Katsuya Shimizu

This study aimed to elucidate the structural evolution of supercritical fluid hydrogen (SCF H2) under high pressure. X-ray diffraction was performed on SCF H2 using synchrotron radiation at room temperature. The structure factor S(Q) was observed in a low-Q region at each pressure, revealing a crossover at around 0.56 GPa, consistent with our previous Raman spectroscopy results. The average volume per molecule was calculated at each pressure using the center-to-center distance between the nearest neighboring H2 molecules, reflecting van der Waals interactions and compression of the internuclear distance. The volumes calculated from the face-centered cubic model were consistent with those derived from the cited equation of state, with a slight discrepancy that increased gradually with the pressure. This finding indicates that further densification following the crossover occurs primarily by intra-atomic/molecular mechanisms rather than intermolecular shortening of distances, which cannot be adequately explained by the close-packed coordination model alone.

Elements have played an essential role in the study of superconductivity, helping us understand this phenomenon. Over the past decade, the progress of experimental and theoretical studies has been significant. The periodic table of superconducting elements added two new superconductors (Rb and Yb). An expanded pressure range of exploration achieved a substantial increase in the highest reported superconducting transition temperatures. Furthermore, we have observed a new direction in searching for new superconducting phases by examining different temperature-pressure paths on energy landscapes. This paper reviews the state of the art in the superconducting properties of elements, highlighting the knowledge gained to enhance superconducting transition temperatures.

In colloidal quantum dots (QDs), excitons are confined within nanoscale dimensions, and the relaxation of hot electrons occurs through Auger cooling. The behavior of hot electrons is evident under ambient pressure. Nanocrystal characteristics, including their size, are key to determining hot electron behavior because they serve as the stage. Applying pressure to materials can effectively modify this stage by providing a means to reversibly control interatomic distances. Unlike the behavior in ambient conditions, the pressure-dependent behavior remains unclear. In this study, InP/ZnS QDs were synthesized, and their pressure-dependent ultrafast carrier dynamics were analyzed using fs-transient absorption spectroscopy. The hot electron relaxation remained nearly constant with pressure up to 3 GPa, suggesting constant interaction between electrons and holes. However, above this threshold, the hot electron relaxation was accelerated by trapping from higher excited states. This study contributes to establishing a fundamental understanding of the pressure-dependent behavior of hot electrons in QDs.

This paper examines the micro-parameters of superconductors. It studies the modulations from weak van der Waals interaction to strong covalence bonding of superconductors. In particular, we studied layered black phosphorus (BP) as a function of pressure. These results reveal a rich scenario of phase transitions and related quantum phenomena, which show that the phases exhibit superconducting states at a pressure higher than 5.0 GPa. In addition, they indicate an angle-dependent upper critical field that demonstrates the dimensional characteristics of superconductivities. This result suggests that the A17 and cubic phases are three-dimensional (3D). The A7 phase shows a two-dimensional (2D) character. The 2D behavior is related to a weakened, distorted, entangled interlayer coupling.

A sharp focus of current research on superconducting superhydrides is to raise their critical temperature T_c at moderate pressures. Here, we report a discovery of giant enhancement of T_c in CeH₉ obtained via random substitution of half Ce by La, leading to equal-atomic (La,Ce)H₉ alloy stabilized by maximum configurational entropy, containing the LaH₉ unit that is unstable in pure compound form. The synthesized (La,Ce)H₉ alloy exhibits T_c of 148–178 K in the pressure range of 97–172 GPa, representing up to 80% enhancement of T_c compared to pure CeH₉ and showcasing the highest T_c at sub-megabar pressure among the known superhydrides. This work demonstrates substitutional alloying as a highly effective enabling tool for substantially enhancing T_c via atypical compositional modulation inside suitably selected host crystal. This optimal substitutional alloying approach opens a promising avenue for synthesis of high-entropy multinary superhydrides that may exhibit further increased T_c at even lower pressures.