Great interests have been attracted for the strongly correlated electron systems, which often show novel physical properties based on the unknown aspect of nature. A high-T_c cuprate and an iron-based superconductor are the examples of such systems and many high-pressure works have been performed because of showing large pressure effect on T_c. In the way of the study of cuprate superconductors, the pressure-induced superconductivity was discovered in spin-ladder type cuprate Sr_14－xCa_xCu₂₄O₄₁, which contributed the study of high-T_c superconductivity. Recently, in case of iron-based superconductors, pressure-induced superconductivity was discovered in the spin-ladder type iron-based compound BaFe₂S₃. This discovery promotes the development of the study of the iron-based superconductors.

In this article, a prototype of high-T_c iron-based superconductors, LaFeAsO_1－xH_x (0≤x≲0.6) was reviewed in the viewpoint of pressure-induced quantum phase transition and quantum criticality. The system undergoes an unusual phase transition from superconducting to antiferromagnetic (AF) phases upon heavily H doping. On applying a pressure of 4.0 GPa, the AF phase for x=0.6 disappears and a “bare” quantum critical point manifests. The present system offers a good opportunity for demonstrating the capability of the NiCrAl hybrid piston-cylinder pressure cell.

We have been investigating superconducting phase diagram in various cuprate superconductors with carrier doping levels and layer numbers of CuO₂ plane to figure out the condition which will realize higher superconducting transition temperature. In this article, we show the result of this study partly, and discuss the way to have a new cuprate with higher transition temperature.

In this article, uniaxial compression effects on cuprate superconductors such as YBa₂Cu₄O₈ [Y-124], Y_0.98Ca_0.02Ba₂Cu₄O₈ [Y_0.98Ca_0.02-124], and Hg_0.83Re_0.18Ba₂Ca_2.4Cu_3.6O₁₄ [Hg_0.83(Re_0.18)-1223] were reviewed. The uniaxial compression effects were compared with the hydrostatic compression effects via measuring the Meissner signal of their single crystals in the setup of using a diamond anvil cell. In Y-124 and Y_0.98Ca_0.02-124, the disappearance of the Meissner signal depended on the style of compression, and the difference in the change in the superconducting transition temperature T_c against initial compression are discussed with the structural symmetry of the CuO₅ pyramid. In Hg_0.83(Re_0.18)-1223, the mechanism of increasing T_c is discussed on the basis of the first principle calculation.

In transition-metal monopnictides, CrAs and MnP, CrAs forms a double-helical magnetic structure below T_N=270 K accompanied by a strong first-order structural transition, while MnP first undergoes a ferromagnetic transition at T_c=290 K and then adopts a similar double-helical order below T_s=50 K. By using high pressure, we observed superconductivity with a maximum superconducting transition temperature of T_sc=2 K and 1 K under a critical pressure of P_c=0.8 and 8 GPa for CrAs and MnP, respectively. In this review article, we will summarize the achievements of superconductivity in CrAs and MnP.

In this article, I introduce the pressure-induced superconductivity in CePtSi₂ with its discovering process. At ambient pressure, the orthorhombic CePtSi₂ shows an antiferromagnetic order below T_N=1.8 K. T_N disappears above 1.2 GPa, and a superconducting phase appears with T_c～0.1 K. Significant decrease of the A coefficient in resistivity and discrepancy between quantum critical point (QCP) and the center of the superconducting phase suggest the valence mediated superconductivity as that observed in CeCu₂ (Si,Ge)₂.