We present broadband infrared ellipsometry measurements of the c-axis conductivity of underdoped RBa_{2}Cu_{3}O_{7-delta} (R=Y, Nd, and La) single crystals. Our data show that separate energy scales are underlying the redistributions of spectral weight due to the normal state pseudogap and the superconducting gap. Furthermore, they provide evidence that these gaps do not share the same electronic states and do not merge on the overdoped side. Accordingly, our data are suggestive of a two gap scenario with a pseudogap that is likely extrinsic with respect to superconductivity.

The air-stable phosphide, Ag6Ge10P12, was synthesized from its elements in gram amounts. As its structure is closely related to high-performance thermoelectric tetrahedrites (Ag6□Ge4Ge6P12 ≡ Cu6SSb4Cu6S12), we studied temperature dependent single-crystal X-ray diffraction experiments, quantum chemical calculations, and thermoelectric transport properties of spark plasma sintered and pristine, single crystalline samples, in order to give a comprehensive picture of its thermoelectric performance and its origin. The semiconducting character of this material is reflected in band structure calculations. Measurements of the thermal diffusivity exhibit a very low thermal conductivity, κ < 1 W m–1 K–1, which is close to a phonon glass-like behavior, and has its origin in a strong local bonding asymmetry, induced by strong bonding of the phosphorus–germanium (Ge4+) covalent framework and weak bonding of lone-pair electrons (Ge2+). This chemical bond hierarchy creates a pronounced anisotropic behavior of the silver atoms leading to low-frequency vibrations and thermal damping. Combining this with a moderate electrical resistivity (ρ ∼ 15 mΩ cm) and a high Seebeck coefficient (S ∼ 380 μV K–1) results in a remarkably high figure of merit (zT) of about 0.6 at 700 K. These results demonstrate that Ag6Ge10P12 is one of the best thermoelectric phosphides and a promising new platform for the future development of thermoelectrics.

A combination of spectroscopic probes was used to develop a detailed experimental description of the transport and magnetic properties of superlattices composed of the paramagnetic metal CaRuO 3 and the antiferromagnetic insulator CaMnO 3 . The charge-carrier density and Ru valence state in the superlattices are not significantly different from those of bulk CaRuO 3 . The small charge transfer across the interface implied by these observations confirms predictions derived from density-functional calculations. However, a ferromagnetic polarization due to canted Mn spins penetrates 3-4 unit cells into CaMnO 3 , far exceeding the corresponding predictions. The discrepancy may indicate the formation of magnetic polarons at the interface.

We have studied the temperature dependence of spectroscopic ellipsometry spectra of an electrically insulating, nearly stoichiometric YTiO 3 single crystal with ferromagnetic Curie temperature T C = 30 K. The optical response exhibits a weak but noticeable anisotropy. Using a classical dispersion analysis, we identify three low-energy optical bands at 2.0, 2.9, and 3.7 eV. Although the optical conductivity spectra are only weakly temperature dependent below 300 K, we are able to distinguish high-and low-temperature regimes with a distinct crossover point around 100 K. The low-temperature regime in the optical response coincides with the temperature range in which significant deviations from a Curie-Weiss mean-field behavior are observed in the magnetization. Using an analysis based on a simple superexchange model, the spectral weight rearrangement can be attributed to intersite d i 1 d j 1 → d i 2 d j 0 optical transitions. In particular, Kramers-Kronig consistent changes in optical spectra around 2.9 eV can be associated with the high-spin-state ͑ 3 T 1 ͒ optical transition. This indicates that other mechanisms, such as weakly dipole-allowed p-d transitions and/or exciton-polaron excitations, can contribute significantly to the optical band at 2 eV. The recorded optical spectral weight gain of the 2.9 eV optical band is significantly suppressed and anisotropic, which we associate with complex spin-orbitlattice phenomena near the ferromagnetic ordering temperature in YTiO 3 .

No abstract

The thermodynamic properties of the ferromagnetic perovskite YTiO3 are investigated by thermal expansion, magnetostriction, specific heat, and magnetization measurements. The low-temperature spin-wave contribution to the specific heat, as well as an Arrott plot of the magnetization in the vicinity of the Curie temperature TC ≃ 27 K, are consistent with a three-dimensional Heisenberg model of ferromagnetism. However, a magnetic contribution to the thermal expansion persists well above TC, which contrasts with typical three-dimensional Heisenberg ferromagnets, as shown by a comparison with the corresponding model system EuS. The pressure dependences of TC and of the spontaneous moment Ms are extracted using thermodynamic relationships. They indicate that ferromagnetism is strengthened by uniaxial pressures p a and is weakened by uniaxial pressures p b, c and hydrostatic pressure. Our results show that the distortion along the a-and b-axes is further increased by the magnetic transition, confirming that ferromagnetism is favored by a large GdFeO3-type distortion. The c-axis results however do not fit into this simple picture, which may be explained by an additional magnetoelastic effect, possibly related to a Jahn-Teller distortion.

Transition metal oxides exhibit a high potential for application in the field of electronic devices, energy storage, and energy conversion. The ability of building these types of materials by atomic layer-by-layer techniques provides a possibility to design novel systems with favored functionalities. In this study, by means of the atomic layer-by-layer oxide molecular beam epitaxy technique, we designed oxide heterostructures consisting of tetragonal KNiF-type insulating LaCuO (LCO) and perovskite-type conductive metallic LaNiO (LNO) layers with different thicknesses to assess the heterostructure-thermoelectric property-relationship at high temperatures. We observed that the transport properties depend on the constituent layer thickness, interface intermixing, and oxygen-exchange dynamics in the LCO layers, which occurs at high temperatures. As the thickness of the individual layers was reduced, the electrical conductivity decreased and the sign of the Seebeck coefficient changed, revealing the contribution of the individual layers where possible interfacial contributions cannot be ruled out. High-resolution scanning transmission electron microscopy investigations showed that a substitutional solid solution of La(CuNi)O was formed when the thickness of the constituent layers was decreased.

Transition metal oxides are promising candidates for thermoelectric applications, because they are stable at high temperature and because strong electronic correlations can generate large Seebeck coefficients, but their thermoelectric power factors are limited by the low electrical conductivity. We report transport measurements on Ca 3 Co 4 O 9 films on various perovskite substrates and show that reversible incorporation of oxygen into SrTiO 3 and LaAlO 3 substrates activates a parallel conduction channel for p-type carriers, greatly enhancing the thermoelectric performance of the film-substrate system at temperatures above 450 °C. Thin-film structures that take advantage of both electronic correlations and the high oxygen mobility of transition metal oxides thus open up new perspectives for thermopower generation at high temperature.