How Cooper pairs vanish approaching the Mott insulator in Bi2Sr2CaCu2O8+d

TL;DR

This study visualizes the electronic structure of Bi2Sr2CaCu2O8+d, revealing that as the Mott insulator state is approached, Cooper pairs in momentum space vanish and are replaced by local symmetry-breaking pseudogap states in real space.

Contribution

It provides direct imaging evidence of the transition from delocalized Cooper pairs to local pseudogap states near the Mott insulator in a cuprate superconductor.

Findings

Low-energy Bogoliubov quasiparticles occupy shrinking k-space regions with decreasing hole density.

Spectral weight shifts to high-energy states that break local symmetries.

Pseudogap states are characterized by local symmetry breaking, replacing Cooper pairs.

Abstract

The antiferromagnetic ground state of copper oxide Mott insulators is achieved by localizing an electron at each copper atom in real space (r-space). Removing a small fraction of these electrons (hole doping) transforms this system into a superconducting fluid of delocalized Cooper pairs in momentum space (k-space). During this transformation, two distinctive classes of electronic excitations appear. At high energies, the enigmatic 'pseudogap' excitations are found, whereas, at lower energies, Bogoliubov quasi-particles -- the excitations resulting from the breaking of Cooper pairs -- should exist. To explore this transformation, and to identify the two excitation types, we have imaged the electronic structure of Bi2Sr2CaCu2O8+d in r-space and k-space simultaneously. We find that although the low energy excitations are indeed Bogoliubov quasi-particles, they occupy only a restricted region of k-space that shrinks rapidly with diminishing hole density. Concomitantly, spectral weight is transferred to higher energy r-space states that lack the characteristics of excitations from delocalized Cooper pairs. Instead, these states break translational and rotational symmetries locally at the atomic scale in an energy independent fashion. We demonstrate that these unusual r-space excitations are, in fact, the pseudogap states. Thus, as the Mott insulating state is approached by decreasing the hole density, the delocalized Cooper pairs vanish from k-space, to be replaced by locally translational- and rotational-symmetry-breaking pseudogap states in r-space.