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

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)PRVDAQ1550-799810.1103/PhysRevD.74.022002]. We evaluated the coating thermal noise in KAGRA and discussed future investigation strategies.