A Neutron Scattering Study on Perovskite Cobaltites

One of the most intriguing phenomena in physics is the anomalous metallic state, such as high T_c superconductivity and colossal magnetoresistivity, that emerges when charge carriers are doped into a Mott insulator. A Mott insulator is a material that is insulating due to strong Coulomb repulsions between electrons, even though it has partially filled electronic bands and band theory would predict it to be metallic. The most important difference from the usual band insulator is that the internal degrees of freedom, spin and orbital, still survive in the Mott insulator. Virtual charge fluctuations in a Mott insulator generate a super-exchange interaction between the spins of unpaired electrons of neighboring ions, which leads to long-range antiferromagnetic order in many materials. Doping (adding charge carriers to) a Mott insulator induces phase transitions to metallic phases whose properties cannot be explained in terms of conventional metal physics.

For instance, a perovskite manganite LaMnO₃ is an antiferromagnetic insulator where the Mn³⁺ (3d⁴) ions surrounded by six oxygens octahedrally are ferromagnetically ordered in the ab-planes and are stacked antiferromagnetically along the c-axis. It is due to an orbital ordering pattern that the e_g elections of the Mn³⁺ ions undertake. When the manganite is doped with holes through chemi- cal substitution such as in La_1-xSr_xMn0₃, Mn⁴⁺ (3d³) ions are formed and the e^g electron of the Mn³⁺ ion can hop to the Mn⁴⁺ site. The hopping of the e_g electron is energetically favored if the spins of the neighboring Mn ions are parallel. This double exchange mechanism leads to the ferromagnetic and metallic phase as the charge carrier density grows. It has been shown theoretically and experimentally that the charge and magnetic inhomogeneity occurs and plays an important role in the physics of the manganites. But theoretical consensus on the nature of the inhomogeneity and its role to the metallicity is yet to come. In this proposed work, we shall address this issue by studying several different perovskites, such as La_1-xSr_xCoO₃, using elastic and inelastic neutron scattering techniques.