  The magnetic excitation spectrum in the two-dimensional (2D) S=1/2 Heisenberg
antiferromagnet copper deuteroformate tetradeuterate (CFTD) has been measured
for temperatures up to T\sim J/2, where J=6.31+-0.02 meV is the 2D exchange
coupling. For T\ll J, a dispersion of the zone boundary energy is observed,
which is attributed to a wavevector dependent quantum renormalization. At
higher temperatures, spin-wave-like excitations persist, but are found to
broaden and soften. By combining our data with numerical calculations, and with
existing theoretical work, a consistent description of the behaviour of the
model system is found over the whole temperature interval investigated.
