Theory of a Strange Metal at the Breakdown of a Heavy Fermi Liquid*

We introduce an effective theory for quantum critical points in heavy fermion systems involving a change in carrier density without symmetry breaking. The new theory captures a strong coupling fixed point, leading to robust marginal Fermi liquid transport phenomenology, within a controlled large N limit. This is contrasted with the conventional so-called "slave boson" theory of the Kondo breakdown, where the large N limit describes a weak coupling fixed point and non-trivial transport behavior may only be obtained through uncontrolled 1/N corrections. We compute the weak field Hall coefficient within the effective model as the system is tuned across the transition. We find that between the two plateaus, reflecting the different carrier densities in the two Fermi liquid phases, the Hall coefficient can develop a peak in the critical crossover regime, consistent with recent experimental findings. In the regime of strong damping of emergent bosonic excitations, the critical point also displays a universal "Planckian" transport lifetime, up to weak logarithmic corrections.