Onset of an insulating zero-plateau quantum Hall state in graphene.

We analyze the dissipative conductance of the zero-plateau quantum Hall state appearing in undoped graphene in strong magnetic fields. Charge transport in this state is assumed to be carried by a magnetic domain wall, which forms by hybridization of two counterpropagating edge states of opposing spin due to interactions. The resulting nonchiral edge mode is a Luttinger liquid of parameter K, which enters a gapped, perfectly conducting state below a critical value K_{c} approximately 1/2. Backscattering in this system involves spin flip, so that interaction with localized magnetic moments generates a finite resistivity Rxx via a "chiral Kondo effect." At finite temperatures T, Rxx(T) exhibits a crossover from metallic to insulating behavior as K is tuned across a threshold K_{MI}. For T --> 0, Rxx in the intermediate regime K_{MI} < K < K_{c} is finite, but diverges as K approaches K_{c}. This model provides a natural interpretation of recent experiments.

Theory of insulator metal transition and colossal magnetoresistance in doped manganites.

The persistent proximity of insulating and metallic phases, a puzzling characteristic of manganites, is argued to arise from the self-organization of the twofold degenerate e(g) orbitals of Mn into localized Jahn-Teller (JT) polaronic levels and broad band states due to the large electron-JT phonon coupling present in them. We describe a new two band model with strong correlations and a dynamical mean-field theory calculation of equilibrium and transport properties. These explain the insulator metal transition and colossal magnetoresistance quantitatively, as well as other consequences of two state coexistence.