arXiv subjects

Wataru Kobayashi

Publications and source records attributed to Wataru Kobayashi.

7 recordsLinked to original sources

arXiv2026

A minimal electrostatic theory for the Seebeck coefficient in liquids

The Seebeck coefficient in liquids often reaches the mV/K range, yet its microscopic origin remains unclear due to the complexity of electrolyte systems. Here we propose a minimal electrostatic theory focusing on solvation entropy. Using a core-shell extension of the Born equation with temperature-dependent dielectric response, we provide an alternative electrostatic interpretation of the mV/K-scale solvent-structuring contribution extracted for a cobalt redox electrolyte. The theory clarifies that large valence, small cationic radius, small dielectric constant, and large magnitude of $\frac{d\varepsilon}{dT}$ are key factors for enhanced liquid Seebeck response.

cond-mat.stat-mech↗

arXiv2024

Analytical expression of negative differential thermal resistance in a macroscopic heterojunction

Heat flux ($J$) generally increases with temperature difference in a material. A differential coefficient of $J$ against temperature ($T$) is called differential thermal conductance ($k$), and an inverse of $k$ is differential thermal resistance ($r$). Although $k$ and $r$ are generally positive, they can be negative in a macroscopic heterojunction with positive $T$-dependent interfacial thermal resistance (ITR). The negative differential thermal resistance (NDTR) effect is an important effect that can realize thermal transistor, thermal memory, and thermal logic gate. In this paper, we examine analytical expressions of $J$, $k$, $r$, and other related quantities as a function of parameters related to thermal conductivity ($κ$) and ITR in a macroscopic heterojunction to precisely describe the NDTR effect.

cond-mat.mes-hall↗

arXiv2021

Thermopower in transition-metal perovskites

High-temperature thermopower is interpreted as entropy that a carrier carries. Owing to spin and orbital degrees of freedom, a transition metal perovskite exhibits large thermopower at high temperatures. In this paper, we revisit the high-temperature thermopower in the perovskites to shed light on the degrees of freedom. Thus, we theoretically derive an expression of thermopower in one-dimensional octahedral-MX6-clusters chain using linear-response theory and electronic structure calculation of the chain based on the tight-binding approximation. The derived expression of the thermopower is consistent with the extended Heikes formula and well reproduced experimental data of several perovskite oxides at high temperatures. In this expression, a degeneracy of many electron states in octahedral ligand field (which is characterized by multiplet term) appears instead of the spin and orbital degeneracies. Complementarity in between our expression and the extended Heikes formula is discussed.

cond-mat.str-el↗

arXiv2013

Spin state of Co$^{3+}$ in LaCo$_{1-x}$Rh$_{x}$O$_{3}$ investigated by structural phenomena

Neutron diffraction for a polycrystalline sample of LaCo$_{0.8}$Rh$_{0.2}$O$_{3}$ and synchrotron x-ray diffraction for polycrystalline samples of LaCo$_{0.9}$Rh$_{0.1}$O$_{3}$ and LaCo$_{0.8}$Rh$_{0.2}$O$_{3}$ have been carried out in order to investigate the structural properties related with the spin state of Co$^{3+}$ ions. We have found that the values of the Co(Rh)-O bond lengths in the Co(Rh)O$_{6}$ octahedron of LaCo$_{0.8}$Rh$_{0.2}$O$_{3}$ are nearly identical at 10 K. The lattice volume for the Rh$^{3+}$ substituted samples decreases with the thermal expansion coefficient similar to that of LaCoO$_{3}$ from room temperature, and ceases to decrease around 70 K. These experimental results favor a mixed state consisting of the high-spin-state and low-spin-state Co$^{3+}$ ions, and suggest that the high-spin-state Co$^{3+}$ ions are thermally excited in addition to those pinned by the substituted Rh$^{3+}$ ions.

cond-mat.str-el↗

arXiv2012

Magnetoresistance, noise properties and the Koshino-Taylor effect in the quasi-1D oxide KRu_4O_8

The low temperature electronic and galvanomagnetic transport properties of the low dimensional oxide KRu_4O_8 are experimentally considered. A quadratic temperature variation of the resistivity is observed to be proportional to the residual resistivity. It shows the role of inelastic electron scattering against impurities, i.e. a large Koshino-Taylor effect, rather than a consequence of strong electronic correlations. In the same temperature range, the Kohler rule is not fulfilled. The resistance noise increases also sharply, possibly due to a strong coupling of carriers with lattice fluctuations in this low dimensional compound.

cond-mat.mtrl-sci↗

arXiv2010

Dominant role of impurity scattering over crystalline anisotropy for magnetotransport properties in the quasi-1D Hollandite Ba1.2Rh8O16

Angular magnetotransport measurements have been performed to tackle the origin of the magnetoresistance in the quasi-1D Hollandite Ba1.2Rh8O16. Three samples of different impurities amount were measured. We observe that the low temperature resistivity upturn is not due to a charge density wave transition, and a dominant role of impurities scattering for low temperature transport properties is instead demonstrated. The components of magnetoresistance were separated by using the Kohler plot and the angular dependency of the resistance under magnetic field. It shows the major contribution of an isotropic, likely spin driven, negative magnetoresistance. Galvanomagnetic characteristics are then consistent with a Kondo effect and appear to be essentially 3D at low temperature.

cond-mat.mtrl-sci↗

arXiv2007

Structure-Property Relationship in the Ordered-Perovskite- Related Oxide Sr3.12Er0.88Co4O10.5

Synchrotron X-ray diffraction patterns were measured and analyzed for a polycrystalline sample of the room-temperature ferromagnet Sr3.12Er0.88Co4O10.5 from 300 to 650 K, from which two structural phase transitions were found to occur successively. The higher-temperature transition at 509 K is driven by ordering of the oxygen vacancies, which is closely related to the metallic state at high temperatures. The lower-temperature transition at 360 K is of first order, at which the ferromagnetic state suddenly appears with exhibiting a jump in magnetization and resistivity. Based on the refined structure, possible spin and orbital models for the magnetic order are proposed.

cond-mat.str-el↗