7.1: The Problem
( \newcommand{\kernel}{\mathrm{null}\,}\)
7.1 The harmonic Hamiltonian
The Hamiltonian for lattice vibrations, in the harmonic approximation, is
H=123N∑i=1mi˙x2i+123N∑i=13N∑j=1xiAijxj.
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
BSB−1
is diagonal. Show that the Hamiltonian can be cast into the form
H=123N∑r=1(˙q2r+Drq2r)
by a linear change of variables. (Clue: As a first step, introduce the change of variable zi=√mixi.)