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What is the "unoccupied band" referring to in this paper?

  1. Aug 11, 2015 #1
    The abstract of this paper ( http://www.sciencemag.org/content/297/5588/1853 ) says

    "The electronic properties of the one-dimensional chains are dominated by an unoccupied electron band"

    I'm not sure what the paper is referring to by an unoccupied electron band. Wouldn't a metal be described by a half-filled band?
     
  2. jcsd
  3. Aug 11, 2015 #2
    They are working with a 1-D chain of 20 Au atoms. At this size scale, you have quantum confinement effects and the 'metal' nanostructure can act as a 1-D quantum well with discrete energy levels.
    Analogously, They are treating the nanostructure as an individual molecule and the mentioned "unoccupied electron band" is essentaily a "LUMO" - Lowest unoccupied Molecular Orbital.
    You can google
    0D 1D 2D 3D Density of States
    and
    HOMO-LUMO gap
     
  4. Aug 12, 2015 #3
    Thanks for the reply.

    What I'm curious about is why the quantum well is empty, as opposed to containing 20 electrons (1 electron for each 6s orbital of the chain atoms).

    I.e. If the quantum well has sinusiodal molecular orbitals φi, I would have thought φ11 would be the LUMO instead of φ1.

    Perhaps the fermi level of the composite "Au chain + substrate" drops below the first molecular orbital, φ1, of the quantum well?
     
    Last edited: Aug 12, 2015
  5. Aug 12, 2015 #4
    Sounds reasonable; The paper says
    "Because the observed electron
    band is localized above the Fermi level, small
    deviations cannot modify the electron occu-
    pation of the band and are ineffective in
    reducing the total energy of the Au chains."

    Then the question is, is the fermi level of the wire determined by the 6s electrons or the 5d and 4f electrons?

    Of note is that they use conductance-based methods to form the DoS picture, and they measure order of 0.1 nA/V current at applied biases of 1 V. So 1E-10 A is still 6E8 elementary charges per second, so 6E8 electrons tuneling per second. seems like weakly bound valence electrons might just be swept away and thus not influence the structure DoS..
     
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