Optical conductivity and the correlation strength of high-temperature copper-oxide superconductors

Since their discovery in 1986, the high-temperature copper-oxide superconductors have been a central object of study in condensed-matter physics. Their highly unusual properties are widely ( although not universally) believed to be a consequence of electron-electron interactions that are so strong that the traditional paradigms of condensed-matter physics do not apply: instead, entirely new concepts and techniques are required to describe the physics. In particular, the superconductivity is obtained by adding carriers to insulating 'parent compounds'. These parent compounds have been identified(1) as 'Mott' insulators, in which the lack of conduction arises from anomalously strong electron-electron repulsion. The unusual properties of Mott insulators are widely(2) believed to be responsible for the high-temperature superconductivity. Here, we present a comparison of new theoretical calculations and published(3-8) optical conductivity measurements, which challenges this belief. The analysis indicates that the correlation strength in the cuprates is not as strong as previously believed, in particular that the materials are not properly regarded as Mott insulators. Rather, antiferromagnetism seems to be necessary to obtain the insulating state. By implication, antiferromagnetism is essential to the properties of the doped metallic and superconducting state as well.