Comment on "Tunable Supermode Dielectric Resonators for Axion Dark-Matter Haloscopes"

We comment on a recently published paper, Phys. Rev. Applied 9, 014028 (2018), which presents frequency-tuning mechanisms for dielectric resonators and demonstrates their potential application to axion haloscopes. One of the schemes introduces a cylindrical dielectric hollow and splits it in the axial direction to tune the frequency. The authors claim that this scheme offers a 1 to 2-order-of-magnitude improvement in axion search sensitivity in exploiting a higher-order resonant mode. We find that their study is based on unrealistic cavity modeling and inappropriate choice of the figure of merit (FOM), which could mislead to the significant improvement in sensitivity. Considering a practical cavity structure and an appropriate FOM, we recalculate the significance of the scheme, which turns out to be not substantial.