arXiv subjects

Tsunehiro Takeuchi

Publications and source records attributed to Tsunehiro Takeuchi.

At least 19 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

Unveiling the Superconducting Ground State of Heusler alloy Pd2ZrIn via muon spin relaxation and rotation measurement

Full Heusler alloys XInPd2 (X= Zr, Hf and Ti) have recently attracted significant attention owing to their symmetry-driven electronic structure and also due to the interplay between disorder and emergent ground states. Within this family, Pd2ZrIn serves as a unique platform to study the effect of disorder on superconducting pairing. This alloy crystallizes in a cubic L21 structure with significant B2-type antisite disorder. Electrical resistivity and magnetic susceptibility studies confirm bulk type-II superconductivity with a transition temperature TC ~ 2.2 K. Zero-field {\mu}SR results reveal no evidence of spontaneous internal magnetic fields below TC, confirming the preservation of time-reversal symmetry. Transverse-field {\mu}SR spectra show the formation of a vortex lattice, consistent with type-II superconductivity, and the superfluid density is well described by a fully gapped, nodeless s-wave state with superconducting gap {\Delta} (0) ~ 0.33 \pm 0.01 meV. Furthermore, the estimated ratio of transition temperature and Fermi temperature (TC/TF) indicates that this alloy lies within the conventional superconducting regime on the Uemura plot. These results establish Pd2ZrIn as a weakly coupled, dirty-limit, type-II superconductor; with a fully gapped, nodeless order parameter and preserved time-reversal symmetry.

cond-mat.supr-con↗

arXiv2025

Anomalous Nernst effect in amorphous Tb-Fe-Co thin films

We conducted a comprehensive study on the compositional dependence of the anomalous Nernst effect (ANE) in amorphous (amo.) Tb-Fe-Co thin films. The anomalous Nernst coefficient strongly depends not only on the Tb composition but also on the transition metal composition, reaching a maximum of 1.8 uV/K for amo. Tb11.0(Fe50.0Co50.0) 89.0. By evaluating the electrical and thermoelectric properties, it was clarified that this maximum is achieved by the superposition of two large contributions: S_1 arising from direct transverse electron conduction due to a temperature gradient, and S_2 resulting from the combined Seebeck and anomalous Hall effects. We discovered that the anomalous Nernst conductivity, which is attributed to Berry curvature, varied significantly with the transition metal, even in an amorphous material lacking long-range crystalline order. Our research indicates that it is possible to control the electronic states that influence thermoelectric properties, even in the amorphous state.

cond-mat.mtrl-sci↗

arXiv2025

Evidence for energy-dependent scattering dominating thermoelectricity in heavy fermion systems

In the field of thermoelectric materials and devices, improving energy conversion efficiency remains a long-standing challenge. As a promising approach to address this issue, utilizing energy-dependent electron-scattering beyond the ordinary constant relaxation time approximation (CRTA) has been proposed. However, direct experimental evidence for an energy-dependent scattering reflected in the Seebeck coefficient is still lacking. Here we demonstrate using angle-resolved photoemission spectroscopy that the relaxation time of heavy fermion quasiparticles is highly dependent on the energy near the Fermi level. The observed energy dependence of the relaxation time is due to the coherent Kondo scattering, describing the sign of the Seebeck coefficient reasonably well, which cannot be deduced from CRTA. Our findings provide not only deeper insight into the understanding of thermoelectricity in correlated materials, but also future perspectives on possible orbital-selective engineering of thermoelectric materials.

cond-mat.str-el↗

arXiv2024

Coupling of magnetism and transport properties to the lattice degrees of freedom in NdBaCo$_2$O$_{5+{\delta}}$ ($\delta \sim 0.65$)

We have studied the origin of zero volume expansion below the Curie temperature (Tc), variable range hopping (VRH) behaviour using structural, magnetic, transport and thermal studies on the oxygen deficient double perovskite NdBaCo$_2$O$_{5+{\delta}}$ ($\delta \sim 0.65$). The valence state of Co ions and the possible properties exhibited by such compound were studied using electronic structure calculations for $\delta = 0.75$. Careful investigation of structure shows that the compound stabilizes in tetragonal structure (P4/mmm) having $2a_p \times 2a_p \times 2a_p$ (222) superstructure, where $a_p$ is the cubic perovskite lattice parameter. The compound exhibits a minimum in resistivity, ferromagnetic and ferrimagnetic transitions around 375 K, 120 K ($T_c$) and 60 K, respectively with signature of Griffiths phase above $T_c$. Our detailed structural analysis suggests signature of the onset of the above magnetic transitions at temperatures well above its stabilisation at long range level thereby leading to VRH behaviour. The observed zero thermal expansion in volume below Tc appears to be due to competing magnetic interactions within and between the magnetic sublattices. Our electronic structure calculations show (a) the importance of electron-electron correlation in Nd $4f$ and Co $3d$ states (b) Co ions stabilize in intermediate spin (IS) state, having oxidation state less than +3 (c) half metallicity. Our results show the possibility of coupling between magnetism and ferroelectricity. We believe that our results especially on the valence state of the Co ion, zero thermal expansion in volume, short range magnetic orderings and the connection between different degrees of freedom will be helpful in clearing the ambiguities existing in literature on the nature of magnetism and thereby aiding in designing new functionalities.

cond-mat.str-el↗

arXiv2024

Significant improvement in sensitivity of an anomalous Nernst heat flux sensor by composite structure

Heat flux sensors (HFS) have attracted significant interest for their potential in managing waste heat efficiently. A recently proposed HFS, that works on the basis of the anomalous Nernst effect (ANE), offers several advantages in its simple structure leading to easy fabrication, low cost, and reduced thermal resistance. However, enhancing sensitivity through traditional material selection is now challenging due to a small number of materials satisfying the required coexistence of a large transverse Seebeck coefficient and low thermal conductivity. In this study, by utilizing composite structures and optimizing the device geometry, we have achieved a substantial improvement in the sensitivity of an ANE-based HFS. We developed composite structures comprised of a plastic substrate with an uneven surface and three-dimensional (3D) uneven TbCo films, fabricated using nanoimprint techniques and sputtering. This approach resulted in a sensitivity that is approximately four times greater than that observed in previous studies. Importantly, this method is independent of the material properties and can significantly enhance the sensitivity. Our findings could lead to the development of highly sensitive HFS devices and open new avenues for the fabrication of 3D devices.

cond-mat.mes-hall↗

arXiv2024

Understanding the anomalous thermoelectric behaviour of Fe-V-W-Al based thin films

We have investigated the thermoelectric and thermal behaviour of Fe-V-W-Al based thin films prepared using radio frequency magnetron sputtering technique at different base pressures (0.1 ~ 1.0 X 10-2 Pa) and on different substrates (n, p and undoped Si). Interestingly, at lower base pressure, formation of bcc type of Heusler structure was observed in deposited samples, whereas at higher base pressure, we have noted the development of non-Heusler amorphous structure in these samples. Our findings indicates that the moderately oxidized Fe-V-W-Al amorphous thin film deposited on n-Si substrate, possesses large magnitude of absoulte S ~ 1098 microvolt per kelvin near room temperature, which is almost the double the previously reported value for thin films. Additionally, the power factor indicated enormously large values ~ 33.9 milliwatt per meter per kelvin sqaure near 320 K. The thermal conductivity of the amorphous thin film is also found to be 2.75 watt per meter per kelvin, which is quite lower compared to bulk alloys. As a result, the maximum figure of merit is estimated to be extremely high i.e. ~ 3.9 near 320 K, which is among one of the highest reported values so far. The anomalously large value of Seebeck coefficient and power factor has been ascribed to formation of amorphous structure and composite effect of thin film and substrate.

cond-mat.mtrl-sci↗

arXiv2022

Lattice effects on the physical properties of half doped perovskite ruthenates

We investigate the unusual phase transitions in SrRuO$_{3}$ and Sr$_{0.5}$Ca$_{0.5}$Ru$_{1-x}$Cr$_{x}$O$_{3}$ (x=0,0.05 and 0.1) employing x-ray diffraction, resistivity, magnetic studies and x-ray photoemission spectroscopy. Our results show the compounds undergo crossover from $itinerant$ ferromagnetism to $localised$ ferromagnetism. The combined studies suggests Ru and Cr to be in 4+ valence state. A Griffith phase and an enhancement in Curie temperature (Tc) from 38 K to 107 K is observed with Cr doping. A shift in the chemical potential towards the valence band is observed with Cr doping. In the metallic samples, interestingly, a direct link between the resistivity and orthorhombic strain is observed. Detailed studies in this direction will be helpful to understand the nature of interactions and hence manoeuvre its properties. In the non metallic samples, the resistivity is mainly governed by disorder and electron-electron correlation effects. The value of the resistivity for the 5% Cr doped sample suggests semi metallic behaviour. Understanding its nature in detail using electron spectroscopic techniques could unravel the possibility of its utility in high mobility transistors at room temperature and its combined property with ferromagnetism will be helpful in making spintronic devices.

cond-mat.str-el↗

arXiv2022

Strange metal dynamics across the phase diagram of Bi$_{2}$Sr$_{2}$CuO$_{6+\delta}$ cuprates

Unlocking the mystery of the strange metal state has become the focal point of high T$_{c}$ research, not because of its importance for superconductivity, but because it appears to represent a truly novel phase of matter dubbed `quantum supreme matter'. Detected originally through high magnetic field, transport experiments, signatures of this phase have now been uncovered with a variety of probes. Our high resolution optical data of the low T$_{c}$ cuprate superconductor, Bi$_{2-x}$Pb$_{x}$Sr$_{2-y}$La$_{y}$CuO$_{6+\delta}$ allows us to probe this phase over a large energy and temperature window. We demonstrate that the optical signatures of the strange metal phase persist throughout the phase diagram. The strange metal signatures in the optical conductivity are two-fold, (i): a low energy Drude response with Drude width on the order of temperature and (ii): a high energy conformal tail with doping dependent power-law exponent. While the Drude weight evolves monotonously throughout the entire doping range studied, the spectral weight contained in the high energy conformal tail appears to be doping and temperature independent. Our analysis further shows that the temperature dependence of the optical conductivity is completely determined by the Drude parameters. Our results indicate that there is no critical doping level inside the superconducting dome where the carrier density starts to change drastically and that the previously observed 'return to normalcy' is a consequence of the increasing importance of the Drude component relative to the conformal tail with doping. Importantly, both the doping and temperature dependence of the resistivity are largely determined by the Drude width.

cond-mat.str-el↗

arXiv2021

Tuning the structural, electronic and magneto-transport properties of spin-orbit Mott insulator Sr2IrO4

We investigate the tunability of the structural, electronic and magneto transport properties of polycrystalline Sr2IrO4 sample. The extent of bifurcation of the magnetisation curves during the field cooled and zero field cooled cycles establishes that the magnetic anisotropy in the as-prepared sample is more as compared to the vacuum annealed one. Based on the behaviours of the structural parameters and the magnetic studies, our results show that the canted AFM structure is stabilised in a larger temperature range in the case of the annealed sample as compared to the as prepared one. At low temperatures, for both the samples, a phase, possibly of glassy nature competes with the canted AFM phase. The temperature extent to which both these phases co-exist for the as-prepared and the annealed one is around 115K an 70K, respectively. The transport studies reveal that for both the samples, in the high temperature region of study, the conduction mechanism is governed by the Arrhenius model. In the intermediate temperature range, variable range hopping(VRH) and Arrhenius models govern the transport in the as-prepared and the annealed one, respectively. At low temperatures, the conduction mechanism occurs through Efros-Shklovskii-VRH and VRH mechanisms for the as-prepared and the annealed samples, respectively. The magneto resistance measurements indicate higher negative magneto resistance in the annealed sample at all temperatures. The field dependence of magneto resistance at 10K suggests a co-existing glassy magnetic phase along with the canted antiferromagnetic structure in the as prepared sample and a suppression of this glassy magnetic phase in the annealed sample. The combined analyses of all the results highlight the role of disorder for the magnetic and trasnport properties of this compound.

cond-mat.str-el↗

arXiv2021

Optical phonon modes assisted thermal conductivity in p-type ZrIrSb Half-Heusler alloy: A combined experimental and computational study

Half Heusler (HH) alloys with 18 valence electron count have attracted significant interest in the area of research related to thermoelectrics. Understanding the novel transport properties exhibited by these systems with semiconducting ground state is an important focus area in this field. Large thermal conductivity shown by most of the HH alloy possesses a major hurdle in improving the figure of merit (ZT). Additionally, understanding the mechanism of thermal conduction in heavy constituents HH alloys is an interesting aspect. Here, we have investigated the high temperature thermoelectric properties of ZrIrSb through experimental studies, phonon dispersion and electronic band structure calculations. ZrIrSb is found to exhibit substantially lower magnitude of resistivity and Seebeck coefficient near room temperature, owing to existence of anti-site disorder between Ir/Sb and vacant sites. Interestingly, in ZrIrSb, lattice thermal conductivity is governed by coupling between the acoustic and low frequency optical phonon modes, which originates due to heavier Ir/Sb atoms. This coupling leads to an enhancement in the Umklapp processes due to the optical phonon excitations near zone boundary, resulting in a lower magnitude of \k{appa}L. Our studies point to the fact that the simultaneous existence of two heavy mass elements within a simple unit cell can substantially decrease the lattice degrees of freedom.

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 {\mu}VK-1 together with a small electrical resistivity of 4.49 m{\Omega}cm, 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

Unravelling the phonon scattering mechanism in Half-Heusler alloys ZrCo1-xIrxSb (x = 0, 0.1, and 0.25)

Insight about the scattering mechanisms responsible for reduction in the lattice thermal conductivity (\k{appa}L) in Half-Heusler alloys (HHA) is imperative. In this context, we have thoroughly investigated the temperature response of thermal conductivity of ZrCo1-xIrxSb (x = 0, 0.1 and 0.25). For ZrCoSb, \k{appa}L is found to be ~15.13 W/m-K at 300 K, which is drastically reduced to ~4.37 W/m-K in ZrCo0.9Ir0.1Sb. This observed reduction is ascribed to softening of acoustic phonon modes and point defect scattering, on substitution of heavier mass. However, no further reduction in \k{appa}L is observed in ZrCo0.75Ir0.25Sb, because of identical scattering parameter. This has been elucidated based on the Klemens Callaway model. Also, in the parent alloy, phonon-phonon scattering mechanism plays a significant role in heat conduction process, whereas in Ir substituted alloys, point defect scattering (below 500 K) and phonon-phonon scattering (above 750 K) are the dominant scattering mechanisms. The minimum \k{appa}L is found to be ~1.73 W/m-K (at 950 K) in ZrCo0.9Ir0.1Sb, which is the lowest reported value till now, for n-type Zr based HHA. Our studies indicate that partial substitution of heavier mass element Ir at Co-site effectively reduces the \k{appa}L of n-type ZrCoSb, without modifying the nature of charge carriers.

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↗

arXiv2020

Anomalous dependence of thermoelectric parameters on carrier concentration and electronic structure in Mn-substituted Fe2CrAl Heusler alloy

We investigate the high temperature thermoelectric properties of Heusler alloys Fe2-xMnxCrAl (0<x<1). Substitution of 12.5% Mn at Fe-site (x = 0.25) causes a significant increase in high temperature resistivity (\r{ho}) and an enhancement in the Seebeck coefficient (S), as compared to the parent alloy. However, as the concentration of Mn is increased above 0.25, a systematic decrement in the magnitude of both parameters is noted. These observations have been ascribed (from theoretical analysis) to a change in band gap and electronic structure of Fe2CrAl with Mn-substitution. Due to absence of mass fluctuations and lattice strain, no significant change in thermal conductivity is seen across this series of Heusler alloys. Additionally, S drastically changes its magnitude along with a crossover from negative to positive above 900 K, which has been ascribed to the dominance of holes over electrons in high temperature regime. In this series of alloys, S and \r{ho} shows a strong dependence on the carrier concentration and strength of d-d hybridization between Fe/Mn and Cr atoms.

cond-mat.mtrl-sci↗

arXiv2019

Distinctive Thermoelectric Properties of Supersaturated Si-Ge-P Compounds: Achieving Figure of Merit ZT > 3.6

The efficiency of energy conversion in thermoelectric generators (TEGs) is directly proportional to electrical conductivity and Seebeck coefficient while inversely to thermal conductivity. The challenge is to optimize these interdependent parameters simultaneously. In this work, the problem is addressed with a novel approach of nanostructuring and constructive electronic structure modification to achieve a very high value of dimensionless figure of merit ZT greater than 3.6 at 1000 K with negative Seebeck coefficient. Supersaturated solid-solutions of Si-Ge containing 1 atomic percent Fe and 10 atomic percent P are prepared by high-energy ball milling. The bulk samples consisting of ultra-fine nano-crystallites 9.7 nm are obtained by the sophisticated low-temperature & high-pressure sintering process. Despite that the electrical resistivity is slightly high due to the localization of electrons is associated with the highly disordered structure and low electrical density of states near the chemical potential, a very low thermal conductivity \k{appa} less than 1 W m-1K-1 and very large magnitude of Seebeck coefficient exceeding 470 uV K-1 are achieved in association with the nanostructuring and the Fe 3d impurity states, respectively, to realize a very large magnitude of ZT.

cond-mat.mtrl-sci↗

arXiv2019

Reduced Hall carrier density in the overdoped strange metal regime of cuprate superconductors

Efforts to understand the microscopic origin of superconductivity in the cuprates are dependent on knowledge of the normal state. The Hall number in the low temperature, high field limit $n_{\rm H}(0)$ has a particular significance because within conventional transport theory it is simply related to the number of charge carriers, and so its evolution with doping gives crucial information about the nature of the charge transport. Here we report a study of the high field Hall coefficient of the single-layer cuprates Tl$_2$Ba$_2$CuO$_{6+\delta}$ (Tl2201) and (Pb/La) doped Bi$_2$Sr$_2$CuO$_{6+\delta}$ (Bi2201) which shows how $n_{\rm H}(0)$ evolves in the overdoped, so-called strange metal, regime of cuprates. We find that $n_{\rm H}(0)$ increases smoothly from $p$ to $1+p$, where $p$ is the number of holes doped into the parent insulating state, over a wide range of doping. The evolution of $n_{\rm H}$ correlates with the emergence of the anomalous linear-in-$T$ term in the low-$T$ in-plane resistivity. The results could suggest that quasiparticle decoherence extends to dopings well beyond the pseudogap regime.

cond-mat.supr-con↗

arXiv2015

Point nodes persisting far beyond Tc in Bi2212

In contrast to a complex feature of antinodal state, suffering from competing order(s), the "pure" pairing gap of cuprates is obtained in the nodal region, which therefore holds the key to the superconducting mechanism. One of the biggest question is whether the point nodal state as a hallmark of d-wave pairing collapses at Tc like the BCS-type superconductors, or it instead survives above Tc turning into the preformed pair state. A difficulty in this issue comes from the small magnitude of the nodal gap, which has been preventing experimentalists from solving it. Here we use a laser ARPES capable of ultrahigh energy resolution, and detect the point nodes surviving far beyond Tc in Bi2212. By tracking the temperature evolution of spectra, we reveal that the superconductivity occurs when the pair breaking rate is suppressed smaller than the single particle scattering rate on cooling, which governs the value of Tc in cuprates.

cond-mat.supr-con↗