University on quantum regime thin film topological insulators developed at Rutgers University. In 2019, a team at the Max Planck Institute for Chemical Physics of Solids published their detection of axion insulators within a Weyl semimetal. An axion insulator is a quasiparticle – an excitation of electrons that behave together as an axion – and its discovery is consistent with the existence of the axion as an elementary particle. Experiments Despite not yet having been found, axion models have been well studied for over 40 years, giving time for physicists to develop insight into axion effects that might be detected. Several experimental searches for axions are presently underway; most exploit axions' expected slight interaction with photons in strong magnetic fields. Axions are also one of the few remaining plausible candidates for dark matter particles, and might be discovered in some dark matter experiments. Direct conversion in a magnetic field Several experiments search for astrophysical axions by the Primakoff effect, which converts axions to photons and vice versa in electromagnetic fields. The Axion Dark Matter Experiment (ADMX) at the University of Washington uses a strong magnetic field to detect the possible weak conversion of axions to microwaves. ADMX searches the galactic dark matter halo for axions resonant with a cold microwave cavity. ADMX has excluded optimistic axion models in the 1.9–3.53 μeV range. From 2013–2018 a series of upgrades were done and it is taking new data, including at 4.9–6.2 µeV. In December 2021 it excluded the 3.3–4.2 μeV range for the KSVZ model. Other experiments of this type include DMRadio, HAYSTAC, CULTASK, and ORGAN. HAYSTAC recently completed the first scanning run of a haloscope above 20 µeV. Polarized light in a magnetic field The Italian PVLAS experiment searches for polarization changes of light propagating in a magnetic field. The concept was first put forward in 1986 by Luciano Maiani, Roberto Petronzio and Emilio Zavattini. A rotation claim in 2006 was excluded by an upgraded setup. An optimized search began in 2014. Light shining through walls Another technique is so called "light shining through walls", where light passes through an intense magnetic field to convert photons into axions, which then pass through metal and are reconstituted as photons by another magnetic field on the other side of the barrier. Experiments by BFRS and a team led by Rizzo ruled out an axion cause. GammeV saw no events, reported in a 2008