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Much progress has been made recently in the study of the effects of electron-phonon (el-ph) coupling in doped insulators using angle-resolved photoemission (ARPES), yielding evidence for the dominant role of el-ph interactions in underdoped cuprates. As these studies have been limited to doped Mott insulators, the important question arises as to how this compares with doped band insulators where similar el-ph couplings should be at work. The archetypical case is that of perovskite SrTiO3 (STO), well known for its giant dielectric constant of 10 000 at low temperatures, exceeding that of La2CuO4 by a factor of 500. Based on this fact, it has been suggested that doped STO should be the archetypical bipolaron superconductor. Here we report an ARPES study from high-quality surfaces of lightly doped STO. In comparison to lightly doped Mott insulators, we find the signatures of only moderate el-ph coupling; a dispersion anomaly associated with the low-frequency optical phonon with a λ′∼0.3 and an overall bandwidth renormalization suggesting an overall λ′∼ 0.7 coming from the higher frequency phonons. Furthermore, we find no clear signatures of the large pseudogap or small-polaron phenomena. These findings demonstrate that a large dielectric constant itself is not a good indicator of el-ph coupling and highlight the unusually strong effects of the el-ph coupling in doped Mott insulators. [ABSTRACT FROM AUTHOR]

The electronic structure of the Cr ions in Zn1− x Cr x Te ( x=0.03 and 0.15) has been investigated using X-ray magnetic circular dichroism (XMCD) and photoemission spectroscopy. Magnetic-field ( H) and temperature ( T) dependences of the Cr L 2,3 XMCD spectra well corresponded to the behavior of the magnetization measured by a SQUID magnetometer. The line shape of the Cr L 2,3 XMCD spectra was independent of H, T, and x, indicating that the ferromagnetism was originated from the same Cr electronic states independent of Cr concentration. The Cr 3 d partial density of states (DOS) showed a peak at the top of the valence band but there was no DOS at the Fermi level, corresponding to their insulating behaviors. [ABSTRACT FROM AUTHOR]

We have investigated the electronic structure of ZnO:Mn and ZnO:Mn,N thin films using x-ray magnetic circular dichroism (XMCD) and resonance-photoemission spectroscopy. From the Mn 2p → 3d XMCD results, it is shown that, while XMCD signals only due to paramagnetic Mn2+ ions were observed in ZnO:Mn, nonmagnetic, paramagnetic, and ferromagnetic Mn2+ ions coexist in ZnO:Mn,N. XMCD signals of ZnO:Mn,N revealed that the localized Mn2+ ground state and Mn2+ state hybridized with ligand hole coexisted, implying p-d exchange coupling. In the valence-band spectra, spectral weight near the Fermi level was suppressed, suggesting that interaction between magnetic moments in ZnO:Mn,N has localized nature. [ABSTRACT FROM AUTHOR]

By means of photoemission and x-ray absorption spectroscopy, we have studied the electronic structure of (Ni,Zn,Fe,Ti)3O4 thin films, which exhibits a cluster glass behavior with a spin-freezing temperature Tf of ∼230 K and photoinduced magnetization (PIM) below Tf. The Ni and Zn ions were found to be in the divalent states. Most of the Fe and Ti ions in the thin films were trivalent (Fe3+) and tetravalent (Ti4+), respectively. While Ti doping did not affect the valence states of the Ni and Zn ions, a small amount of Fe2+ ions increased with Ti concentration, consistent with the proposed charge-transfer mechanism of PIM. [ABSTRACT FROM AUTHOR]

Epitaxial Ba0.5Sr0.5TiO3 thin films were prepared on Nb-doped SrTiO3 (100) substrates by the pulsed laser deposition technique and were studied by measuring the Ti 2p→3d resonant photoemission spectra in the valence-band region as a function of film thickness, both at room temperature and low temperature. The results demonstrated an abrupt variation in the spectral structures between 2.8 nm (∼7 ML) and 2.0 nm (∼5 ML) Ba0.5Sr0.5TiO3 films, suggesting that there exists a critical thickness for phase change in the range of 2.0–2.8 nm. This may be ascribed mainly to the intrinsic size effects. [ABSTRACT FROM AUTHOR]

The authors performed a soft x-ray magnetic circular dichroism (XMCD) study of a Zn1-xVxO thin film, which showed a small ferromagnetic moment. Field and temperature dependences of V 2p XMCD signals indicated the coexistence of Curie-Weiss paramagnetic and antiferromagnetic V ions, quantitatively consistent with the magnetization measurements. The authors attributed the paramagnetic signal to V ions substituting for Zn sites, which are somewhat elongated along the c axis. A possible formation of ferromagnetically dead layers in the surface region of the film is also discussed. [ABSTRACT FROM AUTHOR]

Application of the Luttinger theorem to the Kondo lattice YbRh2Si2 suggests that its large 4f-derived Fermi surface (FS) in the paramagnetic (PM) regime should be similar in shape and volume to that of the divalent local-moment antiferromagnet (AFM) EuRh2Si2 in its PM regime. Here we show by angle-resolved photoemission spectroscopy that paramagnetic EuRh2Si2 has a large FS essentially similar to the one seen in YbRh2Si2 down to 1 K. In EuRh2Si2 the onset of AFM order below 24.5 K induces an extensive fragmentation of the FS due to Brillouin zone folding, intersection and resulting hybridization of the Fermi-surface sheets. Our results on EuRh2Si2 indicate that the formation of the AFM state in YbRh2Si2 is very likely also connected with similar changes in the FS, which have to be taken into account in the controversial analysis and discussion of anomalies observed at the quantum critical point in this system. Heavy-fermion materials have unusual electronic behavior due to a dual localized-itinerant character of 4f electrons. Here, by studying divalent EuRh2Si2, the authors gain insight into the electronic states of the trivalent heavy fermion system YbRh2Si2 and show that it experimentally demonstrates Luttinger's theorem. [ABSTRACT FROM AUTHOR]