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.
Strain control is one of the most promising avenues to search for new emergent phenomena in transition-metal-oxide films. Here, we investigate the strain-induced changes of electronic structures in strongly correlated LaNiO3 (LNO) films, using angle-resolved photoemission spectroscopy and the dynamical mean-field theory. The strongly renormalized eg-orbital bands are systematically rearranged by misfit strain to change its fermiology. As tensile strain increases, the hole pocket centered at the A point elongates along the kz-axis and seems to become open, thus changing Fermi-surface (FS) topology from three- to quasi-two-dimensional. Concomitantly, the FS shape becomes flattened to enhance FS nesting. A FS superstructure with Q1 = (1/2,1/2,1/2) appears in all LNO films, while a tensile-strained LNO film has an additional Q2 = (1/4,1/4,1/4) modulation, indicating that some instabilities are present in metallic LNO films. Charge disproportionation and spin-density-wave fluctuations observed in other nickelates might be their most probable origins.
We have performed a high-resolution angle-resolved photoemission spectroscopy ͑ARPES͒ on antiferromagnetic USb to study the electronic structure near the Fermi level. We found that USb has a metallic band structure with the fully occupied Sb 5p bands in contrast to semimetallic CeSb that has the partially filled Sb 5p bands. This suggests that the magnetic phase transition of USb is not understood within the framework of the p-f mixing model. This difference in the electronic structure between USb and CeSb is ascribed to the energy position of the respective bare f level with respect to the Sb 5p band. The observed fully occupied Sb 5p bands in USb is consistent with the band calculation based on the itinerant U 5 f model, but different from that of the localized model. On the other hand, we found two dispersionless bands just below E F in ARPES spectra of USb, which are well described in terms of the 5 f 2 -final-state multiplet structure calculated based on the localized 5 f model. These experimental results suggest the dual ͑itinerant and localized͒ character of 5 f electrons that characterizes anomalous properties of USb.
High-resolution photoemission spectroscopy ͑HRPES͒ was performed on V 2Ϫy O 3 (yϭ0.00, 0.03, and 0.04͒, which has been considered as the prototype of the Mott-Hubbard model for the metal-insulator transition ͑MIT͒. The density of states at the Fermi level (E F ) observed by HRPES is one order of magnitude smaller than that of the band calculation based on the local-density approximation. We observed no distinguishable sharp structure at E F for hole-doped V 1.96 O 3 at 15 K as well as negligible temperature-dependence of the HRPES spectrum of V 2 O 3 between 200 and 300 K. The analysis of the V 3d spectrum with a local self-energy gives a too large quasiparticle effective mass compared to the specific-heat measurement, which implies the importance of spatial fluctuations near the MIT. All these facts strongly question the applicability of the dynamical mean-field theory of the Hubbard model to the V 2Ϫy O 3 system.
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