We have studied the electronic structure of MnO by photoemission spectroscopy. Mn 3d–derived emission is found to be confined within ∼10 eV of the top of the valence band, and we find no evidence for an intrinsic satellite at higher binding energies. The photoemission intensity of the whole valence band is enhanced for photon energies in the Mn 3p→3d core absorption region. These observations suggest that ${\mathit{d}}^4$ and ${\mathit{d}}^5$L photoemission final states (L denotes a ligand hole) are close in energy and are strongly hybridized with each other. The spectra are analyzed in terms of configuration-interaction theory using a ${\mathrm{MnO}}_6$ cluster model. We thus find that the ligand-to-Mn d charge-transfer energy Δ is comparable to the intra-atomic Coulomb energy U≃7.5 eV. Namely, MnO is indeed close to the boundary between the Mott-Hubbard (U<Δ) and the charge-transfer (U>Δ) regimes in the Zaanen-Sawatzky-Allen phase diagram, and the ${\mathit{d}}^4$ and ${\mathit{d}}^5$L states are nearly degenerate. The large Δ value leads to a highly ionic character in the Mn-O bonding.

The electronic properties of substitutional 3d transition-metal impurities in II-VI semiconductors have been studied using the cluster and Anderson impurity models with configuration interaction. It is shown that the photoemission and inverse-photoemission spectra, d-d optical-absorption spectra, exchange interaction between the 3d magnetic moment and the host band states, and donor and acceptor ionization energies can be reproduced with the same set of parameters, which show systematic variation with expected chemical trends. The importance of multiplet effects in the formation of donor and acceptor levels within the band gap is demonstrated.

We studied LaAlO${}_3$/SrTiO${}_3$ interfaces for varying LaAlO${}_3$ thickness by core-level photoemission spectroscopy. In Ti $2p$ spectra for conducting “$n$-type” interfaces, Ti${}^{3+}$ signals appeared, which were absent for insulating “$p$-type” interfaces. The intensity of the Ti${}^{3+}$ signals was significantly higher than that expected for the transport carrier densities, indicating that part of the carriers at the interfaces are localized. The Ti${}^{3+}$ signals increased with LaAlO${}_3$ thickness, but started well below the critical thickness of 4 unit cells for metallic transport. Core-level shifts with LaAlO${}_3$ thickness were much smaller than predicted by the polar catastrophe model. We attribute these observations to surface defects and/or adsorbates providing charges to the interface even below the critical thickness.

Quantum phase transitions play an important role in shaping the phase diagram of high-temperature cuprate superconductors. These cuprates possess intertwined orders which interact strongly with superconductivity. However, the evidence for the quantum critical point associated with the charge order in the superconducting phase remains elusive. Here, we reveal the short-range charge orders and the spectral signature of the quantum fluctuations in ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ (LSCO) near the optimal doping using high-resolution resonant inelastic x-ray scattering. On performing calculations through a diagrammatic framework, we discover that the charge correlations significantly soften several branches of phonons. These results elucidate the role of charge order in the LSCO compound, providing evidence for quantum critical scaling and discommensurations associated with charge order.

Thin films of the ferromagnetic metal ${\mathrm{SrRuO}}_3$ (SRO) show a varying easy magnetization axis depending on the epitaxial strain, and undergo a metal-to-insulator transition with decreasing film thickness. We have investigated the magnetic properties of SRO thin films with varying thicknesses fabricated on ${\mathrm{SrTiO}}_3$(001) substrates by soft x-ray magnetic circular dichroism at the Ru $M_{2,3}$ edge. Results have shown that, with decreasing film thickness, the film changes from ferromagnetic to nonmagnetic at around 3 monolayer thickness, consistent with previous magnetization and magneto-optical Kerr effect measurements. The orbital magnetic moment perpendicular to the film was found to be $\sim 0.1{\mu }_{\text{B}}/\mathrm{Ru}$, and remained nearly unchanged with decreasing film thickness while the spin magnetic moment decreases. A mechanism for the formation of the orbital magnetic moment is discussed based on the electronic structure of the compressively strained SRO film.

The layered crystal structures of cuprates enable collective charge excitations fundamentally different from those of three-dimensional metals. Acoustic plasmons have been observed in electron-doped cuprates by resonant inelastic x-ray scattering (RIXS); in contrast, the characteristics of acoustic plasmons in hole-doped cuprates are under debate, despite extensive measurements. This contrast led us to investigate the charge excitations of hole-doped cuprate ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$. Here we present incident-energy-dependent RIXS measurements and calculations of collective charge response via the loss function to reconcile the issues above. Our results provide evidence for the acoustic plasmons of Zhang-Rice singlet (ZRS), which has a character of the Cu $3d_{x^2-y^2}$ strongly hybridized with the O $2p$ orbitals; the metallic behavior is implied to result from the movement of ZRS rather than the simple hopping of O $2p$ holes.