Infrared Spectroscopic Evidences of Strong Electronic Correlations in (Sr1-xLax)3Ir2O7

Revelation of Mott insulating state in layered honeycomb lattice Li2RuO3

We investigate the role of electron correlation in the electronic structure of honeycomb lattice Li₂RuO₃using photoemission spectroscopy and band structure calculations. Monoclinic Li₂RuO₃having Ru network as honeycomb lattice undergoes magneto-structural transition atT_c∼ 540 K from high temperature phaseC2/mto low temperature dimerized phaseP2₁/m. Room temperature valence band photoemission spectra reveal an insulating ground state with no intensity at Fermi level (E_F). Ru 4dband extracted from high and low photon energy valence band photoemission spectra reveal that the surface and bulk electronic structures are very similar in this system. Band structure calculations using generalized gradient approximation leads to metallic ground state while screened hybrid (YS-PBE0) functional reveals opening up of a gap in almost degenerated_zx/d_yzorbitals, whereasd_xyorbital is already gapped. Ru 3dcore level spectra with prominent unscreened feature provides direct evidence of strong electron correlation among Ru 4delectrons which is also manifested by |E-E_F|²dependence of spectral density of states in the vicinity ofE_Fin the high-resolution spectra, establishing Li₂RuO₃as Mott insulator.

Doping and temperature evolutions of optical response of Sr3(Ir1-xRux)2O7

We report on optical spectroscopic study of the Sr₃(Ir_1-xRu_x)₂O₇ system over a wide doping regime. We find that the changes in the electronic structure occur in the limited range of the concentration of Ru ions where the insulator-metal transition occurs. In the insulating regime, the electronic structure associated with the effective total angular momentum J_eff = 1/2 Mott state remains robust against Ru doping, indicating the localization of the doped holes. Upon entering the metallic regime, the Mott gap collapses and the Drude-like peak with strange metallic character appears. The evolution of the electronic structure registered in the optical data can be explained in terms of a percolative insulator-metal transition. The phonon spectra display anomalous doping evolution of the lineshapes. While the phonon modes of the compounds deep in the insulating and metallic regimes are almost symmetric, those of the semiconducting compound with x = 0.34 in close proximity to the doping-driven insulator-metal transition show a pronounced asymmetry. The temperature evolution of the phonon modes of the x = 0.34 compound reveals the asymmetry is enhanced in the antiferromagnetic state. We discuss roles of the S = 1 spins of the Ru ions and charge excitations for the conspicuous lineshape asymmetry of the x = 0.34 compound.

Infrared probe of pseudogap in electron-doped Sr2IrO4

We report on infrared spectroscopy experiments on the electronic response in (Sr_1-x La _x )₂IrO₄ (x = 0, 0.021, and 0.067). Our data show that electron doping induced by La substitution leads to an insulator-to-metal transition. The evolution of the electronic structure across the transition reveals the robustness of the strong electronic correlations against the electron doping. The conductivity data of the metallic compound show the signature of the pseudogap that bears close similarity to the analogous studies of the pseudogap in the underdoped cuprates. While the low energy conductivity of the metallic compound is barely frequency dependent, the formation of the pseudogap is revealed by the gradual suppression of the featureless conductivity below a threshold frequency of about 17 meV. The threshold structure develops below about 100 K which is in the vicinity of the onset of the short-range antiferromagnetic order. Our results demonstrate that the electronic correlations play a crucial role in the anomalous charge dynamics in the (Sr_1-x La _x )₂IrO₄ system.