Open questions in CMR manganites, relevance of clustered states and analogies with other compounds including the cuprates

Abstract

This is an informal paper that contains a list of 'things we know' and 'things we do not know' in manganites and other compounds. It is adapted from the Conclusions chapter of a recent book by the author, Nanoscale Phase Separation and Colossal Magnetoresistance. The Physics of Manganites and Related Compounds (Berlin: Springer-Verlag; 2002), but it also contains a summary of some of the most important recent results in the field. It is argued that the current main theoretical and experimental frameworks to rationalize the results of recent manganite investigations are based on the discovery of tendencies towards nanoscale inhomogeneous states, both in experiments and in simulations of models. The colossal magnetoresistance effect appears to be closely linked to these mixed-phase tendencies, although considerably more work is needed to fully confirm these ideas. The paper also includes information on cuprates, diluted magnetic semiconductors, relaxor ferroelectrics, cobaltites and organic and heavy fermion superconductors. These materials potentially share some common phenomenology with the manganites, such as a temperature scale T* above the ordering temperature where anomalous behaviour starts. Many of these materials also present low-temperature phase competition. The possibility of colossal-like effects in compounds that do not involve ferromagnets is briefly discussed. In particular, colossal effects in cuprates are explained. Overall, it is concluded that inhomogeneous 'clustered' states should be considered as a new paradigm in condensed matter physics, since their presence appears to be far more common than previously anticipated.