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Electrostatics and band structures?

  1. Jun 12, 2007 #1
    Hi all,
    Recently got to thinking about electrostatics and electron densities and band structures. If you have an electrostatically charged piece of material, say for example, a metal, does the density of states of that metal change because of the depletion of electrons? Alternately, if you have an excess of electrons in a metal does it mean that the metal will have an increased density of states in the conduction band?

    Basically, does charging a sample (metal/polymer etc.) change the electroninc properties of the sample?

    Many thanks for any any input
     
  2. jcsd
  3. Jun 12, 2007 #2

    ZapperZ

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    Under normal circumstances, you do not change the band structure. All you are doing is raising or lowering the Fermi level.

    However, as is normal in this field of study, there are always caveats because it depends on how "exotic" a material you are talking about. If the amount of charges that you have either added or remove causes a noticeable change structurally, then yes, it will now affect the band structure. In strongly correlated system, there is something called the U/W ratio, which is the ratio of the on-site coulombic repulsion to the bandwidth of the density of states. So if we look at, let's say, the Mott-Hubbard-type insulator for example, a significant change of this ratio might change the gap between the lower and upper Hubbard bands, thus changing the band structure. But again, this is something that you do not often see for ordinary metals and removal of electrons within typical working conditions.

    Zz.
     
  4. Jun 12, 2007 #3
    Thanks fo the reply, It is now apparent that under 'normal' electrostatic conditions the band structure is not affected (generally).

    However, in a negatively charged piece of material, will the excess of electrons cause a decrease in the electron mean free path because of the increased number of electrons in the conduction band? I.e if you stimulate(e.g. with a photon) an electron that is present in a negatively charged sample, will that electron have a shorter mean free path due to increased collisions? (assuming that the photon energy is below the work function for that particular material)
     
  5. Jun 12, 2007 #4

    ZapperZ

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    In a standard metal, the electron mean free path is predominantly dominated by scattering with the lattice, at least for wide-ranging temperatures. Electron-electron scattering doesn't show up until at sufficiently low temperatures. Furthermore, you have to stuff A LOT of electrons here for such a thing to start being affected, and I would think such a metal would be unstable (think of breakdown effects) before you can actually start seeing the effects of the added electrons.

    Zz.
     
  6. Jun 13, 2007 #5
    Thanks again for the reply ZapperZ.
    This brings me to another question, will an excess/depletion of electrons cause a change in the refelection/absorption mechanism of a given material?
    Also, to practically achieve and measure the electrostatic potential in a sample in the lab, what do you need. I see a commom source of electrostatic electricity is the Van de Graff machine, and an electroscope is used to measure the potential. Are these instruments used in modern studies of electrostatics, i.e. how does on go about achieving an overall net charge in a sample?
     
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