Crystalline Corundum-Structured Oxides as a Potential Alternative for Mirror Coatings of Gravitational Wave Interferometers

Crystalline coatings have emerged as a promising alternative to amorphous mirror coatings for planned upgrades of second-generation detectors such as Advanced Virgo and for future 3G observatories. In this work, we investigate the structural and mechanical properties of epitaxial Cr2O3 thin films grown on c-plane sapphire by molecular beam epitaxy, with the objective of evaluating corundum-structured oxides as a new class of crystalline coating materials for gravitational wave interferometers. We report, for the first time, cryogenic mechanical loss measurements of Cr2O3 coatings and relate the measured dissipation to their crystalline quality. The structural and mechanical properties of the coatings were evaluated by XRD, RHEED, AFM, and GeNS measurements. Our findings show that the highest-quality chromia layers exhibit mechanical losses at 6 K as low as (5±1) × 10−6 rad, while poorer crystalline quality is associated with significantly higher losses. These results demonstrate the potential of corundum-structured oxides as mirror coatings for next-generation interferometers. The wide availability of corundum-structured oxides (X2O3 with X = Al, Ga, Fe, V, Cr, Ti, …) and their epitaxial compatibility with sapphire, including the possibility of forming solid solutions, further highlights the potential of this largely unexplored materials class.