We report on infrared spectroscopic studies of the electronic response of the (Sr1−xLax)3Ir2O7 system. Our experiments revealed hallmarks of strong electronic correlations in the evolution of the electronic response across the filling-controlled insulator-metal transition. We observed a collapse of the Jeff = 1/2 Mott gap accompanying the transfer of the spectral weight from the high-energy region to the gap region with electron doping. The intraband conductivity at the metallic side of the transition was found to consist of coherent Drude-like and incoherent responses. The sum rule and the extended Drude model analyses further indicated a large mass enhancement. Our results demonstrate a critical role of the electronic correlations in the charge dynamics of the (Sr1−xLax)3Ir2O7 system.

We report a combined infrared and angle-resolved photoemission study of the electronic response of Sr 3 (Ir 1-x Ru x ) 2 O 7 (x=0, 0.22, 0.34). The low-temperature optical conductivities of the three compounds exhibit the characteristic feature of the effective total angular momentum J eff =1/2 antiferromagnetic Mott state. As the temperature increases across the antiferromagnetic ordering temperature T N , the indirect gap gradually closes whereas the direct gap remains open. In the optical conductivity of Sr 3 (Ir 0.66 Ru 0.34 ) 2 O 7 which shows a thermally driven insulator-metal transition at T N , a Drude-like response from itinerant carriers is registered in the paramagnetic phase. We observe in angle-resolved photoemission data of Sr 3 (Ir 0.66 Ru 0.34 ) 2 O 7 that the valence band shifts continuously toward the Fermi energy with the weakening of the antiferromagnetic order and crosses the Fermi level in the paramagnetic phase. Our findings demonstrate that the temperature-induced metalinsulator transition of the Sr 3 (Ir 1-x Ru x ) 2 O 7 system should be attributed to a magnetically driven band shift. * These two authors contributed equally. † yeongkwan@kaist.ac.kr ‡ soonjmoon@hanyang.ac.kr A discovery of the relativistic Mott state in Sr 2 IrO 4 [1,2] suggested that the Mott physics can be applicable in 5d transition metal oxides and stimulated extensive studies on the nature of their metal-insulator transitions. While the electromagnetic properties of Sr 2 IrO 4 were successfully explained in terms of an effective total angular momentum J eff =1/2 Mott state [1-5], a number of experimental and theoretical studies suggested that its ground state should instead be envisioned as a Slater insulator or as an intermediate phase between the Mott and Slater insulators [6-9]. In the Slater picture, the metal-insulator transition occurs at antiferromagnetic ordering temperature T N via a continuous opening of the band gap due to the appearance of a magnetic supercell [10]. Pyrochlore iridates R 2 Ir 2 O 7 (R=Nd, Sm, and Eu) which have attracted much attention as potential candidates for realizing correlated topological insulators/semimetals [11-13] exhibit a continuous metal-insulator transition accompanying the onset of antiferromagnetic order [14]. A recent angle-resolved photoemission spectroscopy (ARPES) experiment on Nd 2 Ir 2 O 7 [15] observed a gap opening at T N with an energy shift of quasiparticle peaks in a fashion similar to the Slater transition. The continuous metal-insulator transitions at T N in Cd 2 Os 2 O 7 andNaOsO 3 were also attributed to the Slater transition in early studies [16][17][18][19]. Recently, however, the metalinsulator transitions of the two osmates were revisited and ascribed to the Lifshitz-type transition [20][21][22][23].Density-functional-theory calculations showed that the metal-insulator transitions of Cd 2 Os 2 O 7 [20] and NaOsO 3 [21] involved a continuous shift of the bands away from the Fermi level and the resulting vanishing of the Fermi surface with decreasing the t...

Ferromagnetic insulators have great potential for spintronic applications. For such applications, it is essential to find materials with a robust and controllable ferromagnetic insulating phase. However, because ferromagnetism in functional transition metal oxides is usually coupled to metallicity, ferromagnetic insulators are very rare and independent control of their magnetic and electrical properties is difficult. In this study, the electrical, magnetic, and optical properties of (LaCoO 3) n /(SrCoO 2.5) n superlattice films are investigated for the manipulation of the ferromagnetic insulating phase. Whilst the superlattices remain insulating irrespective of the periodicity n, the electronic structure and magnetic state vary drastically. Superlattices with large periodicities n of 10 and 20 show a ferromagnetic transition at a critical temperature T C of ~80 K. With decreasing periodicity and increasing interface density of the superlattices, systems with n = 4 become almost nonmagnetic, while in systems with n = 2 and 1, a reentrant ferromagnetic phase is observed at T C of ~180 and ~225 K, respectively. Optical spectroscopy reveals that the fine control of the magnetic ground state is achieved by the modified electronic structure associated with the spin-state transition. Our results suggest an important design principle to create and manipulate the ferromagnetic insulating properties of Co-based oxide thin films.

We report on infrared spectroscopy experiments on the electronic response in (Sr1−xLax)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 (Sr1−xLax)2IrO4 system.

Applications of correlated vanadium dioxides Vo 2 (A) and Vo 2 (B) in electrical devices are limited due to the lack of effective methods for tuning their fundamental properties. We find that the resistivity of Vo 2 (A) and Vo 2 (B) is widely tunable by doping them with tungsten ions. When x < 0.1 in V 1−x W x o 2 (A), the resistivity decreases drastically by four orders of magnitude with increasing x, while that of V 1− x W x o 2 (B) shows the opposite behaviour. Using spectroscopic ellipsometry and X-ray photoemission spectroscopy, we propose that correlation effects are modulated by either chemical-strain-induced redistribution of V−V distances or electron-doping-induced band filling in V 1−x W x o 2 (A), while electron scattering induced by disorder plays a more dominant role in V 1−x W x o 2 (B). The tunable resistivity makes correlated Vo 2 (A) and Vo 2 (B) appealing for next-generation electronic devices.