arXiv subjects

Takao Mori

Publications and source records attributed to Takao Mori.

At least 19 recordsLinked to original sources

arXiv2026

Observation of universal thermopolarization effect in insulators

Heat-to-charge conversion has traditionally been realized via the Seebeck effect in conductors and pyroelectricity in polar insulators. Here, we demonstrate that temperature gradients generate electrical polarization, namely thermopolarization, in a wide range of insulators through a thermomechanical pathway. We identify a mechanism where thermal expansion under a temperature gradient produces strain gradients that induce polarization via the flexoelectric effect. Using a device with an on-chip heater, we detect the heat-induced polarization in crystalline, polymeric, and amorphous systems, including MgO, Al$_2$O$_3$, MnO, mica, PET, PEN, polyimide, and soda-lime glass. The magnitude of the response exhibits a robust scaling with the coefficient of thermal expansion, which is reproduced by finite-element simulations. Furthermore, we identify two routes to enhance the response: reducing the sample thickness and exploiting structural instabilities such as glass and antiferromagnetic phase transitions, where more than an order-of-magnitude enhancement is observed. These results establish a symmetry-independent route for heat-to-charge conversion in insulators and provide a device-compatible platform for electrically probing lattice responses, with potential for enhancement in nanoscale systems such as two-dimensional materials.

cond-mat.mtrl-sci↗

arXiv2026

On-chip detection of anisotropic thermopolarization in quartz

Temperature gradients are widely used to drive and probe transport phenomena in solids, forming the basis of heat-to-charge conversion processes. In typical experiments, local heating is introduced to generate a temperature gradient, and the resulting electrical response is detected by separate electrodes. Such measurements usually regard heating purely as a source of thermal excitation. Here, we show that heating inherently generates mechanical stress through thermal expansion, which in turn produces measurable electrical signals via electromechanical coupling. Using quartz as a model piezoelectric system, we demonstrate that heat can be converted to electrical currents via thermally generated stress. The on-chip device used in our experiment enables us to probe the anisotropy of the piezoelectric tensor through the thermally generated current, exhibiting twofold and threefold responses for X-cut and Z-cut crystals, respectively. We further show that the response can be detected in both current and voltage modes. These results reveal a thermomechanical pathway for heat-to-charge conversion and establish a general platform for electrically probing thermomechanical responses in insulating materials.

cond-mat.mtrl-sci↗

arXiv2025

$P$-type Ru$_2$Ti$_{1-x}$Hf$_x$Si full-Heusler bulk thermoelectrics with $zT = 0.7$

Heusler compounds have emerged as important thermoelectric materials due to their combination of promising electronic transport properties, mechanical robustness and chemical stability -- key aspects for practical device integration. While a wide range of XYZ-type half-Heusler compounds have been studied for high-temperature applications, X$_2$YZ-type full-Heuslers, often characterized by narrower band gaps, may offer potential advantages at different temperature regimes but remain less explored. In this work, we report the discovery of $p$-type Ru$_2$Ti$_{1-x}$Hf$_x$Si full-Heusler thermoelectrics, exhibiting a high figure of merit $zT \sim 0.7$ over a broad range of temperatures $700-1000$ K. These results not only represent the largest values known to date among full-Heusler materials but confirm earlier theoretical predictions that $p$-type Ru$_2$TiSi systems would be superior to their $n$-type counterparts. Moreover, using a two-band model, we unveil electronic structure changes induced by the Hf substitution at the Ti site and outline strategies to further improve $zT$ up to $zT > 1$. Our findings highlight the untapped potential of new semiconducting full-Heusler phases and the crucial need for continued exploration of this rich materials class for thermoelectric applications.

cond-mat.mtrl-sci↗

arXiv2025

Pisarenko's Formula for the Thermopower

The thermopower $α$ (also known as the Seebeck coefficient) is one of the most fundamental material characteristics for understanding charge carrier transport in thermoelectric materials. Here, we revisit the Pisarenko formula for the thermopower, which was traditionally considered valid only for non-degenerate semiconductors. We demonstrate that regardless of the dominating scattering mechanism, the Pisarenko formula describes accurately enough the relationship between thermopower $α$ and charge carrier concentration $n$ beyond the non-degenerate limit. Moreover, the Pisarenko formula provides a simple thermopower-conductivity relation, $α= \pm \frac{k_{\mathrm{B}}}{e} (b - \ln σ)$, valid for materials with $α> 90$ $μ$V K$^{-1}$ when acoustic phonon scattering is predominant. This offers an alternative way to analyze electron transport when Hall measurements are difficult or inaccessible. Additionally, we show how the Pisarenko formula can be used to estimate the maximum power factor of a thermoelectric material from the weighted mobility of a single, not necessarily optimized, sample at any given temperature.

cond-mat.mtrl-sci↗

arXiv2025

SeeBand: A highly efficient, interactive tool for analyzing electronic transport data

Linking the fundamental physics of band structure and scattering theory with macroscopic features such as measurable bulk thermoelectric transport properties is indispensable to a thorough understanding of transport phenomena and ensures more targeted and efficient experimental research. Here, we introduce SeeBand, a highly efficient and interactive fitting tool based on Boltzmann transport theory. A fully integrated user interface and visualization tool enable real-time comparison and connection between the electronic band structure (EBS) and microscopic transport properties. It allows simultaneous analysis of data for the Seebeck coefficient $S$, resistivity $ρ$ and Hall coefficient $R_\text{H}$ to identify suitable EBS models and extract the underlying microscopic material parameters and additional information from the model. Crucially, the EBS can be obtained by directly fitting the temperature-dependent properties of a single sample, which goes beyond previous approaches that look into doping dependencies. Finally, the combination of neural-network-assisted initial guesses and an efficient subsequent fitting routine allows for a rapid processing of big datasets, facilitating high-throughput analyses to identify underlying, yet undiscovered dependencies, thereby guiding material design.

physics.data-an↗

arXiv2025

Energy filtering-induced ultrahigh thermoelectric power factors in Ni$_3$Ge

Traditional thermoelectric materials rely on low thermal conductivity to enhance their efficiency but suffer from inherently limited power factors. Novel pathways to optimize electronic transport are thus crucial. Here, we achieve ultrahigh power factors in Ni$_3$Ge through a new materials design principle. When overlapping flat and dispersive bands are engineered to the Fermi level, charge carriers can undergo intense interband scattering, yielding an energy filtering effect similar to what has long been predicted in certain nanostructured materials. Via a multi-step DFT-based screening method developed herein, we discover a new family of L1$_2$-ordered binary compounds with ultrahigh power factors up to 11 mW m$^{-1}$ K$^{-2}$ near room temperature, which are driven by an intrinsic phonon-mediated energy filtering mechanism. Our comprehensive experimental and theoretical study of these new intriguing materials paves the way for understanding and designing high-performance scattering-tuned metallic thermoelectrics.

cond-mat.mtrl-sci↗

arXiv2024

Thermoelectric transport in Ru$_2$TiSi full-Heusler compounds

Heusler compounds with six valence electrons per atom have attracted interest as thermoelectric materials owing to their semimetallic and semiconducting properties. Here, we theoretically and experimentally investigate electronic transport in Ru$_2$TiSi-based full-Heuslers. We show that electronic transport in this system can be well captured by a two-parabolic band model. The larger band gap of Ru$_2$TiSi promises a higher thermoelectric performance, compared to its isovalent family member Fe$_2$VAl, which has been studied as a thermoelectric material for over two decades. Additionally, we identify $p$-type Ru$_2$TiSi as far more efficient than previously studied $n$-type compounds and demonstrate that this can be traced back to much lighter and more mobile holes originating from dispersive valence bands. Our findings demonstrate that an exceptionally high dimensionless figure of merit $zT > 1$ can be realized in these $p$-type compounds around 700 K upon proper reduction of the lattice thermal conductivity, e.g., by substituting Zr or Hf for Ti.

cond-mat.mtrl-sci↗

arXiv2024

Defect engineering-induced Seebeck coefficient and carrier concentration decoupling in CuI by noble gas ion implantation

Copper(I) iodide, CuI, is the leading $p$-type non-toxic and earth-abundant semiconducting material for transparent electronics and thermoelectric generators. Defects play a crucial role in determining the carrier concentration, scattering process, and therefore thermoelectric performance of a material. A result of defect engineering, the power factor of thin film CuI was increased from $332\pm32$ μWm$^{-1}$K$^{-2}$ to $578\pm58$ μWm$^{-1}$K$^{-2}$ after implantation with noble gas ions (Ne, Ar, Xe). The increased power factor is due to a decoupling of the Seebeck coefficient and electrical conductivity identified through a changing scattering mechanism. Ion implantation causes the abundant production of Frenkel pairs, which were found to suppress compensating donors in CuI, and which scenario was also supported by density functional theory calculations. The compensating donor suppression led to a significantly improved Hall carrier concentration, increasing from $6.5\times10^{19}\pm0.1\times10^{19}$ cm$^{-3}$ to $11.5\times10^{19}\pm0.4\times10^{19}$ cm$^{-3}$. This work provides an important step forward in the development of CuI as a transparent conducting material for electronics and thermoelectric generators by introducing beneficial point defects with ion implantation.

cond-mat.mtrl-sci↗

arXiv2024

Decoupled charge and heat transport for high-performance Fe$_2$VAl composite thermoelectrics

Decoupling charge and heat transport is essential for optimizing thermoelectric materials. Strategies to inhibit lattice-driven heat transport, however, also compromise carrier mobility, limiting the performance of most thermoelectrics, including Fe$_2$VAl Heusler compounds. Here, we demonstrate an innovative approach, which bypasses this tradeoff: via liquid-phase sintering, we incorporate the archetypal topological insulator Bi$_{1-x}$Sb$_{x}$ between Fe$_2$V$_{0.95}$Ta$_{0.1}$Al$_{0.95}$ grains. Structural investigations alongside extensive thermoelectric and magneto-transport measurements reveal distinct modifications in the microstructure, and a reduced lattice thermal conductivity and enhanced carrier mobility are simultaneously found. This yields a huge performance boost $-$ far beyond the effective-medium limit $-$ and results in one of the highest figure of merits among both half- and full-Heusler compounds, $z\approx 1.6\times 10^{-3}\,$K$^{-1}$ ($zT\approx 0.5$) at 295 K. Our findings highlight the potential of secondary phases to decouple charge and heat transport and call for more advanced theoretical studies of multiphase composites.

cond-mat.mtrl-sci↗

arXiv2024

Enhanced Thermoelectric Performance of $p$-type BiSbTe Through Incorporation of Magnetic CrSb

There is evidence that magnetism can potentially increase the thermopower of materials, most likely due to magnon scattering, suggesting the incorporation of intrinsic magnetic semiconductors in non-magnetic thermoelectric materials. Here, samples of $\textit{p}$-type Bi$_{0.5}$Sb$_{1.5}$Te$_{3}$ with 10 at.% excess Te are ball-milled with varying ratio of the antiferromagnetic semiconductor CrSb (0, 0.125, 0.5, and 1 wt.%) to prepare bulk samples by spark plasma sintering technique. The thermopower of samples containing CrSb is increased due to an increase in the effective mass of the charge carriers, indicating that there is a drag effect originating from the magnetic particles. However, this was at the expense of reduced electrical conductivity caused by reduced charge carrier mobility. While overall only marginal improvements in power factors were observed, these samples exhibited significantly lower thermal conductivity compared to the single-phase material. As a result, a peak $\textit{zT}$ value of $\sim$1.4 was achieved at 325 K for the sample with 0.125 wt.% CrSb. These results highlight the potential of incorporating magnetic secondary phases to enhance the thermoelectric performance of materials.

cond-mat.mtrl-sci↗

arXiv2024

Iterative composition optimization in Fe$_2$VAl-based thin-film thermoelectrics using single-target sputtering

Magnetron sputtering inherently exhibits the advantage of dislodging particles from the target in a ratio equivalent to the target stoichiometry. Nevertheless, film compositions often deviate due to element-dependent scattering with the working gas, necessitating the adjustment of the sputtering process. In this work, we explore an unconventional approach of addressing this issue, involving the employment of an off-stoichiometric target. The required composition is obtained through an iterative process, which is demonstrated by Fe$_2$VAl and Fe$_2$V$_{0.9}$Ti$_{0.1}$Al films as case studies. Ultimately, the correct stoichiometry is obtained from Fe$_{1.86}$V$_{1.15}$Al$_{0.99}$ and Fe$_{1.88}$V$_{1.02}$Ti$_{0.13}$Al$_{0.97}$ targets, respectively. Despite the thermoelectric properties falling below expectations, mainly due to imperfect film crystallization, the strategy successfully achieved the desired stoichiometry, enabling accurate film synthesis without the need of advanced sputtering setups.

cond-mat.mtrl-sci↗

arXiv2024

High thermoelectric power factor through topological flat bands

Thermoelectric (TE) materials are useful for applications such as waste heat harvesting or efficient and targeted cooling. While various strategies towards superior thermoelectrics through a reduction of the lattice thermal conductivity have been developed, a path to enhance the power factor is pressing. Here, we report large power factors up to 5 mW m$^{-1}$ K$^{-2}$ at room temperature in the kagome metal Ni$_3$In$_{1-x}$Sn$_x$. This system is predicted to feature almost dispersionless flat bands in conjunction with highly dispersive Dirac-like bands in its electronic structure around the Fermi energy $E_\text{F}$ [L. Ye et al., Nature Physics 1-5 (2024)]. Within this study, we experimentally and theoretically showcase that tuning this flat band precisely below $E_\text{F}$ by chemical doping $x$ boosts the Seebeck coefficient and power factor, as highly mobile charge carriers scatter into the flat-band states. Our work demonstrates the prospect of engineering extremely flat and highly dispersive bands towards the Fermi energy in kagome metals and introduces topological flat bands as a novel tuning knob for thermoelectrics.

cond-mat.str-el↗

arXiv2024

Thermoelectric properties of marcasite-type compounds MSb$_2$ (M = Ta, Nb): A combined experimental and computational study

Here, we investigate the thermoelectric properties of the marcasite-type compounds MSb$_2$ (M = Ta, Nb) in the temperature range of 310-730 K. These compounds were synthesized by a solid-state reaction followed by the spark plasma sintering process. The Rietveld refinement method confirms the monoclinic phase with space group C2/m for both compounds. The observed values of Seebeck coefficients exhibit non-monotonic behaviour in the studied temperature range, with the maximum magnitude of -14.4 and -22.7 $μ$V K$^{-1}$ for TaSb$_2$ and NbSb$_2$, respectively at ~444 K. The negative sign of S in the full temperature window signifies the n-type behaviour of these compounds. Both electrical and thermal conductivities show an increasing trend with temperature. The experimentally observed thermoelectric properties are understood through the first-principles DFT and Boltzmann transport equation. A pseudogap in the density of states around the Fermi level characterizes the semimetallic behaviour of these compounds. The multi-band electron and hole pockets were found to be mainly responsible for the temperature dependence of transport properties. The experimental power factors are found to be ~0.09 and ~0.42 mW m$^{-1}$ K$^{-2}$ at 310 K for TaSb2 and NbSb2, respectively. From the DFT-based calculations, the maximum possible power factors for p-type conduction are predicted as ~1.14 and ~1.74 mW m$^{-1}$ K$^{-2}$, while these values are found to be ~1.16 and ~1.80 mW m$^{-1}$ K$^{-2}$ for n-type TaSb$_2$ and NbSb$_2$, respectively at 300 K with the corresponding doping concentrations. The present study suggests that the combined DFT and Boltzmann transport theory are found to be reasonably good at explaining the experimental transport properties, and moderate power factors are predicted.

cond-mat.mtrl-sci↗

arXiv2023

Evaluation of insulating behavior by Pb Substitution and Pressure-Induced Superconductivity in La2O2Bi3-xPb1+xS6

We report synthesis, characterization, and physical properties of layered bismuth-sulfide compounds La2O2Bi3-xPb1+xS6. We synthesized a new La2O2Bi3PbS6 compound, whose crystal structure is similar to those of the La2O2Bi3AgS6 superconductor or La2O2Bi2Pb2S6 (insulator) with a four-layer-type conducting layer. The crystal structure of La2O2Bi3PbS6 is characterized in a tetragonal P4/nmm space group, and the lattice parameters are a = 4.084(1) Å and c = 19.71(2) Å. The obtained La2O2Bi3PbS6 sample exhibits narrow gap semiconductor (metallic-like) transport behavior with an anomaly near the 160 K. A metallic-like to insulator transition has been observed through Pb substitution, with increasing x in La2O2Bi3-xPb1+xS6. Furthermore, we applied external pressure for x = 0 and observed pressure-induced superconductivity. The onset of superconductivity appeared at 0.93 GPa, and the highest transition temperature was 3.1 K at 2.21 GPa.

cond-mat.supr-con↗

arXiv2023

Planar-type silicon thermoelectric generator with phononic nanostructures for 100 μW energy harvesting

Energy harvesting is essential for the internet-of-things networks where a tremendous number of sensors require power. Thermoelectric generators (TEGs), especially those based on silicon (Si), are a promising source of clean and sustainable energy for these sensors. However, the reported performance of planar-type Si TEGs never exceeded power factors of 0.1 $μ Wcm^{-2} K^{-2}$ due to the poor thermoelectric performance of Si and the suboptimal design of the devices. Here, we report a planar-type Si TEG with a power factor of 1.3 $μ Wcm^{-2} K^{-2}$ around room temperature. The increase in thermoelectric performance of Si by nanostructuring based on the phonon-glass electron-crystal concept and optimized three-dimensional heat-guiding structures resulted in a significant power factor. In-field testing demonstrated that our Si TEG functions as a 100-$μW$-class harvester. This result is an essential step toward energy harvesting with a low-environmental load and cost-effective material with high throughput, a necessary condition for energy-autonomous sensor nodes for the trillion sensors universe.

physics.app-ph↗

arXiv2023

High thermoelectric performance in metallic NiAu alloys

Thermoelectric (TE) materials seamlessly convert thermal into electrical energy and vice versa, making them promising for applications such as power generation or cooling. Although historically the TE effect was first discovered in metals, state-of-the-art research mainly focuses on doped semiconductors with large figure of merit, $zT$, that determines the conversion efficiency of TE devices. While metallic alloys have superior functional properties, such as high ductility and mechanical strength, they have mostly been discarded from investigation in the past due to their small Seebeck effect. Here, we realize unprecedented TE performance in metals by tuning the energy-dependent electronic scattering. Based on our theoretical predictions, we identify binary NiAu alloys as promising candidate materials and experimentally discover colossal power factors up to 34 mWm$^{-1}$K$^{-2}$ (on average 30 mWm$^{-1}$K$^{-2}$ from 300 to 1100 K), which is more than twice larger than in any known bulk material above room temperature. This system reaches a $zT$ up to 0.5, setting a new world record value for metals. NiAu alloys are not only orders of magnitude more conductive than heavily doped semiconductors, but also have large Seebeck coefficients originating from an inherently different physical mechanism: within the Au s band conduction electrons are highly mobile while holes are scattered into more localized Ni d states, yielding a strongly energy-dependent carrier mobility. Our work challenges the common belief that good metals are bad thermoelectrics and presents an auspicious paradigm for achieving high TE performance in metallic alloys through engineering electron-hole selective s-d scattering.

cond-mat.mtrl-sci↗

arXiv2022

Strong phase coherence and vortex matter in a fractal system with proximity-induced superconductivity

The proximity effect in normal/superconductor heterostructures is an intriguing phenomenon in that the normal side takes on the properties of a superconductor with an induced gap. However, the structural and pinning properties of vortices inside the normal regions remain poorly understood. Here, we report structure and superconducting properties of a proximity-coupled Mg/MgO/MgB2 system with ~30 vol. % of superconducting MgB2, in which MgB2 nanograins are distributed in a fractal manner to form a proximity network with clean interfaces. Conductivity and magnetic measurements demonstrate that this proximity-coupled system acts as a fully phase coherent superconductor with isotropic pinning. Magneto-optical imaging also reveals a rather homogeneous flux density distribution with no apparent granularity. Furthermore, we observe quantized proximity vortices and their clustering behavior by scanning superconducting quantum interface device microscopy. These results show that in contrast to the case of conventional granular superconductors, the grain boundaries in the present sample carry high critical currents and have high vortex pinning efficiency, resulting in a robust phase coherent state irrespective of the low volume fraction of the MgB2 nanograins. This finding not only reveals the features of proximity-induced vortices, but also demonstrates an excellent phase-locked capability of the proximity-coupled fractal system.

cond-mat.supr-con↗

arXiv2022

Thermoelectric properties of Sm-doped BiCuSeO oxyselenides fabricated by two-step reactive sintering

Among layered oxygen-containing compounds, BiCuSeO is one of the most promising candidates for thermoelectric applications due to its intrinsically low thermal conductivity and good thermal stability. However, the rather poor electrical conductivity of pristine BiCuSeO hinders its potential. Further enhancement of the thermoelectric performance by single doping at Bi site is limited mainly due to dramatic decrease of carrier mobility. Thus, new strategies, such as dual doping or doping with variable-valence elements seem to be promising. Along with that, the development of a fast and scalable synthesis route is essential for the industrial-scale fabrication of thermoelectric materials. Hence, in this paper, Bi$_{1-x}$Sm$_{x}$CuSeO samples (0 $\leq$ $x$ $\leq$ 0.08) have been synthesized with a simple and scalable reactive sintering process. For comparison, Bi$_{1-x}$Sm$_{x}$CuSeO oxyselenides were also obtained by the conventional solid-state route. Our results highlight that, Sm for Bi substitution increases the electrical conductivity by 1.5 - 2 times and decreases the Seebeck coefficient by ~1.4 times at 873 K for both series. Overall, considering the increase of lattice thermal conductivity upon doping and not optimized power factor, the figure of merit $zT$ is reducing upon doping.

cond-mat.mtrl-sci↗