



Search for axions as quasi-particles in condensed matter

An important step towards a possible direct detection of axions

The behavior of electrons in the Weyl semi-metal showed a dynamic identical to that predicted for axions. Credits: J. Gooth

Bibliography:



Article: Axionic charge-density wave in the Weyl semimetal (TaSe4)2I



Authors: J. Gooth, B. Bradlyn, S. Honnali, C. Schindler, N. Kumar, J. Noky, Y. Qi, C. Shekhar, Y. Sun, Z. Wang, B. A. Bernevig & C. Felser



Nature 575, 315–319 (2019)



doi:10.1038/s41586-019-1630-4

Proposed as a solution in 1977 to the problem of CP symmetry in quantum chromodynamics, axions are hypothetical neutrals and very low mass particles, considered today as potential candidates for dark matter. While several experiments aim to detect these particles, a team of physicists has recently discovered clues that axions may well exist. Although these results do not directly prove the existence of axions, they are an important step in the search for particles.Physicists have found clues about the existence of the axion, an elusive particle that rarely interacts with normal matter. The axion was first predicted more than 40 years ago, but has never been observed until now.Physicists have suggested that dark matter could be composed of axions. But rather than searching for axions in space, they discovered the mathematical signatures of an axion in a particular material here on Earth. The results were published in the journal Nature.The newly discovered axion is not quite a usual particle: it acts as an electron wave in a supercooled material called semi-metal. This strange particle could also help solve a long-standing physical puzzle, known as a strong CP problem. For some reason, the laws of physics seem to act in the same way on particles and their antimatter partners, even when their spatial coordinates are reversed.This phenomenon is known as charge-parity symmetry, but the Standard Model does not say anything about the origin of this symmetry. Unexpected symmetry can be explained by the existence of a special field (axionic field); to detect an axion would prove that this field exists, thus solving this mystery.Because physicists believe that the particle hardly interacts with ordinary matter, they assumed that it would be difficult to detect it using existing space telescopes. The researchers turned to condensed matter.Condensed matter experiments, such as the one conducted here, have been used to highlight predisposed particles that are elusive in several well-known cases, including that of fermion majorana.The particles are not detected in the usual sense, but are in the form of collective vibrations in materials that behave and respond exactly as a particle would. It is therefore quasi-particles.The research team worked with a Weyl (TaSe 4 ) 2 I semi-metal , a special material in which electrons behave as if they had no mass, did not interact and split in two types: right-handed and left-handed.The property of being right-handed or left-handed is called chirality; the chirality in Weyl's semi-metal is conserved, which means that there is an equal number of left and right electrons. Cooling the semi-metal to minus 11 ° C allowed the electrons to interact and condense to form a crystal of their own.Vibration waves propagating through the crystals are called phonons. Since the strange laws of quantum mechanics dictate that particles can also behave in waves, some phonons have the same properties as classical quantum particles, such as electrons and photons.Gooth and his colleagues observed phonons in the electron crystal, which responded to electric and magnetic fields exactly as predicted for axions.In addition, these quasi-particles did not have an equal number of right and left particles (physicists also predicted that axions would break the conservation of chirality).Frank Wilczek (Nobel Prize in Physics), who did not participate in the present study, also suggested that a material such as Weyl's semi-metal could one day be used as a sort of "antenna" to detect fundamental axions, or axions that exist in their own way as particles in the Universe, rather than as collective vibrations.While the search for the axion as a single, independent particle will continue, experiments like this one are helping more traditional detection experiments by providing boundaries and estimates of the properties of the particle, such as mass. This gives other experimenters a better idea of ​​where to look for these particles. It also convincingly demonstrates that the existence of the particle is possible.