7.1: The Problem
- Page ID
- 6371
7.1 The harmonic Hamiltonian
The Hamiltonian for lattice vibrations, in the harmonic approximation, is
\[ \mathcal{H}=\frac{1}{2} \sum_{i=1}^{3 N} m_{i} \dot{x}_{i}^{2}+\frac{1}{2} \sum_{i=1}^{3 N} \sum_{j=1}^{3 N} x_{i} A_{i j} x_{j}.\]
Notice that this Hamiltonian allows the possibility that atoms at different lattice sites might have different masses. Accept the fact that any real symmetric matrix S can be diagonalized through an orthogonal transformation, i.e. that for any such S there exists a matrix B whose inverse is its transpose and such that
\[ \mathrm{BSB}^{-1}\]
is diagonal. Show that the Hamiltonian can be cast into the form
\[ \mathcal{H}=\frac{1}{2} \sum_{r=1}^{3 N}\left(\dot{q}_{r}^{2}+D_{r} q_{r}^{2}\right)\]
by a linear change of variables. (Clue: As a first step, introduce the change of variable \(z_{i}=\sqrt{m_{i}} x_{i}\).)