We discuss non-Fermi liquid and quantum critical behaviour in heavy-fermion
materials, focusing on the mechanism by which the electron mass appears to
diverge at the quantum critical point. We ask whether the basic mechanism
for the transformation involves electron diffraction off a quantum critical
spin-density wave, or whether a break-down in the composite nature of the
heavy electron takes place at the quantum critical point. We show that the
Hall constant changes continuously in the first scenario, but may 'jump' di
scontinuously at a quantum critical point where the composite character of
the electron quasiparticles changes.