Transport anomalies in a simplified model for a heavy electron quantum critical point

We discuss the transport anomalies associated with the development of heavy electrons out of a neutral spin fluid using the large-N treatment of the Kondo-Heisenberg lattice model. At the phase transition in this model the spin excitations suddenly acquire charge. The Higgs process by which this takes place causes the constraint gauge field to loosely "lock" together with the external, electromagnetic gauge field. From this perspective, the heavy-fermion phase is a Meissner phase in which the field representing the difference between the electromagnetic and constraint gauge fields is excluded from the material. We show that at the transition into the heavy-fermion phase, both the linear and Hall conductivities jump together. However, the Drude weight of the heavy-electron fluid does not jump at the quantum critical point, but instead grows linearly with the distance from the quantum critical point, forming a kind of "gossamer" Fermi liquid.