax)3Ir2O7. Despite the fact that the Raman probe size is greater than the charge ordering length, we establish that the charge ordering breaks the fourfold rotational symmetry of the underlying tetragonal crystal lattice into twofold, as well as the translational symmetry, and forms short-range domains with 90 ∘ rotated charge order wave vectors, as soon as the charge order sets in below TCO = ∼ 200 K and across the doping-induced insulator metal transition. We observe that this charge order mode frequency remains nearly constant over a wide temperature range and up to the highest doping level. These features above are highly reminiscent of the ubiquitous unidirectional charge order in underdoped high-TC copper-oxide-based superconductors (cuprates). We further resolve that the charge order damping rate diverges when approaching TCO from below and increases significantly as increasing the La-doping level, which resembles the behaviors for a disorder-interrupted ordered phase and has not been observed for the charge order in cuprates.