The Glass transition in the polaron dynamics of manganites

Dimitri Argyriou

Hahn-Meitner-Institut, Berlin, Germany

Nanoscale fluctuations of charge and orbital ordering within a paramagnetic or ferromagnetic matrix lies at the heart of the CMR effect in manganites. These fluctuations highlighting the strong tendency for charge and/or orbital ordering in manganites which occur on a macroscopic scale for compositions such as x=0, 1/8, 1/2 etc. Using neutron scattering we have shown that these nanoscale fluctuations are purely dynamic in nature at sufficiently high temperatures and exhibit correlations. The correlation are characterized by a wavevector characteristic to the charge and orbital ordering found in half doped manganites such as La0.5Ca0.5MnO3. The correlations freeze upon cooling to a temperature T*~310 K with a character akin to a glass transition. This glass transition suggests that the paramagnetic/insulating state arises from an inherent orbital frustration that inhibits the formation of a long-range orbital- and charge-ordered state. This frustrated state initially competes with ferromagnetism but melts below the ferromagnetic ordering temperature Tc. We have been able to demonstrate this behavior both in double layered La2-2xSr1+2xMn2O7 and in the more traditional CMR manganite such as La0.7Ca0.3MnO3. In this talk we shall review the neutron scattering measurements that provide evidence for the glass transition in manganites and present new results on the polaron dynamics in La0.7Ca0.3MnO3.