Spectroscopic Fingerprint of Phase-Incoherent Superconductivity in the Underdoped Bi 2 Sr 2 CaCu 2 O 8+δ

Below the transition temperature, an energy gap opens in superconductors, which effectively protects the superconducting phase. Above the transition temperature, the gap closes, creating excitations and a loss of superconductivity. In the high-temperature superconducting cuprates, however, the gap persists above the transition temperature. Understanding the electronic structure of this pseudogap region is important in understanding the mechanism of superconductivity in the cuprates. Lee et al. (p. 1099 ; see the Perspective by Norman ) use high-resolution, temperature-dependent scanning tunneling microscopy to reveal that the pseudogap regime is an incoherent (or phase-disordered) d-wave superconductor. Scanning tunnelling spectroscopy reveals the pseudogap regime of the cuprates to be an incoherent d-wave superconductor. 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 T c , and it survives up to at least temperature T ~ 1.5 T c . 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.