No abstract

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 how this compares with doped band insulators where similar el-ph couplings should be at work. The archetypical case is the perovskite SrTiO 3 (STO), well known for its giant dielectric constant of 10000 at low temperature, exceeding that of La 2 CuO 4 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 SrTiO 3 . Comparing to lightly doped Mott insulators, we find the signatures of only moderate electron-phonon 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. Further, 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.Strong energy-momentum dispersion in the lightly doped SrTiO 3 2

We have performed angle-resolved photoemission spectroscopy (hϭ21.2,40.8 eV), Ce 3d-4 f resonant photoemission spectroscopy (hϳ881 eV), and Ce 3d x-ray absorption spectroscopy studies on the layered cerium compounds CeTIn 5 (TϭRh and Ir͒, which show competition between superconductivity and antiferromagnetism. The results suggest that the Ce 4 f electrons in both compounds are nearly localized. We have found that although the Ce 4 f electrons in the superconducting CeIrIn 5 are more delocalized than those in the antiferromagnetic CeRhIn 5 , their electronic structures are very similar to each other.

The half-Heusler compounds NiXSn (X = Ti, Zr, Hf) are regarded as examples of materials possessing excellent thermoelectric properties. We have studied their electronic structures by photoemission spectroscopy excited by soft and hard X-ray synchrotron radiation. Although the overall features of the experimental valence band spectra are well explained by a calculated density of states,''in-gap'' states close to the Fermi energy emerge due to atomic disorder. These electronic states were further confirmed by core-level photoemission and appear to be key for excellent thermoelectric behavior. #