arXiv subjects

Takahiro Ishikawa

Publications and source records attributed to Takahiro Ishikawa.

15 recordsLinked to original sources

arXiv2026

Search for high-pressure phases of yttrium via a data assimilation approach

We investigate the distorted face-centered-cubic (dfcc) phase of yttrium (Y) using a data-assimilation-based structure search that combines high-resolution powder x-ray diffraction (XRD) data with machine-learning interatomic potentials. By exploring supercells containing up to 128 atoms, we identify three low-enthalpy phases: the previously reported $I4_1/a$ structure and two additional structures, $Ibam$ and $R\overline{3}$. No data-assimilation-derived structure relaxes to the previously proposed $R\overline{3}m$ phase. Phonon calculations show that $I4_1/a$, $Ibam$, and $R\overline{3}$ are dynamically stable, whereas $R\overline{3}m$ exhibits imaginary modes near the $Γ$ point, indicating dynamical instability. Enthalpy calculations using both PBE and r$^{2}$SCAN place the four candidate structures within about 10 meV/atom, indicating a complex energy landscape with multiple competing minima, although $R\overline{3}m$ is consistently highest in enthalpy and r$^{2}$SCAN favors $I4_1/a$ throughout the dfcc pressure range. Rietveld refinements of the powder XRD profile at 60 GPa further narrow the viable structural models to $I4_1/a$ and $Ibam$, both of which reproduce the experimental data better than $R\overline{3}m$ and $R\overline{3}$. Taken together with the energetic ordering and dynamical stability, these results identify $I4_1/a$ as the most plausible structure of the dfcc phase of Y, with $Ibam$ remaining a close competing candidate, particularly toward the high-pressure side of the dfcc region.

cond-mat.mtrl-sci↗

arXiv2024

Data-Assimilated Crystal Growth Simulation for Multiple Crystalline Phases

To determine crystal structures from an X-ray diffraction (XRD) pattern containing multiple unknown phases, a data-assimilated crystal growth (DACG) simulation method has been developed. The XRD penalty function selectively stabilizes the structures in the experimental data, promoting their grain growth during simulated annealing. Since the XRD 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 C (graphite and diamond) and SiO$_2$ (low-quartz and low-cristobalite) systems, demonstrating that the DACG simulation successfully reproduced multiple crystal structures.

cond-mat.mtrl-sci↗

arXiv2023

Evolutionary search for superconducting phases in the lanthanum-nitrogen-hydrogen system with universal neural network potential

Recently, Grockowiak $\textit{et al.}$ reported "hot superconductivity" in ternary or multinary compounds based on lanthanum hydride [A. D. Grockowiak $\textit{et al.}$, Front. Electron. Mater. $\textbf{2}$, 837651 (2022)]. In this paper, we explored thermodynamically stable phases and superconducting phases in the lanthanum-nitrogen-hydrogen system (La$_{x}$N$_{y}$H$_{1-x-y}$, $0 \leq x \leq 1$, $0 \leq y \leq 1$) at pressure of 20$\,$GP. We rapidly and accurately constructed the formation-enthalpy convex hull using an evolutionary construction scheme based on density functional theory calculations, extracting the candidates for stable and moderately metastable compounds by the universal neural network potential calculations. The convex hull diagram shows that more than fifty compounds emerge as stable and moderately metastable phases in the region of $ΔH \leq 4.4$$\,$mRy/atom. In particular, the compounds are concentrated on the line of $x = 0.5$ connecting between LaH and LaN. We found that the superconductivity is gradually enhanced due to N doping for LaH and the superconducting critical temperature $T_{\rm c}$ reaches 8.77$\,$K in La$_2$NH with $y = 0.25$. In addition, we predicted that metastable La$_2$NH$_2$ shows the highest $T_{\rm c}$ value, 14.41$\,$K, of all the ternary compounds predicted in this study. These results suggest that it is difficult to obtain the hot superconductivity in the La-H compounds with N at 20$\,$GPa.

cond-mat.supr-con↗

arXiv2022

High-pressure synthesis of superconducting Sn$_{3}$S$_{4}$ using diamond anvil cell with boron-doped diamond heater

High-pressure techniques open exploration of functional materials in broad research fields. An established diamond anvil cell with a boron-doped diamond heater and transport measurement terminals has performed the high-pressure synthesis of a cubic Sn$_{3}$S$_{4}$ superconductor. X-ray diffraction and Raman spectroscopy reveal that the Sn$_{3}$S$_{4}$ phase is stable in the pressure range of P>5 GPa in a decompression process. Transport measurement terminals in the diamond anvil cell detect a metallic nature and superconductivity in the synthesized Sn$_{3}$S$_{4}$ with a maximum onset transition temperature of 13.3 K at 5.6 GPa. The observed pressure-T$_{c}$ relationship is consistent with that from the first-principles calculation. The observation of superconductivity in Sn$_{3}$S$_{4}$ opens further materials exploration under high temperature and pressure conditions.

cond-mat.supr-con↗

arXiv2021

ChainerRL: A Deep Reinforcement Learning Library

In this paper, we introduce ChainerRL, an open-source deep reinforcement learning (DRL) library built using Python and the Chainer deep learning framework. ChainerRL implements a comprehensive set of DRL algorithms and techniques drawn from state-of-the-art research in the field. To foster reproducible research, and for instructional purposes, ChainerRL provides scripts that closely replicate the original papers' experimental settings and reproduce published benchmark results for several algorithms. Lastly, ChainerRL offers a visualization tool that enables the qualitative inspection of trained agents. The ChainerRL source code can be found on GitHub: https://github.com/chainer/chainerrl.

cs.LG↗

arXiv2021

Lattice dynamics effects on finite-temperature stability of $R_{1-x}$Fe$_{x}$ ($R$ = Y, Ce, Nd, Sm, and Dy) alloys from first principles

We report the effects of lattice dynamics on thermodynamic stability of binary $R_{1-x}$Fe$_x$ $(0<x<1)$ compounds ($R$: rare-earth elements, Y, Ce, Nd, Sm, and Dy) at finite temperature predicted by first-principles calculation based on density functional theory (DFT). We first demonstrate that the thermodynamic stability of $R_{1-x}$Fe$_x$ $(0<x<1)$ alloys cannot be predicted accurately by the conventional approach, where only the static DFT energy at $T = 0$ K is used. This issue can be overcome by considering the entropy contribution, including electronic and vibrational free energies, and we obtained convex hull plots at finite temperatures that successfully explain the thermodynamic stability of various known compounds. Our systematic calculation indicates that vibrational entropy helps stabilize various $R_{1-x}$Fe$_x$ compounds with increasing temperature. In particular, experimentally reported $R_2$Fe$_{17}$ compounds are predicted to become thermodynamically stable above $\sim$800 K. We also show that thermodynamic stability is rare-earth dependent and discuss its origin. Besides the experimentally reported structures, the stability of two new monoclinic $R$Fe$_{12}$ structures found by Ishikawa \textit{et al.} [Phys. Rev. Mater.~\textbf{4}, 104408 (2020)] based on a genetic algorithm are investigated. These monoclinic phases are found to be dynamically stable and have larger magnetization than the ThMn$_{12}$-type $R$Fe$_{12}$. Although they are thermodynamically unstable, the formation energies decrease significantly with increasing temperature, indicating the possibility of synthesizing these compounds at high temperatures.

cond-mat.mtrl-sci↗

arXiv2021

Evolutionary search for cobalt-rich compounds in the yttrium-cobalt-boron system

Modern high-performance permanent magnets are made from alloys of rare earth and transition metal elements, and large magnetization is achieved in the alloys with high concentration of transition metals. We applied evolutionary search scheme based on first-principles calculations to the Y-Co-B system and predicted 37 cobalt-rich compounds with high probability of being stable. Focusing on remarkably cobalt-rich compounds, YCo$_{16}$ and YCo$_{20}$, we found that, although they are metastable phases, the phase stability is increased with increase of temperature due to the contribution of vibrational entropy. The magnetization and Curie temperature are higher by 0.22 T and 204 K in YCo$_{16}$ and by 0.29 T and 204 K in YCo$_{20}$ than those of Y$_{2}$Co$_{17}$ which has been well studied as strong magnetic compounds.

cond-mat.mtrl-sci↗

arXiv2020

Monoclinic YFe$_{12}$ phases predicted from first principles

We searched for stable crystal structures of YFe$_{12}$ using a crystal structure prediction technique based on a genetic algorithm and first-principles calculations. We obtained two monoclinic $C2/m$ structures as metastable phases that are different from the well-known ThMn$_{12}$ structure. These two phases have advantages in their magnetism over the ThMn$_{12}$ structure: The total magnetization $M$ is increased from 25.6 $μ_{\text{B}}$/f.u. up to 26.8 $μ_{\text{B}}$/f.u. by the transformations. We also calculated Curie temperature $T_{\text{C}}$ for these structures within the mean-field approximation and predicted the increase of $T_{\text{C}}$ from 792 K up to 940 K, which is mainly caused by the increase of intersite magnetic couplings within the distance of 2.3--3.1Å. The similar enhancements of $M$ and $T_{\text{C}}$ are also obtained in the pseudo-binary system Y(Fe$_{1-x}$Co$_{x}$)$_{12}$ with $x$ of 0--0.7.

cond-mat.mtrl-sci↗

arXiv2020

Beryllium Polyhydride Be4H8(H2)2 Synthesized at HP/HT

We report the XRD and Raman scattering measurements in combination with DFT calculations that reveal the formation of beryllium polyhydride Be4H8(H2)2 by laser heating Be/H2 mixture to above 1700 K at pressures between 5 GPa and 8 GPa. The Be4H8(H2)2 crystallizes in a P6_3/mmc structure and consists of corner-sharing BeH4 tetrahedrons and H2 molecules that are in an interstitial site. The Be4H8(H2)2 is stable at least to 14 GPa on compression and stable down to 4 GPa at room temperature. Our ab-initio calculations suggest that the Be4H8(H2)2 is a meta-stable phase of Be-H system.

cond-mat.mtrl-sci↗

arXiv2020

Evolutionary construction of formation energy convex hull: Practical scheme and application to carbon-hydrogen binary system

We present an evolutionary construction technique of formation energy convex hull to search for thermodynamically stable compounds. In this technique, candidates with a wide variety of chemical compositions and crystal structures are created by systematically applying evolutionary operators, "mating", "mutation", and "adaptive mutation", to two target compounds, and the convex hull is directly updated through the evolution. We applied the technique to carbon-hydrogen binary system at 10 GPa and obtained 15 hydrocarbons within the convex hull distance less than 0.5 mRy/atom: graphane, polybutadiene, polyethylene, butane, ethane, methane, three molecular compounds of ethane and methane, and six molecular compounds of methane and hydrogen. These results suggest that our evolutionary construction technique is useful for the exploration of stable phases under extreme conditions and the synthesis of new compounds.

cond-mat.mtrl-sci↗

arXiv2019

Hydrogen-Storing Salt NaCl(H$_2$) Synthesized at High Pressure and High Temperature

X-ray diffraction and Raman scattering measurements, and first-principles calculations are performed to search for the formation of NaCl-hydrogen compound. When NaCl and H$_{2}$ mixture is laser-heated to above 1500 K at pressures exceeding 40 GPa, we observed the formation of NaClH$_{\textit{x}}$ with $\textit{P}$6$_{3}$/$\textit{mmc}$ structure which accommodates H$_{2}$ molecules in the interstitial sites of NaCl lattice forming ABAC stacking. Upon the decrease of pressure at 300 K, NaClH$_\textit{x}$ remains stable down to 17 GPa. Our calculations suggest the observed NaClH$_{\textit{x}}$ is NaCl(H$_{2}$). Besides, a hydrogen-richer phase NaCl(H$_{2}$)$_{4}$ is predicted to become stable at pressures above 40 GPa.

cond-mat.mtrl-sci↗

arXiv2019

Materials informatics based on evolutionary algorithms: Application to search for superconducting hydrogen compounds

We present materials informatics approach to search for superconducting hydrogen compounds, which is based on a genetic algorithm and a genetic programming. This method consists of four stages: (i) search for stable crystal structures of materials by a genetic algorithm, (ii) collection of physical and chemical property data by first-principles calculations, (iii) development of superconductivity predictor based on the database by a genetic programming, and (iv) discovery of potential candidates by regression analysis. By repeatedly performing the process as (i) $\rightarrow$ (ii) $\rightarrow$ (iii) $\rightarrow$ (iv) $\rightarrow$ (i) $\rightarrow$ $\dots$, the superconductivity of the discovered candidates is validated by first-principles calculations, and the database and predictor are further improved, which leads to an efficient search for superconducting materials. We applied this method to hypothetical ternary hydrogen compounds and predicted KScH$_{12}$ with a modulated hydrogen cage showing the superconducting critical temperature of 122 K at 300 GPa and GaAsH$_{6}$ showing 98 K at 180 GPa.

cond-mat.supr-con↗

arXiv2018

First-principles study on superconductivity of P- and Cl-doped H$_3$S

The recent reports on 203 K superconductivity in compressed hydrogen sulfide, H$_3$S, has attracted great interest in sulfur-hydrogen system under high pressure. Here, we investigated the superconductivity of P-doped and Cl-doped H$_3$S using the first-principles calculations based on the supercell method, which gives more reliable results on the superconductivity in doped systems than the calculations based on the virtual crystal approximation reported earlier. The superconducting critical temperature is increased from 189 to 212 K at 200 GPa in a cubic $Im\bar{3}m$ phase by the 6.25 % P doping, whereas it is decreased to 161 K by the 6.25 % Cl doping. Although the Cl doping weakens the superconductivity, it causes the $Im\bar{3}m$ phase to be stabilized in a lower pressure region than that in the non-doped H$_3$S.

cond-mat.supr-con↗

arXiv2015

Crystal Structure of 200 K-Superconducting Phase of Sulfur Hydride System

This article reports the experimentally clarified crystal structure of a recently discovered sulfur hydride in high temperature superconducting phase which has the highest critical temperature Tc over 200 K which has been ever reported. For understanding the mechanism of the high superconductivity, the information of its crystal structure is very essential. Herein we have carried out the simultaneous measurements electrical resistance and synchrotron x-ray diffraction under high pressure, and clearly revealed that the hydrogen sulfide, H2S, decomposes to H3S and its crystal structure has body-centered cubic symmetry in the superconducting phase.

cond-mat.supr-con↗

arXiv2015

Optical freezing of charge motion in an organic conductor

Dynamical localization, i.e., reduction of the intersite electronic transfer integral t by an alternating electric field, E(omega) , is a promising strategy for controlling strongly correlated systems with a competing energy balance between t and the Coulomb repulsion energy. Here we describe a charge localization induced by the 9.3 MV/cm instantaneous electric field of a 1.5 cycle (7 fs) infrared pulse in an organic conductor alpha-(bis[ethylenedithio]-tetrathiafulvelene)_2I_3. A large reflectivity change of > 25% and a coherent charge oscillation along the time axis reflect the opening of the charge ordering gap in the metallic phase. This optical freezing of charges, which is the reverse of the photoinduced melting of electronic orders, is attributed to the 10% reduction of t driven by the strong, high-frequency (omega>t/h_bar) electric field.

cond-mat.str-el↗