Phase Separation, Charge Segregation, and Superconductivity in Layered Cuprates

Abstract

Experimental observations suggesting inhomogeneity in the layered cuprates have been reported since shortly after the discovery of superconductivity in these materials [1]. There are at least two forms of inhomogeneity that have, by now, been fairly well established. One involves the phase separation of oxygen interstitials in La2CuO4+δ . The interstitials are mobile near room temperature, giving a homogeneous phase, while cooling to lower temperatures can lead to segregation into phases with distinct oxygen concentrations. Early work focussed on samples with δ < 0.05, in which case the two phases present at low temperature are the nearly-stoichiometric an-tiferromagnet (described in Chapter 4) and an oxygen-rich superconducting phase. For larger δ, multiple superconducting phases have been discovered, with the T c varying with δ. It has also been shown that, within a particular oxygen-rich phase, the interstitials order in a layered fashion. Neutron diffraction has played a crucial role in establishing the nature of phase separation and order in La2CuO4+δ , and that work is the subject of the first half of this chapter.