Thermoelectric Property in Orthorhombic-Domained SnSe Film.

Single-crystal SnSe exhibits extremely high thermoelectric properties, and fabrication of SnSe films is promising for practical application and basic research on properties. However, the high thermoelectric properties have not yet been reported in SnSe films and their thermoelectric properties and nanostructure have not yet been analyzed in detail. In the present study, a-axis-oriented epitaxial SnSe films were prepared to discuss the thermoelectric properties of the SnSe films. While the electrical conductivity of the films was orders of magnitude smaller than that in the single crystals at room temperature, surprisingly, the thermoelectric property (power factor) of the films was slightly higher than that in the single crystals at high temperatures (∼300 °C). The SnSe films contained orthorhombic domain boundaries with a spacing of several hundred nanometers. The orthorhombic domain boundaries caused carrier scattering and degraded the mobility of the films at room temperature, but their effect decreased with increasing temperature. Thus, the carrier scattering at domain boundaries results in characteristic temperature dependence of thermoelectric properties in the SnSe films. High thermoelectric properties at high temperatures were successfully achieved in the SnSe films in spite of the existence of domain boundaries, demonstrating the possibility of high-performance of SnSe thermoelectric films.

Structural Evolution Induced by Interfacial Lattice Mismatch in Self-Organized YBa2Cu3O7-δ Nanocomposite Film.

Intriguing properties of self-organized nanocomposites of perovskite oxides are usually derived from the complex interface of constituent material phases. A sophisticated control of such a system is required for a broad range of energy and device applications, which demand a comprehensive understanding of the interface at the atomic scale. Here, we visualized and theoretically modeled the highly elastically strained nanorod, the interface region with misfit dislocations and heterointerface distortion, and the matrix with strain-induced oxygen vacancies in the self-organized YBa2Cu3O7-δ nanocomposite films with Ba perovskite nanorods. Large misfit strain was elastically accommodated in the nanocomposites, but since the elastic strain was mainly accommodated by the nanorods, the concentration of strain-induced oxygen vacancies was small enough for the matrix to keep high critical temperature (>85 K). The interfacial bonding distorted the atomic structure of YBa2Cu3O7-δ, but the thickness of distortion was limited to a few unit cells (less than the coherence length) due to the electron screening. The effect of volume fraction on elastic strain and the electron screening are crucial for strong vortex pinning without significant degradation of both the elementary pinning force and critical temperature in the nanocomposites. Thus, we comprehensively clarified the self-organized nanocomposite structure for on-demand control of superconductivity and oxide functionality in the nanocomposite engineering of perovskite oxides.

Strain-controlled critical temperature in REBa2Cu3Oy-coated conductors.

Recently, we succeeded in detwinning REBa2Cu3O7 (RE123, RE = rare-earth elements)-coated conductors by annealing under an external uniaxial strain. Using the untwinned RE123 tapes, the uniaxial-strain dependencies of the critical temperature Tc along the a and b crystal axes were investigated over a wide strain region from compression to tension. We found that the strain dependencies of Tc for the a and b axes obey a power law but exhibit opposite slopes. In particular, the maximum value of Tc is obtained when the CuO2 plane becomes a square, and its lattice constant is close to 0.385 nm. It is suggested that a tetragonal structure with a ≈ 0.385 nm is the optimum condition for a high critical temperature in high-Tc cuprates.