arXiv subjects

Akitaka Nakanishi

Publications and source records attributed to Akitaka Nakanishi.

7 recordsLinked to original sources

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↗

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

Computational materials design of attractive Fermion system with large negative effective $U$ in the hole-doped Delafossite of CuAlO$_2$, AgAlO$_2$ and AuAlO$_2$

In order to realize super-high-critical temperature $(T_c)$ superconductors ($T_c$>1,000 K) based on general design rules for negative effective $U$ $(U_{eff})$ systems by controlling purely-electronic and attractive Fermion mechanisms, we perform computational materials design for the negative $U_{eff}$ system in hole-doped two-dimensional (2D) Delafossite CuAlO$_2$, AgAlO$_2$ and AuAlO$_2$ from ${\it ab\ initio}$ calculations. It is found that the large negative $U_{eff}$ in the hole-doped attractive Fermion systems for CuAlO$_2$ ($U_{eff}$ = -4.53 eV), AgAlO$_2$ ($U_{eff}$ = -4.88 eV), AuAlO$_2$ ($U_{eff}$ = -4.14 eV). These values are 10 times larger than that in hole-doped three-dimensional (3D) CuFeS$_2$ ($U_{eff}$ = -0.44 eV). For future calculations of the $T_c$ and phase diagram by quantum Monte Carlo simulations, we propose the negative $U_{eff}$ Hubbard model with the anti-bonding single $π$-band model for CuAlO$_2$, AgAlO$_2$ and AuAlO$_2$ by using the parameters obtained from ${\it ab\ initio}$ electronic structure calculations. The behavior of $T_c$ in the 2D Delafossite of CuAlO$_2$, AgAlO$_2$ and AuAlO$_2$ and 3D Chalcopyrite of CuFeS$_2$ shows the interesting chemical trend, ${\it i.e.,}$ $T_c$ increases exponentially in the weak coupling regime $|U_{eff}| < W$ ($\sim$ 2 eV) (where $W$ is the band width of Hubbard model) for the hole-doped CuFeS$_2$, and then $T_c$ goes through a maximum when $|U_{eff}| \sim W$ (2.8 eV, 3.5 eV) for the hole-doped AgAlO$_2$ and AuAlO$_2$, and finally $T_c$ decreases with increasing $|U_{eff}|$ in the strong coupling regime, where $|U_{eff}| > W$ (1.7 eV), for the hole-doped CuAlO$_2$. In this strong coupling regime, one can expect that $T_c$ = 1,000 $\sim$ 2,000 K by assuming the relation of the very strong coupling as $2Δ/ k_{\rm B}T_c$ = 50 $\sim$100 and the superconducting gap $Δ\sim |U_{eff}|$ = 4.53 eV $\sim$ 50,000 K.

cond-mat.supr-con↗

arXiv2014

Chemical Trend of Superconducting Transition Temperature in Hole-doped CuBO$_2$, CuAlO$_2$, CuGaO$_2$ and CuInO$_2$

We calculated the superconducting transition temperature $T_{\rm c}$ of hole-doped CuBO$_2$, CuAlO$_2$, CuGaO$_2$ and CuInO$_2$ using first-principles. The calculated $T_{\rm c}$ are about 50 K for CuAlO$_2$, 10 K for CuBO$_2$ and CuGaO$_2$ and 1 K for CuInO$_2$ at maximum in the optimum hole-doping concentration. The low $T_{\rm c}$ of CuInO$_2$ is attributed to the weak electron-phonon interaction caused by the low covalency and heavy atomic mass.

cond-mat.supr-con↗

arXiv2012

Chemical Trend of Superconducting Transition Temperature in Hole-doped Delafossite of CuAlO_2, AgAlO_2 and AuAlO_2

We have performed the first-principles calculations about the superconducting transition temperature T_c of hole-doped delafossite CuAlO_2, AgAlO_2 and AuAlO_2. Calculated T_c are about 50 K(CuAlO_2), 40 K(AgAlO_2) and 3 K(AuAlO_2) at maximum in the optimum hole-doping concentration. The low T_c of AuAlO_2 is attributed to the weak electron-phonon interaction caused by the low covalency and heavy atomic mass.

cond-mat.supr-con↗

arXiv2011

Computational Materials Design for High Critical Temperature Superconductivity in Hole-Doped Delafossite CuAlO$_2$: Transparent Superconductors

We have calculated the superconducting critical temperature $T_c$ of hole-doped delafossite CuAlO$_2$ based on the first-principles calculations. According our calculation, $0.2\sim0.3$ hole-doped CuAlO$_2$ can become a phonon-mediated high-$T_c$ superconductor with $T_c\sim50$ K. In the hole-doped CuAlO$_2$, the A$_1$L$_1$ phonon mode that stretches O-Cu-O dumbbell has a strong electron-phonon interaction with electrons of the flat band in Cu 3d$_{3z^2-r^2}$ and the O 2p$_z$ anti-bonding $π$-band.

cond-mat.supr-con↗

arXiv2008

General Rule and Materials Design of Negative Effective U System for High-T_c Superconductivity

Based on the microscopic mechanisms of (1) charge-excitation-induced negative effective U in s^1 or d^9 electronic configurations, and (2) exchange-correlation-induced negative effective U in d^4 or d^6 electronic configurations, we propose a general rule and materials design of negative effective U system in itinerant (ionic and metallic) system for the realization of high-T_c superconductors. We design a T_c-enhancing layer (or clusters) of charge-excitation-induced negative effective $U$ connecting the superconducting layers for the realistic systems.

cond-mat.supr-con↗