arXiv subjects

Osamu Sugino

Publications and source records attributed to Osamu Sugino.

At least 19 recordsLinked to original sources

arXiv2026

Open Quantum System Theory of Muon Spin Relaxation in Materials

We present a non-Markovian theory of muon spin relaxation that treats the implanted muon as an open quantum spin coupled to a temporally correlated local magnetic environment. Using a Schwinger-Keldysh influence-functional formulation, we derive a stochastic equation of motion for the muon spin, in which the fluctuation kernel is fixed by the local-field correlation tensor, while the retarded memory torque is introduced through an effective phenomenological backaction kernel. In the appropriate limits, the theory reduces to standard Kubo-Toyabe descriptions. This enables quantitative, global analysis of zero-field (ZF) and weak longitudinal-field (LF) $μ$SR spectra beyond the strong-collision approximation. Applied to $\mathrm{Li}_{0.73}\mathrm{CoO}_2$, the model supports a decomposition into a quenched width and a Li-driven dynamical component within the adopted parametrization, and yields fluctuation rates approximately consistent with activated behavior over the intermediate-temperature window. The fitted memory parameter is most visible in the crossover between quasi-static and fast-fluctuation limits.

cond-mat.mtrl-sci↗

arXiv2026

Functional renormalization group for classical liquids without recourse to hard-core reference systems: A study of three-dimensional Lennard-Jones liquids

In our previous work [Phys. Rev. E 104, 014124 (2021)], we developed a method for analyzing classical liquids using the functional renormalization group (FRG) without relying on a hard-core reference system. In this paper, we extend this method to three-dimensional liquids. We describe an efficient approach for performing the spatial integrals that appear in the renormalization group equations, which is essential for realizing numerical calculations in three dimensions. As a demonstration, we present its application to the Lennard-Jones liquids. Through calculations of thermodynamic quantities, we find that FRG preserves thermodynamic consistency (TC) better than traditional integral-equation methods such as the hypernetted-chain, Percus-Yevick, and Kovalenko-Hirata closures. Taking the molecular dynamics results as a benchmark, we also show that FRG can achieve an accuracy comparable to that of integral-equation methods that incorporate TC, such as the Rogers-Young closure. We further assess the accuracy of the pair distribution function and examine whether our method remains applicable below the critical temperature. Our results demonstrate that FRG provides a new method for describing classical liquids with accuracy comparable to modern liquid theories.

cond-mat.soft↗

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

Exploring functionalized Zr$_2$N and Sc$_2$N MXenes as superconducting candidates with $\textit{ab initio}$ calculations

We study new superconductor candidates in functionalized MXenes Zr$_2$NS$_2$, Zr$_2$NCl$_2$, and Sc$_2$NCl$_2$ with $\textit{ab initio}$ calculations based on density functional theory for superconductors (SCDFT). The superconducting transition temperature $(T_c)$ at ambient pressure is predicted to reach 9.48 K (Zr$_2$NS$_2$), with potential further improvements under applied strain. We note that the changes in the profiles of superconducting gap $(Δ)$ and electron-phonon coupling $(λ)$ across the Fermi surface may be influenced by their modified electronic bandstructure components.

cond-mat.supr-con↗

arXiv2025

Dynamics with Simultaneous Dissipations to Fermionic and Bosonic Reservoirs

We introduce a non-phenomenological framework based on the influence functional method to incorporate simultaneous interactions of particles with fermionic and bosonic thermal reservoirs. In the slow-motion limit, the electronic friction kernel becomes Markovian, enabling an analytical expression for the friction coefficient. The framework is applied to a prototypical electrochemical system, where the metal electrode and solvent act as fermionic and bosonic reservoirs, respectively. We investigate quantum vibrational relaxation of hydrogen on metal surfaces, showing that dissipation to electron-hole pairs reduces the relaxation time. Additionally, in solvated proton discharge, electronic friction prolongs charge transfer by delaying proton transitions between potential wells. This study provides new insights into the interplay of solvent and electronic dissipation effects, with direct relevance to electrochemical processes and other systems involving multiple thermal reservoirs.

cond-mat.other↗

arXiv2024

Magnetic phases of electron-doped infinite-layer Sr$_{1-x}$La$_x$CuO$_2$ from first-principles density functional calculations

The magnetic phases of electron-doped infinite-layer Sr$_{1-x}$La$_x$CuO$_2$ are elucidated by first-principles density functional calculations. The antiferromagnetic parent state, metallic transition, as well as lattice evolution with doping and pressure are found to be consistent with experiments. The specific heat coefficient $γ$, magnetic exchange coupling $J$, as well as the density of states at Fermi level $N(0)$ of low-energy states with multiple magnetic configurations are investigated. We highlight a subset of such states in which we note an increase in $N(0)$ to suggest the interesting effects of magnetic fluctuations and La substitution on the electronic structure of this material.

cond-mat.supr-con↗

arXiv2024

Time-dependent electron transfer and energy dissipation in condensed media

We study a moving adsorbate interacting with a metal electrode immersed in a solvent using the time-dependent Newns-Anderson-Schmickler model Hamiltonian. We have adopted a semiclassical trajectory treatment of the adsorbate to discuss the electron and energy transfers that occur between the adsorbate and the electrode. Using the Keldysh Green's function scheme, we found a non-adiabatically suppressed electron transfer caused by the motion of the adsorbate and coupling with bath phonons that model the solvent. The energy is thus dissipated into electron-hole pair excitations, which are hindered by interacting with the solvent modes and facilitated by the applied electrode potential. The average energy transfer rate was discussed in terms of the electron friction coefficient and given an analytical expression in the slow-motion limit.

cond-mat.mes-hall↗

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↗

arXiv2022

First-principles electronic structure investigation of HgBa$_{2}$Ca$_{n-1}$Cu$_{n}$O$_{2n+2+x}$ with the SCAN density functional

We perform first-principles calculation to study the electronic structure of HgBa$_{2}$Ca$_{n-1}$Cu$_{n}$O$_{2n+2+x}$ copper oxides up to $n = 6$ for the undoped parent compound $(x = 0)$ and up to $n = 3$ for the doped compound $(x > 0)$ by means of the SCAN meta-GGA density functional. Our calculations predict an antiferromagnetic insulator ground state for the parent compounds with an energy gap that decreases with the number of CuO$_{2}$ planes. We report structural, electronic and magnetic order evolution with $x$ which agree with experiments. We find an enhanced density of states at Fermi level at $x \approx 0.25$ for the single-layered compound manifesting in a peak of the Sommerfeld parameter, which recently has been discussed as a possible signature of quantum criticality generic to all cuprates.

cond-mat.str-el↗

arXiv2021

Optical representation of thermal nuclear fluctuation effect on band-gap renormalization

The bandgap of insulating materials is renormalized in various ways by the electron-phonon interaction owing to the dynamical and quantum fluctuations of nuclei. These fluctuation effects are considered in the perturbative Allen-Heine-Cardona theory using the formulae for the Fan-Migdal and Debye-Waller terms. However, the material dependence is not clear in the formulae. Thus, in this study, we focus on the analytical form of the Debye-Waller term and find that the term can be reformulated using the momentum matrix. In addition, the optical selection rule is found to play a role. For diamond-type materials, the Debye-Waller term can be approximately decomposed into a product of the optical transition energy, the mean square displacement of nuclei, and the dipole transition probability. The decomposition can also be applied with an additional approximation to zinc-blende-type materials, as revealed by our first-principles calculation. The magnitudes of the Debye-Waller term of several materials can thus be estimated using basic physical quantities prior to performing the calculation of the electron-phonon interaction.

cond-mat.mtrl-sci↗

arXiv2021

Machine-Learning-Based Exchange-Correlation Functional with Physical Asymptotic Constraints

Density functional theory is the standard theory for computing the electronic structure of materials, which is based on a functional that maps the electron density to the energy. However, a rigorous form of the functional is not known and has been heuristically constructed by interpolating asymptotic constraints known for extreme situations, such as isolated atoms and uniform electron gas. Recent studies have demonstrated that the functional can be effectively approximated using machine learning (ML) approaches. However, most ML models do not satisfy asymptotic constraints. In this study, by applying a novel ML model architecture, we demonstrate a neural network-based exchange-correlation functional satisfying physical asymptotic constraints. Calculations reveal that the trained functional is applicable to various materials with an accuracy higher than that of existing functionals, even for materials whose electronic properties are different from the properties of materials in the training dataset. Our proposed approach thus improves the accuracy and generalization performance of the ML-based functional by combining the advantages of ML and analytical modeling.

cond-mat.mtrl-sci↗

arXiv2021

Functional-renormalization-group approach to classical liquids with short-range repulsion: a scheme without repulsive reference system

The renormalization-group approaches for classical liquids in previous works require a repulsive reference such as a hard-core one when applied to systems with short-range repulsion. The need for the reference is circumvented here by using a functional renormalization group approach for integrating the hierarchical flow of correlation functions along a path of variable interatomic coupling. We introduce the cavity distribution functions to avoid the appearance of divergent terms and choose a path to reduce the error caused by the decomposition of higher order correlation functions. We demonstrate using an exactly solvable one-dimensional models that the resulting scheme yields accurate thermodynamic properties and interatomic distribution at various densities when compared to integral-equation methods such as the hypernetted chain and the Percus-Yevick equation, even in the case where our hierarchical equations are truncated with the Kirkwood superposition approximation, which is valid for low-density cases.

cond-mat.stat-mech↗

arXiv2020

First-principles calculations of thermal electron emission from H$^-$ in silicon

Thermal electron emission process of a hydrogen impurity is an important topic of fundamental semiconductor physics. Despite of decades-long study, theory is not established yet. Here, we study the process of $\mathrm{H}^{-}$ in silicon, $\mathrm{H^{-}} \to \mathrm{H^{0}} + e^{-}$, using a first-principles calculation. Our calculation indicates that the process consists of two steps: slow diffusion of H$^{-}$ from a tetrahedral site to a bond-center site, which is the rate-limiting step, and faster nonradiative transition from $\mathrm{H}^{-}$ to $\mathrm{H}^{0} + e^{-}$ that occurs subsequently at the body-center site. The calculated rate is consistent with a deep level transient spectroscopy experiment

cond-mat.mtrl-sci↗

arXiv2019

Completing density functional theory by machine-learning hidden messages from molecules

Kohn-Sham density functional theory is the base of modern computational approaches to electronic structures. Their accuracy vitally relies on the exchange-correlation energy functional, which encapsulates electron-electron interaction beyond the classical one. The functional provides a way to obtain the density and energy without solving the many-body equation and can, in principle, be determined to reproduce the exact ones universally. However, the past approaches are dependent on the theoretical development, which limits the possibility of the functional to human's intuition. Here, we demonstrate a systematic way to machine-learn a functional from a database, without complicated assumptions. The density and energy are related with a flexible feed-forward neural network, which is trained to reproduce accurate dataset, and the KS-DFT is solved by taking the functional derivatives with the back-propagation technique. Surprisingly, a trial functional, trained for just a few molecules, has been shown to be applicable to hundreds of molecular systems with comparable accuracy to the standard functionals. Also, by adding the nodes connected to the hidden layers, a non-local term is straightforwardly included to improve accuracy, which has been hitherto impractically difficult. Utilizing the strategy of rapidly advancing machine learning techniques, this novel approach is expected to enrich the DFT framework by constructing a functional just from a database for materials conventionally difficult to calculate accurately.

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↗

arXiv2018

Tensor decomposition methods for correlated electron pairs

We analyze wave functions constructed as a sum of product of two-electron functions, or as a polynomial of geminals, to investigate their ability to represent the ground state of a strongly correlated few-body system. The known difficulty associated with variational determination of the total energy is overcome by applying a tensor decomposition method called Waring decomposition. Convergence speed of the total energy is compared for various polynomial types. The result provides information bridging between geminal product wave functions and the full-CI in the strongly correlated regime, thereby enriching knowledge on the hierarchy of molecular orbital theories of electron pairs.

physics.chem-ph↗

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↗