Properties of an extended periodic Anderson model with the f-c Coulomb interaction U fcis studied as a model for CeCu 2 Ge 2 and related compounds which is cosidered to exhibit a sharp valence change under pressure. The problem is treated by extending the slave-boson and large-N expansion method to treat the present system. It is shown that, as the f-level ǫ f is increased relative to the Fermi level, the sharp valence change is caused by the effect of U fc with moderate strength of the order of the bandwith of conduction electrons. The superconducting transition temperature T c due to the valence-fluctuation exchange is estimated on the slaveboson fluctuation approximation. In the model with spherical Fermi surface, T c exhibits sharp peak as a function of ǫ f , simulating the effect of pressure, for the d-wave pairing channel.CeCu 2 Si 2 is well known not only as the firstly discovered unconventional superconductor with T c ∼ 0.7 K at ambient pressure, 1) but as a remarkable pressure dependence of T c exhibiting a pronounced peak at P ≃ 17 GPa. 2) The isostructural compound, CeCu 2 Ge 2 , has a similar phase diagram in P -T plane in which one can see the superconductivity appears after the magnetism is suppressed by pressure at P ∼ 8 GPa. 3-5) These two compounds have similar physical properties, if the scale of the pressure for CeCu 2 Si 2 is shifted by 7.6 GPa. 6) The mechanism of such prominent enhancement of T c has not been understood so far. A purpose of this paper is to discuss it from the view point that such phenomena is based on the sharp valence change of Ce ion.Apart from the pronounced peak of T c , remarkable properties under pressure of these compounds are as follows: 3) 1) The residual resistivity ρ 0 also exhibits a peak at around the pressure where * Present address: NEC Corporation, 4-1-1 Miyazaki, Miyamae-ku, Kawasaki, Kanagawa 216-8555, Japan.

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It is shown that a new type of superconductivity is possible in one-dimensional (1D) and quasi-one-dimensional (Q1D) metals where the pairing interaction is mediated by both spin and charge fluctuations. The gap function inevitably changes sign in the radial direction near the Fermi surface; it vanishes at the Fermi points for 1D, and for Q1D it has line nodes on the Fermi surface as in usual anisotropic pairings. The transition temperature T c is generally higher for the singlet pairing than for the triplet, but only by a few times. This implies that T c for the triplet pairing can exceed that of the singlet under a moderately large Zeeman magnetic field, i.e., field-induced triplet pairing. This feature seems to explain the anomalous behaviours of H c2 and the Knight shift observed in (TMTSF) 2 PF 6 .