The search for the axion has recently become an intense research field, as this hypothetical elementary particle, postulated by the Peccei–Quinn theory in 1977, can single-handedly solve two mysteries in physics. It could resolve the strong CP problem in quantum chromodynamics, and be the constituent of the dark matter that fills 27% of the universe.

In the presence of a magnetic field, the axion may decompose into a photon through the strong force interaction. The resulting photon will have a frequency proportional to the mass of the axion. Since the mass of the axion is unknown, and can span several orders of magnitude, a myriad of experiments world-wide are probing different axion mass ranges. The RADES project uses a haloscope [1] to search for axions with a mass in the range of the tens of meV (cold dark matter). The haloscope is a resonant cavity that can detect an axion produced photon with n_res = n_g (≈8 GHz inthe RADES case), with a detection efficiency that goes as P a QB_e², where Q is the quality factor of the cavity and B_e the external applied magnetic field. Therefore, these cavities work at low temperatures (4.2K) and under very high B_e. For this reason, haloscopes use copper instead of low temperature superconductor as a high conductance material to have low-surface resistance and hence a high in-field Q value.

We have demonstrated that under the working conditions of a haloscope, REBCO coated conductors (CCs) can outperform copper in terms surface resistance by a factor of 5 or more [2,3]. In this work we present a method to coat haloscopes, including curved surface cavities, that can use any CC in the market. The first cavity coated within our collaboration with RADEs has yielded a larger-than-copper Q value at high magnetic fields. We will also show that we are in the position to further improve the Q factor of the future cavities, potentially achieving a Q-factor 5 times that of a Cu cavity. Finally, we explore the advantages and pitfalls of using REBCO on cavities operating at lower frequencies than that of the RADES project.

[1] P. Sikivie, PRL 51 1983
[2] Puig T, et al Supercond. Sci. Technol. 32 2019
[3] Romanov A, et al Sci. Rep. 10 2020

Figure caption: Photo of the two halves of the axion cavity from RADES (CERN) after coating.

Keywords: REBCO, Coated conductors, Vortex dynamics, Dark matter