Although the binding of electrons into Cooper pairs is essential in forming
the superconducting state, its remarkable properties-zero resistance and p
erfect diamagnetism-require phase coherence among the pairs as well. When c
oherence is lost at the transition temperature T-c, pairing remains, togeth
er with phase correlations which are finite in space and time. In conventio
nal metals, Cooper pairs with short-range phase coherence survive no more t
han 1 K above T-c. In underdoped high-T-c copper oxides, spectroscopic evid
ence for some form of pairing is found up to a temperature T*, which is rou
ghly 100 K above T-c (refs 1-3). How this pairing and Cooper-pair formation
are related is a central problem in high-T-c superconductivity. The nature
of this relationship hinges on the extent to which phase correlations acco
mpany pairing in the normal state(4). Here we report measurements of high-f
requency conductivity that track the phase-correlation time tau in the norm
al state of the Bi2Sr2CaCu2O8+delta,s family of underdoped copper oxide sup
erconductors. Just above T-c,we find that tau reflects the motion of therma
lly generated topological defects in the phase, or vortices(5,6). However,
vortex proliferation reduces tau to a value indistinguishable from the life
time of normal-state electrons at 100 K, well below T*.