Superconductivity was first discovered in metallic systems and has evolved into copper oxides, iron systems, hydrides, and nickel oxides. The metallic, copper oxide, and iron-based systems were discovered first through experiments, while hydride and nickel oxide were discovered first through theory. This is a kind of game change in research. The discovery of superconductivity in hydrides and nickel oxides required high pressure. In other words, theoretical prediction is essential for a labor-intensive experiment such as high pressure. In this article, we will discuss the “electrode fabricated diamond anvil,” an item in the search for superconductivity under high pressure and briefly introduce the recently discovered nickel oxide superconductors.

This paper investigates the superconducting properties of the bilayer nickelate La₂PrNi₂O₇ and its superconducting properties under high pressure. The polycrystalline samples were synthesized by the sol-gel method and measured using multi-anvil high-pressure techniques. In the pressure range of 18–20 GPa, superconductivity is strongly enhanced, with T_c^onset reaching 82.5 K and T_c^zero attaining 60 K. AC magnetic susceptibility measurements confirmed bulk superconductivity, showing a remarkably high superconducting shielding volume fraction of ~97 %. The introduction of Pr³⁺ ions effectively suppresses the intergrowth with other competing Ruddlesden-Popper phases and stabilizes the bilayer Ni–O octahedral layers, thereby enabling robust superconducting behavior. These findings provide compelling experimental evidence for pressure-induced high-T_c bulk superconductivity in bilayer nickelates and contribute to understanding the structure-property relationship in this emerging class of superconductors.

In this article we review ¹³⁹La-nuclear magnetic/quadrupole resonance (NMR/NQR) studies on the bilayer nickelate La₃Ni₂O₇ at ambient pressure, which distinguish two inequivalent La sites, La(1) between the NiO₂ planes and La(2) on the out-of-plane blocking layers. The ¹³⁹La(2)-NQR in La₃Ni₂O₇ further distinguishes the La(2)a site of the ideal La₃Ni₂O₇ from the La(2)b site close to local defects. The ¹³⁹La(2)-NQR spectrum analyses revealed the appearance of two intrinsic La(2)a sites with zero and finite internal fields below T*~150 K. It is consistently explained by the spin density wave order with the single spin-spinless stripe and the moderately reduced Ni moment that is antiferromagnetically aligned. It will give us a clue to understand the relationship with the novel high-T_c superconducting states under high pressure.

We report recent experimental results on multiple superconducting phases in the spin-triplet superconductor UTe₂, revealed by ac calorimetry under pressure. When a small hydrostatic pressure is applied, the superconducting transition splits into two distinct temperatures, indicating the emergence of multiple superconducting phases. Furthermore, magnetic fields applied along either the easy or hard magnetization axis induce a rich variety of superconducting phases, reflecting the spin and orbital degrees of freedom inherent to the spin-triplet state.

The kagome superconductor AV₃Sb₅ (A = K, Cs, Rb) exhibits unconventional charge-density-wave (CDW) order with broken time-reversal and rotational symmetries. However, the superconducting symmetry remains unclear. We investigate the superconducting state of CsV₃Sb₅ by studying impurity effects through electron irradiation and magnetic penetration depth measurements. We found that non-magnetic impurities change from an anisotropic superconducting gap to an isotropic one without inducing a nodal state. Furthermore, transport measurements under high pressure show that the double superconducting dome persists even after introducing impurities. Our results indicate that CsV₃Sb₅ hosts a non-chiral, anisotropic s-wave superconducting state without sign change of the order parameter both at ambient and under pressure, and supports a new type of unconventional superconductivity due to bond-order fluctuations.

In this article, we review recently developed piezoelectric-driven uniaxial stress apparatus, and associated methodology. The apparatus enables precise control of uniaxial stress, and measurement of samples’ stress-strain relationships. This technique not only facilitates well-calibrated uniaxial stress tuning experiments, but also serves as a thermodynamic probe of strain-induced changes in electronic structure. As a representative example, we highlight recent studies on the strongly correlated system Sr₂RuO₄, where measurement of stress-strain curves revealed a pronounced softening of the Young’s modulus with decreasing temperature near a Lifshitz transition, reflecting an increase in the entropy of the conduction electrons.