arXiv subjects

Yoshikazu Mizuguchi

Publications and source records attributed to Yoshikazu Mizuguchi.

At least 19 recordsLinked to original sources

arXiv2026

Ideal band structures for high-performance thermoelectric materials with band convergence

We investigate optimal band structures in band-converged systems to achieve high zT using numerical calculations based on a virtual spectral conductivity model. We consider a two parabolic band system, in which multiple band parameters can be independently controlled. Despite its simplicity, this model provides theoretical validation of empirical trends observed in thermoelectric materials. Our results provide a physically transparent set of design principles for band-structure engineering, offering quantitative design guidelines for the development of a wide range of thermoelectric materials. The main conclusions are as follows: (i) When a band does not cross the chemical potential and |μ-E_edge |>5k_B T, the contribution of the band to zT is negligibly small; (ii) To suppress the bipolar effect, a band gap E_g satisfying E_g>5k_B T_op, where T_op is the operating temperature, is required; (iii) In band-converged systems, the energy separation between the band edge ΔE should satisfy ΔE~0 to maximize zT when interband scattering is insignificant; (iv) Achieving high spectral conductivity Σ (high band degeneracy N, density of states effective mass m_DOS^*, and relaxation time τ) near the band edge is essential for achieving high zT.

cond-mat.mtrl-sci↗

arXiv2026

Observation of giant nonvolatile magneto-thermal switching in superconductor-ferromagnet hybrids

Magneto-thermal switch is a crucial thermal component which enables heat transfer control by the application of an external magnetic field. Recently, a nonvolatile behavior in magneto-thermal conductivity at zero magnetic field was observed in type-II and phase-separated superconductors owing to magnetic flux pinning nature, leading to an energy-efficient thermal control technology. However, the nonvolatile magneto-thermal switching ratio has been much lower than the volatile one in conventional materials. Here, we demonstrate a giant nonvolatile magneto-thermal switching in ferromagnetic Fe-superconducting Pb hybrids. The dispersion of pure Fe particles realizes increased electron and decreased phonon contributions in the thermal conductivity, which enhances the magneto-thermal switching ratio at the superconducting-to-normal conducting phase transition. Furthermore, in concert with trapped magnetic flux by supercurrent, ferromagnetic moment of Fe breaks the superconductivity of Pb matrix at zero magnetic field, enabling a significantly large nonvolatility even with a slight amount of Fe inclusions. Consequently, the nonvolatile magneto-thermal switching ratio reaches 719% in maximum at the Fe ratio of 8.7 vol%, which is more than twice the previous record value observed in Pb-Sn composites and the volatile one in pure Pb. This work broadens the exploration space and strategy for giant nonvolatile magneto-thermal switching materials.

cond-mat.supr-con↗

arXiv2026

Distinct Roles of Hydrogen in Superconducting and Ferromagnetic Phases of CoZr$_{2}$H$_{x}$

Hydrogenation offers a versatile route to tuning the physical properties of intermetallic compounds. In this study, we synthesized CoZr$_{2}$H$_{x}$ with different hydrogen contents and found that hydrogen is incorporated in two distinct concentration regimes separated by a wide composition gap: a low-concentration hydrogenated superconducting phase ($x$ = 0-0.054) and a high-concentration hydrogenated ferromagnetic phase ($x$ = 2.786). Hydrogen plays fundamentally different roles in the two concentration regimes. In the high hydrogen concentration phase, the Zr-H interactions substantially modify the metallic bands crossing the Fermi level, leading to the emergence of ferromagnetism. In contrast, in the low hydrogen concentration phase, hydrogen behaves as a nonmagnetic impurity without altering the electronic band structure. Despite the nearly identical Debye temperatures across the low-concentration series, the superconducting transition temperature ($T_{\mathrm{c}}$) is progressively suppressed with increasing hydrogen content.The observed $T_{\mathrm{c}}$ suppression is quantitatively described by the Abrikosov-Gor'kov pair-breaking theory, indicating that the superconducting gap of CoZr$_{2}$ is anisotropic or multigap rather than a fully isotropic $s$-wave symmetry.

cond-mat.supr-con↗

arXiv2026

Low-temperature magnetic-field-driven thermal oscillator based on metal-superconductor joint

Thermal control is one of the important technologies for fundamental science and thermal management. Among them, thermal oscillators have been in demands in the field of materials science and device application. In general, flexible frequency, amplitude, and waveform are needed for useful thermal oscillator, and the stability of the average temperature is also highly required. However, thermal oscillators based on an AC-current-driven heater require complicated control of input power to achieve the above-mentioned flexibility and stability of the outputs. Here, we demonstrate that magnetically-driven thermal oscillators fabricated using a metal-superconductor (Cu-Pb) joint achieve those requirements easily by tuning the applied magnetic field (H). A DC-current-driven heater is attached on the metal (Cu) side, and the superconductor (Pb) edge is attached to thermal bath. We use a sharp and huge change in thermal conductivity at the superconducting transition of the Pb wire to generate thermal oscillation at the Cu-wire side. A sine-shaped thermal oscillation with an amplitude of 180 mK and a frequency of 0.17 Hz is observed with highly stable average temperature. Furthermore, a larger amplitude is achieved in a square-shaped oscillation with a larger H amplitude. Our thermal oscillator with temperature stability, large amplitude, and relatively high frequency will work as a flexible AC heat source at cryogenic temperatures.

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↗

arXiv2026

Liquid-state structural asymmetry governs species-selective crystallization in multicomponent systems

Multicomponent crystals are often assumed to form nearly random solid solutions when thermodynamically stable. However, crystal growth proceeds from structurally heterogeneous liquids, raising the possibility that the liquid state may influence which species are incorporated into the growing crystal. Here we demonstrate that liquid-state structural asymmetry can induce species-selective crystallization in multicomponent systems. Using molecular dynamics simulations of a multivalent rocksalt-type model (AgPbBiTe$_3$), we find that cations with higher valence readily form locally crystal-compatible coordination environments in the liquid and are efficiently incorporated into the growing lattice, whereas lower-valence cations exhibit more disordered liquid coordination and attach less efficiently at the crystal-liquid interface. This asymmetry leads to species-selective incorporation and slower crystal growth. Depth-resolved photoelectron spectroscopy measurements on AgPbBiTe$_3$ further reveal enhanced Ag concentration near grain-boundary and surface regions, consistent with the selective incorporation predicted by the simulations. These results demonstrate that structural compatibility between liquid-state structure and the target crystal motif governs selective incorporation during crystallization, providing a general kinetic mechanism by which compositional heterogeneity can emerge during growth of multicomponent crystals.

cond-mat.soft↗

arXiv2026

Enhanced $T_\mathrm{c}$ in eutectic high-entropy alloy superconductors Hf-Nb-Sc-Ti-Zr

The present investigation into the superconducting properties of eutectic high-entropy alloy (HEA) Hf-Nb-Sc-Ti-Zr systems reveals an enhanced superconducting critical temperature ($T_\mathrm{c}$) in body-centered cubic (bcc) phases compared to typical quinary bcc HEAs. In Hf$_{10}$Nb$_{25}$Sc$_{25}$Ti$_{20}$Zr$_{20}$, Hf$_{5}$Nb$_{45}$Sc$_{20}$Ti$_{15}$Zr$_{15}$, and Hf$_{5}$Nb$_{45}$Sc$_{10}$Ti$_{5}$Zr$_{35}$ systems, which span a broad range of valence electron concentration per atom, lattice strain and the presence of partial or absent eutectic phases are characteristic features at lower annealing temperatures. The eutectic regions expand rapidly following annealing at 600$^{\circ}$C in all systems. The $T_\mathrm{c}$ of each system increases markedly with rising annealing temperatures from 400$^{\circ}$C to 600$^{\circ}$C, reaching a maximum value of 9.93 K in the Hf$_{5}$Nb$_{45}$Sc$_{10}$Ti$_{5}$Zr$_{35}$ sample annealed at 800$^{\circ}$C. Nearly all samples can be classified as strong-coupling superconductors. The sample annealed at 500$^{\circ}$C in the Hf$_{5}$Nb$_{45}$Sc$_{10}$Ti$_{5}$Zr$_{35}$ system exhibits a critical current density ($J_\mathrm{c}$) exceeding the practical threshold of 10$^{5}$ A/cm$^{2}$ up to approximately 4 T at 4.2 K and 6 T at 2 K. The elevated $J_\mathrm{c}$ is attributed to significant lattice strain and phase instability. The underlying mechanism for the enhanced $T_\mathrm{c}$ in Hf-Nb-Sc-Ti-Zr systems is examined through specific heat data analysis, suggesting that the expansion of the eutectic regions induced by thermal annealing plays a pivotal role.

cond-mat.supr-con↗

arXiv2026

Insight into high-entropy effect in body-centered cubic superconducting alloys

We have characterized the superconducting critical temperature ($T_\mathrm{c}$), the Debye temperature ($θ_\mathrm{D}$), the electronic specific heat coefficient, and the Vickers microhardness of HfNbTiVZr, NbTiZr, HfNbTi, HfNbZr, and HfNbTa, all possessing a body-centered cubic (bcc) structure. By compiling a comparable dataset for other equiatomic quinary bcc high-entropy alloy (HEA) superconductors, we have examined the validity of the hypothesis regarding the high-entropy effect in bcc HEA superconductors, as proposed in our previous work. This hypothesis attributes the observed negative correlation between the electron-phonon coupling constant ($λ_\mathrm{e-p}$) and $θ_\mathrm{D}$ to a reduced phonon lifetime at higher $θ_\mathrm{D}$, arising from the uncertainty principle in highly disordered quinary alloys. However, a pronounced change in this negative correlation is not evident in equiatomic ternary alloys with a lower degree of atomic disorder, thereby providing limited support for the hypothesis. Alternatively, by assembling the full dataset of bcc alloys spanning binary through senary systems, we have identified a universal negative correlation between $λ_\mathrm{e-p}$ and $θ_{D}$. This result would be useful for the materials design of bcc superconducting alloys. We further propose that the Vickers microhardness offers an alternative means to evaluate $θ_{D}$ and may serve as a rapid screening metric for identifying bcc alloys with desired properties.

cond-mat.supr-con↗

arXiv2026

Anharmonic lattice dynamics study of phonon transport in layered and molecular-crystal indium iodides

Indium iodides, which adopt layered or molecular-crystal-like arrangements depending on composition, are expected to exhibit low lattice thermal conductivity because of their heavy constituent atoms and weak In-I bonding. In this work, we employed first-principles anharmonic lattice dynamics calculations to systematically investigate phonon transport in indium iodides from particle- and wave-like perspectives. The calculated lattice thermal conductivities of both materials remained below 1 W/m-K over a broad temperature range. Notably, the influence of wave-like phonon transport differed by composition: in InI3, the wave-like contribution became comparable to the particle-like Peierls contribution, whereas it remained negligible in InI. We also investigated the thermal transport properties of the experimentally reported high-pressure phase of InI3. Motivated by experimental indications of stacking faults and partial disorder in indium site occupancy within the rhombohedral phase, we constructed several ordered structural models with different stacking sequences. These stacking sequences exhibited no significant energetic preference and had similar lattice thermal conductivities, suggesting that in-plane thermal transport is largely governed by the vibrational properties of the In2I6 layers themselves rather than by the specific stacking sequence. These findings provide insight into phonon transport in layered and molecular-crystal systems with structural complexity and contribute to a broader understanding of thermal transport mechanisms in layered and molecular-crystal-like materials.

cond-mat.mtrl-sci↗

arXiv2026

Time-Temperature-Transformation (TTT) Diagrams to rationalize the nucleation and quenchability of metastable $α$-Li$_3$PS$_4$

$α$-Li$_3$PS$_4$ is a promising solid-state electrolyte with the highest ionic conductivity among its polymorphs. However, its formation presents a thermodynamic paradox: the $α$-phase is the equilibrium phase at high temperature and transforms to the stable $γ$-Li$_3$PS$_4$ polymorph when cooled to room temperature; however, $α$-Li$_3$PS$_4$ can be synthesized and quenched in a metastable state via rapid heating at relatively low temperatures. The origin of this synthesizability and anomalous stability has remained elusive. Here, we resolve this paradox by establishing a comprehensive time-temperature-transformation (TTT) diagram, constructed from a computational temperature-size phase diagram and experimental high-time-resolution isothermal measurements. Our density functional theory calculations reveal that at the nanoscale, the $α$-phase is stabilized by its low surface energy, which drastically lowers the nucleation barrier across a wide temperature range. This size-dependent stabilization is directly visualized using in-situ synchrotron X-ray diffraction and electron microscopy, capturing the rapid nucleation of nano-sized $α$-phase and its subsequent slow transformation. This work presents a generalizable framework that integrates thermodynamic and kinetic factors for understanding nucleation and phase transformation mechanisms, providing a rational strategy for the targeted synthesis of functional metastable materials.

cond-mat.mtrl-sci↗

arXiv2025

Thermal rectification in jointless Pb solid wire

Thermal rectification is observed in jointless Pb wires at temperatures near the superconducting transition of Pb under magnetic fields. Using different magnetic-field (H) response of temperature dependence of thermal conductivity (\k{appa}-T) under H parallel to J and H perpendicular to J where J is heat flow, we fabricated a jointless thermal diode. Thermal rectification is observed with the thermal rectification ratio (TRR) of 1.5 and the difference in \k{appa} of 330 W m-1 K-1 at T = 5.11 K under H = 400 Oe for a Pb wire with a 50%-bent (H perpendicular to J) and 50%-straight (H parallel to J) structure. The peak temperature of TRR can be tuned by the strength of applied magnetic field. By changing bent ratio to 40%-bent, a higher TRR exceeding 2 was observed. The Pb-jointless thermal diode will be a useful material for thermal management at cryogenic temperatures.

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

Uniaxial negative thermal expansion in a weak-itinerant-ferromagnetic phase of CoZr$_{2}$H$_{3.49}$

We discovered unique uniaxial negative thermal expansion (NTE) behavior for a weak-itinerant-ferromagnetic phase of CoZr$_{2}$H$_{3.49}$. CoZr$_{2}$ is known as a superconductor exhibiting uniaxial NTE along the $c$-axis, which is called anomalous thermal expansion (ATE). Additionally, CoZr$_{2}$ is also known as a well-absorbent of hydrogen, and hydrogen insertion raises weak-itinerant ferromagnetism instead of superconductivity. However, the influence of hydrogen insertion on ATE behavior in this system is still unclear. To investigate it, we performed powder synchrotron X-ray diffraction (SXRD) for CoZr$_{2}$H$_{3.49}$. Through Arrott plots analysis, we determined the Curie temperature ($T_{\mathrm{C}}$) to be 139 K, and the Rhodes-Wohlfarth ratio was estimated to be 3.49, which clearly exceeds 1, suggesting the itinerancy of emerging ferromagnetism. Temperature dependencies of lattice constants $a$ and $c$ were extracted from powder SXRD analyses, and we revealed that lattice constant $c$ exhibited NTE behavior below $T_{\mathrm{C}}$. The uniaxial NTE behavior along the $c$-axis can be understood by sharpening an antibonding Co3$dz^{2}$ partial density of states near the Fermi level, linked to the expansion of a one-dimensional Co-Co chain running parallel to the $c$-axis.

cond-mat.mtrl-sci↗

arXiv2025

Huge anisotropic magneto-thermal switching in high-purity polycrystalline compensated metals

Magneto-thermal transport is a promising physical property for thermal management applications. Magneto-thermal switching enables active control of heat flows, and a high switching ratio is desirable for improving performance. Here, we report on the observation of a huge magneto-thermal switching (MTS) effect in high-purity (5N) Pb polycrystalline wires, where magnetic fields perpendicular to the heat current direction are applied at low temperatures. At T = 3 K and B = 0.1 T, the measured thermal conductivity (\k{appa}) of the Pb wire is about 2500 W m-1 K-1 but is reduced to ~150 and ~5 W m-1 K-1 at B = 1 and 9 T, respectively. This strong suppression is attributed to magnetoresistance in compensated metals. Although the huge magnetoresistance has been studied in single crystals with field along the selected orbitals, our results demonstrate that a huge MTS can similarly be realized even in flexible polycrystalline wires. This finding highlights the practical potential of magneto-thermal control in low-temperature thermal management, including applications in space environments where temperatures are around 3 K.

cond-mat.mtrl-sci↗

arXiv2025

High critical current densities of body-centered cubic high-entropy alloy superconductors: recent research progress

High-entropy alloy (HEA) superconductors have garnered significant attention due to their unique characteristics, such as robust superconductivity under extremely high pressure and irradiation, the cocktail effect, and the enhancement of the upper critical field. A high critical current density is another noteworthy feature observed in HEAs. Several body-centered cubic (bcc) HEAs have exhibited critical current densities comparable to those of Nb-Ti superconducting alloys. Such HEAs hold potential for applications as multifunctional superconducting wires, a capability rarely achieved in conventional alloys. In this context, we review recent advancements in research on critical current densities in bcc HEA superconductors, including Ta$_{1/6}$Nb$_{2/6}$Hf$_{1/6}$Zr$_{1/6}$Ti$_{1/6}$, (TaNb)$_{0.7}$(HfZrTi)$_{0.5}$, NbScTiZr, and others. Comparative analyses among these HEAs reveal that both eutectic microstructures, which accompany lattice strain, and nanosized precipitates play pivotal roles in achieving elevated critical current densities across wide magnetic field ranges. Furthermore, we propose several future directions for research. These include elucidating the origin of lattice strain, exploring more fine eutectic microstructures, artificially introducing nanoscale pinning sites, improving the superconducting critical temperature, and investigating the mechanical properties of these materials.

cond-mat.supr-con↗

arXiv2025

Direct Observation of Vortices and Antivortices Generation in Phase-Separated Superconductor Sn-Pb Solder

Quantized vortices in type-II superconductors provide insights into the mechanisms of superconductivity. However, the generation of antivortices, characterized by magnetization antiparallel to the external magnetic field, remains less understood. In this study, we investigate Sn-Pb solder, a superconductor with phase-separated Sn and Pb phases, and report the observation of both vortices and antivortices. Scanning SQUID (superconducting quantum interference device) microscopy revealed the presence of both vortices and antivortices, while magneto-optical imaging demonstrated flux avalanches. Our results demonstrate that Sn in Sn-Pb solder behaves as a type-II superconductor when magnetic fluxes are trapped, despite bulk Sn being a type-I superconductor with a transition temperature (TcSn) of 3.7 K. Our findings suggest that the size effect and proximity effect with Pb contribute synergistically to induce type-II superconductivity in Sn Notably, vortices were observed at temperatures as high as 5 K, exceeding the bulk TcSn. Furthermore, the interplay between the type-I superconducting Pb phase and the type-II superconducting Sn phase results in the generation of antivortices, providing a mechanism to accommodate excess magnetic flux. This study shed light on new research on composites combining type-I and type-II superconductors.

cond-mat.supr-con↗

arXiv2024

Investigation of superconducting gap of high-entropy telluride AgInSnPbBiTe5

We performed transverse-field muon spin relaxation/rotation (TF-μSR) on a high-entropy-type (HE-type) superconductor AgInSnPbBiTe5. The emergence of bulk superconducting states was confirmed from magnetic susceptibility, specific heat, and μSR. The superconducting gap 2Δ(0) estimated from μSR was clearly larger than that expected from conventional weak-coupling phonon-mediated model, suggesting the strong-coupling nature of superconductivity. In addition, a long penetration depth of 3.21(7) μm was obtained. The strong-coupling nature of superconductivity and the long penetration depth are similar to the trends observed in the other HE-type superconductors (HE alloys and transition-metal zirconides), which may be universal feature of HE-type superconductors.

cond-mat.supr-con↗

arXiv2024

Nonvolatile magneto-thermal switching driven by vortex trapping in commercial In-Sn solder

Magneto-thermal switching (MTS) is a key technology for efficient thermal management. Recently, large MTS with nonvolatility has been observed in Sn-Pb solders [H. Arima et al., Commun. Mater. 5, 34 (2024)] where phase separation, different superconducting transition temperature (Tc) of Sn and Pb, and magnetic-flux trapping are the causes of the nonvolatile MTS. To further understand the mechanism and to obtain the strategy for enhancing switching ratio, exploration of new phase-separated superconductors with nonvolatile MTS is needed. Here, we show that the In52-Sn48 commercial solder is a phase-separated superconducting composite with two Tc and traps vortices after field cooling. A clear signature of nonvolatile MTS was observed at T = 2.5 K. From specific heat analyses, we conclude that the vortices are mainly trapped in the lower-Tc phase (γ-phase) after field cooling, which is the evidence that vortex trapping also works on achieving nonvolatile MTS in phase-separated superconducting composites.

cond-mat.supr-con↗