Singular Fermi liquid behavior in the underscreened Kondo model

Using the Schwinger boson spin representation, we reveal a new aspect to the physics of a partially screened magnetic moment in a metal, as described by the spin-S Kondo model. We show that the residual ferromagnetic interaction between a partially screened spin and the electron sea destabilizes the Landau Fermi liquid, forming a singular Fermi liquid with a 1/[Tln⁴(TK/T)] divergence in the low-temperature specific heat coefficient CV/T. A magnetic field B tunes this system back into Landau Fermi liquid with a Fermi temperature proportional to Bln²(TK/B). We discuss a possible link with field-tuned quantum criticality in heavy-electron materials.