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Unformatted text preview: Physics 545 Problem Set 5 Due l. Vibrations of square lattice. We consider transverse vibrations of a planar square
lattice of rows and columns of identical atoms, and let all,” denote the displacement
normal to the plane of the lattice of the atom in the lth column and mth row (Fig. 13). The mass of each atom is M . and C is the force constant for nearest neighbor atoms.
(a) Show that the equation of motion is M(d2uim/dtz) = Cl:(ul+l,m + ul—l,m — 2“(111) + (ul,nl+l + ”Lml — 2“im)]  Figure 13 Square array oflattice constant a. The
displacements considered are normal to the plane
of the lattice. ‘ 0)) Assume solutions of the form
uh" = “(0) expll‘iUKxa + mea “‘ (Him I where a is the spacing between nearestneighbor atoms. Show that the equation of
motion is satisfied if (02M = 2C(2  cos lga  cos Kya) . This is the dispersion relation for the problem. (0) Show that the region of K space for
which independent solutions exist may be taken as a square ofside 277/(1. This is the first Brillouin zone of the square lattice. Sketch a) versus K for K = K. with K,,.= 0,. '
and For K. = Ky. (d) For Ka < 1, show that l J .' w = '(Ca2/M)”2(K3 + K3)“ = (Caz/MWEK , so that in this limit the velocity is constant. ‘ 2. Kittel p. 102, #1 ' 3. 'Cons‘ider'a)‘q ys qfor a diatomic linear chain with in (light) and M (heavy) atoms
in the unit cell. The alternate m’s and M’s are equidistant, the force constant between
m and M being ﬂ and spaced ‘a’ apatt. l 1 l l 2 4sin2ka 2 2 PD th 4 . ' a) 2 .._+_.._ i ___+____ _W I ‘m WWW“ q ﬂ[m M] ﬂ im Mi mM (i) Calculate expressions for a) q (+) and a) q (~) as a function of q, i.e., the optical and the acoustic branches, and display them in the lSt Brillouin Zone.
(ii) Mark the a) (1’3 at the zone center and zone boundary in the reduced zone scheme. (iii) Calculate the dispersion curves form = M and show them on the above sketch. ...
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 Spring '09
 CARLSON

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