TiO2 doping effect on reflective coating mechanical loss for gravitational wave detection at low temperature

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We measured the mechanical loss of a dielectric multilayer reflective coating (ion-beam-sputtered SiO₂ and Ta₂O₅) with and without TiO₂ mixed into the Ta₂O₅ layers on sapphire disks between 6 and 77 K. The measured loss angle exhibited a temperature dependence, and the local maximum was found at approximately 20 K. This maximum was 7.0×10^-4 (with TiO₂) and 7.7×10^-4 (without TiO₂), although the previous measurement for the coating on sapphire disks showed almost no temperature dependence [Phys. Rev. D 74, 022002 (2006)]. We evaluated the coating thermal noise in KAGRA and discussed future investigation strategies.