  We discuss the peak at 400 cm-1, which is seen in c-axis conductivity spectra
of underdoped high temperature superconductors. The model of van der Marel and
Munzar, where the peak is the result of a transverse plasmon arising from a low
frequency conductivity mode between the closely spaced planes, fits our data
well. Within the model we find that the temperature dependence of the peak
amplitude is controlled by in-plane scattering processes. The temperature range
where the mode can be seen coincides with Ts, the spin gap temperature, which
is lower than T*, the pseudogap temperature. As a function of temperature, the
amplitude of the mode tracks the amplitude of the 41 meV neutron resonance and
the spin lattice relaxation time, suggesting to us that the mode is controlled
by magnetic processes and not by superconducting fluctuations which have
temperature scale much closer to Tc, the superconducting transition
temperature.
