arXiv subjects

Kenta Hongo

Publications and source records attributed to Kenta Hongo.

At least 19 recordsLinked to original sources

arXiv2026

Application and Performance Assessment of Annealing Methods for Electrostatic-Energy-Based Configuration Search in Mixed Crystals

In first-principles design of solid solutions and disordered materials, exhaustive evaluation of all possible substitutional configurations is often impractical because the number of site occupations increases exponentially. Here, we develop a framework for pre-screening mixed-crystal configurations using annealing methods, where the Ewald electrostatic energy is used as the objective function. Substitutional occupations are represented by binary variables, allowing the Ewald energy to be mapped onto an Ising-type Hamiltonian and the search for low-energy configurations to be formulated as a combinatorial optimization problem. We implement this formulation using simulated annealing (SA) and quantum annealing (QA), and benchmark their performance against exhaustive search. For the small-scale system CaYAlO$_4$, SA achieved a speed-up of about 30 times, while QA achieved a speed-up of more than 100 times; both methods identified all lowest-energy configurations. For the medium-scale system $\beta$-KSbF$_4$ and the large-scale Ba-doped SiAlON system, SA achieved speed-ups of about 200-300 times while robustly identifying the lowest-energy structures. In contrast, QA was effective for the small-scale case but showed limited speed-up for medium-scale problems and missed some low-energy configurations due to chain breaks. These results indicate that SA is currently the most robust and general-purpose approach for rapid pre-screening of mixed-crystal configurations based on electrostatic energy. The proposed formulation can be implemented automatically using publicly available libraries and provides a practical route for accelerating candidate-structure generation before first-principles calculations.

cond-mat.mtrl-sci↗

arXiv2026

High-Tc Superconductivity in Functionalized Out-of-Plane Ordered Double Transition Metal MXenes

Two-dimensional (2D) superconductors attracted growing interest in condensed-matter physics research. In this work, we explore the superconducting properties of surface-functionalized, out-of-plane ordered double transition-metal MXenes (o-MXenes), which exhibit distinctive structural and electronic characteristics. Using first-principles calculations, we investigate the effects of electronic structure, electron-phonon coupling (EPC), anharmonicity, and anisotropy effect in superconductivity properties of o-MXenes. We examine a wide range of o-MXene systems, M$_{2}$M$^\prime$X$_{2}$T$_{2}$ (M = Mo, W; M$^\prime$ = Sc, Ti, V, Mo, Zr, Nb, Ta; X = C, N), functionalized with F, O, Cl, and H groups. Out of 128 candidates, 32 compounds are found to be mechanically, dynamically, and thermodynamically stable, exhibiting superconducting transition temperatures (T$_{c}$) from 0.1 K to 52 K. Notably, the Mo$_{2}$ScN$_{2}$O$_{2}$ compound achieves the highest T$_{c}$ of 52 K, with a superconducting gap of $\sim$10 meV. Solving the anisotropic Eliashberg equation reveals that Mo$_{2}$ScN$_{2}$O$_{2}$ is an anisotropic two-gap superconductor, and incorporating anharmonic effects decreases its T$_{c}$ slightly. We further analyze flat-band-induced EPC enhancement and present EPC matrix elements as functions of phonon wavevector q for distinct vibrational modes that show anharmonic behavior of these materials.

cond-mat.mtrl-sci↗

arXiv2025

Empowering Neural Network-based Quantum Monte Carlo with Local Pseudopotentials

Neural Network-based Quantum Monte Carlo (NNQMC), an emerging method for solving many-body quantum systems with high accuracy, has been mainly applied to small systems due to demanding computation requirements. In this work, we introduce a framework based on local pseudopotentials to break through such limitation, improving the computational efficiency and scalability of NNQMC. The incorporation of local pseudopotentials reduces the number of electrons treated in neural network and also achieves better relative energy accuracy than all electron NNQMC calculations for complex systems. This counterintuitive outcome is made possible by the distinctive characteristics inherent to NNQMC. Notably, by avoiding costly integration terms, this approach is also substantially more efficient than its widely used semilocal counterparts. Our approach enables the reliable treatment of large and challenging systems, such as the $\text{Fe}_4 \text{S}_4 (\text{SCH}_3)_4$ iron-sulfur cluster. Overall, our findings demonstrate that the synergy between NNQMC and local pseudopotentials substantially expands the scope of accurate ab initio calculations.

physics.chem-ph↗

arXiv2025

Electronic transport properties of titanium nitride grown by molecular beam epitaxy

This study investigates the molecular beam epitaxial (MBE) growth of titanium nitride (TiN) thin films, achieving a high residual resistivity ratio (RRR) of 15.8. We observed a strong correlation between growth temperature and crystalline quality, as reflected in both RRR values and lattice parameter variations. Characterization of superconductivity yielded a Ginzburg-Landau coherence length of 60.4 $\pm$ 0.6 nm, significantly higher than typical sputtered films, suggesting improved superconducting coherence. First-principles calculations, in conjunction with experimental data, provided detailed insights into the electronic structure and transport properties of the TiN films. Temperature-dependent Hall coefficient measurements further revealed the influence of anisotropic scattering mechanisms. These findings establish a promising route for the development of nitride-based superconducting materials for advanced quantum computing technologies.

cond-mat.supr-con↗

arXiv2025

Ab initio screening for BCS-type superconductivity in ThCr$_2$Si$_2$-type compounds

In this study, we applied ab initio $T_\mathrm{c}$ calculations to compounds with the ThCr$_2$Si$_2$-type structure to search for BCS superconductor candidates. From the 1883 compounds registered in the Inorganic Crystal Structure Database, we excluded those whose chemical compositions would inhibit the emergence of BCS-type superconductivity by giving rise to magnetism or heavy-fermionic behavior. We then focused on 66 compounds confirmed to be dynamically stable through phonon calculations. Among these, for the 24 systems with experimentally reported $T_\mathrm{c}$ values, we verified that the ab initio $T_\mathrm{c}$ calculations exhibit excellent predictive reliability. For the remaining 42 compounds lacking experimental $T_\mathrm{c}$ values, our predictions identified several new BCS-type superconductor candidates, including SrPb$_2$Al$_2$ $\left(T_c^\mathrm{calc}=2.2\,\mathrm{K}\right)$.

cond-mat.supr-con↗

arXiv2023

Locality Error Free Effective Core Potentials for 3d Transition Metal Elements Developed for the Diffusion Monte Carlo Method

Pseudopotential locality errors have hampered the applications of the diffusion Monte Carlo (DMC) method in materials containing transition metals, in particular oxides. We have developed locality error free effective core potentials, pseudo-Hamiltonians, for transition metals ranging from Cr to Zn. We have modified a procedure published by some of us in [M.C. Bennett et al, JCTC 18 (2022)]. We carefully optimized our pseudo-Hamiltonians and achieved transferability errors comparable to the best semilocal pseudopotentials used with DMC but without incurring in locality errors. Our pseudo-Hamiltonian set (named OPH23) bears the potential to significantly improve the accuracy of many-body-first-principles calculations in fundamental science research of complex materials involving transition metals.

cond-mat.mtrl-sci↗

arXiv2022

First-Principles-Based Insight into Electrochemical Reactivity in a Cobalt-Carbonate-Hydrate Pseudocapacitor

Cobalt carbonate hydroxide (CCH) is a pseudocapacitive material with remarkably high capacitance and cycle stability. Previously, it was reported that CCH pseudocapacitive materials are orthorhombic in nature. Recent structural characterization has revealed that they are hexagonal in nature; however, their H positions still remain unclear. In this work, we carried out first-principles simulations to identify the H positions. Through the simulations, we could consider various fundamental deprotonation reactions inside the crystal and computationally evaluate the electromotive forces (EMF) of the deprotonation ($V_\mathrm{dp}$). Compared with the experimental potential window of the reaction ($< 0.6$ V (vs. saturated calomel electrode (SCE))), the computed $V_\mathrm{dp}$ (vs. SCE) value ($3.05$ V) was beyond the potential window, indicating that deprotonation never occurred inside the crystal. This may be attributed to the strongly formed hydrogen-bonds (H-bonds) in the crystal, thereby leading to the structural stabilization. We further investigated the crystal anisotropy in an actual capacitive material by considering the growth mechanism of the CCH crystal. By associating our X-ray diffraction (XRD) peak simulations with experimental structural analysis, we found that the H-bonds formed between CCH $\{(\bar{1}\bar{1}\bar{1}), (2\bar{1}\bar{1}), (2\bar{1}1)\}$ planes (approximately parallel to $ab$-plane) can result in 1-D growth (stacked along with $c$-axis).

cond-mat.mtrl-sci↗

arXiv2022

Towards chemical accuracy using the Jastrow correlated antisymmetrized geminal power ansatz

Herein, we report accurate atomization energy calculations for 55 molecules in the Gaussian-2 (G2) set using lattice regularized diffusion Monte Carlo (LRDMC). We compare the Jastrow-Slater determinant ansatz with a more flexible JsAGPs (Jastrow correlated antisymmetrized geminal power with singlet correlation) ansatz. AGPs is built from pairing functions, which explicitly include pairwise correlations among electrons and hence, this ansatz is expected to be more efficient in recovering the correlation energy. The AGPs wave functions are first optimized at the variational Monte Carlo (VMC) level, which includes both the Jastrow factor and the nodal surface optimization. This is followed by the LRDMC projection of the ansatz. Remarkably, for many molecules, the LRDMC atomization energies obtained using the JsAGPs ansatz reach chemical accuracy ($\sim$1 kcal/mol) and for most other molecules, the atomization energies are accurate within $\sim$5 kcal/mol. We obtained a mean absolute deviation of 1.6 kcal/mol with JsAGPs and 3.2 kcal/mol with JDFT (Jastrow factor + Slater determinant with DFT orbitals) ansatz. This work shows the effectiveness of the flexible AGPs ansatz for atomization energy calculations and electronic structure simulations in general.

physics.chem-ph↗

arXiv2022

(La,Th)H$_{10}$: the potential high-$T_{c}$ superconductors stabilized thermodynamically below 200 GPa

The recent high-pressure experimental discovery of superconductivity in (La,Y)H$_{10}$, (La,Ce)H$_{9}$, (La,Ce)H$_{10}$, (Y,Ce)H$_{9}$, and (La,Nd)H$_{10}$ shows that the ternary rare-earth clathrate hydride can be promising candidate for high-temperature superconductor. In this work, we theoretically demonstrate that the combination of actinide-metal thorium (Th) and rare-earth-metal lanthanum (La) with hydrogen can also form some ternary hydrides with cage-like structures to be stable at 200 GPa. Using the evolutionary algorithms combined with the first-principles calculations, we have predicted the pressure-dependent ternary phase diagram of La$_{x}$Th$_{y}$H$_{z}$, particularly including the case of (La$_{1-x}$Th$_{x}$)H$_{n}$ [or designated as (La,Th)H$_{n}$ for simplicity]. Our calculations show that the hydrogen-rich phases such as (La,Th)H$_{9}$ (only including $P\bar{6}m2$-LaThH$_{18}$) and (La,Th)H$_{10}$ (including $I4/mmm$-La$_{3}$ThH$_{40}$, $R\bar{3}m$-LaThH$_{20}$, and $I4/mmm$-LaTh$_{3}$H$_{40}$) with H$_{29}$ and H$_{32}$ cages can be thermodynamically stable below 200 GPa. However, the phase decomposition can happen to only (La,Th)H$_{9}$ when the pressure is above 150 GPa. More importantly, the electron-phonon coupling (EPC) calculations show that the (La,Th)H$_{10}$ series could the potential superconductors, of which $I4/mmm$-La$_{3}$ThH$_{40}$ at 200 GPa exhibits the large EPC constant $\lambda$ = 2.46 with a highest transition temperature ($T_\mathrm{c}$) of 210 K. Since there are few previous studies on ternary hydrides composed of actinide metals, the present work would greatly stimulate the further discovery of this type of ternary hydrides and provide useful guidance for the high-pressure experimental studies on them.

cond-mat.supr-con↗

arXiv2022

High-pressure BaCN$_2$ phases explored by genetic algorithm

Polymers containing nitrogen have attracted much attention in connection with their application to high energy density materials (HEDMs), in which energy is inherent in the triple bond. It is an interesting question whether such polymerized phases appear in the high-pressure phase of metal carbodiimide MCN$_2$, of which synthesis have been reported in recent years, but few studies have investigated the crystal structure at high pressure. We have adopted a structure search based on the genetic algorithm coupled with ab initio electronic structure calculations to investigate possible crystal structures that may appear in the high-pressure phase of BaCN$_2$. The structure search successfully reproduced the previously reported crystal structures in the lower pressure range. With confirmed reliability of its predictive ability, the genetic search further predicts a polymerized phase with Ima2 appearing at higher pressure above 42 GPa. The polymerized phase takes the structure of a linear network of CN$_3$ planar triangular units. It is understood that the anion site units CN$_2$, which are close to each other under high pressure, form covalent bonds directly with each other and stabilize the phase.

cond-mat.mtrl-sci↗

arXiv2022

Electronic structure and effective mass analysis of doped TiO$_2$ (anatase) systems using DFT+$U$

In this work, electronic structure of several doped TiO$_2$ anatase systems is computed using DFT+$U$. Effective masses of charge carriers are also computed to quantify how the dopant atoms perturb the bands of the host anatase material. $U$ is computed systematically for all the dopants using the linear response method rather than using fitting procedures to physically known quantities. A combination of $d$ and $f$ block elements (Nb, Ta, V, Mo, W, Cr, La, Cu, Co and Ce) are considered as dopants. Depending upon the energies of their outer $d$ or $f$ electrons, the dopants are found to form defect states at various positions in the band structure of host anatase system. Some dopants like Cr, Mo etc. form mid-gap states, which could reduce transparency. Other dopants like Nb, Ta and W are found to have the Fermi levels positioned near the conduction band edge, indicating these systems to exhibit $n$-type conductivity. From the effective mass analysis, dopants are found to increase the effective mass of charge carriers and the non-parabolic nature of bands. Based on electronic structure and effective mass analysis, Nb, Ta and W are identified to exhibit higher transparency and conductivity as compared to the other dopants considered here. The theoretical results presented here, increase our understanding and show the potential of dopants to alter the properties in anatase TiO$_2$.

cond-mat.mtrl-sci↗

arXiv2022

High-$T\rm_{c}$ superconductivity of clathrate Y$_{3}$EuH$_{24}$

Recent progress on theoretical predictions of ternary superhydrides and their subsequent experimental confirmations have introduced us with a new generation of superconductors, having the potential to realize the synthesis of the most anticipated room temperature superconductors. Motivated by the recent high pressure experiment on YH$_6$ and EuH$_6$, we have theoretically examined the stability as well as superconductivity by mixing the two compounds. Here we identified four phases capable of maintaining thermodynamic stability in the pressure range of 150-300 GPa, namely $Fm\bar{3}m$-Y$_{3}$EuH$_{24}$, $Cmmm$-YEuH$_{12}$, $Cmmm$-YEu$_{3}$H$_{24}$,$Immm$-YEu$_{3}$H$_{24}$. Among them, only $Fm\bar{3}m$-Y$_{3}$EuH$_{24}$ can maintain dynamically stability in the searched pressure range. The Allen-Dynes modified McMillan formula was used to further check the superconductivity of this phase, and the results reveal that the predicted superconducting transition temperature $T\rm_{c}$ of Y$_{3}$EuH$_{24}$ is approaching 220 K at 200 GPa. $Fm\bar{3}m$-Y$_{3}$EuH$_{24}$ has a nearly identical logarithmic average phonon frequency $\omega_{log}$ to YH$_{6}$, but its electron-phonon coupling (EPC) constant $\lambda$ is somewhat smaller than YH$_{6}$'s 2.56, resulting in a lower ultimate $T_{c}$ value.

cond-mat.supr-con↗

arXiv2022

Potential high-$T_{c}$ superconductivity in YCeH$_{x}$ and LaCeH$_{x}$ under pressure

Lanthanum, yttrium, and cerium hydrides are the three most well-known superconducting binary hydrides (La-H, Y-H, and Ce-H systems), which have gained great attention in both theoretical and experimental studies. Recent studies have shown that ternary hydrides composed of lanthanum and yttrium can achieve high superconductivity around 253 K. In this study we employ the evolutionary-algorithm-based crystal structure prediction (CSP) method and first-principles calculations to investigate the stability and superconductivity of ternary hydrides composed of (Y, Ce) and (La, Ce) under high pressure. Our calculations show that there are multiple stable phases in Y-Ce-H and La-Ce-H systems, among which $P4/mmm$-YCeH$_{8}$, $P\bar{6}m2$-YCeH$_{18}$, $R\bar{3}m$-YCeH$_{20}$, $P4/mmm$-LaCeH$_{8}$, and $R\bar{3}m$-LaCeH$_{20}$ possessing H$_{18}$, H$_{29}$ and H$_{32}$ clathrate structures can maintain both the thermodynamic and lattice-dynamic stabilities. In addition, we also find that these phases also maintain a strong resistance to decomposition at high temperature. Electron-phonon coupling calculations show that only three of these five phases can exhibit high-temperature superconductivity. The superconducting transition temperatures ($T_\mathrm{c}$) of $R\bar{3}m$-YCeH$_{20}$, $R\bar{3}m$-LaCeH$_{20}$, and $P\bar{6}m2$-YCeH$_{18}$ are predicted using the Allen-Dynes-modified McMillan formula to be 122 K at 300 GPa, 116 K at 250 GPa, and 173 K at 150 GPa, respectively. Moreover, the pressure to stabilize $P\bar{6}m2$-YCeH$_{18}$ can be lowered to 150 GPa, suggesting an accessible condition for its high-pressure synthesis.

cond-mat.supr-con↗

arXiv2021

Application of canonical augmentation to the atomic substitution problem

A common approach for studying a solid solution or disordered system within a periodic ab-initio framework is to create a supercell in which a certain amount of target elements is substituted with other ones. The key to generating supercells is determining how to eliminate symmetry-equivalent structures from the large number of substitution patterns. Although the total number of substitutions is on the order of trillions, only symmetry-inequivalent atomic substitution patterns need to be identified, and their number is far smaller than the total. A straightforward solution would be to classify them after determining all possible patterns, but it is redundant and practically unfeasible. Therefore, to alleviate this drawback, we developed a new formalism based on the {\it canonical augmentation}, and successfully applied it to the atomic substitution problem. Our developed \verb|python| software package, which is called \textsc{SHRY} (\underline{S}uite for \underline{H}igh-th\underline{r}oughput generation of models with atomic substitutions implemented by p\underline{y}thon), enables us to pick up only symmetry-inequivalent structures from the vast number of candidates very efficiently. We demonstrate that the computational time required by our algorithm to find $N$ symmetry-inequivalent structures scales {\it linearly} with $N$ up to $\sim 10^9$. This is the best scaling for such problems.

cond-mat.mtrl-sci↗

arXiv2021

Diffusion Monte Carlo Study on Relative Stabilities of Boron Nitride Polymorphs

Although Boron nitride (BN) is a well-known compound widely used for engineering and scientific purposes, the phase stability of its polymorphs, one of its most fundamental properties, is still under debate. The ab initio determination of the ground state of the BN polymorphs, such as hexagonal and zinc-blende, is difficult because of the elusive Van der Waals interaction, which plays a decisive role in some of the polymorphs, making quantitative prediction highly challenging. Hence, despite multiple theoretical studies, there has been no consensus on the ground state yet, primarily due to contradicting reports. In this study, we apply a state-of-the-art ab initio framework - fixed-node diffusion Monte Carlo (FNDMC), to four well known BN polymorphs, namely hexagonal, rhombohedral, wurtzite, and zinc-blende BNs. Our FNDMC calculations show that hBN is thermodynamically the most stable among the four polymorphs at 0 K as well as at 300K. This result agrees with the experimental data of Corrigan~{\it et al.} and Fukunaga. The conclusions are consistent with those obtained using other high-level methods, such as coupled cluster. We demonstrate that the FNDMC is a powerful method to address polymorphs that exhibit bonds of various forms. It also provides valuable information, like reliable reference energies, when reliable experimental data are missing or difficult to access. Our findings should promote the application of FNDMC for other van der Waals materials.

cond-mat.mtrl-sci↗

arXiv2021

Candidate structure for the H$_2$-PRE phase of solid hydrogen

Experimental progress finally reached the metallic solid hydrogen phase, which was predicted by Wigner and Huntington over 80 years ago. However, the different structures in the phase diagram are still been debated due to the difficulty of diffraction experiments for high-pressured hydrogen. The determination of crystal structures under extreme condition is both of the basic condensed matter physics, and in planetary science: the behavior of giant gaseous planets (e.g. Jupiter, Saturn...) strongly depends on the properties of inner high-pressured hydrogen. This work describes new possible structures appearing under high pressures of 400$\sim$600 GPa. We applied a structural search using particle swarm optimization with density functional theory (DFT) to propose several candidate structures. For these structures, we performed fixed-node diffusion Monte Carlo simulations combined with DFT zero-point energy corrections to confirm their relative stability. We found $P2_{1}/c$-8 as a promising candidate structure for the H$_2$-PRE phase. $P2_{1}/c$-8 is predicted the most stable at 400 and 500~GPa. $P2_{1}/c$-8 reproduces qualitatively the IR spectrum peaks observed in the H$_2$-PRE phase.

cond-mat.mtrl-sci↗

arXiv2021

Order-Disorder Competitive Cooperation in Equiatomic 3d-Transition-Metal Quaternary Alloys: Phase Stability and Electronic Structure

We use high-throughput first-principles sampling to investigate competitive factors that determine the crystal structure of high-entropy alloys (HEAs) and the energetics dependence of the stable phase on the atomic configuration of fully ordered L1$_2$, D0$_{22}$, and random solid solution (RSS) phases of equiatomic quaternary alloys comprising four of the six constituent elements (Cr, Mn, Fe, Co, Ni, and Cu). Considering the configurational entropy, we demonstrate that valence electron concentration (VEC) and temperature are crucial to determine the phase stability of HEAs at finite temperatures, wherein the ordered phases are energetically more favorable than RSS phases. Some D0$_{22}$ phases with high VEC are energetically more stable than L1$_2$ phases, though both phases are metastable. Further, we explore magnetic configurations to identify the origin of the enthalpy term. The calculations reveal that ordered phases comprising antiferromagnetic atoms surrounded by ferromagnetic atoms are energetically stable. The quantitative structure--property relationship is also discussed.

cond-mat.mtrl-sci↗

arXiv2021

Anionic Ordering in Pb$_2$Ti$_4$O$_9$F$_2$ Revisited by Nuclear Magnetic Resonance and Density Functional Theory

A combination of $^{19}$F magic angle spinning (MAS) nuclear magnetic resonance (NMR) and density functional theory (DFT) were used to study the ordering of F atoms in Pb$_2$Ti$_4$O$_9$F$_2$. This analysis revealed that F atoms predominantly occupy two of the six available inequivalent sites in a ratio of 73:27. DFT-based calculations explained the preference of F occupation on these sites and quantitatively reproduced the experimental occupation ratio, independent of the choice of functional. We concluded that the Pb atom's 6$s^2$ lone pair may play a role ($\sim$0.1 eV/f.u.) in determining the majority and minority F occupation sites with partial density of states and crystal orbital Hamiltonian population analyses applied to the DFT wave functions.

cond-mat.mtrl-sci↗