Abstract
The origin of the gap state in YbB12 and SmB6 has been studied in detail, in particular for YbB12. It is shown that, due to the Coulomb interaction, an electron created by the mixing with the 4f state, and the hole created in the conduction band to screen the 4f hole, form a bound state in low-carrier-density systems. Various mysterious properties are explained very well by this model.