arXiv subjects

Kenjiro Okawa

Publications and source records attributed to Kenjiro Okawa.

6 recordsLinked to original sources

arXiv2026

Universal Magnetoresistance Scaling in Layered Pd-based Multiband Metals Beyond Compensated Semimetal Regime

We systematically investigated the magnetoresistance ratio (MR) of single-crystalline, nonmagnetic layered Pd-based metals, including centrosymmetric PdTe2, PdPb2, and beta-PdBi2 and noncentrosymmetric alpha-PdBi. Our study identifies a distinct class of large MR in multiband, high-carrier-density systems. Unlike well-studied extremely large MR materials, such as Dirac and Weyl semimetals described by simple compensated-carrier models, these compounds possess complex Fermi surfaces, as validated by our first-principles calculations. Nevertheless, they exhibit a remarkably simple MR scaling governed by carrier mobility, manifested in systematic dependencies on magnetic field, temperature, and the residual resistivity ratio (RRR). The validity of Kohler's rule in high-RRR crystals indicates that MR is governed by a single effective scattering time, even in these multiband systems. The field and RRR dependences of MR follow an intermediate power-law behavior between linear and quadratic, attributable to imperfect carrier compensation and a distribution of carrier mobilities. Among the studied compounds, alpha-PdBi exhibits the largest MR, reaching 1500% (2 K, 7 T), owing to its exceptionally high RRR (approximately 660). However, when compared on an equal-RRR basis, its MR is smaller than that of its centrosymmetric counterparts. This trend suggests that additional scattering channels arising from spin-orbit-induced band splitting in noncentrosymmetric systems reduce the effective carrier mobility. Our results establish a new class of large MR in clean multiband metals where complex electronic structures give rise to emergent single-parameter scaling, highlighting the interplay between disorder, mobility, and symmetry.

cond-mat.mtrl-sci↗

arXiv2026

Quantitative comparison of heat flow, guarded-heater and AC Harman methods for thermoelectric module efficiency

The evaluation of thermoelectric conversion efficiency remains challenging owing to the lack of internationally standardized measurement protocols. Commonly used techniques -including the heat flow, guarded heater, and AC Harman methods-differ fundamentally in their operating principles and sensitivity to heat losses. In this study, we benchmark three module-level efficiency measurement techniques -the heat-flow, guarded heater, and AC Harman methods- using commercial Bi2Te3-based modules with different module architectures. The conversion efficiencies obtained using the heat flow and guarded heater methods showed closely consistent central values and similar temperature-dependent trends over the investigated range. In contrast, the efficiency derived using the AC Harman method was systematically lower by approximately 16 % to 30 %, depending on the module architecture. Steady-state finite-element calculations of heat conduction and radiation indicated that the open thermal boundary condition used in the Harman configuration produces module-architecture-dependent internal temperature distributions and effective temperature differences, consistent with the experimentally observed trend. These results demonstrate that module-level efficiency estimated using the AC Harman method can be affected by nonideal thermal environments and emphasize the necessity of accounting for radiative and substrate-related heat losses. Nevertheless, the AC Harman method remains useful for rapid performance screening, provided that its module- and boundary-condition-dependent systematic bias is appropriately considered. Our results provide a quantitative benchmark for major measurement techniques and support the development of best practices, method-selection guidelines, and future methodological standardization in module-level thermoelectric metrology.

physics.app-ph↗

arXiv2026

Reverse heat flow with Peltier-induced thermoinductive effect

The concept of "thermal inductance" expands the options of thermal circuit design. However, the inductive component is the only missing components in thermal circuits, unlike their electromagnetic counterparts. Herein, we report an electrically controllable reverse heat flow, in which heat flows from a low-temperature side to a high-temperature side locally and temporarily in a single material by imposing thermal inertia and an ac current. This effect can be regarded as an equivalent of the "thermoinductive" effect induced by the Peltier effect. We derive an exact solution indicating that this reverse heat flow occurs universally in solid-state systems and that it is considerably enhanced by thermoelectric properties. A local cooling of 25 mK is demonstrated in (Bi,Sb)2Te3, which is explained by our exact solution. This effect can be directly applied to the potential fabrication of a "thermoinductor" in thermal circuits.

physics.app-ph↗

arXiv2025

Direct observation of the surface superconducting gap in the topological superconductor candidate β-PdBi2

β-PdBi2 is one of the candidates for topological superconductors with a superconducting (SC) transition temperature (Tc) of 5.3 K, in which parity mixing of spin singlet and spin triplet has been anticipated, being crucial for the further understanding of relationship with inversion symmetry and parity mixing in the superconductivity. In this work, we measured the SC gap in high-quality single crystal of β-PdBi2 by using high-resolution laser angle-resolved photoemission spectroscopy below Tc. We found the isotropic SC gaps in momentum space for multiple bands, and observed that the difference between the SC gap of the topological surface bands and the bulk bands is about 0.1 meV, consistent with other experimental results. These direct and quantitative experimental results support the possibility of β-PdBi2 as a topological superconductor, characterized by unique crystal and electronic band structures.

cond-mat.supr-con↗

arXiv2024

Accurate determination of thermoelectric figure of merit using ac Harman method with a four-probe configuration

The ac Harman method has been used for the direct estimation of dimensionless thermoelectric figure of merit (zT) through ac/dc resistance measurements. However, accurate zT estimation with a four-probe configuration is difficult owing to the occurrence of a thermal phase-delay in the heat flow with a low frequency current. This study reports an exact solution for zT estimation by solving the heat conduction equation. The analysis can explain the reverse heat flow, which is the main source of the error in the four-probe configuration, and the experimentally obtained behavior of the frequency dependence of zT of (Bi,Sb)$_2$Te$_3$. Approximately 20 % of the error is caused by a thermal phase-delay, unless an appropriate current frequency and voltage-terminal position are chosen. Thus, an accurate zT evaluation using a four-probe configuration at any voltage terminal position is achieved. These findings can lead to interesting thermoelectric metrology and could serve as a powerful tool to search for promising thermoelectric materials.

cond-mat.mtrl-sci↗

arXiv2020

Large-scalable fabrication of improved Bi-Te-based flexible thermoelectric modules using a semiconductor manufacturing process

Among the several flexible thermoelectric modules in existence, sintered Bi-Te-based modules represent a viable option because of their high output power density and flexibility, which enables the use of arbitrary heat sources. We have fabricated Bi-Te-based modules with a large-scalable fabrication process and improved their output performance. The reduction in the interconnection resistance, using thick electrodes of the flexible printed circuit, significantly improves the module's output power to 87 mW/cm$^{2}$ at $ΔT$ = 70 K, which is 1.3-fold higher than a previous prototype module. Furthermore, the establishment of the fabrication for the top electrodes by using the surface mount technology makes it possible to realize a high-throughput manufacturing of the module. Our durability tests reveal that there is no significant change in the internal resistance of the module during 10000 cycles of mechanical bending test and 1000 cycles of thermal stress test.

physics.app-ph↗