We investigated electronic structure of 5d transition-metal oxide Sr 2 IrO 4 using angle-resolved photoemission, optical conductivity, and x-ray absorption measurements and first-principles band calculations. The system was found to be well described by novel effective total angular momentum J eff states, in which relativistic spin-orbit (SO) coupling is fully taken into account under a large crystal field. Despite of delocalized Ir 5d states, the J eff -states form so narrow bands that even a small correlation energy leads to the J eff = 1/2 Mott ground state with unique electronic and magnetic behaviors, suggesting a new class of the J eff quantum spin driven correlated-electron phenomena.

The nutrition transition in South Korea is unique. National efforts to retain elements of the traditional diet are thought to have shaped this transition in South Korea in the midst of rapid economic growth and the introduction of Western culture.

Strong charge-spin coupling is found in a layered transition-metal trichalcogenide NiPS_{3}, a van der Waals antiferromagnet, from studies of the electronic structure using several experimental and theoretical tools: spectroscopic ellipsometry, x-ray absorption, photoemission spectroscopy, and density functional calculations. NiPS_{3} displays an anomalous shift in the optical spectral weight at the magnetic ordering temperature, reflecting strong coupling between the electronic and magnetic structures. X-ray absorption, photoemission, and optical spectra support a self-doped ground state in NiPS_{3}. Our work demonstrates that layered transition-metal trichalcogenide magnets are useful candidates for the study of correlated-electron physics in two-dimensional magnetic materials.

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We investigated the metal-insulator transition (MIT) driven by all-in-all-out (AIAO) antiferromagnetic ordering in the 5d pyrochlore Cd 2 Os 2 O 7 using optical spectroscopy and first-principles calculations. We showed that the temperature evolution in the band-gap edge and free carrier density were consistent with rigid upward (downward) shifts of electron (hole) bands, similar to the case of Lifshitz transitions. The delicate relationship between the band gap and free carrier density provides experimental evidence for the presence of an AIAO metallic phase, a natural consequence of such MITs. The associated spectral weight change at high energy and first-principles calculations further support the origin of the MIT from the band shift near the Fermi level. Recently, MITs and antiferromagnetic ordering in 5d pyrochlores, A 2 B 2 O 7 (B ¼ Ir, Os), have received much attention due to their strong correlation and the key role they play in novel phases. The MITs in these compounds [3][4][5][6][7][8] reportedly are prompted by the emergence of magnetic ordering. Such magnetism-driven MITs are expected to induce exotic phenomena, including a Weyl semimetal [9][10][11][12] and quantum criticality [13] in rare-earth pyrochlore iridates. However, questions remain as to how magnetic ordering induces MITs in 5d pyrochlores.The peculiar antiferromagnetic structure in 5d pyrochlores makes the MITs distinct from the Slater type [14], a wellknown magnetic-driven MIT. In the original Slater picture, collinear antiferromagnetic ordering induces a doubling of the lattice periodicity, resulting in a BCS-like band gap at the folded zone boundaries. However, recent studies support that the magnetic ground states in 5d pyrochlores are noncollinear all-in-all-out (AIAO) magnetic orderings [9][10][11][15][16][17], where all spins in transition metal ions align inward or outward of tetrahedra. Since AIAO ordering maintains the unit cell of pyrochlores, a new paradigm beyond the Slater picture is essential to understanding MITs in 5d pyrochlores.Cd 2 Os 2 O 7 is a good candidate for the study of such MITs, in that it shows a continuous MIT at an antiferromagnetic ordering temperature T N (¼ 227 K) [18,19]. Until its magnetic ground state was revealed, this material was best known as one of a few examples of a Slater insulator [20]. However, recent experimental [19,21] and theoretical [22,23] results showed that the magnetic ground state in Cd 2 Os 2 O 7 is AIAO ordering. Investigations on Cd 2 Os 2 O 7 should provide an understanding of how AIAO ordering causes MITs in 5d pyrochlores.In this Letter, we report on the detailed temperature (T) evolution of the optical spectra of Cd 2 Os 2 O 7 to reveal how the emergence of AIAO ordering affects the electronic structure and induces the MITs. The absorption edge analyses suggest the Lifshitz-type MIT where a rigid shift of the bands away from the Fermi level E F gradually annihilates the Fermi surface [22,24]. The model based on the Lifshitz-type MIT reproduces the delicate relationship betw...