Takahiro Ishikawa

To verify ‘hot superconductivity’ recently proposed in lanthanum hydride-based compounds, we explored thermodynamically stable and superconducting phases in the lanthanum (La)-platinum (Pt)-hydrogen (H) ternary system at 20 GPa using a computational framework that integrates an evolutionary construction scheme of formation-enthalpy convex hulls, machine learning potential calculations, and density functional theory calculations. Although we found no evidence of the hot superconductivity in this ternary system at the pressure, we predicted a unique compound, LaPtH ₆ . In this compound, PtH ₆ coordination units are formed, and the hydrogen atoms are arranged in an equilateral triangular pattern that nearly constitutes a two-dimensional kagome lattice between the La and Pt layers, leading to superconductivity at 18.67 K. This structure is dynamically stable from ambient pressure to at least 200 GPa and the superconducting critical temperature increases from 13.51 to 40.63 K.

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.

To determine crystal structures from a powder x-ray diffraction (PXRD) pattern containing multiple unknown phases, a data-assimilated crystal growth (DACG) simulation method has been developed. The PXRD penalty function selectively stabilizes the structures in the experimental data, promoting their grain growth during simulated annealing. Since the PXRD pattern is calculated as the Fourier transform of the pair distribution function, the DACG simulation can be performed without prior determination of the lattice parameters. We applied it to carbon (graphite and diamond) and SiO2 (low-quartz, low-cristobalite, and coesite) systems, demonstrating that the DACG simulation successfully reproduced multiple crystal structures.

In this paper, we present a methodology of laser ablation simulation including electronic entropy effects. Ablation by ultrashort laser pulses is suitable for micromachining because of its small thermal damage. To further enhance its effectiveness, it is crucial to explore both theoretically and experimentally the desirable conditions for processing. The density functional theory calculations predict that hot electrons make crystalline lattice thermodynamically unstable at electronic temperature roughly above 20 000 K due to the electronic entropy-driven mechanism. In our simulations, equations combining hydrodynamic equations and the two-temperature model are modeled and are numerically solved, where the electronic entropy effects are included as the electronic temperature-dependent latent heat, boiling point, and melting point. For metal targets, Al and Cu, the calculated ablation rates for 100 fs laser pulses agree with the experimental one at laser fluences up to 10 J/cm2. The heat-affected zone (HAZ) is numerically evaluated from the thickness of the molten layer. Short pulses produce a high ablation rate and small HAZ because of the suppression of diffusion of the deposited pulse energy. Electronic entropy effects make a clear difference between 100 fs and 10 ps pulses. The calculation method presented in this paper serves as a useful tool for exploring nonequilibrium between electrons and ions in the equation of states for metals such as Al and Cu.

In this paper, we explored stable states in the system of 2,4,6-trinitrotoluene (TNT) crystal with a few additional hydrogen radicals (H_add's) using a structure-search scheme based on first-principles calculations and an evolutionary algorithm (EA) to get insights into the decomposition process of TNT. We introduced three evolutionary operators acting on H_add's and transforming only local structures of TNT molecules: "displacement", "permutation", and "mating". We searched for stable structures by increasing the number of H_add's (n) from 1 to 2, 3, 4, 6, and 8 and constructed a convex-hull diagram for the formation energy from solid TNT and solid hydrogen. We showed that the system of n = 6 had the largest energy reduction, in which five of the eight TNT molecules in the calculation cell were transformed into NO, H₂O, C₂H₃N, C₂NO₃H₃, C₈N₂O₄H₇, C₉N₂O₈H₅, and C₁₄N₇O₁₂H₁₁. Analysis of the structural transformations observed during the EA search indicates that (1) the H_add's approaching the TNT molecules react with C, forming a six-membered ring, and with N and O in nitro groups, leaving the TNT molecules as NO, H₂O, C₂H₃N, and C₂NO₃H₃, and (2) the partially decomposed TNT molecules are bonded to one another via C, N, and O.