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

[Morberticus]

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?

 

[Hyo X]

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

 

[Morberticus]

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 φ_i, 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?

 

[Hyo X]

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 φ_i, 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?

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..