arXiv subjects

Keisuke Hirata

Publications and source records attributed to Keisuke Hirata.

7 recordsLinked to original sources

arXiv2026

Semiclassical thermoelectric transport in disordered Dirac electron system Ag2Te

We investigated the thermoelectric effects of the Dirac electron system Ag2Te under magnetic field. Our anal-ysis based on the Boltzmann semiclassical model associated the disorder with the unconventional magnetic field responses such as linear magnetoresistance, linear Nernst effect, step-like Nernst effect, and sign change in Nernst effect. The analysis also revealed the impurity band near the Fermi energy. We simultaneously clari-fied the serious impact of the thermal Hall effect on the measurement of the Nernst effect, and we proposed the definitive solution. Our careful measurement and analysis will be the standard for the thermoelectric study under magnetic field.

cond-mat.mtrl-sci↗

arXiv2026

Realizing record-high transverse thermoelectric figure of merit at room temperature in artificially tilted multilayers based on high power factor NiFe alloy

Transverse thermoelectric conversion using artificially tilted multilayers (ATMLs) offers a versatile device architecture that circumvents the structural limitations of conventional longitudinal thermoelectrics. However, achieving competitive room-temperature thermoelectric performance without an external magnetic field remains a critical challenge. Here, we report a record-high transverse thermoelectric figure of merit $z_{yx}T$ of 0.36 in Ni$_{50}$Fe$_{50}$/Bi$_{0.2}$Sb$_{1.8}$Te$_{3}$-based ATML at room temperature without an external magnetic field. Leveraging the longitudinal high power factor in a Ni$_{50}$Fe$_{50}$ alloy and the sharp contrast in electrical and thermal transport properties between $n$-type Ni$_{50}$Fe$_{50}$ and $p$-type Bi$_{0.2}$Sb$_{1.8}$Te$_{3}$, we engineer an anisotropic structure that simultaneously exploits high electrical conductivity, large transverse thermopower, and low thermal conductivity to maximize $z_{yx}T$ in ATML. Through the direct measurements of these thermoelectric transport parameters, we obtained $z_{yx}T$ of 0.36 in Ni$_{50}$Fe$_{50}$/Bi$_{0.2}$Sb$_{1.8}$Te$_{3}$-based ATML, which is in excellent agreement with the analytical prediction of 0.36 owing to the low interfacial electrical and thermal resistances at the Ni$_{50}$Fe$_{50}$/Bi$_{0.2}$Sb$_{1.8}$Te$_{3}$ junctions. These results pave the way for the practical implementation of transverse thermoelectric materials around room temperature.

cond-mat.mtrl-sci↗

arXiv2026

Revealing nonvolatile behaviors in magneto-thermal switching using microstructure-controlled superconducting composites

Thermal conductivity in a conductor changes by the application of an external magnetic field, which functions as a magneto-thermal switch. For superconductors, a large magneto-thermal switching can occur through a superconducting-to-normal conducting phase transition due to the change in the electron contribution in thermal conductivity. Arima et al. recently reported a nonvolatile nature of the magneto-thermal switching for superconducting solders, which consist of phase-separated Sn and Pb domains. Although they clarified that magnetic flux trapping is required to induce the nonvolatile magneto-thermal switching, a rule for such material design is still unclear. Here, we investigate the microstructure dependence of magneto-thermal switching in superconducting Sn/Pb multilayered composites, which are created by an accumulative roll bonding method. The thickness of each layer, that is the scale of microstructure, can be systematically controlled by the repetition number of roll bonding while the whole sample size and average composition are unchanged. We find that, as the formation of micro-scaled Sn domains proceeds by increasing the repetition number, a nonvolatility in the magneto-thermal conductivity gradually appears in correlation with the remanent magnetization. This study directly confirms that the inclusions with a size comparable to or less than the magnetic vortex in superconducting matrix is essential for magnetic flux trapping, enabling the nonvolatile magneto-thermal switching in superconducting composites.

cond-mat.supr-con↗

arXiv2025

Magneto-Tunable Thermal Diode Based on Bulk Superconductor

Thermal diode is a growing technology and important for active thermal flow control. Since the theoretical designing of thermal diode in 2004, various kinds of solid-state thermal diodes have been theoretically and experimentally investigated. Here, we report on the observation of thermal rectification in bulk-size superconductor-normal metal junctions. High-purity (5N) wires of Pb and Al are soldered, and thermal conductivity (\k{appa}) of the junctions is measured in two different directions of the heat flow, forward (\k{appa}F) and reverse (\k{appa}R) directions. Thermal rectification ratio (\k{appa}F / \k{appa}R) of 1.75 is obtained at T ~ 5.2 K with H = 400 Oe. The merit of the Pb-Al junction is a large difference of \k{appa} in an order of several hundred W m-1 K-1 and magneto-tunability of the working temperature.

cond-mat.supr-con↗

arXiv2025

Trans-scale spin Seebeck effect in nanostructured bulk composites based on magnetic insulator

The spin Seebeck effect (SSE) enables thermoelectric conversion through thermally generated spin currents in magnetic materials, offering a promising transverse geometry for scalable devices. However, conventional SSE devices are confined to nanoscale thin-film architectures, with significantly restricted output power due to the intrinsic constraints of spin and magnon diffusion lengths. Here, we demonstrate a trans-scale SSE using nano-structured bulk composites composed of Pt-coated yttrium iron garnet powders fabricated via dynamic powder sputtering and low-temperature sintering. The resulting three-dimensional composites exhibit continuous Pt channels and robust mechanical integrity. Transverse thermoelectric measurements confirm isotropic SSE signals at the bulk scale. Power analysis indicates that the three-dimensional architecture enables scalable volumetric thermoelectric power generation beyond diffusion-limited thin-film SSE geometries. This work establishes a scalable platform for bulk SSE-based thermoelectrics, bridging nanoscale spin caloritronics with macroscopic device integration.

cond-mat.mtrl-sci↗

arXiv2021

Synergetic enhancement of power factor and suppression of lattice thermal conductivity via electronic structure modification and nanostructuring on Ni and B co-doped p-type Si-Ge alloy

For simultaneously achieving the high-power factor and low lattice thermal conductivity of Si-Ge based thermoelectric materials, we employed, in this study, constructively modifying the electronic structure near the chemical potential and nano-structuring by low temperature and high-pressure sintering on nano-crystalline powders. Nickel was doped to create the impurity states near the edge of the valence band for enhancing the power factor with boron for tuning the carrier concentration. The nanostructured samples with the nominal composition of Si0.65-xGe0.32Ni0.03Bx (x = 0.01, 0.02, 0.03, and 0.04) were synthesized by the mechanical alloying followed low-temperature and high-pressure sintering process. A large magnitude of Seebeck coefficient reaching 321 μVK-1 together with a small electrical resistivity of 4.49 mΩcm, leads to a large power factor of 2.3 Wm-1K-2 at 1000 K. With successfully reduced thermal conductivity down to 1.47 Wm-1K-1, a large value of ZT ~1.56 was obtained for Si0.65-xGe0.32Ni0.03B0.03 at 1000 K

cond-mat.mtrl-sci↗

arXiv2021

Capacitor Type Thin-Film Heat Flow Switching Device

We developed a capacitor type heat flow switching device, in which electron thermal conductivity of the electrodes is actively controlled through the carrier concentration varied by an applied bias voltage. The devices consist of an amorphous p-type Si-Ge-Au alloy layer, an amorphous SiO$_2$ as the dielectric layer, and a n-type Si substrate. Both amorphous materials are characterized by very low lattice thermal conductivity, less than 1 Wm-1K-1. The Si-Ge-Au amorphous layer with 40 nm in thickness was deposited by means of molecular beam deposition technique on the 100 nm thick SiO$_2$ layer formed at the top surface of Si substrate. Bias voltage-dependent thermal conductivity and heat flow density of the fabricated device were evaluated by a time-domain thermoreflectance method at room temperature. Consequently, we observed a 55 percent increase in thermal conductivity.

physics.app-ph↗