E. R. Ylvisaker

Publications (5)40.21 Total impact

• Source

  Available from: Claus M. Schneider

  Article: Probing bulk electronic structure with hard X-ray angle-resolved photoemission.

  A X Gray, C Papp, S Ueda, B Balke, Y Yamashita, L Plucinski, J Minár, J Braun, E R Ylvisaker, C M Schneider, W E Pickett, H Ebert, K Kobayashi, C S Fadley
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  ABSTRACT: Traditional ultraviolet/soft X-ray angle-resolved photoemission spectroscopy (ARPES) may in some cases be too strongly influenced by surface effects to be a useful probe of bulk electronic structure. Going to hard X-ray photon energies and thus larger electron inelastic mean-free paths should provide a more accurate picture of bulk electronic structure. We present experimental data for hard X-ray ARPES (HARPES) at energies of 3.2 and 6.0 keV. The systems discussed are W, as a model transition-metal system to illustrate basic principles, and GaAs, as a technologically-relevant material to illustrate the potential broad applicability of this new technique. We have investigated the effects of photon wave vector on wave vector conservation, and assessed methods for the removal of phonon-associated smearing of features and photoelectron diffraction effects. The experimental results are compared to free-electron final-state model calculations and to more precise one-step photoemission theory including matrix element effects.
  Nature Material 08/2011; 10(10):759-64. · 32.84 Impact Factor
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  Available from: ArXiv

  Article: NaAlSi: a self-doped semimetallic superconductor with free electrons and covalent holes

  H. B. Rhee, S. Banerjee, E. R. Ylvisaker, W. E. Pickett
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  ABSTRACT: The layered ternary sp conductor NaAlSi, possessing the iron-pnictide "111" crystal structure, superconducts at 7 K. Using density functional methods, we show that this compound is an intrinsic (self-doped) low-carrier-density semimetal with a number of unusual features. Covalent Al-Si valence bands provide the holes, and free-electron-like Al 3s bands, which propagate in the channel between the neighboring Si layers, dip just below the Fermi level to create the electron carriers. The Fermi level (and therefore the superconducting carriers) lies in a narrow and sharp peak within a pseudogap in the density of states. The small peak arises from valence bands which are nearly of pure Si, quasi-two-dimensional, flat, and coupled to Al conduction bands. Isostructural NaAlGe, which is not superconducting above 1.6 K, has almost exactly the same band structure except for one missing piece of small Fermi surface. Certain deformation potentials induced by Si and Na displacements along the c-axis are calculated and discussed. It seems likely that the mechanism of pairing is related to that of several other lightly doped two-dimensional nonmagnetic semiconductors (TiNCl, ZrNCl, HfNCl), which is not well understood but apparently not of phonon origin.
  01/2010;
• Source

  Available from: ArXiv

  Article: Orbital order, stacking defects and spin-fluctuations in the $p$-electron molecular solid RbO$_2$

  E. R. Ylvisaker, R. R. P. Singh, W. E. Pickett
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  ABSTRACT: We examine magnon and orbiton behavior in localized O$_2$ anti-bonding molecular $\pi^*$ orbitals using an effective Kugel-Khomskii Hamiltonian derived from a two band Hubbard model with hopping parameters taken from {\em ab initio} density functional calculations. The considerable difference between intraband and interband hoppings leads to a strong coupling between the spin wave dispersion and the orbital ground state, providing a straightforward way of experimentally determining the orbital ground state from the measured magnon dispersion. The near degeneracy of different orbital ordered states leads to stacking defects which further modulate spin-fluctuation spectra. Proliferation of orbital domains disrupts long-range magnetic order, thus causing a significant reduction in the observed N\'eel temperature. Comment: 5 pages, 2 figures
  12/2009;
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  Available from: ArXiv

  Article: Charge fluctuations and the valence transition in Yb under pressure.

  E R Ylvisaker, J Kunes, A K McMahan, W E Pickett
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  ABSTRACT: We present a dynamical mean-field theory study of the valence transition (f;{14} --> f;{13}) in elemental, metallic Yb under pressure. Our calculations reproduce the observed valence transition as reflected in the volume dependence of the 4f occupation. The transition is advanced by heating, and suggests quasiparticle or Kondo-like structure in the spectra of the trivalent end state, consistent with the early lanthanides. Results for the local charge fluctuations and susceptibility, however, show novel signatures uniquely associated with the valence transition itself, indicating that Yb is a fluctuating valence material in contrast with the intermediate valence behavior seen in the early trivalent lanthanides Ce, Pr, and Nd.
  Physical Review Letters 06/2009; 102(24):246401. · 7.37 Impact Factor
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  Available from: arxiv.org

  Article: Comparison of the Electronic Structures of Two Non-cuprate Layered Transition Metal Oxide Superconductors

  E. R. Ylvisaker, K. -W. Lee, W. E. Pickett
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  ABSTRACT: Comparison is made of the electronic structure of the little-studied layered transition metal oxide LiNbO$_2$ with that of Na$_x$CoO$_2$, which has attracted tremendous interest since superconductivity was discovered in its hydrate. Although the active transition metal $d$ states are quite different due to different crystal fields and band filling, both systems show a strong change of electronic structure with changes in the distance between the transition metal ion layer and the oxygen layers. The niobate is unusual in having a large second-neighbor hopping amplitude, and a nearest neighbor hopping amplitude that is sensitive to the Nb-O separation. Li$_x$NbO$_2$ also presents the attractive simplicity of a single band triangular lattice system with variable carrier concentration that is superconducting.
  01/2006;