New trend of superconductivity in strongly correlated electron systems

A new trend in the theory of heavy-fermion superconductivity is reviewed, paying attention to the role of critical valence fluctuations (CVFs). First of all, the trends of superconducting mechanisms of electronic (repulsive) origin are briefly summarized. Secondly, it is discussed that the pressure induced enhancement of the superconducting transition temperature T_c and associated anomalies observed in CeCu₂(Ge,Si)₂ can be understood in a unified way by a single assumption that the systems are subject to the critical valence transition of the Ce ion by the pressure tuning. Thirdly, it is shown that the extended periodic Anderson model with the repulsion between f and conduction electrons really has the potentiality of such a sharp valence transition of Ce ions containing f electrons. In particular, enhancement of the residual resistivity, the Sommerfeld constant and T_c, and T-linear resistivity, are shown to be concluded from this model. Fourthly, the locality of this CVF is briefly discussed. Fifthly, it is argued that CVF is enhanced by the magnetic field when the system is located near the critical point of valence transition in relation to the case of CeCu₆. Finally, it is argued that the CVF mechanism is a rather universal one which may be related to the physics of Ce115 and Pu115 compounds as well.