Thermoelectric properties of layered oxyselenides La1–xSrxCuOSe (x = 0 to 0.2)

Thermoelectric properties of layered oxyselenides La1–xSrxCuOSe (x = 0.00 to 0.20) were investigated to evaluate the potential as thermoelectric material. Temperature dependence of the electrical conductivity and Seebeck coefficient measured in a temperature range of 373 to 673 K indicated that nondoped LaCuOSe was a p-type degenerate semiconductor due to Cu vacancies, while Sr-doped materials with x = 0.05 to 0.20 were p-type metals. The electrical conductivity increased and Seebeck coefficient decreased with increasing Sr concentration up to x = 0.10 in La1–xSrxCuOSe, suggesting that the effective hole carriers increase with increasing Sr content up to x = 0.10. Thermoelectric power factors were drastically enhanced by the Sr doping, and the value reached 1.0–1.4×10–4 W m–1 K–2 for La0.95Sr0.05CuOSe. Thermal conductivities measured for the materials with x = 0.00 and 0.05 were 2.1 W m–1 K–1 and 2.3 W m–1 K–1 at room temperature, respectively. These results lead to an estimation of Z value of 4.4×10–5 K–1 for La0.95Sr0.05CuOSe

Stripe antiferromagnetic correlations in LaFeAsO1-xFx probed by 75As NMR

The anisotropy of the nuclear spin-lattice relaxation rate $1/T_{1}$ of $^{75}$As was investigated in the iron-based superconductor LaFeAs(O$_{1-x}$F$_{x}$) ($x = 0.07, 0.11$ and 0.14) as well as LaFeAsO. While the temperature dependence of the normal-state $1/T_1T$ in the superconducting (SC) $x = 0.07$ is different from that in the SC $x = 0.11$, their anisotropy of $1/T_1$, $R \equiv (1/T_{1})_{H \parallel ab}/(1/T_{1})_{H \parallel c}$ in the normal state is almost the same ($\simeq$ 1.5). The observed anisotropy is ascribable to the presence of the local stripe correlations with $Q = (\pi, 0)$ or $(0, \pi)$. In contrast, $1/T_1$ is isotropic and $R$ is approximately 1 in the overdoped $x = 0.14$ sample, where superconductivity is almost suppressed. These results suggest that the presence of the local stripe correlations originating from the nesting between hole and electron Fermi surfaces is linked to high-$T_c$ superconductivity in iron pnictides.Comment: 4 pages, 3 figures, Accepted for publication in Phys. Rev.

Thermoelectric properties of delafossite-type layered oxides AgIn1–xSnxO2

The thermoelectric properties of delafossite-type layered oxides AgIn1–xSnxO2 that consist of alternating layers of Ag and In1–xSnxO2 were investigated to elucidate their potential as a thermoelectric material. Polycrystalline materials of the AgIn1–xSnxO2 were prepared by a cation exchange reaction between NaIn1–xSnxO2 and AgCl. The solubility limit of the Sn atoms on the In sites was approximately x=0.05. The electrical conductivity and Seebeck coefficient were measured between 373 and 673 K in air. Undoped AgInO2 was an n-type semiconductor with conductivities of 10–4–10–2 –1 cm–1, and the electron carriers were generated via the formation of oxygen vacancies. AgIn0.95Sn0.05O2 was an n-type degenerate semiconductor with conductivities of 100–101 –1 cm–1 where the Sn atoms acted as electron donors. This drastic increase in the electrical conductivity increased the thermoelectric power factor by approximately two orders of magnitude to 10–6–10–5 W m–1 K–2

Heteroepitaxial growth and optoelectronic properties of layered iron oxyarsenide, LaFeAsO

Epitaxial thin films of LaFeAsO were fabricated on MgO (001) and mixed-perovskite (La, Sr)(Al, Ta)O3 (001) single-crystal substrates by pulsed laser deposition using a Nd:YAG second harmonic source and a 10 at.% F-doped LaFeAsO disk target. Temperature dependences of the electrical resistivities showed no superconducting transition in the temperature range of 2-300 K, and were similar to those of undoped polycrystalline bulk samples. The transmittance spectrum exhibited a clear peak at ~0.2 eV, which is explained by ab-initio calculations.Comment: Submission: 31st July 2008, Accepted for publication in Appl. Phys. Let

Single-atomic-layered quantum wells built in wide-gap semiconductors LnCuOCh (Ln=lanthanide, Ch=chalcogen)

LnCuOCh (Ln=lanthanide, Ch=chalcogen) layered oxychalcogenides are wide-gap p-type semiconductors composed of alternately stacked (Ln2O2)2+ oxide layers and (Cu2Ch2)2- chalcogenide layers. Energy band calculations revealed that Cu-Ch hybridized bands only spread in the (Cu2Ch2)2- layers, which suggests that hole carriers in these bands are confined by the potential barriers formed by the (Ln2O2)2+ layers. Stepwise absorption spectra of a series of LnCuOCh experimentally verified that an exciton in the (Cu2Ch2)2- layers shows a two-dimensional behavior. These theoretical and experimental results indicate that LnCuOCh has “natural multiple quantum wells” built into its layered structure