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Due to its excellent optical properties, such as low absorption and scattering, amorphous ${\text{Ta}}_2{\mathrm{O}}_5$is commonly used as an optical coating material, often in combination with ${\mathrm{SiO}}_2$layers to produce a highly reflective stack. However, the high mechanical loss of ${\text{Ta}}_2{\mathrm{O}}_5$limits the thermal-noise performance of such coatings when used in precision measurement applications. Doping with ${\mathrm{TiO}}_2$has previously been shown to slightly reduce the mechanical loss, but it is still very high compared to many other materials, particularly at low temperatures. In this paper, we present a detailed study of different heat treatment temperatures and of Ti concentrations of up to nominally 75%. We show a significant mechanical-loss reduction for the mixture with the highest Ti cation content, which crystallized after heat treatment at 500°C. The resulting loss is much lower than that of pure ${\mathrm{TiO}}_2$or that of ${\text{Ta}}_2{\mathrm{O}}_5$after crystallization, making further studies highly interesting, in particular investigations of scattering which may pose a major drawback for optical applications. 

The mirror suspensions in gravitational wave detectors demand low mechanical loss jointing to ensure good enough detector performance and to enable the detection of gravitational waves. Hydroxide catalysis bonds have been used in the fused silica suspensions of the GEO600, Advanced LIGO, and Advanced Virgo detectors. Future detectors may use cryogenic cooling of the mirror suspensions and this leads to a potential change of mirror material and suspension design. Other bonding techniques that could replace or be used alongside hydroxide catalysis bonding are of interest. A design that incorporates repair scenarios is highly desirable. Indeed, the mirror suspensions in KAGRA, which is made from sapphire and operated at cryogenic temperatures, have used a combination of hydroxide catalysis bonding and gallium bonding. This Letter presents the first measurements of the mechanical loss of a gallium bond measured between 10 K and 295 K. It is shown that the loss, which decreases with temperature down to the level of $(1.8\pm 0.3)\times {10}^{-4}$at 10 K, is comparable to that of a hydroxide catalysis bond. Published by the American Physical Society2024