Mixed valence insulators with neutral Fermi surfaces

A number of recent experiments have suggested the possibility that the low-temperature insulating bulk of mixed valence systems like SmB6 and YbB12 hosts electrically neutral gapless excitations. I will 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". I will describe the mechanism responsible for the formation of such excitons, discuss the phenomenology of the composite exciton Fermi-liquid and discuss existing and possible future experiments.