  Brownian thermal noise associated with highly-reflective mirror coatings is a
fundamental limit for several precision experiments, including
gravitational-wave detectors. Recently, there has been a worldwide effort to
find mirror coatings with improved thermal noise properties that also fulfill
strict optical requirements such as low absorption and scatter. We report on
the optical and mechanical properties of ion-beam-sputtered niobia and
titania-niobia thin films, and we discuss application of such coatings in
current and future gravitational-wave detectors. We also report an updated
direct coating thermal noise measurement of the HR coatings used in Advanced
LIGO and Advanced Virgo.
