Discontinuous Transition from a Real Bound State to Virtual Bound State   in a Mixed-Valence State of SmS
  Golden SmS is a paramagnetic, mixed-valence system with a pseudogap. With increasing pressure across a critical pressure Pc, the system undergoes a discontinuous transition into a metallic, anti-ferromagnetically ordered state. By using a combination of thermodynamic, transport, and magnetic measurements, we show that the pseudogap results from the formation of a local bound state with spin singlet. We further argue that the transition Pc is regarded as a transition from an insulating electron-hole gas to a Kondo metal, i.e., from a spatially bound state to a Kondo virtually bound state between 4f and conduction electrons. 
Keiichiro Imura, Shinkichi Kanematsu, Kazuyuki Matsubayashi, Hiroyuki   S. Suzuki, Kazuhiko Deguchi, Noriaki K. Sato
cond-mat.str-el