Impurity atoms on view in cuprates

24 ISSN:1369 7021 © Elsevier Science Ltd 2002 Conventional superconductors Bardeen-Cooper-Schrieffer (BCS) theory provides a microscopic picture of how attractive electron-electron interactions in the degenerate electron gas of a metal result in the formation of Cooper-pairs1. Each Cooper-pair can be considered a bound state of two electrons with opposite spin and momentum. The superconducting ground-state then essentially represents condensation of these Cooper-pairs into a single macroscopic quantum state – a process analogous to Bose-Einstein condensation. In conventional superconductors, the order parameter (OP) associated with this BCS ground-state typically has s-wave symmetry. The formation of Cooper-pairs opens a gap of width ∆ in the density of electronic states at the Fermi level. This is referred to as the superconducting energy gap. Above this gap, a new spectrum of excited states called Bogoliubov quasiparticles appears. Near the Fermi level, these quasiparticles have a density of states and dispersion relation very different from those of the Landau quasiparticles of the metallic parent state. They are of central interest here because single particle tunneling experiments can be used to measure the energy dependence of their density of states2.