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 ~ 1.5
Tc. In this 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.
