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

We construct a measurement system of the absolute thermoelectric power, and present experimental results of the Seebeck coefficient S (T, P) of samarium mono-sulphide (SmS) at ambient and high pressure. At ambient pressure, S (T ) of SmS exhibits negative sign and its absolute value |S | steeply increases with lowering temperature, indicating that the dominant charge carrier is an electron and the energy gap opens at Fermi level. When increasing pressure at room temperature, S (P) sharply changes from a negative to positive value at the black-to-golden phase transition, indicating that the dominant carrier changes from electrons to holes possibly associated with the discontinuous collapse of the band gap at the transition.

The resistivity, magnetic susceptibility, magnetization and specific heat under magnetic fields were measured on a Eu0.7Na0.3Fe2As1.4P0.6 single crystal. A decrease in resistivity to zero, which shifted toward lower temperatures under a magnetic field, was observed at Tc ∼ 25 K due to a superconducting transition. The electrical and magnetic anisotropy was quite small. In low-field magnetization, helimagnetism appeared at THM ∼ 19 K, which changed to ferromagnetism in higher fields. Therefore, superconductivity coexists with helimagnetism in a low field but with ferromagnetism in a high field. The specific heat showed anomalies due to the helimagnetic and superconducting transitions as well as the temperature dependence caused by ferromagnetic spin waves in the low temperature regions. The Sommerfeld constant γ was extraordinarily large in the normal and superconducting states. The superconducting and magnetic transitions under magnetic fields with the -plane and -axis configurations were summarized in a phase diagram.

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