arXiv subjects

Yoshiki Kubota

Publications and source records attributed to Yoshiki Kubota.

3 recordsLinked to original sources

arXiv2026

Survival of the metallic state in a single-hole multiband $p$-orbital molecular system

Strong correlations and ferromagnetic Hund's coupling lead to diverse electronic phenomena in transition-metal oxides that sensitively depend on the $d$-orbital electron filling. Fullerides, their $p$-electron counterparts, exhibit effective antiferromagnetic Hund's coupling in a different energy range. At half-filling ($n=3$, three electrons in triply degenerate orbitals), both $d-$ and $p$-electron systems are Mott insulators due to strong correlations and Hund's coupling. Away from half-filling, in single-electron/hole ($n=1,5$) $d$-orbital systems, Hund's coupling opposes the correlations, reducing the Mott gap and allowing survival of metallicity. Here we report a single-hole multiorbital correlated $p$-electron system, orthorhombic-structured Yb$_2$CsC$_{60}$ comprising pentavalent C$_{60}^{5-}$ anions, which also exhibits a robust metallic state with no Mott transition, just like in the metastable single-electron cubic-structured CsC$_{60}$. We assert that particle-hole symmetry holds well in ($n=1,5$) fullerides and that their $p$-electron-derived states are analogous to those in $d$-orbital solids, providing impetus for further study of these correlated systems.

cond-mat.str-el↗

arXiv2025

A Noninvasive and Dispersive Framework for Estimating Nonuniform Conductivity of Brain Tumor in Patient-Specific Head Models

We propose a noninvasive and dispersive framework for estimating the spatially nonuniform conductivity of brain tumors using MR images. The method consists of two components: (i) voxel-wise assignment of tumor conductivity based on reference values fitted to the Cole-Cole model using empirical data from the literature and (ii) fine-tuning of a deep learning model pretrained on healthy participants. A total of 67 cases, comprising both healthy participants and tumor patients and including 9,806 paired T1- and T2-weighted MR images, were used for training and evaluation. The proposed method successfully estimated patient-specific conductivity maps, exhibiting smooth spatial variations that reflected tissue characteristics, such as edema, necrosis, and rim-associated intensity gradients observed in T1- and T2-weighted MR images. At 10 kHz, case-wise mean conductivity values varied across patients, ranging from 0.132 to 0.512 S/m in the rim (defined as the region within 2 mm of the tumor boundary), from 0.132 to 0.608 S/m in the core (the area inside the rim), and from 0.141 to 0.542 S/m in the entire tumor. Electromagnetic simulations for transcranial magnetic stimulation in individualized head models showed substantial differences in intratumoral field distributions between uniform assignments and the proposed nonuniform maps. Furthermore, this framework demonstrated voxel-wise dispersive mapping at 10 kHz, 1 MHz, and 100 MHz. This framework supports accurate whole-brain conductivity estimation by incorporating both individual anatomical structures and tumor-specific characteristics. Collectively, these results advance patient-specific EM modeling for tumor-bearing brains and lay the groundwork for subsequent microwave-band validation.

physics.med-ph↗

arXiv2023

Unusual nanoscale coexistence of polar-nonpolar domains underlying oxygen storage properties in Ho(Mn, Ti)O$_{3+δ}$

Hexagonal manganese oxides RMnO$_3$ show intriguing topological ferroelectric-domain walls with variable conductivity, leading to domain wall engineering. Despite the numerous experimental studies on the polar nanoscale structures, controlling ferroelectric domains has not been sufficiently investigated. Here, we reveal the unprecedented coexistence of polar-nonpolar nanoscale domains that can be formed by substituting Ti ions in HoMnO$_3$. Unusual polar nanoscale domains are embedded in nonpolar domains with different crystallographic symmetry. This polar-nonpolar coexisting structure is naturally assembled by adjusting the lattice length during a solid-state reaction process. Furthermore, the comprehensive study reveals that the reversible microstructural change with a nonpolar-polar transition is strongly correlated with the oxygen storage properties in Ho(Mn, Ti)O$_{3+δ}$. The present results provide important insight into the nanoscale polar-nonpolar domain coexistence in functional rare-earth manganese oxides, RMnO$_3$.

cond-mat.mtrl-sci↗