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

States in energy band

  1. Mar 18, 2015 #1
    It's known that the number of states in a band is equal to the number of unit cells in crystal.

    Here is my problem (confusion with trivial concepts)

    Bloch function is a electronic state, or orbital.
    The number of orbitals in a band inside the first zone is equal to the number os units cells in crystal.

    each state can accommodate at most two electrons. So the max number of electrons that can occupy a single band is 2N

    But here is my stupid question:

    I know that a state can be occupied with two electrons with opposite spins, but why two?

    In my mind (i know that im thinking wrong) one state should be only occupied by one electron.

    Is like having N chairs for 2N people (N male and N female). We are saying that one chair can be occupied by one male and one female simultaneously . But one chair is made for one individual and not two individuals.

    Other question:

    If the Pauli exclusion principle would not be valid for electrons, then what is the max number of electrons that can occupy a single band? infinite?
  2. jcsd
  3. Mar 19, 2015 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    It has to do with how the wavefunctions of fermions interact. I believe that with two possible spin states, two wavefunctions can overlap completely without destructively interfering with each other.
  4. Mar 19, 2015 #3
    Actually, two electrons do not occupy the exact same state, the difference being the spin. What happens is that the "states" are spin-degenerated, that is, there are states with same quantum numbers, and differ only in spin. Think about the atomic levels for example: neglecting spin interaction, the 1s electrons occupy have same energy, angular momentum, and the only difference is spin.

    Yes, particles that are bosons do not obbey the Pauli exclusion principle and therefore an inifinite number of particles can occupy the EXACT (including spin) state. This difference is the basis of the difference between Fermi-Dirac and Bose-Einstein statistics.
    About many particles in the same state, take a look about the Bose-Einstein Condensates.
  5. Mar 19, 2015 #4
    (Neglecting spin interaction) if we have a sate with two electrons, same band, same wave vector, but opposite spins and i want to "plot" the wave function of that state, what is the result?

    A single wave function that resulted on the overlapping of the two electrons?

    Or two wave functions, with different spins.
  6. Mar 21, 2015 #5
    Well...about details related to spin I might commit errors becaue I have no deep knowledge of relativistic quantum mechanics, which is where spin is accounted more precisely as far as I know.
    But in non-relativistic quantum mechanics (at least for spin 1/2, which is the case of the electron), when you treat different spin components it appears as a different componnent, that is, the wave function of the system has one spin +1/2 component and one spin -1/2 component. Think of it like a 2D vector: you can decompose it in x and y components.
    That means in the case you ask about, the two wave functions would be the same if plotted.
  7. Mar 21, 2015 #6
    There are only two bands : the valence band and the conduction band, each containing many states. Be careful not to confuse band and state.
    A photon is a particle to which the Pauli exclusion principle does not apply. There is not limit to the number of photons in a system that can be in the same state.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook