  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 ueV, resulting in the establishment of new exclusion
limits for axion-photon coupling greater than approximately 10^-13 GeV^-1.
These findings signify a breakthrough, demonstrating that our tuning mechanism
facilitates the practical utilization of higher-order modes for cavity
haloscope searches.
