Researcher profile

Yu Kumagai

Yu Kumagai contributes to research discovery and scholarly infrastructure.

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Published work

6 published item(s)

preprint2026arXiv

Alternating Target-Path Planning for Scalable Multi-Agent Coordination

The concurrent target assignment and pathfinding (TAPF) problem extends multi-agent pathfinding (MAPF) by asking planners to allocate distinct targets and collision-free paths to agents. Prior work on TAPF has relied exclusively on Conflict-Based Search (CBS), which tightly couples target assignment and pathfinding, resulting in compute-intensive, non-scalable solutions. In contrast, we propose an iterative refinement framework that decouples target assignment from pathfinding. Our framework builds on modern, fast, suboptimal MAPF solvers, such as LaCAM. Specifically, within a given time budget, it repeatedly solves MAPF for the current target assignment, identifies bottleneck agents via MAPF feedback, and refines the assignment. Empirical results show that feedback-driven reassignment loop is effective, enabling our framework to scale well beyond the reach of the state-of-the-art CBS-based solver while maintaining decent solution quality. This represents a solid step toward practical, large scale TAPF suitable for real-world setups.

preprint2022arXiv

Switchable Electric Dipole from Polaron Localization in Dielectric Crystals

Ferroelectricity in crystals is associated with the displacement of ions or rotations of polar units. Here we consider the dipole created by donor doping ($D^+$) and the corresponding bound polaron ($e^-$).A dipole of 6.15 Debye is predicted, from Berry phase analysis, in the Ruddlesden-Popper phase of ${\rm Sr_3Ti_2O_7}$. A characteristic double-well potential is formed, which persists for high doping densities. The effective Hubbard $U$ interaction can vary the defect state from metallic, a two-dimensional polaron, through to a zero-dimensional polaron. The ferroelectric-like behavior reported here is localized and distinct from conventional spontaneous lattice polarization.

preprint2020arXiv

Data-Mining Element Charges in Inorganic Materials

Oxidation states are well-established in chemical science teaching and research. We data-mine more than 168,000 crystallographic reports to find an optimal allocation of oxidation states to each element. In doing so we uncover discrepancies between text-book chemistry and reported charge states observed in materials. We go on to show how the oxidation states we recommend can significantly facilitate materials discovery and heuristic design of novel inorganic compounds.

preprint2020arXiv

Finite-size corrections for defect-involving vertical transitions in supercell calculations

A correction method for vertical transition levels (VTLs) involving defect states calculated with a supercell technique is formulated and its effectiveness is systematically verified with ten defects in prototypical materials: cubic-BN, GaN, MgO, and 3C-SiC. Without any corrections, the absolute errors are around 1 eV with moderate size supercells in most cases. In contrast, when our correction method is adopted, the absolute errors are reduced and become less than 0.12 eV in all the cases. Our correction scheme is general and will have the potential for wide application as it is adaptive for evaluating various quantities at fixed geometry, as represented by those relevant to the generalized Koopmans' theorem.

preprint2020arXiv

Phonon scattering limited mobility in the representative cubic perovskite semiconductors SrGeO$_3$, BaSnO$_3$ and SrTiO$_3$

Cubic perovskite oxides are emerging high-mobility transparent conducting oxides (TCOs), but Ge-based TCOs had not been known until the discovery of metastable cubic SrGeO$_3$. $0.5 \times 0.4 \times 0.2$-mm$^3$ large single crystals of the cubic SrGeO$_3$ perovskite were successfully synthesized employing the high-pressure flux method. The phonon spectrum is determined from the IR optical reflectance and Raman-scattering analysis to evaluate the electron transport governed by optical phonon scattering. A calculated room-temperature mobility on the order of $3.9 \times 10^2$ cm$^2$V$^{-1}$s$^{-1}$ is obtained, identifying cubic SrGeO$_3$ as one of the most promising TCOs. Employing classical phonon theory and a combined experimental-theoretical approach, a comprehensive analysis of the intrinsic electron mobility in the cubic perovskite semiconductors SrGeO$_3$, BaSnO$_3$, and SrTiO$_3$ is provided based on the magnitude of polarization and eigenfrequency of optically active phonons.

preprint2017arXiv

Unusual magnetic structure of high-pressure synthesized perovskites ACrO3(A=Sc, In, Tl)

Magnetic structures of metastable perovskites ScCrO3, InCrO3 and TlCrO3, stabilized under high-pressure and high-temperature conditions, have been studied by neutron powder diffraction. Similar to the other orthochromites LnCrO3 (Ln=lanthanide or Y), these materials crystallize into the orthorhombic structure with Pnma10 symmetry. The spin configuration of the metastable perovskites has been found to be C-type, contrasting with the G-type structure usually observed in LnCrO3. First-principles calculations demonstrate that the Ctype structure found in ScCrO3 and InCrO3 is attributed to a ferromagnetic (FM) nearest-neighbor interaction, while in TlCrO3, this type of magnetic ordering is stabilized by a strong next-nearest-neighbor antiferromagnetic (AFM) exchange. The spins in the C-type magnetic structure line up along the orthorhombic b-axis, yielding the Pnma magnetic symmetry. The dominant mechanism controlling this spin direction has been concluded to be the single ion anisotropy imposed by a uniaxial distortion of CrO6 octahedra.