  Superconducting RadioFrequency (SRF) cavities have been developed for modern
particle accelerator projects in the world. These cavities are mainly made of
bulk niobium operated in superfluid or normal fluid helium. Due to the price
increase in niobium material as well as helium, superconductors beyond the
niobium are investigated for more sustainable accelerators. In this paper, we
consider the most fundamental loss mechanism of microwaves inside various
superconductors caused by thermally excited quasi-particles in the Meissner
state.
