Mixed-valence insulators with neutral Fermi-surfaces

Samarium hexaboride is a classic three-dimensional mixed valence system with a high-temperature metallic phase that evolves into a paramagnetic charge insulator below 40 kelvin. A number of recent experiments have suggested the possibility that the low-temperature insulating bulk hosts electrically neutral gapless fermionic excitations. In this work we describe a new phase of matter---composite exciton Fermi liquid--- a mixed valence insulator with a three dimensional fermi-surface of a neutral fermion, that we name the "composite exciton". We describe the mechanism responsible for the formation of such excitons, discuss the phenomenology of the composite exciton Fermi-liquids and make comparison to experiments in SmB6.