DescriptionWe have studied Kondo insulator states in the single and two-channel Kondo models. The Kondo insulator state shows remarkable optical properties. Specifically, a dielectric constant which is ≈ 100 times higher than for a regular insulator. Moreover, the diamagnetic permeability does not seem to be affected by the onset of the Kondo effect. This suggest that while the Kondo effect is very effective in suppressing the longitudinal current fluctuations, it is largely ineffective in its influence on diamagnetic currents.
In the two channel Kondo model, we observe the formation of a half-Kondo insulator, where the spontaneous breaking of channel symmetry leads to the formation of a Kondo insulator in one channel, while the other channel remains metallic. We discuss, how this phenomenon can be understood using the concept of order fractionalization, in which the channel magnetization breaks up into an emergent spinor order parameter. By integrating out the fermions we derive an effective action that describes this symmetry breaking and its emergent collective modes. A remarkable observation is that topological defects in the order parameter carry a U(1) flux, manifested in the Aharonov-Bohm phase picked by electrons that orbit the defect. By studying the effective action, we argue that the phase diagram contains a non-magnetic transition between a large and a small Fermi surface.