Optical properties of MgH2 measured in situ in a novel gas cell for ellipsometry/spectrophotometry

The dielectric properties of alpha-MgH2 are investigated in the photon energy range between 1 and 6.5 eV. For this purpose, a novel sample configuration and experimental setup are developed that allow both optical transmission and ellipsometric measurements of a transparent thin film in equilibrium with hydrogen. We show that alpha-MgH2 is a transparent, colour neutral insulator with a band gap of 5.6 +/- 0.1 eV. It has an intrinsic transparency of about 80% over the whole visible spectrum. The dielectric function found in this work confirms very recent band structure calculations using the GW approximation by Alford and Chou [J.A. Alford and M.Y. Chou (unpublished)]. As Pd is used as a cap layer we report also the optical properties of PdHx thin films.Comment: REVTeX4, 15 pages, 12 figures, 5 table

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Civil Engineering and Geoscience

Crystal-structure transformations and magnetic-ordering phenomena in GdCu1–xGax

Electrical resistivity, magnetic susceptibility, thermal expansion, x-ray diffraction, and scanning calorimetry measurements have been performed over wide temperature ranges on the pseudobinary compounds GdCu1–xGax. In GdCu, which forms in the CsCl crystal structure at room temperature when prepared from the melt, a crystal-structure transformation into a low-temperature noncubic phase occurs at 250 K. Large hysteresis effects are observed with the transformation back into the CsCl structure occurring around 600 K, probably due to a stress-relieving process. In GdCu1–xGax the CsCl crystal structure is stabilized for x ≥ 0.03. The CsCl pseudobinary compound GdCu1–xGax exhibits a kink-like anomaly in the magnetic susceptibility for 0.03 ≤ x ≤ 0.17. This is ascribed to a complicated type of long-range antiferromagnetic ordering. For 0.17 < x < 0.29 we suggest a mixed or canted low-temperature magnetic state for the ordered Gd moments. Over the whole 0.03 ≤ x < 0.29 concentration region a negative temperature coefficient is observed in the measured electrical resistivity at the ordering temperature. For x ≥ 0.29 ferromagnetic long-range order is found. A simple model with only nearest-neighbor magnetic interactions, depending on the local Cu and Ga surroundings, is proposed to describe the x dependence of the paramagnetic Curie temperature. Finally a structural and magnetic phase diagram is constructed for the GdCu1–xGax system with 0 ≤ x ≤ 0.5.