Elasticity theory for the CDW is considerably simplified by the strong
anisotropy. In particular, since the electrons in the CDW are
essentially confined on the chains, we need only keep track of
displacements in the x-direction, i.e. along the most conducting
axis. Because of this simplicity, we may define instead a single,
dimensionless phase by 
, where 
is
the displacement along the x direction. By a rescaling of axes, we
may then write the elastic CDW Hamiltonian as
where K is the elastic stiffness.
