Electronic inhomogeneity appears to be a hallmark of strongly correlated electron materials, but it occurs over widely differing length scales. The length scale itself appears to be one of the most important parameters determining the ultimate properties of a particular material. Just how the length scale is set, or might be controlled, has not been established and in many ways has been a neglected issue. Here we present a comparative study of two doped Mott insulator systems each of which can be doped in two different ways: a parent compound for the cuprate superconductors, La 2 CuO 4 , which can be doped either by immobile cation substitution of Sr onto La sites or the incorporation of mobile, excess oxygen; and a perovskite magnet SrCoO 3 which can also be doped either by immobile cation substitution of La for Sr or by controlling the concentration of mobile oxygen vacancies. In both cases, doping via cation substitution leads to nano-scale charge inhomogeneity and glassy properties while doping via the control of oxygen concentration leads to the coexistence of large separated regions with different electronic or magnetic properties. Despite being apparently quite different materials systems, aspects of their phase diagrams are remarkably similar and charge ordering seems to be associated with the stable phases. We use the comparison to identify themes that might broadly control the physics of doped Mott insulators.
Work on superconductors supported by USDOE-BES through contract DE-FG02-00ER45801. Work on ferromagnetic oxides is supported by the United States NSF through grant DMR-0907197.