  We review the contribution of infrared spectroscopy to the study of the
pseudogap in high temperature superconductors. The pseudogap appears as a
depression of the frequency dependent conductivity in the c-axis direction and
seems to be related to a real gap in the density of states. It can also be seen
in the Knight shift, photoemission and tunneling experiments. In underdoped
samples it appears near room temperature and does not close with temperature.
Another related phenomenon that has been studied by infrared is the depression
in the ab-plane scattering rate. Two separate effects can be discerned. At high
temperatures there is broad depression of scattering below 1000 cm-1 which may
be related to the gap in the density of states. At a lower temperature a
sharper structure is seen, which appears to be associated with scattering from
a mode at 300 cm-1, and which governs the carrier life time at low
temperatures. This mode shows up in a number of other experiments, as a kink in
ARPES dispersion, and a resonance at 41 meV in magnetic neutron scattering.
Since the infrared technique can be used on a wide range of samples it has
provided evidence that the scattering mode is present in all high temperature
cuprates and that its frequency in optimally doped materials scales with the
superconducting transition temperature. The lanthanum and neodymium based
cuprates do not follow this scaling and appear to have depressed transition
temperatures.
