Search for Dark Matter Axions with Tunable TM_{020} Mode.

Axions are hypothesized particles believed to potentially resolve two major puzzles in modern physics: the strong CP problem and the nature of dark matter. Cavity-based axion haloscopes represent the most sensitive tools for probing their theoretically favored couplings to photons in the microelectronvolt range. However, as the search mass (or frequency) increases, the detection efficiency decreases, largely due to a decrease in cavity volume. Despite the potential of higher-order resonant modes to preserve experimental volume, their practical application in searches has been limited by the challenge of maintaining a high form factor over a reasonably wide search bandwidth. We introduce an innovative tuning method that uses the unique properties of auxetic materials, designed to effectively tune higher modes. This approach was applied to the TM_{020} mode for a dark matter axion search exploring a mass range from 21.38 to 21.79  μeV, resulting in the establishment of new exclusion limits for axion-photon coupling greater than approximately 10^{-13}  GeV^{-1}. These findings should allow use of higher-order modes for cavity haloscope searches.