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Homework Help: Crystal Vibrations: Impurity in a monoatomic chain

  1. Oct 17, 2007 #1
    1. The problem statement, all variables and given/known data

    Hello everyone, i'm a bit stuck on this particular problem and i was hoping somebody could give me a couple tips or hints!

    "consider a linear monoatomic chain where all atoms have a mass M except one which has a small mass m. The force constant is C."
    First i'm asked to write the equation of motion for the relevant atoms. Then, i'm asked to solve it assuming a displacment function for atom n in the form of

    Un = Aexp(-ax)exp(i((omega)t - Kx))

    Let the chain be:

    vs-2 -- vs-1 --- u --- vs --- vs+1

    where u is the impurity

    3. The attempt at a solution

    I'm currently lost at the beginning of the problem, finding the equation of motion. What i first thought was to treat this almost like it were the case of a diatomic chain.

    Mdvs/dt = C(vs+1 + u - 2vs)

    mdu/dt = C(vs-1 + vs - 2u)

    however, upon continuing in the problem i'm not sure that this is correct. Could anyone offer any advice as to how i should start this problem?
  2. jcsd
  3. Oct 20, 2007 #2


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    Staff Emeritus
    Science Advisor
    Gold Member

    Your notation is a little distracting. Simply call the displacement of the n'th atom (from its equilibrium position) [itex]u_n[/itex]. Let the impurity be at n=k.

    Secondly. you've made a mistake in constructing your equation of motion, which is a second order linear DE (not first order).

    See that you should get something more like :

    [tex]m_n \ddot{u_n} = c(u_{n+1} -2u_n+u_{n-1}) [/tex]

    From here, I'm not sure what's the best approach. One possibility is to fourier transform to k-space and hope the equations become decoupled. Odds are they will. Also, you will likely find that the solution in k-space is the FT of the localized wavepacket provided to you.
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