Superconducting Resonator Studies for Axion Dark Matter Searches

The axion has emerged as a leading dark matter candidate over the past several decades. It elegantly solves the strong CP problem while also having properties that are consistent with dark matter. Recent theoretical calculations for the post-inflation axion predict the mass to be in the 10-100 GHz frequency range (> 40 μeV). However, such high frequencies present a significant challenge for traditional cavity haloscopes, which become prohibitively small as the axion mass increases.  An appealing solution to this conundrum are wire-array metamaterials whose plasma frequency can be engineered and tuned, without sacrificing their volume.  But higher frequencies require arrays of greater wire density, and thus a decreased quality factor Q with normal conducting wires. In this talk, we discuss the development of superconducting resonators using MgB₂ coatings, a high-temperature superconductor with a favorably high critical temperature T_c and upper critical field H_c2 for axion haloscopes.