arXiv subjects

Hiroshi Kitagawa

Publications and source records attributed to Hiroshi Kitagawa.

6 recordsLinked to original sources

arXiv2026

Substitution effects in RuO$_2$ single crystals

RuO$_2$ has emerged as one of the leading candidates for investigating altermagnetism. Recent quantum oscillation and angle-resolved photoemission spectroscopy measurements found the absence of altermagnetism or antiferromagnetism in pure RuO$_2$ crystals. The continuing debate over intrinsic magnetic order in RuO$_2$ may reflect its proximity to an altermagnetic instability. In this work, we grow single crystals of Ru$_{0.9}$V$_{0.1}$O$_2$ and investigate their structural, transport, and magnetic properties. X-ray photoelectron spectroscopy reveals an average V oxidation state near $+4$. The paramagnetic susceptibility remains nearly unchanged up to room temperature, with no evidence of magnetic ordering. Thus, the 10\% V-substitution in RuO$_2$ does not induce altermagnetism. Electronic structure calculations for the V-substituted systems using two methods suggest that a higher level of V-substitution leads to a significant change in the density of states. These findings underscore the potential of nonmagnetic substitution in RuO$_2$ as an attractive candidate for probing altermagnetic transitions and their experimental signatures.

cond-mat.mtrl-sci↗

arXiv2025

High-pressure electronic states in semiconductors studied by infrared spectroscopy: metallization and band gap tuning in Mg$_2$Si, InAs and InSb

In this article, a brief introduction is first given on infrared studies of materials at high pressures using a diamond anvil cell. Then, our recent results of high-pressure infrared studies are described for Mg$_2$Si, InAs, and InSb. For Mg$_2$Si, pressure-induced metallization at pressures near 10 GPa were clearly demonstrated for both carrier-doped and undoped Mg$_2$Si by large increases of reflectivity. For InAs and InSb, their band gap ($E_g$) increased rapidly and almost linearly with pressure with linear coefficients of $dE_g/dP$=84.6 and 112 meV/GPa, respectively. Obtained values of $E_g$ versus lattice parameter at high pressures are compared with those for other IIl-V semiconductors at ambient pressure, giving unique insight into effects of physical and chemical pressures on $E_g$. Above the structural transition pressures of 7 and 3 GPa for InAs and InSb, respectively, they exhibit highly metallic characteristics accompanied by high reflectivity.

cond-mat.mtrl-sci↗

arXiv2024

Breakdown of Kubo relation in Pt-Cu nanoparticle

Nanoparticles were predicted to exhibit unique physical properties due to quantum size effects, but their identification remains difficult. According to Kubo's theory, the gap size is inversely correlated with both the density of states at the Fermi energy and the number of atoms in the particle. Previously, we confirmed that the particle size and magnetic field dependence of NMR anomaly temperature is consistent with the estimated "Kubo" gap. Here, we investigated the density-of-states dependence in the Pt$_{1-x}$Cu$_{x}$ nanoparticles. While an enhancement of nuclear spin-lattice relaxation rate $1/T_1$ at low temperatures was clearly observed for the Pt-rich nanoparticles, such behavior was abruptly suppressed in the Cu-rich nanoparticles. Furthermore, the NMR anomaly temperature is nearly unchanged with varying the density of states. Our findings indicate that the quantum size effect contains more profound physics than just the ones predicted by Kubo.

cond-mat.mes-hall↗

arXiv2020

Magnetic-Field Dependence of Novel Gap Behavior Related to the Quantum-Size Effect

$^{195}$Pt-NMR measurements of Pt nanoparticles with a mean diameter of 4.0 nm were performed in a high magnetic field of approximately $μ_0 H = 23.3$ T to investigate the low-temperature electronic state of the nanoparticles. The characteristic temperature $T^*$, below which the nuclear spin-lattice relaxation rate $1/T_1$ deviates from the relaxation rate of the bulk, shows a magnetic-field dependence. This dependence supports the theoretical prediction of the appearance of discrete energy levels.

cond-mat.mtrl-sci↗

arXiv2020

NMR-based gap behavior related to the quantum size effect

We conducted$^{195}$Pt-nuclear magnetic resonance measurements on various-diameter Pt nanoparticles coated with polyvinylpyrrolidone in order to detect the quantum size effect and the discrete energy levels in the electron density of states, both of which were predicted by Kubo more than 50 years ago. We succeeded in separating the signals arising from the surface and interior regions and found that the nuclear spin-lattice relaxation rates in both regions show the metallic behavior at high temperatures. Surprisingly, the magnetic fluctuations in both regions exhibited anomalous behavior below the same temperature $T^*$, which points to a clear size dependence and is well scaled with $δ_\mathrm{Kubo}$. These results suggest that a size-tunable metal-insulator transition occurs in the Pt nanoparticles as a result of the Kubo effect.

cond-mat.mes-hall↗

arXiv2019

Towards an understanding of the chemisorption and catalytic activity of Pd$_{X}$Ru$_{1-X}$ nanoparticles using photoelectron spectroscopy

Chemisorption process and catalytic activity of CO to CO2 conversion on PdXRu1-X (X : 0, 0.1, 0.3, 0.5, 0.7, 0.9 and 1) extended as a benchmark for further investigation of very complex nanoparticle (NP) structures have been checked into thoroughly by using the synchrotron-based hard X-ray photoelectron spectroscopy (HAXPES). Assessment and diagnostic of core levels and valence bands data highlight valuable information regarding the surface interaction and structure of PdRu bimetallic nanoparticles (BM-NPs). Core level shift observed from the C 1s clearly emphasized that the Pd0.5Ru0.5 NPs which provide the highest catalytic efficiency (CO oxidation) exhibits a preferential adsorption of the CO molecule at the top site. Otherwise, our results also display that remaining NPs exhibit two components assigned to the bridge and hollow CO sites. Combination of Pd 3d5/2 and Ru 3p3/2 core levels results points out a transition from alloy structures to core-shell configuration with increasing the molar ratio of Ru. Valence-band maximum (VBM) demonstrates a position dependent to the molar composition of Ru. Additionally, by digging into the electronic structures (shape and width) of each NPs very deeply, we clearly emphasise the valuable relationship between composition, atomic arrangement, d-band and catalytic properties of bimeatllic PdRu. According to the HAXPES results, the strong and clear evidence for the enhanced CO to CO2 conversion ability of Pd0.5Ru0.5-NP may be assigned to the existence of a most favourable spatial d-band extension and an optimum balance between the core-shell and alloy structures. Electronic and chemical data and interpretations are consistent with the PdRu catalytic performances. This present prospection gives a new step towards a better knowledge on the catalytic surface interaction of binary, ternary and multimetallic NPs with reactants.

cond-mat.mtrl-sci↗