Evidence of a Pair Density Wave Statein the Cuprate Pseudogap Phase*

In cuprate superconductors a mysterious quantum fluid designated as the pseudogap is produced by electron-density depletion in the CuO₂ antiferromagnetic insulator. This phase appears to be the parent state to the highest temperature ambient superconductors but its nature remains unidentified. The possibility that the pseudogap of cuprates is a pair density wave (PDW) state, in which the density of electron pairs modulates spatially, has come into sharp and intense research focus. By studying strongly underdoped Bi₂Sr₂CaDyCu₂O₈ in the superconductive phase, we detect the 8a₀-periodic  modulations in real-space signifying a PDW coexisting with superconductivity. We develop a new theory for the quasiparticle interference signature (QPI) that provides a 'fingerprint' Λ(q) specifically for an 8a₀-periodic PDW. By visualizing the evolution with temperature of the electronic structure from the superconducting into the pseudogap phase, we find the temperature dependence of the QPI signature Λ(q) that is predicted for the PDW. These observations are consistent with theory for the transition from a PDW state coexisting with d-wave superconductivity to a pure PDW state in the Bi₂Sr₂CaDyCu₂O₈ pseudogap phase.