The electronic structure of BiVO4 has been studied by x-ray photoelectron, x-ray absorption, and x-ray emission spectroscopies, in comparison with density functional theory calculations. Our results confirm both the direct band gap of 2.48 eV and that the Bi 6s electrons hybridize with O 2p to form antibonding “lone pair” states at the top of the valence band. The results highlight the suitability of combining s2 and d0 cations to produce photoactive ternary oxides.

We present a detailed study of the electronic structure of europium nitride (EuN), comparing spectroscopic data to the results of advanced electronic structure calculations. We demonstrate the epitaxial growth of EuN films, and show that in contrast to other rare-earth nitrides successful growth of EuN requires an activated nitrogen source. Synchrotron-based x-ray spectroscopy shows the samples contain predominantly Eu 3+ , but with a small and varying quantity of Eu 2+ that we associate with defects, most likely nitrogen vacancies. X-ray absorption and x-ray emission spectroscopies (XAS and XES) at the nitrogen K-edge are compared to several different theoretical models, namely LSDA+U (local spin density functional theory with Hubbard U corrections), dynamic mean field theory in the Hubbard-I approximation, and QSGW (quasiparticle self-consistent GW ) calculations. The DMFT and QSGW models capture better the density of conduction band states than LSDA+U . Only the Hubbard-I model contains a correct description of the Eu 4f atomic multiplets and locates their energies relative to the band states, and we see some evidence in XAS for hybridization between the conduction band and the the lowest lying 8 S multiplet. The Hubbard-I model is also in good agreement with purely atomic mutliplet calculations for the Eu M-edge XAS. LSDA+U and DMFT find a metallic ground state, while QSGW predicts a direct band gap at X for EuN of about 0.9 eV that matches closely an absorption edge seen in optical transmittance at 0.9 eV and a smaller indirect gap. Overall, the combination of theoretical methods and spectroscopies provides insights into the complex nature of the electronic structure of this material. The results imply that EuN is a narrow band-gap semiconductor that lies close to the metal-insulator boundary, where the close proximity to the Fermi level of an empty Eu 4f multiplet raises the possibility of tuning both the magnetic and electronic states in this system.

The strong spin-orbit interaction in the rare-earth elements ensures that even within a ferromagnetic state there is a substantial orbital contribution to the ferromagnetic moment, in contrast to more familiar transition metal systems, where the orbital moment is usually quenched. The orbital-dominant magnetization that is then possible within rare-earth systems facilitates the fabrication of entirely new magnetic heterostructures, and here we report a study of a particularly striking example comprising interfaces between GdN and SmN. Our investigation reveals a twisted magnetization arising from the large spin-only magnetic moment in GdN and the nearly zero, but orbital-dominant, moment of SmN. The unusual twisted phase is driven by (i) the similar ferromagnetic Gd-Gd, Sm-Sm and Gd-Sm exchange interactions, (ii) a SmN Zeeman interaction 200 times weaker than that of GdN, and (iii) the orbital-dominant SmN magnetic moment. The element specificity of X-ray magnetic circular dichroism (XMCD) is used in seperate modes probing both bulk and surface regions, revealing the depth profile of the twisting magnetization. PACS numbers: 75.25-j, 75.47.-m, 75.50.Pp 1 arXiv:1504.04425v1 [cond-mat.mtrl-sci] 17 Apr 2015 I. INTRODUCTION An inhomogeneous, twisted magnetic ordering commonly occurs near interfaces between ferromagnetic materials, due to competing interactions which favor opposing alignments of the magnetization. These phases are types of engineered domain walls, and thus have important implications for spintronics applications, where current-driven domain wall motion is an active area of research. 1-4 So far, twisted phases are known to manifest in diverse magnetic systems, 5-12 however these all fall under the conventional spin-dominant paradigm of magnetism where the orbital moment plays no significant role. Competing interactions in the presence of a dominant orbital moment have so far remained unexplored, yet the opportunity now exists within the rare-earth nitride (REN) series, where orbital-dominant magnetism is possible due to strong spin-orbit coupling of the 4f electrons. Forming a series of mostly intrinsic ferromagnetic semiconductors, 13-19 the RENs are already integrated within spintronic devices, 20,21 and thus provide a novel system for studying competing interactions. The rare-earth elements, comprising the series across which the 4f shell is filled, have been of interest for nearly a century. They are most commonly found in the trivalent state in a wide range of compounds, including the RENs. The 4f shell, with l = 3, comprises seven distinct orbital states, −3 m l 3, and with the spin degeneracy a total of 14 single-electron states. Gd 3+ has a half filled shell, for which Hund's rules state that the seven electrons fill all of the orbital states with spin-up electrons; L = 0 and S = J = 7/2. It thus has a purely spin moment of 7 µ B . The indirect exchange interaction aligns the spins below a Curie temperature of about 50 K, rising to 70 K under heavy donor doping, 22 but the spherical symmetry of...

Soft x-ray spectroscopy is used to investigate the strain dependence of the metal-insulator transition of VO2. Changes in the strength of the V 3d -O 2p hybridization are observed across the transition, and are linked to the structural distortion. Furthermore, although the V-V dimerization is well-described by dynamical mean-field theory, the V-O hybridization is found to have an unexpectedly strong dependence on strain that is not predicted by band theory, emphasizing the relevance of the O ion to the physics of VO2.