Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Several Questions

  1. Mar 20, 2008 #1
    *Why are electrons confined to bands where they can posses a partial quantum of energy (right?) as opposed to descrete energy levels in quantum dots?
    *Why do electrons behave differently in crystaline structures?
    *How do electrons behave in crystaline structure? (like bouncing back and forth) and why?
    *How and why are the energy levels changed when spatial restriction occurs?
    *Why does spatial restriction occur?
    *What coordinate systems are used for the space and time coordinates of the Schrodinger equation?
    Last edited: Mar 20, 2008
  2. jcsd
  3. Mar 24, 2008 #2
    The basic issue with electrons in lattices is that the usual translational symmetry of space is broken. You cannot translate (or rotate) by any arbitrary amount, but only by a lattice vector. This basic fact is formally called Bloch's theorem, which gives a constraint on the form of the wavefunction for an electron in a periodic potential. How electrons "behave" is not a particularly well-posed problem. Like any quantum particle, trying to visualise their behaviour as a billiard ball bouncing back and forth is going to run into problems. The bands come from the fact that you have a macroscopic (so effectively infinite) single electron states with the same energy --- one from each atom --- which mix together to produce a macroscopic number of multi-electron states, each slightly differing in energy. Thus you get a continuum of energies, occasionally separated by finite energy gaps.

    Hope that's not too confusing.
  4. Mar 24, 2008 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Not right.

    Why should they behave the same way is 2 different environments?

    Deifferently from what?

    Read genneth's post. The so-called behavior is encoded in the electron wavefunction. You need to learn how the wavefunction of an electron in a crystal lattice is figured, from Bloch's theorem to nearly free electron approximations to the tight binding model (to the mott-hubbard model).

    Hand waving answer: spatial restriction affects the momenta through the Uncertainty relation. Alternatively, spatial restriction affects the wave vector of bound states, which in turn affects the energy through the dispersion.

    Because you put in a confining potential barrier.

    You choose a co-ordinate system depending on the geometry of the physical system; e.g., for a cubical quantum dot, you will use Cartesian co-ordinates.
    Last edited: Mar 24, 2008
  5. Mar 24, 2008 #4
    According to Wikipedia:
    So why do bands occur in quantum dots containing a hundred to a few thousand atoms?
    Last edited: Mar 24, 2008
  6. Mar 24, 2008 #5
    The key is the symmetry. In quantum dots you arrange the structure such that you don't have a regular lattice. There's also the issue of temperature...
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook