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-Tc 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 Tc. In the way of the study of cuprate superconductors, the pressure-induced superconductivity was discovered in spin-ladder type cuprate Sr14－xCaxCu24O41, which contributed the study of high-Tc 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-Tc iron-based superconductors, LaFeAsO1－xHx (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], Y0.98Ca0.02Ba2Cu4O8 [Y0.98Ca0.02-124], and Hg0.83Re0.18Ba2Ca2.4Cu3.6O14 [Hg0.83(Re0.18)-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 Y0.98Ca0.02-124, the disappearance of the Meissner signal depended on the style of compression, and the difference in the change in the superconducting transition temperature Tc against initial compression are discussed with the structural symmetry of the CuO5 pyramid. In Hg0.83(Re0.18)-1223, the mechanism of increasing Tc 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 TN=270 K accompanied by a strong first-order structural transition, while MnP first undergoes a ferromagnetic transition at Tc=290 K and then adopts a similar double-helical order below Ts=50 K. By using high pressure, we observed superconductivity with a maximum superconducting transition temperature of Tsc=2 K and 1 K under a critical pressure of Pc=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 TN=1.8 K. TN disappears above 1.2 GPa, and a superconducting phase appears with Tc～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.