New V2O5 polymorphs have risen to prominence as a result of their open framework structures, cation intercalation properties, tunable electronic structures, and wide range of applications. The application of these materials and the design of new, useful polymorphs requires understanding their defining structure-property relationships. We present a characterization of the band gap and electronic structure of nanowires of the novel ζ-phase and the orthorhombic α-phase of V2O5 using X-ray spectroscopy and density functional theory calculations. The band gap is found to decrease from 1.90 ± 0.20 eV in the α-phase to 1.50 ± 0.20 eV in the ζ-phase, accompanied by the loss of the α-phase's characteristic split-off dxy band in the ζ-phase. States of dxy origin continue to dominate the conduction band edge in the new polymorph but the inequivalence of the vanadium atoms and the increased local symmetry of [VO6] octahedra results in these states overlapping with the rest of the V 3d conduction band. ζ-V2O5 exhibits anisotropic conductivity along the b direction, defining a 1D tunnel, in contrast to α-V2O5 where the anisotropic conductivity is along the ab layers. We explain the structural origins of the differences in electronic properties that exist between the α- and ζ-phase.

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By studying Fe-doped ZnO pellets and thin films with various x-ray spectroscopic techniques, and complementing this with density functional theory calculations, we find that Fedoping in bulk ZnO induces isovalent (and isostructural) cation substitution (Fe 2+ → Zn 2+ ).In contrast to this, Fe-doping near the surface produces both isovalent and heterovalent substitution (Fe 3+ → Zn 2+ ). The calculations performed herein suggest that the most likely defect structure is the single or double substitution of Zn with Fe, although, if additional oxygen is available, then Fe substitution with interstitial oxygen is even more energetically favourable.Furthermore, it is found that ferromagnetic states are energetically unfavourable, and ferromagnetic ordering is likely to be realized only through the formation of a secondary phase (i.e.ZnFe 2 O 4 ), or codoping with Cu.

Herein we systematically study a range of dopants (Cr, Fe, Ni, Cu, and an MnCo alloy) in ZnO and TiO2 using several X-ray spectroscopic techniques. We identify the dopant’s local environment and interaction with the host lattice by employing crystal field multiplet calculations and hence clarify their potential applicability for spintronic technologies. Our density functional theory (DFT) calculations predict a decreasing probability of direct cation (Zn/Ti) substitution by dopant atoms as atomic number increases, as well as a much greater likelihood of metallic clustering in TiO2. Our spectroscopic measurements confirm that in all cases, except Mn, metallic clusters of dopant atoms form in the TiO2 crystal lattice, thus making it unfit for spintronic capabilities. On the other hand, in ZnO, the dopants substitute directly into zinc sites, which is promising for spintronic technologies.

Using inelastic X-ray scattering beyond the dipole limit and hard X-ray photoelectron spectroscopy we establish the dual nature of the U5felectrons in UM2Si2(M = Pd, Ni, Ru, Fe), regardless of their degree of delocalization. We have observed that the compounds have in common a local atomic-like state that is well described by the U5f2configuration with theΓ1(1)andΓ2quasi-doublet symmetry. The amount of the U 5f3configuration, however, varies considerably across the UM2Si2series, indicating an increase of U 5f itineracy in going from M = Pd to Ni to Ru and to the Fe compound. The identified electronic states explain the formation of the very large ordered magnetic moments inUPd2Si2andUNi2Si2, the availability of orbital degrees of freedom needed for the hidden order inURu2Si2to occur, as well as the appearance of Pauli paramagnetism inUFe2Si2. A unified and systematic picture of the UM2Si2compounds may now be drawn, thereby providing suggestions for additional experiments to induce hidden order and/or superconductivity in U compounds with the tetragonal body-centeredThCr2Si2structure.

Dilute magnetic semiconductors (DMSs) show great promise for applications in spin-based electronics, but in most cases continue to elude explanations of their magnetic behavior. Here, we combine quantitative x-ray spectroscopy and Anderson impurity model calculations to study ferromagnetic Fe-substituted In2O3 films, and we identify a subset of Fe atoms adjacent to oxygen vacancies in the crystal lattice which are responsible for the observed room temperature ferromagnetism. Using resonant inelastic x-ray scattering, we map out the near gap electronic structure and provide further support for this conclusion. Serving as a concrete verification of recent theoretical results and indirect experimental evidence, these results solidify the role of impurity-vacancy coupling in oxide-based DMSs.The enigmatic nature of dilute magnetic semiconductors (DMSs) has provided an intriguing and popular topic in materials science and condensed matter physics research for well over a decade [1]. Research interest in DMSs is fueled by the desire to find suitable ferromagnetic semiconductors for spintronics applications which, if realized on a large scale, would revolutionize computing capabilities [2]. Experimental reports of the desirable room temperature ferromagnetism (RTFM) obtained by substituting transition metals into semiconducting and insulating oxides are now prevalent (see, for example, Refs. 3-8). What is not prevalent, however, is a universally accepted description of the mechanism which mediates the often unexpected ferromagnetic behavior. Such an understanding is necessary to develop and optimize effective spintronic devices using these materials.Much of the original interest in DMSs centered on ptype materials, motivated by the discovery of magnetism at low temperatures in Ga 1−x Mn x As [9, 10]. Further interest was stimulated by a theoretical prediction of attainable RTFM in Mn-substituted, p-type DMS materials [11]. Recent studies of Ga 1−x Mn x As have yielded some important developments regarding the magnetic mechanisms and electronic structure [12][13][14][15][16]. For this material, the FM seems to be intricately linked to the tightly bound [14] holes in the Mn-induced impurity band which overlaps the Fermi level [15,16]. Unfortunately, however, to date all variations of Ga 1−x Mn x As only exhibit FM at low temperatures. Thus, while much can be learned from studying the physics of this material, it is not an ideal candidate for spintronics applications.Recently, the n-type (In 1−x Fe x ) 2 O 3 , along with other oxide-based materials, has generated significant interest due to numerous independent reports of RTFM [17][18][19][20][21][22][23][24][25][26]. In search of an explanation of the magnetism, some of these studies have provided indirect evidence that oxygen site vacancies (V O ) are important for the ferromagnetism. In some cases, annealing cycles between oxygen and vacuum environments have been reported to respectively destroy and restore the room temperature ferromagnetic ordering [20,23,24]. Oxygen...

Topological insulators have become one of the most prominent research topics in materials science in recent years. Specifically, Bi2Te3 is one of the most promising for technological applications due to its conductive surface states and insulating bulk properties. Herein, we contrast the bulk and surface structural environments of dopant ions Cr, Mn, Fe, Co, Ni, and Cu in Bi2Te3 thin films in order to further elucidate this compound. Our measurements show the preferred oxidation state and surrounding crystal environment of each 3d-metal atomic species, and how they are incorporated into Bi2Te3. We show that in each case there is a unique interplay between structural environments, and that it is highly dependant on the dopant atom. Mn impurities in Bi2Te3 purely substitute into Bi sites in a 2+ oxidation state. Cr atoms seem only to reside on the surface and are effectively not able to be absorbed into the bulk. Whereas for Co and Ni, an array of substitutional, interstitial, and metallic configurations occur. Considering the relatively heavy Cu atoms, metallic clusters are highly favourable. The situation with Fe is even more complex, displaying a mix of oxidation states that differ greatly between the surface and bulk environments.

X-ray photoelectron spectroscopy (XPS) and resonant x-ray emission spectroscopy (RXES) measurements of pellet and thin film forms of TiO 2 with implanted Fe ions are presented and discussed. The findings indicate that Fe-implantation in a TiO 2 pellet sample induces heterovalent cation substitution (Fe 2+ → Ti 4+ ) beneath the surface region. But in thin film samples, the clustering of Fe atoms is primarily detected. In addition to this, significant amounts of secondary phases of Fe 3+ are detected on the surface of all doped samples due to oxygen exposure. These experimental findings are compared with density functional theory (DFT) calculations of formation energies for different configurations of structural defects in the implanted TiO 2 :Fe system. According to our calculations, the clustering of Fe-atoms in TiO 2 :Fe thin films can be attributed to the formation of combined substitutional and interstitial defects. Further, the differences due to Fe doping in pellet and thin film samples can ultimately be attributed to different surface to volume ratios.