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.

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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 examined the temperature (T ) evolution of the optical conductivity spectra of Sr 3 Ir 2 O 7 over a wide range of 10-400 K. The system was barely insulating, exhibiting a small indirect bandgap of 0.1 eV. The low-energy features of the optical d-d excitation ( ω < 0.3 eV) evolved drastically, whereas such evolution was not observed for the O K-edge x-ray-absorption spectra. This suggests that the T evolution in optical spectra is not caused by a change in the bare (undressed) electronic structure, but instead presumably originates from an abundance of phonon-assisted indirect excitations. Our results showed that the low-energy excitations were dominated by phonon-absorption processes which involved, in particular, the optical phonons. This implies that phonon-assisted processes significantly facilitate the charge dynamics in barely insulating Sr 3 Ir 2 O 7 .

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...

We investigated the electronic structure and lattice dynamics of multiferroic MnWO 4 by optical spectroscopy. With variation of polarization, temperature, and magnetic field, we obtained optical responses over a wide range of photon energies. The electronic structure of MnWO 4 near to the Fermi level was examined, with inter-band transitions identified in optical conductivity spectra above a band-gap of 2.5 eV. As for the lattice dynamics, we identified all the infrared transverse optical phonon modes available according to the group-theory analysis. Although we did not observe much change in global electronic structure across the phase transition temperatures, an optical absorption at around 2.2 eV showed an evident change depending upon the spin configuration and magnetic field. The behavior of this band-edge absorption indicates that spin-orbit coupling plays an important role in multiferroic MnWO 4 .

Japan 2We investigate infrared manifestations of the pseudogap in the prototypical cuprate and pnictide superconductors: YBa 2 Cu 3 O y and BaFe 2 As 2 (Ba122) systems. We find remarkable similarities between the spectroscopic features attributable to the pseudogap in these two classes of superconductors. The hallmarks of the pseudogap state in both systems include a weak absorption feature at about 500 cm -1 followed by a featureless continuum between 500 and 1500 cm -1 in the conductivity data and a significant suppression in the scattering rate below 700 -900 cm -1 . The latter result allows us to identify the energy scale associated with the pseudogap  PG . We find that in the Ba122-based materials the superconductivity-induced changes of the infrared spectra occur in the frequency region below 100 -200 cm -1 , which is much lower than the energy scale of the pseudogap. We performed theoretical analysis of the scattering rate data of the two compounds using the same model which accounts for the effects of the pseudogap and electron-boson coupling. We find that the scattering rate suppression in Ba122-based compounds below  PG is solely due to the pseudogap formation whereas the impact of the electron-boson coupling effects is limited to lower frequencies. The magnetic resonance modes used as inputs in our modeling are found to evolve with the development of the pseudogap, suggesting an intimate correlation between the pseudogap and magnetism.