Direct phase-sensitive identification of a d-form factor density wave in underdoped cuprates

High-temperature superconductivity emerges when holes are introduced into the antiferromagnetic, insulating CuO₂ plane of the cuprates. Intervening between the insulator and the superconductor is the mysterious pseudogap phase. Evidence has been accumulating that this phase supports an exotic density wave state that may be key to its existence. By introducing visualization techniques that discriminate the electronic structure at the two oxygen sites with each CuO₂ unit cell, we demonstrate that this density wave consists of periodic modulations maintaining a phase difference of π between every such pair of oxygen sites. Therefore, the cuprate pseudogap phase contains a previously unknown electronic state—a density wave with a d-symmetry form factor.