Search for Axion Dark Matter around \texorpdfstring{$15.6\mu \text{eV}$}{15.6 microeV} with a Broadly Tunable High-Temperature Superconducting Cavity

We report an axion dark matter search in the mass range of $15.30\text{–}15.85\mu \text{eV}$ using a broadly tunable high-temperature superconducting (HTS) haloscope. By soldering substrate-stripped rare-earth barium copper oxide (REBCO) films onto a copper shell, we achieved a quality factor 3–4 times higher than in copper cavities across its tuning band in an 8$\text{-T}$ magnetic field. We set robust Frequentist 90%~confidence-level exclusion limits on the axion–photon coupling down to 1.3 times the KSVZ coupling, while a complementary Bayesian analysis achieves KSVZ-level sensitivity.