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, antiferromagnetically 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.PACS numbers: 65.40. De, 71.27.+a, 75.30 -6], which are ascribed to the interplay between 4f and conduction electrons. In these mixedvalence compounds, Coulomb repulsion between 4f electrons gives rise to a gap formation on the order 10 meV at the Fermi level. While such materials behave like a Kondo metal at temperatures (T ) higher than the gap energy, they show insulating behavior at low T . For SmB 6 , the collapse of the insulating gap and the emergence of a magnetic order occur at the same pressure [7,8], which suggests that the gap formation competes with the longrange magnetic ordering.Nonalloyed SmS undergoes a phase transition under pressure (P ). With increasing P , the system undergoes a valence transition at a critical pressure of approximately P = 0.7 GPa at T = 300 K from divalence to mixed valence, accompanied by a color change from black to golden yellow [9]. In this mixed-valence phase (named golden SmS), two configurations of 4f 6 and 4f 5 are energetically degenerate:This is also described formally as (1 − ǫ) Sm 2+ + ǫ Sm
3+with 0 ≤ ǫ ≤1, which leads to the definition of the mean valence as ν = 2(1 − ǫ) + 3ǫ. Note that the Sm 2+ ion has no magnetic moment owing to the vanishment of total angular momentum. As seen from Fig. 1(a), ν estimated from high-T experiments shows a continuous evolution with P [10-12]. A similar smooth P -variation is observed in the conductivity σ measured at T = 150 K [ Fig. 1(b)]. From these results, it is inferred that the number of electrons e on the right-hand side of eq. (1) monotonically * Present address: UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan.E-mail address: imura@ims@ac.jp † E-mail address: kensho@cc.nagoya-u.ac.jp increases with P . As seen from the phase diagram of Fig. 1(c), there is no phase transition as a function of P at high T .At low T , by contrast, a magnetic transition occurs at P c ∼ 1.8 GPa [13,14]. Magnetic susceptibility revealed that the ground state at P > P c is antiferromagnetic