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SrCoO2 .5 (SCO) bulk is found to be a charge-transfer insulator with a positive value of charge-transfer energy (Δ) and 3d6 ground state, which achieves the antiferromagnetic nature through conventional Co3+–O–Co3+ superexchange interaction. However, in the epitaxial SCO thin film, it is observed, with the help of resonant photoemission spectroscopy and X-ray absorption spectroscopy, that substrate-induced strain modifies the ground state of SCO film to 3d7 L (L: O-2p hole), causing a negative value of Δ. The presence of the O-2p hole in the negative Δ SCO film arises because of strong O 2p–Co 3d hybridization and induces charge disproportionation (CD) in the system. Consequently, the Co3+–O–Co3+ superexchange interaction is modified, and a hole-mediated unconventional ferromagnetic (or ferrimagnetic) ordering is observed in the SCO film. The magnetic moment is found to depend on the values of Δ and CD, which are controlled by the lattice-induced strain. This is manifested from the strain-dependent valence band and conduction band spectra of the SCO films. Because of the negative Δ and CD, additional spectral features appear in the Co L-edge of the films, which are absent in its bulk counterpart. The energy positions and intensities of such features vary with the film thickness, divulging the role of strain in modifying Δ (and CD). Tuning SCO from the positive Δ regime in the bulk to the negative Δ regime in the thin film provides an opportunity to modulate the electronic structure vis-à-vis magnetic property via strain engineering with huge technological potential applications.

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Temperature-dependent photoluminescence (PL) of titanium oxide (TiO2) shows an evolution of blue emission when exposed to 50 keV Ar+ ions. The origin of observed PL has been examined by X-ray absorption near-edge spectroscopy (XANES) at Ti-K,L and O-K edges, revealing the reduction of ligand field splitting owing to the formation of oxygen vacancies (OVs) by destroying TiO6 octahedral symmetry. Detailed PL and XANES analyses suggest that the fluence (ions/cm2) dependent increase in OVs not only boosts the conduction electrons but also increases the density of holes in localized self-trapped exciton (STE) states near the valence band. Based on these observations, we propose a model in which doped conduction electrons are recombining radiatively with the holes in STE states for blue light emission.

The electronic structure of CeAg 2 Ge 2 single crystal has been investigated by using valence-band photoemission at different photon energies ranging from 110 to 150 eV. Resonant photoemission has been observed near the 4d threshold of Ce at 121 eV. The constant initial-state spectra shows two photoemission features having 4f character near the Fermi level at −0.4 and −1.7 eV which exhibits Fano-type sharp resonance character. The experimental spectra have been interpreted with the help of calculations based on full-potential linearized augmented plane-wave method using density-functional theory. Excellent agreement has been obtained between the theory and the experiment. The origin of the feature near to Fermi level is related to the Ce 4f states and the feature at −1.7 eV is related to the strong hybridization between the Ce 4f and 5d, Ag 4d and Ge 4p states.Resonant photoemission spectroscopy ͑RPES͒ has emerged as a very powerful tool to understand the electronic states of rare earths. From last few decades, Ce-based intermetallic compounds have attracted much attention for their various ground-state properties, such as the magnetism, nonmagnetic heavy Fermion, and quantum criticality, etc. 1-6 The diverse ground states are due to the competition between the Ruderman-Kittel-Kasuya-Yosida interaction and the Kondo effect. 7 CeAg 2 Ge 2 belongs to a wide class of cerium compounds that crystallize in the ThCr 2 Si 2 structure having a body-centered tetragonal lattice. 6 At room temperature it exhibits a paramagnetic phase while at low temperature an antiferromagnetic phase with T N = 7 K is reported. 5,6 More recently, detailed studies were performed on a single crystal, which has shown that CeAg 2 Ge 2 orders antiferromagnetically at 4.6 K. 8 The neutron-diffraction experiment shows a sine modulated structure with a magnetic moment equal to 1.85 B at T = 1.5 K. 6 In the Ce based intermetallic alloys it is believed that the ground-state properties depend on the strength of hybridization of the f electrons with the delocalized band states and this motivated us to understand the electronic structure of CeAg 2 Ge 2 . In the present work, we investigate the occupied electronic states of CeAg 2 Ge 2 at room temperature and try to provide a clear understanding of the hybridization between the local Ce 4f electrons and the itinerant conduction electrons by the systematic Ce 4d-4f RPES study.CeAg 2 Ge 2 single crystal was grown by the self-flux method. 8 For the PE measurements the sample has been mechanically polished to mirror finish using quarter micron diamond paste. The RPES measurements on this sample were carried out at the angle-integrated PE beamline on the Indus-1 synchrotron radiation source. 9 The valence-band ͑VB͒ photoemission spectra were recorded using a photon energy of 110-130 eV in very small steps of 1 eV. The energy analyzer from Omicron ͑EA125͒ is used to measure the spectra at room temperature. The spectra were normalized by the photon flux estimated from the photocurrent from the post mirror of th...

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We have investigated the electronic and magnetic properties of the pulsed laser deposited epitaxial thin films of undoped and Fe doped (4 at. %) anatase TiO2−d by photoemission, magnetization measurements, and ab-initio band structure calculations. These films show room temperature magnetic ordering. It is observed that Fe ions hybridize with the oxygen vacancy induced Ti3+ defect states. Our study reveals the formation of local magnetic moment at Ti and Fe sites to be responsible for magnetic ordering. A finite density of states at the Fermi level in both undoped and Fe doped films is also observed, suggesting their degenerate semiconducting nature.