  A possible explanation for the existence of the cuprate "pseudogap" state is
that it is a d-wave superconductor without quantum phase rigidity. Transport
and thermodynamic studies provide compelling evidence that supports this
proposal, but few spectroscopic explorations of it have been made. One
spectroscopic signature of d-wave superconductivity is the particle-hole
symmetric "octet" of dispersive Bogoliubov quasiparticle interference
modulations. Here we report on this octet's evolution from low temperatures to
well into the underdoped pseudogap regime. No pronounced changes occur in the
octet phenomenology at the superconductor's critical temperature Tc, and it
survives up to at least temperature T ~ 1.5Tc. In the pseudogap regime, we
observe the detailed phenomenology that was theoretically predicted for
quasiparticle interference in a phase-incoherent d-wave superconductor. Thus,
our results not only provide spectroscopic evidence to confirm and extend the
transport and thermodynamics studies, but they also open the way for
spectroscopic explorations of phase fluctuation rates, their effects on the
Fermi arc, and the fundamental source of the phase fluctuations that suppress
superconductivity in underdoped cuprates.
