arXiv subjects

Shusuke Kasamatsu

Publications and source records attributed to Shusuke Kasamatsu.

13 recordsLinked to original sources

arXiv2025

Benchmarking of Oxygen Adsorption using TPD Spectroscopy for Accurate DFT Prediction of ORR on Anatase Titanium Dioxide (101)

Despite the wide use of first-principles calculations to elucidate catalytic reaction mechanisms, the reliability of the theory remains unknown for reactions that initiate with O$_2$ adsorption, as few papers in the literature have systematically verified the accuracy of the calculations. Here, we have overcome this issue by comparing calculated O$_2$ adsorption energy ($E_{\text{ads}}$) to simulated $E_{\text{ads}}$ distribution from experimental temperature-programmed desorption (TPD) spectrum. The distribution obtained based on equilibrium thermodynamic arguments is in good agreement with the calculated $E_{\text{ads}}$ from modelling O$_2$ adsorption on anatase-TiO$_2$(101) using the generalized gradient approximation and van der Waals density functionals (vdW-DFs) with Hubbard correction, which is particularly the case when vdW-DF3-opt2 is used. On this basis, we concluded that the oxygen reduction reaction (ORR) initiates from a physisorbed O$_2$ thermodynamically as stable as that in the gas phase. In contrast, the calculated $E_{\text{ads}}$ for Pt(111) exhibit a significant overestimation of the O$_2$ adsorption due to excessive vdW correction. Therefore, a systematic investigation based on TPD spectroscopy can be used to diagnose the reliability of theoretical predictions of ORR and is expected to lead to improvements in exchange-correlation functionals for catalysts.

physics.chem-ph↗

arXiv2025

X-ray free-electron laser observation of giant and anisotropic magnetostriction in $\beta$-O$_{2}$ at 110 Tesla

In strong magnetic fields beyond 100 T, the significant Zeeman energy competes with the lattice interactions, where a considerable magnetostriction is expected. However, the microscopic observation of the magnetostriction above 100 T has been hindered due to the short pulse duration of $\mu$-seconds and the coil's destruction. Here, we report the observation of the giant and anisotropic magnetostriction of $\sim 1$ % at 110 T in the spin-controlled crystal, $\beta$-O$_{2}$, by combining the single-shot diffraction of x-ray free-electron laser (XFEL) and the newly developed portable 100 T generator (PINK-02). The very soft and anisotropic response of $\beta$-O$_{2}$ should originate in the competing van der Waals force and exchange interaction, and also the frustration of spin and lattice on the triangular network. The XFEL experiment above 100 T using PINK-02 enables microscopic investigations on materials' properties at high magnetic fields, providing insights into how spins contribute to the stability of crystal structures.

cond-mat.str-el↗

arXiv2024

Comparison of intermediate-range order in GeO$_2$ glass: molecular dynamics using machine-learning interatomic potential vs.\ reverse Monte Carlo fitting to experimental data

The short and intermediate-range order in GeO$_2$ glass are investigated by molecular dynamics using machine-learning interatomic potential trained on ab initio calculation data and compared with reverse Monte Carlo fitting of neutron diffraction data. To characterize the structural differences in each model, the total/partial structure factors, coordination number, ring size and shape distributions, and persistent homology analysis were performed. These results show that although the two approaches yield similar two-body correlations, they can lead to three-dimensional models with very different short and intermediate-range ordering. A clear difference was observed especially in the ring distributions; RMC models exhibit a broad distribution in the ring size distribution, while neural network potential molecular dynamics yield much narrower ring distributions. This confirms that the density functional approximation in the ab initio calculations determines the preferred network assembly more strictly than RMC with simple coordination constraints and neutron diffraction data with isotope substitution.

cond-mat.mtrl-sci↗

arXiv2024

Structural Analysis of Amorphous GeO$_2$ under High Pressure Using Reverse Monte Carlo Simulations

The structural properties of amorphous GeO$_2$, a prototypical network glass, were investigated under ambient to high pressure using reverse Monte Carlo simulations based on reported structure factors from in situ high-pressure neutron diffraction experiments with isotopic substitution. The results indicate the retention of the topological structure containing predominantly tetrahedral GeO$_4$ units up to ca.~\SI{4}{\giga\pascal} ($ρ/ρ_0 = 1.15$), which is explained by the reduction of cavity volumes. With further application of pressure, an increase in the number of GeO$_5$ units is first observed, which is then followed more gradually by an increase in the number of GeO$_6$ units.

cond-mat.mtrl-sci↗

arXiv2023

Theoretical analysis of zirconium oxynitride/water interface using neural network potential

Zr oxides and oxynitrides are promising candidates to replace precious metal cathodes in polymer electrolyte fuel cells. Oxygen reduction reaction activity in this class of materials has been correlated with the amount of oxygen vacancies, but a microscopic understanding of this correlation is still lacking. To address this, we simulate a defective Zr$_7$O$_8$N$_4$/H$_2$O interface model and compare it with a pristine ZrO$_2$/H$_2$O interface model. First, ab initio replica exchange Monte Carlo sampling was performed to determine defect segregation at the surface in the oxynitride slab model, then molecular dynamics accelerated by neural network potentials was used to perform 1000 of 500 ps-long simulations to attain sufficient statistical accuracy of the solid/liquid interface structure. The presence of oxygen vacancies on the surface was found to clearly modify the local adsorption structure: water molecules were found to adsorb preferentially on Zr atoms surrounding oxygen vacancies, but not on the oxygen vacancies themselves. The fact that oxygen vacancy sites are free from poisoning by water molecules may explain the activity enhancement in defective systems. The layering of water molecules was also modified considerably, which should influence the proton and O$_2$ transport near the interfaces which is another parameter that determines the overall activity.

cond-mat.mtrl-sci↗

arXiv2023

Configuration sampling in multi-component multi-sublattice systems enabled by ab Initio Configuration Sampling Toolkit (abICS)

Simulation of the intermediate levels of disorder found in multi-component multi-sublattice systems in various functional materials is a challenging issue, even for state-of-the-art methodologies based on first-principles calculation. Here, we introduce our open-source package ab Initio Configuration Sampling Toolkit (abICS), which combines high-throughput first-principles calculations, machine learning, and parallel extended ensemble sampling in an active learning setting to enable such simulations. The theoretical background is reviewed in some detail followed by brief notes on usage of the software. In addition, our recent applications of abICS to multi-component ionic systems and their interfaces for energy applications are reviewed as demonstration of the power of this approach.

cond-mat.mtrl-sci↗

arXiv2023

Fermi surface reconstruction due to the orthorhombic distortion in Dirac semimetal YbMnSb$_2$

Dirac semi-metal with magnetic atoms as constituents delivers an interesting platform to investigate the interplay of Fermi surface (FS) topology, electron correlation, and magnetism. One such family of semi-metal is YbMn$Pn_2$ ($Pn$ = Sb, Bi), which is being actively studied due to the intertwined spin and charge degrees of freedom. In this Letter, we investigate the relationship between the magnetic/crystal structures and FS topology of YbMnSb$_2$ using single crystal x-ray diffraction, neutron scattering, magnetic susceptibility, magnetotransport measurement and complimentary DFT calculation. Contrary to previous reports, the x-ray and neutron diffraction reveal that YbMnSb$_2$ crystallizes in an orthorhombic $Pnma$ structure with notable anti-phase displacement of the magnetic Mn ions that increases in magnitude upon cooling. First principles DFT calculation reveals a reduced Brillouin zone and more anisotropic FS of YbMnSb$_2$ compared to YbMnBi$_2$ as a result of the orthorhombicity. Moreover, the hole type carrier density drops by two orders of magnitude as YbMnSb$_2$ orders antiferromagnetically indicating band folding in magnetic ordered state. In addition, the Landau level fan diagram yields a non-trivial nature of the SdH quantum oscillation frequency arising from the Dirac-like Fermi pocket. These results imply that YbMnSb$_2$ is an ideal platform to explore the interplay of subtle lattice distortion, magnetic order, and topological transport arising from relativistic quasiparticles.

cond-mat.mtrl-sci↗

arXiv2022

Facilitating {\it ab initio} configurational sampling of multicomponent solids using an on-lattice neural network model and active learning

We propose a scheme for {\it ab initio} configurational sampling in multicomponent crystalline solids using Behler-Parinello type neural network potentials (NNPs) in an unconventional way: the NNPs are trained to predict the energies of relaxed structures from the perfect lattice with configurational disorder instead of the usual way of training to predict energies as functions of continuous atom coordinates. An active learning scheme is employed to obtain a training set containing configurations of thermodynamic relevance. This enables bypassing of the structural relaxation procedure which is necessary when applying conventional NNP approaches to the lattice configuration problem. The idea is demonstrated on the calculation of the temperature dependence of the degree of A/B site inversion in three spinel oxides, MgAl$_2$O$_4$, ZnAl$_2$O$_4$, and MgGa$_2$O$_4$. The present scheme may serve as an alternative to cluster expansion for `difficult' systems, e.g., complex bulk or interface systems with many components and sublattices that are relevant to many technological applications today.

physics.comp-ph↗

arXiv2018

Direct coupling of first-principles calculations with replica exchange Monte Carlo sampling of ion disorder in solids

We demonstrate the feasibility of performing sufficient configurational sampling of disordered oxides directly from first principles without resorting to the use of fitted models such as cluster expansion. This is achieved by harnessing the power of modern-day cluster supercomputers using the replica exchange Monte Carlo method coupled directly with structural relaxation and energy calculation performed by density functional codes. The idea is applied successfully to the calculation of the temperature-dependence of the degree of inversion in the cation sublattice of MgAl$_2$O$_4$ spinel oxide. The possibility of bypassing fitting models will lead to investigation of disordered systems where cluster expansion is known to perform badly: for example, systems with large lattice deformation due to defects, or systems where long-range interactions dominate such as electrochemical interfaces.

cond-mat.mtrl-sci↗

arXiv2017

First-principles investigation of polarization and ion conduction mechanisms in hydroxyapatite

We report first-principles simulation of polarization mechanisms in hydroxyapatite to explain the underlying mechanism behind the reported ion conductivities and polarization under electrical poling at elevated temperatures. It is found that ion conduction occurs mainly in the column of OH$^-$ ions along the $c$-axis through a combination of the flipping of OH$^-$ ions, exchange of proton vacancies between OH$^-$ ions, and the hopping of the OH$^-$ vacancy. The calculated activation energies are consistent with those found in conductivity measurements and thermally stimulated depolarization current measurements.

cond-mat.mtrl-sci↗

arXiv2017

First-principles description of van der Waals-bonded spin-polarized systems using vdW-DF$+U$ method---application to solid oxygen at low pressure

The description of the molecular solid phase of O$_2$, especially its ground-state antiferromagnetic insulating phase, is known to be quite unsatisfactory within the conventional local and semilocal density functional approximations used in the Kohn-Sham formalism of density functional theory. The recently-developed van der Waals functionals that take into account nonlocal correlations have also shown subpar performance in this regard. The difficulty lies in the subtle balance between the van der Waals interactions and the exchange coupling between the antiferromagnetic and ferromagnetic molecule pairs in the molecular crystal. Here, we report that the DFT$+U$ approach used in combination with the vdW-DF functional performs surprisingly well in this regard, and discuss the reasoning behind this behavior. We also apply this approach to study the recently-reported magnetic field-induced $θ$ phase of solid O$_2$.

cond-mat.mtrl-sci↗

arXiv2017

Discovery of two-dimensional Dirac nodal line fermions in monolayer Cu2Si

Topological nodal line semimetals, a novel quantum state of materials, possess topologically nontrivial valence and conduction bands that touch at a line near the Fermi level. The exotic band structure can lead to various novel properties, such as long-range Coulomb interaction and flat Landau levels. Recently, topological nodal lines have been observed in several bulk materials, such as PtSn4, ZrSiS, TlTaSe2 and PbTaSe2. However, in two-dimensional materials, experimental research on nodal line fermions is still lacking. Here, we report the discovery of two-dimensional Dirac nodal line fermions in monolayer Cu2Si based on combined theoretical calculations and angle-resolved photoemission spectroscopy measurements. The Dirac nodal lines in Cu2Si form two concentric loops centred around the Γ point and are protected by mirror reflection symmetry. Our results establish Cu2Si as a new platform to study the novel physical properties in two-dimensional Dirac materials and provide new opportunities to realize high-speed low-dissipation devices.

cond-mat.mtrl-sci↗

arXiv2015

Configuration Interaction with Antisymmetrized Geminal Powers

To avoid the combinatorial computational cost of configuration interaction (CI), we have previously introduced the symmetric tensor decomposition CI (STD-CI) method, where we take advantage of the antisymmetric nature of the electronic wave function and express the CI coefficients compactly as a series of Kronecker product states (STD series) [W. Uemura and O. Sugino, Phys. Rev. Lett. 109, 253001 (2012)]. Here we extend the variational degrees of freedom by using different molecular orbitals for different terms in the STD series. This scheme is equivalent to the linear combination of the Hartree-Fock-Bogoliubov state or the antisymmetrized geminal powers (AGP). The total energy converges very rapidly within 0.72 $μ$Hartree taking only 10 terms for the water molecule, and the convergence is likewise fast for Hubbard tetramers. The computational cost scales as the fifth power of the number of electrons and the square of the number of terms in the STD series, indicating the promise of this AGP-based scheme for highly accurate and efficient computation of quantum systems.

physics.chem-ph↗