Pseudogap formation near at the border of an insulator-metal transition in SmS

We measured the thermal expansion and heat capacity of the mixed-valence compound SmS that shows an insulator to metal phase transition under pressure. We report a convincing evidence for the existence of not only an electronic specific heat coefficient gamma but also a Schottky like gap A in the metal phase, generally referred to as golden phase. As the pressure increases, gamma steeply increases while A monotonically decreases. Nevertheless, the product gamma Delta/N is kept constant, where N denotes the number of Sm3+ ion. We argue that a novel bound state is formed in golden phase.