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Physics LibreTexts

7.1: The Problem

( \newcommand{\kernel}{\mathrm{null}\,}\)

7.1 The harmonic Hamiltonian

The Hamiltonian for lattice vibrations, in the harmonic approximation, is

H=123Ni=1mi˙x2i+123Ni=13Nj=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

BSB1

is diagonal. Show that the Hamiltonian can be cast into the form

H=123Nr=1(˙q2r+Drq2r)

by a linear change of variables. (Clue: As a first step, introduce the change of variable zi=mixi.)


This page titled 7.1: The Problem is shared under a CC BY-SA license and was authored, remixed, and/or curated by Daniel F. Styer.

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