Tsunehiro Takeuchi

An Oxide Single Crystal with High Thermoelectric Performance in Air.

An oxide single-crystalline whisker with high thermoelectric properties at temperatures (T) higher than 600 K in air has been discovered. This whisker is assigned to Ca2Co2O5 phase (abbreviated to Co-225 whiskers) and has a layered structure in which Co–O layers of two different kinds alternate in the direction of the c-axis. Seebeck coefficient of the whiskers is higher than 100 µVK-1 at 100 K and increases with temperature up to 210 µVK-1. Temperature dependence of electric resistivity shows a semiconducting-like behavior. These results indicate that the electric carriers are transported via hopping conduction. Using thermal conductivity of a Co-225 polycrystalline sample, figure of merit (ZT) of the Co-225 whiskers is estimated 1.2–2.7 at T≥873 K. This compound is characterized with regard to low mobility and high density of carriers, which contradicts the conventional materials with high thermoelectric properties.

Improvement of thermoelectric properties for half-Heusler TiNiSn by interstitial Ni defects

We have synthesized off-stoichiometric Ti-Ni-Sn half-Heusler thermoelectrics in order to investigate the relation between randomly distributed defects and thermoelectric properties. A small change in the composition of Ti-Ni-Sn causes a remarkable change in the thermal conductivity. An excess content of Ni realizes a low thermal conductivity of 2.93 W/mK at room temperature while keeping a high power factor. The low thermal conductivity originates in the defects generated by an excess content of Ni. To investigate the detailed defect structure, we have performed first-principles calculations and compared with x ray photoemission spectroscopy measurement. Based on these analyses, we conclude that the excess Ni atoms randomly occupy the vacant sites in the half-Heusler structure, which play as phonon scattering centers, resulting in significant improvement of the figure of merit without any substitutions of expensive heavy elements, such as Zr and Hf.

Fermi Surface and Pseudogap Evolution in a Cuprate Superconductor

Under the Dome The superconducting transition temperature Tc of copper oxides has a dome-shaped dependence on chemical doping. Whether there is a quantum critical point (QCP) beneath the dome, and whether it is related to the enigmatic pseudogap, has been heavily debated. Two papers address this question in two different families of Bi-based cuprates. In (Bi,Pb)2(Sr,La)2CuO6+δ, He et al. (p. 608) found that the Fermi surface (FS) undergoes a topological change as doping is increased, which points to the existence of a QCP at a doping close to the maximum in Tc, seemingly uncorrelated with the pseudogap. Fujita et al. (p. 612) studied a range of dopings in Bi2Sr2CaCu2O8+δ to find an FS reconstruction simultaneous with the disappearance of both rotational and translational symmetry breaking, the latter of which has been associated with the pseudogap. These findings point to a concealed QCP. Scanning tunneling microscopy is used to provide evidence for a quantum critical point beneath the superconducting dome. The unclear relationship between cuprate superconductivity and the pseudogap state remains an impediment to understanding the high transition temperature (Tc) superconducting mechanism. Here, we used magnetic field–dependent scanning tunneling microscopy to provide phase-sensitive proof that d-wave superconductivity coexists with the pseudogap on the antinodal Fermi surface of an overdoped cuprate. Furthermore, by tracking the hole-doping (p) dependence of the quasi-particle interference pattern within a single bismuth-based cuprate family, we observed a Fermi surface reconstruction slightly below optimal doping, indicating a zero-field quantum phase transition in notable proximity to the maximum superconducting Tc. Surprisingly, this major reorganization of the system’s underlying electronic structure has no effect on the smoothly evolving pseudogap.

Thermoelectric properties of tungsten-substituted Heusler Fe2VAl alloy

A Heusler Fe2V1-xWxAl sintered alloy was synthesized to evaluate the effect of W substitution on thermoelectric properties of the Heusler alloy. The Seebeck coefficient and the electrical conductivity are simultaneously enhanced through electron injection resulting from the W substitution. Comparison with the Si-substituted Fe2VAl alloy reveals that the additional electronic states derived from W 5d orbital in the vicinity of pseudogap are likely to degrade the Seebeck coefficient. Thermal conductivity is effectively reduced by the W substitution because of the large atomic mass and volume of W compared to the constituent elements of Fe2VAl alloy. The appreciable reduction of thermal conductivity, without a serious deterioration in electrical conduction, enhances the thermoelectric figure of merit in the Heusler alloy.

Point nodes persisting far beyond Tc in Bi2212

In contrast to a complex feature of antinodal state, suffering from competing orders, the 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.

Thermoelectric properties of supersaturated Re solid solution of higher manganese silicides

In this study, we developed a higher manganese silicide (HMS) that possesses a high dimensionless figure of merit ZT exceeding unity. HMSs containing a larger amount of Re than its solubility limit were prepared by the liquid quenching technique, and the obtained metastable HMSs showed good thermal stability to enable pulse current sintering at 1240 K. The lattice thermal conductivity was effectively reduced with increasing Re concentration, whereas the electron transport properties were not greatly affected. Consequently, the ZT of p-type HMS increased to 1.04 at 6 at. % Re from 0.4 of the Re-free sample.

Thermoelectric properties of Al-(Mn,X)-Si C54-phase (X = Ru and Re)

The lattice thermal conductivity of the Al-Mn-Si C54-phase, which possesses a large magnitude of the Seebeck coefficient and metallic electrical conduction, was greatly reduced by partial substitution of the heavy elements Ru and Re for Mn; these were selected by first principles calculations so as not to produce impurity states near the chemical potential. The heavy element substituted Al-Mn-Si-based C54-phase showed a drastic increase in the dimensionless of figure of merit ZT due to the reduction of the lattice thermal conductivity together with the unchanged electrical properties. The maximum ZT value of the p-type sample and the n-type sample reached 0.12 at 520 K and 0.38 at 540 K, respectively. These values are about five times larger than that of the heavy element-free Al-Mn-Si C54-phase.

Formation of Gapless Fermi Arcs and Fingerprints of Order in the Pseudogap State of Cuprate Superconductors

We use angle-resolved photoemission spectroscopy and a new quantitative approach based on the partial density of states to study properties of seemingly disconnected portions of the Fermi surface (FS) that are present in the pseudogap state of cuprates called Fermi arcs. We find that the normal state FS collapses very abruptly into Fermi arcs at the pseudogap temperature (T*). Surprisingly, the length of the Fermi arcs remains constant over an extended temperature range between T* and T(pair), consistent with the presence of an ordered state below T*. These arcs collapse again at the temperature below which pair formation occurs (T(pair)) either to a point or a very short arc, whose length is limited by our experimental resolution. The tips of the arcs span between points defining a set of wave vectors in momentum space, which are the fingerprints of the ordered state that causes the pseudogap.

Atomic structure determination, electronic structure calculations and interpretation of electron transport properties of various 1/1-1/1-1/1 approximants

The atomic structure of various 1/1– 1/1– 1/1 approximants has been extensively studied in the last decade to bridge the gap to that of the corresponding quasicrystal. Reliable information about electronic states in their valence bands has also been accumulated by using the atomic structure determined theoretically and experimentally. In this article, we have classified 1/1– 1/1– 1/1 approximants into three groups in terms of the structure type (P or F) and space group (Im or P m) and discussed characteristic features of both atomic and electronic structures of 1/1– 1/1– 1/1 approximants in each group. The electrical resistivity behaviour is then discussed on the basis of the atomic and electronic structures thus determined.

Thermoelectric properties of Al?Mn?Si C40 phase containing small amount of W or Ta

The lattice thermal conductivity of the Al–Mn–Si C40 phase was effectively reduced by substituting a small amount of W for Mn while maintaining a large Seebeck coefficient |S| > 100 µV/K and a low electrical resistivity ρ < 1.7 mΩ cm. The Al–Mn–Si-based C40 phase containing a small amount of W showed essentially the same behaviors of both Seebeck coefficient and electrical resistivity as those of a W-free sample at the same carrier concentration. This experimental result was consistent with the first principles calculations that predicted the absence of impurity states in the energy gap in association with the W atom at the Mn site. The lattice thermal conductivity, on the other hand, markedly decreased owing to the heavy element impurity scattering effect of phonons. Consequently, the ZT of the n-type Al27.5Mn29.0W3.0Fe1.0Si39.5 C40 phase increased to a value that was ~4 times larger than that of the W-free sample.