Coexisting charge modulation and ferromagnetism produces long period phases in manganites: new example of electronic soft matter

摘要

The phenomenon of colossal magnetoresistance in manganites is generallyagreed to be a result of competition between crystal phases with differentelectronic, magnetic, and structural order; a competition which can be strongenough to cause phase separation between metallic ferromagnet and insulatingcharge modulated states. Nevertheless, closer inspection of phase diagrams inmany manganites reveals complex phases where the two order parameters ofmagnetism and charge modulation unexpectedly coexist. Here we show that suchexperiments can be naturally explained within a phenomenologicalGinzburg-Landau theory. In contrast to models where phase separation originatesfrom disorder or as a strain induced kinetic phenomenon, we argue that magneticand charge modulation coexist in new thermodynamic phases. This leads to a richdiagram of equilibrium phases, qualitatively similar to those seen inexperiment. The success of this model argues for a fundamental reinterpretationof the nature of charge modulation in these materials from a localised to amore extended "charge density wave" picture. The same symmetry considerationsthat favour textured coexistance of charge and magnetic order may apply to manyelectronic systems with competing phases. The resulting "Electronically soft"phases of matter with incommensurate, inhomogeneous and mixed order may begeneral phenomena in correlated systems.