Bulk nature of layered perovskite iridates beyond the Mott scenario: An approach from a bulk-sensitive photoemission study

Abstract

We present genuine bulk Ir 5d j eff states of layered perovskite iridates obtained by hard-x-ray photoemission spectroscopy (HAXPES) with s-and p-polarized lights. HAXPES spectra of Sr 2 IrO 4 and Ba 2 IrO 4 are well reproduced by the quasiparticle density of states calculated by the local density approximation with dynamical mean-field theory (LDA + DMFT). It is demonstrated that the insulating nature of the iridates is triggered by antiferromagnetic correlation (Slater type) combined with electron correlation (Mott type). The extremely-lowenergy bulk-sensitive photoemission spectroscopy reveals "bad metallic" states in the paramagnetic phase of the iridates, suggesting strongly renormalized metallic states above the Néel temperature as predicted by the LDA + DMFT. Mott physics, which explains the insulating nature in materials triggered by strong electron correlations, has been vigorously studied for several decades from both experimental and theoretical points of view [1]. Owing to these efforts, new mechanisms for the metal-insulator transition (MIT) were proposed in some "Mott" systems, for instance, orbital switching for VO 2 and the essential role of antiferromagnetic (AF) correlation effects for La 2 CuO 4 [2,3].Layered perovskite iridates A 2 IrO 4 (A = Sr, Ba) have an insulating state, the origin of which was mysterious in the early stage of the research [4]. Detailed spectroscopic and theoretical studies recently demonstrated Sr 2 IrO 4 to be a spin-orbit (SO) -driven Mott insulator [5]. According to that scenario, the electronic structure near the Fermi level (E F ) is characterized by Kramers-doublet Ir 5d j eff = 1/2 states, which are eigenstates of the SO Hamiltonian within the t 2g subspace. For the iridate with five 5d electrons, the lower j eff = 3/2 bands are fully occupied and the higher half-filled j eff = 1/2 band split into the upper and lower Hubbard bands due to the relatively weak on-site Coulomb interaction (U ). This simple picture has been widely accepted since the results of experiments, such as optical conductivity spectra, seemed to be well explained [5,6]. In addition, the results of angle-resolved ultraviolet photoemission spectroscopy were well reproduced by the local density approximation (LDA) + U + SO band structure calculations [5]. For 5d transition-metal systems, however, the nature of atom-band duality, which is a characteristic feature of heavy elements, makes the interpretation of optical conductivity spectra complicated [7]. In addition, we point out here that the photoemission spectra excited by several tens-eV photons are strongly affected by the surface electronic structures modified by stronger electron correlation effect on the surface than in the bulk even for quasi-two-dimensional compounds [8,9].Recently, the LDA study with dynamical mean-field theory (LDA + DMFT) pointed out that both Sr 2 IrO 4 and Ba 2 IrO 4 should be classified as Slater insulators, in which the AF correlation mainly contributed to realizing the insulating states in these materials rather ...

Spin dynamics in NaFeAs and NaFe0.53Cu0.47As probed by resonant inelastic x-ray scattering

“…It is interesting to note the RIXS spectra of BaFe 2 Se 3 are dominated by ddexcitations, which are typically strongly suppressed in the iron pnictides [47], this suggests a larger U in BaFe 2 Se 3 compared to the iron pnictides. Similar crossover between Mott-type and Slater-type insulating behavior has been discussed for the iridates Sr 2 IrO 4 [79][80][81] and Nd 2 Ir 2 O 7 [82].…”

Spin dynamics in NaFeAs and NaFe$_{0.53}$Cu$_{0.47}$As probed by resonant inelastic X-ray scattering

“…It is interesting to note the RIXS spectra of BaFe 2 Se 3 are dominated by dd-excitations, which are typically strongly suppressed in the iron pnictides [47], this suggests larger U in BaFe 2 Se 3 compared to the iron pnictides. Similar crossover between Mott-type and Slater-type insulating behavior has been discussed for the iridates Sr 2 IrO 4 [80][81][82] and Nd 2 Ir 2 O 7 [83].…”

Anomalous High-Energy Waterfall-Like Electronic Structure in 5 d Transition Metal Oxide Sr2IrO4 with a Strong Spin-Orbit Coupling

“…A number of experimental results are consistent with this scenario 7 10 11 12 13 14 15 16 17 18 . Moreover, in addition to the J eff = 1/2 Mott scenario 7 , the slater-type mechanism is also found to be important in the formation of the insulating ground state in Sr 2 IrO 4 20 21 22 . A consensus that has been reached is Sr 2 IrO 4 being a typical system where the Mott- and slater-type behaviors coexist.…”