I. Science of magnetic systems in biological/chemical environment by quantum simulation

Magnetic systems such as magnetic multilayers or alloys, carbon structures, reactants and certain metal surfaces have now been used in many kinds of industrial applications. In particular, the oxygen reaction on surfaces has been a vital reaction in CO oxidation, oxygen-reduction-reaction (ORR) and corrosion, among others. The fundamental theoretical study of this reaction with explicit treatment of the magnetic states of the reactants is needed to elucidate the mechanisms of various observed phenomena. In this research, first-principles methods such as density functional theory and Monte-Carlo simulations are employed to draw the reactivity as well as stability mechanisms of these catalytic-magnetic systems. Examples are given below:


合金、金属酸化物、スピン偏極炭素、 遷移金属(3d)といった磁気システムは、その資源量が豊富であるため近年工業的利用が注目されている。特に、これらのシステムでの酸素の反応は、エネルギーデバイス(燃料電池、リチウム電池等)や電子デバイスの重要な反応の一つである。また、磁気システムは、生物学的システムの一部である。このような化学環境中における磁気システムの基礎研究(原子レベル)により、安定性・構造、相互作用メカニズム、界面特性、磁気相転移、ダイナミクスに関する極めて重要な情報を得ることが期待される。本研究では、反応物質の磁性状態の理論的検討を可能にする密度関数理論(DFT)やモンテカルロシミュレーション(MC)のような第一原理法を用いることにより、このような情報に関する詳細な検討を実施している。得られた結果は、種々の実験的観察の補完や、より優れた素材開発のための新しい設計指針の提案に活用できると考えている。研究例は、以下の通りである。

A. Stability of magnetic alloys under oxygen environment

Experiments have shown that components of metal alloys can degrade under oxygen environment. Using spin-polarized density functional theory, the mechanism of the oxygen-induced degradation of metal alloys such as Pt-Co is elucidated. This mechanism is further used to create new ternary alloy design concepts leading to more stable ORR nanoparticle catalysts.



Mechanism of Effect of Oxygen on Pt/Co Stability via Quantum Simulation

New ternary alloy design concepts

B. Reactivity of spin-polarized systems

Graphene is inert to many reactants including oxygen. However, due to its abundance, ease of production and high conductivity, various means to enhance its reactivity have been studied. Conventionally, geometric effects (doping , cutting to form semi-infinite nanoribbons, straining) have been imposed to alter its chemical reactivity. In this research, the exchange splitting of unstrained and uncut (infinite) graphene using ferromagnetic substrate is used to alter its electronic structure in favor for better reaction. This method allows all carbon atoms to act as reactive sites. The electronic mechanism of the enhanced reactivity of graphene is drawn using spin-polarized density functional theory.



C. Spin-dependent oxygen adsorption energetics on ferromagnetic surface

Experimental investigation of spin-dependent adsorption of oxygen requires its complete polarization using a ferromagnetic material. Potential energy surface (PES) corresponding to different spin states of oxygen on a ferromagnetic surface is so far non-existent. First-principles methods are being conducted to obtain the PES and to elucidate the mechanism of spin-dependent O2 adsorption. In particular, the constrained density-functional theory is employed to restrict the molecule’s and surface’s spin moment directions throughout the reaction pathway. Monte-Carlo simulation is used to determine the sticking probabilities.


酸素のスピン依存性吸着の実証研究には、磁性物資を用いた完全偏極が不可欠である。そのため、磁気面における酸素の異なるスピン状態(三重項および一重項)と一致するポテンシャルエネルギー面(PES)が必要となる。 PESを得るため、またスピン依存酸素吸着構造の解明のため第一原理法(Constrained DFT)を用いて検討を行っている。また、検討結果を実験と直接比較するためにモンテカルロシミュレーション等を用いて付着確率を決定している。

Spin-dependent energetics and sticking probability of oxygen on magnetic surface using constrained DFT

D. Magnetic phase transition of magnetic multilayers/alloys/metal oxides

More often, the application of magnetic systems in real devices requires understanding of their magnetic phase transition under chemical environments. Here, we study such property of multilayers/alloys/metal oxides using DFT +CLM + MC simulations.

Magnetic phase transition of Fe and Co substrates and the Pt layer deposited on them.

II. Investigation of photoconductivity by sub-band gap optical excitations

Photoconductivity by sub-bandgap optical excitations (SOEP) can be used for low-noise photoconductive detector and efficient photoconductive emitter for THz radiation [1]. The low-temperature molecular beam epitaxy-grown GaAs (LT-GaAs) is a typical semiconductor, which exhibit SOEP, however mechanisms are still elusive. Here, we calculate the density of states (DOS) of LT GaAs with and without defects using quantum simulation and determine the origin of proposed mid-gap states and the photoconductivity. We first show below the DOS of bulk GaAs in comparison with photoemission experiment [2].

半導体のサブバンドギャップ光励起による光伝導度の研究 (遠赤外領域開発研究センター共同研究)

Calculated DOS obtained by quantum simulation (a) and experimental DOS obtained by HARPES [2] (b) for bulk GaAs

[1] M. Tani, et al, Appl. Phys. Lett. 77, 1396 (2000). [2] A.X. Gray et al, Nat. Mater. 10 759 (2011).