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The microwave properties of single crystalline LaAlO/sub 3/, LSAT ((LaAlO/sub 3/)/sub 0.3/-(Sr/sub 2/AlTaO/sub 6/)/sub 0.7/), rutile (TiO/sub 2/), and polycrystalline Al/sub 2/O/sub 3/ were investigated employing dielectric resonator techniques. LaAlO/sub 3/ (/spl epsi//sub r/=23.7) exhibits a frequency dependent maximum of tan/spl delta/ at about 70 K, which can be explained by defect dipole relaxation. This leads to a variation of tan/spl delta/ at 9 GHz and 77 K from 5/spl middot/10/sup -6/ to 2/spl middot/10/sup -5/, the lowest values were achieved with Verneuil grown crystals. LSAT (/spl epsi//sub r/=22.8) exhibits an even more pronounced maximum with absolute values in the 10/sup -4/ range. For rutile (/spl epsi//sub r/=108) we measured a monotonous increase of tan/spl delta/ with temperature, which can be explained by intrinsic losses due to phonons. The loss tangent of Ti-doped polycrystalline, sintered Al/sub 2/O/sub 3/ was found to be only 4/spl middot/10/sup -6/ at 10 GHz and 77 K. Except LSAT the materials investigated so far provide a large potential for applications as dielectric resonators shielded by high temperature superconducting (HTS) films.
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Zuccaro et al. (1997) studied this question.
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