High temperature superconductivity in planar copper oxide compounds remains heavily studied because it is the most dramatic among many interesting properties in a poorly understood class of materials; those with strongly correlated conduction band electrons (or holes). The common understanding of these materials is that a uniform hole density determines the particular material properties. There is a standard temperature-doping phase diagram that describes an insulating, antiferromagnetic parent compound that is made superconducting by adding the appropriate density of holes. However, there has always been some indication that the hole density might not be uniform and that a smoothly evolving phase diagram may not completely reflect the underlying physics of these materials. Early theory had predicted that the doped holes would clump together, leading to a phase separation where parts of a sample would remain an insulating antiferromagnet while the rest becomes an optimally doped superconductor. While this was not observed experimentally, a peculiar charge-ordered phase with alternating magnetic and conducting stripes was interpreted as a kind of phase separation. More recently, many experiments have indicated that the hole concentration of the copper oxides is not homogenous over length scales of a few nanometers. Our recent work focuses on a special case, La 2‑x Sr x CuO 4 doped with excess oxygen, where this charge inhomogeneity more fully develops. In this case we detect clear two-phase coexistence with separated magnetic and superconducting regions of a length scale from tens to hundreds of nanometers, somewhat like the early theory. These results suggest that the underlying phase diagram really consists of only a few stable line-phases.
This work is supported by the US-DOE through contract DE-FG02-00ER45801.