Three Odd Crystals Could Make Dark Matter Detectors 100 to 1,000 Times More Sensitive, and Two of Them Would Show the Signal Rising and Falling as Earth Turns.

Dark matter is thought to make up about 85% of the matter in the universe, and every attempt to catch a particle of it directly has come up empty. Part of the problem may be the material inside the detectors. An international team reports in Physical Review Letters that three unconventional crystals could sense the lightest dark matter candidates far more effectively than anything now in use, in the best case by two to three orders of magnitude, and that two of them would carry a built-in signature to separate a real detection from noise.

The work comes from Yonit Hochberg and Rotem Ovadia at the Hebrew University of Jerusalem, Dino Novko at the Institute of Physics in Croatia, and Antonio Politano at the University of L'Aquila, a collaboration spanning particle physics, condensed matter theory and materials science. Their targets are "light" dark matter particles, far less massive than the WIMPs that most large underground experiments were built to find. A light particle passing through a detector deposits only a minuscule amount of energy, and finding a material that responds strongly to such a tiny nudge is the whole game.

The three candidates are titanium diselenide, strontium ruthenate and diamond doped with electron "holes." What they share is a population of low-energy plasmons, collective ripples in the material's sea of electrons that can be set in motion by a very small energy transfer. Using first-principles quantum mechanical simulations, the team calculated how each crystal would respond to a dark matter particle across a range of masses and found that all three outperform today's benchmark detector materials over broad regions of the possible parameter space. Titanium diselenide stood out, with a predicted improvement in sensitivity of as much as a factor of 100 to 1,000.

Two of the materials offer a second advantage. Their response depends on the direction the incoming particle is traveling relative to the crystal's structure. Because Earth rotates once a day through the galaxy's presumed dark matter halo, a detector made from such a crystal would see its signal rise and fall on a 24-hour cycle, a pattern no terrestrial background is likely to mimic. That daily modulation has long been on physicists' wish lists as a way to confirm a discovery rather than merely claim one.

The practical case is also strong. All three materials can be synthesized with existing techniques, and titanium diselenide in particular lends itself to scalable production; paired with existing low-threshold sensor technology, the authors argue, they could anchor a new generation of experiments. "Dark matter remains one of the greatest mysteries in physics, and discovering its nature requires us to rethink not only the particles we are searching for, but also the materials we use to search for them," the researchers said. They suspect the three crystals are only the beginning: many quantum materials host low-energy collective excitations that nobody has yet examined as dark matter targets, which turns a question for particle physicists into one for materials scientists as well.