Ultrafast collective dynamics of short-range charge/orbital ordering fluctuations in colossal magnetoresistive oxides

Abstract

The "colossal magnetoresistive" (CMR) manganites are highly correlated systems with a strong coupling between spin, charge, orbital, and lattice degrees of freedom, which leads to complex phase diagrams and to the coexistence of various forms of ordering. For example, nanoscale charge/orbital ordering (CO) fluctuations appear to cooperate with Jahn-Teller (JT) distortions of the MnO$_{6} octahedra in CMR manganites and compete with the electron itinerancy favored by double exchange. However, access to the ordered dynamical state has been challenging, mostly due to intrinsic experimental difficulties in measuring fast short-range correlations. Here, we report on a strongly damped low-energy collective mode originating from fast short-range CO fluctuations in La}$_{0.67}Ca$_{0.33}Mn{O}$_{3}(LCMO) single crystal and thin films. We elucidate the collective mode in terms of its dispersion relation and dependence on average A-site ion radius, r$_{A}, and hole-doping concentration. Our results show for the first time that dynamical short-range CO correlations in CMR manganites can be detected with high momentum resolution by coherent ultrafast optical techniques.