Yu Kumagai 교수

First-principles defect formation energy calculations can include errors up to several eV. Improving on a previous method for correcting these errors by Freysoldt et al (PRL 102, 016402), the authors obtain defect energies with errors less than 0.2 eV for 17 defects in 10 compounds; the method is widely applicable to a range of defects.

Lately binary tin sulfides, being composed of inexpensive earth-abundant elements, have come under intense scrutiny for applications spanning photovoltaics, thermoelectrics, valleytronics, batteries, and photocatalysis. The authors use first-principles methods to examine these materials' point defects, which can play crucial roles in electronic applications. This comprehensive study expands the technological horizons for the tin sulfides, particularly the less-studied Sn${}_{2}$S${}_{3}$, which

Oxygen vacancies play significant roles in various properties of oxide materials. Therefore, insights into the oxygen vacancies can facilitate the discovery of better oxide materials. To achieve this, we developed codes for high-throughput point-defect calculations and applied them to characterize oxygen vacancies in 937 oxides. From the resulting large dataset, we analyzed the vacancy structures and formation energies and constructed machine-learning regression models to predict vacancy formati

Chalcopyrite ${\mathrm{ZnSnP}}_{2}$ is an alternative photoabsorber material for solar cells because of its controllable band gap, high absorption coefficient, and earth abundant constituents. In this study we systematically investigate its native point defects including vacancies, interstitials, and antisites using first-principles calculations with the Heyd-Scuseria-Ernzerhof hybrid functional. We evaluate the defect formation energies and defect single-particle levels at the dilute limit usin

The controversy regarding the ferroelectric behavior of hexagonal InMnO${}_{3}$ is resolved by using a combination of x-ray diffraction (XRD), piezoresponse force microscopy (PFM), second harmonic generation (SHG), and density functional theory (DFT). While XRD data show a symmetry-lowering unit-cell tripling, which is also found in the multiferroic hexagonal manganites of $P{6}_{3}cm$ symmetry, PFM and SHG do not detect ferroelectricity at ambient or low temperature, in striking contrast to the

Scandium nitride holds great promise for applications in several fields, based on its thermoelectric, piezoelectric, spintronic, and optoelectronic properties. Applications particularly in the last areas are strongly associated with this compound's point defects. The authors comprehensively investigate ScN in terms of the properties of its point defects (native defects, unintentional impurities, and $p$-type dopants), electronic structure, and chemical stability. The physical understanding offer

$\text{Co-}{L}_{2,3}$ x-ray-absorption near-edge structures (XANES) and electron-energy-loss near-edge structures (ELNES) are calculated from first principles, taking into account the configuration interactions among molecular orbitals for model clusters obtained using fully-relativistic one-electron calculations. The spectra for ${\text{LaCoO}}_{3}$ at low temperature and for ${\text{LiCoO}}_{2}$, both of which have low-spin ${\text{Co}}^{3+}$ ions in similar local environments, are well reprod

The semiconductor Zn${}_{3}$N${}_{2}$, composed of inexpensive, abundant, nontoxic elements, shows a very high electron mobility and is quite appealing for many applications. However, real-world samples remain puzzling, with reported band gaps ranging from 0.85 to 3.2 eV. Using advanced first-principles calculations, the authors solve this puzzle: Gaps of up to 2 eV are mainly due to either hydrogen interstitials or oxygen substitution at nitrogen sites, while larger values seem to be due to fla

p-Type doping in Cu(I)-based semiconductors is pivotal for solar cell photoabsorbers and hole transport materials to improve the device performance. Impurity doping is a fundamental technology to overcome the intrinsic limits of hole concentration controlled by native defects. Here, we report that alkali metal impurities are prominent p-type dopants for the Cu(I)-based cation-deficient hole conductors. When the size mismatch with Cu⁺ in the host lattice is increased, these isovalent i

The phase diagrams and band gaps in CuInSe${}_{2}$-CuGaSe${}_{2}$ (CIS-CGS) and CuInSe${}_{2}$-CuAlSe${}_{2}$ (CIS-CAS) pseudobinary systems are determined using a combination of first-principles calculations based on a hybrid Hartree-Fock density functional approach, cluster expansion, and Monte Carlo simulations. The CIS-CGS and CIS-CAS systems show phase-separation critical temperatures of 420 and 460 K, respectively. For both CuIn${}_{1\ensuremath{-}x}$Ga${}_{x}$Se${}_{2}$ (CIGS) and CuIn${}

Although n-type transparent conducting oxides (TCOs) with visible-light transparency and electric conductivity have been used in industry for decades, their p-type counterparts have not been commercialized. To search for promising p-type TCOs, computational screening was applied by several research groups in the last decade. However, screening of a wide material space is mainly based on the approximated physical quantities, such as underestimated band gaps, and band-alignment techniques. In part

Since the ionization potential (IP) is one of the fundamental quantities in a solid, ruling the physical and chemical properties and electronic device performances, many researchers have quantified the IPs using first-principles calculations of slab models recently. However, the breakdown into bulk and surface contributions has remained a contentious issue. In this study, we discuss how to decompose the IP into the bulk and surface contributions by using the macroscopic average technique. Althou