Fractionalized Fermi liquid in square-lattice Kondo-Heisenberg model

The Kondo-Heisenberg model on a (un-frustrated ) square lattice is studied via a controlled large-N (Sp(2N)) approach to demonstrate on a general ground the existence of a peculiar metallic state so-called “fractionalized Fermi liquid (FL*)” with unbroken translational symmetry and a Fermi surface volume not controlled by the total electron density. Close to half-filling, we show that the nesting of Fermi surface favors the Kondo hybridization to form between conduction electrons and local resonant-valence-bond spin-liquid fermions, which stabilizes FL* state with well-defined quasi-particle and gapped spinon excitations. The system develops an ordered phase at ground state (charge density wave or pair density wave superconducting). Our result generalizes the FL* state previously proposed in Phys. Rev. Lett. 90, 216403 (2003), which is restricted to frustrated Kondo lattice systems.