Abstract

Perovskite oxide manganites, generally known for their Colossal Magnetoresistance (CMR) show a number of interesting ground states depending on the hole concentration. At optimum hole concentration these materials show a metallic state that is ferromagnetic. At lower hole concentration these materials show a very interesting phase where it is ferromagnetic and insulating. This talk is an investigation into the ferromagnetic insulating (FMI) state. We show that in this phase the material is distinct from a band insulator. It has a finite density of states at the Fermi level (though it is insulating) as shown by the finite linear heat capacity. Unlike the optimal hole doping where the material shows CMR , in the FMI state the magnetoresistance collapses even up to a field of 14T. The low temperature transport shows a Shklovskii Efros variable range hopping showing existence of a Coulomb gap. The FMI state can be made completely metallic by application of high hydrostatic pressure (~7GPa) yet the metallic state show created is distinct from the metallic state seen in the CMR phase. We show, by measuring the Nyquist Noise (=4kBRTe) which measures the electron temperature (Te) that the lattice temperature gets decoupled form the electron temperature when the material is cold through the FMI transition and below the transition temperature the electrons show a slow response much like a glassy state. We argue that these experiments show that the electrons in the FMI state get into an electron glass state due to Coulomb interactions.