Pictorial view of the combined atomic position and electronic charge density of a charge density wave, compared with a normal metal. The amplitude of the modulations is exaggerated in scale for illustration. Image: David Le Bolloc'h, LPS, Orsay France.The axion is a particle that physicists have hypothesized for decades. It has garnered attention in recent years for its potential involvement in dark matter, but its very existence remains in dispute.
Now, Russian and French researchers say that an experimental approach for demonstrating the existence of an axionic behavior in specific materials may not have found it, as previously reported. In Applied Physics Letters, the multinational team was unable to detect the expected increase in magnetoconductivity in the charge density wave of a compound made of tantalum, selenium and iodine – (TaSe4)2I.
These findings come three years after a paper in Nature seemed to provide sufficient evidence for axionic behavior using a similar approach.
"A null result, as we report, is in its own an interesting result," said co-author Pierre Monceau from University Grenoble Alpes in France. "The non-reproducibility in the results could be the source of a controversy but, more importantly, may open a scientific debate to uncover experimental ways for investigating this new field."
The concept of an axion in particle physics was proposed in the 1970s to explain why the strong nuclear force does not exhibit parity violation. Such a violation would lead, for instance, to an electric dipole moment in the neutron, which has not so far been detected experimentally.
Evidence for the axion lies in the specific way it couples to its electromagnetic field. Recently, it was theorized that a similar coupling, involving electrons, occurs in Weyl semimetals such as (TaSe4)2I. Under the right conditions, these electron interactions can induce a charge density wave.
Charge density waves can take on axionic characteristics if they include electrons with spiral-like movement that mirror one another in a magnetic field. The key finding that would suggest this behavior is an increase in a wave's magnetoconductivity. (TaSe4)2I possesses the right conditions for this behavior because of its structure, which can separate electron states according to their helix formation.
"Our surprise was to detect no effect in the conductance in a magnetic field," Monceau said. "We think that, given the lack of further evidence, it is premature to assert that (TaSe4)2I harbors an axionic charge density wave."
According to Monceau, charge density wave excitations with axionic characteristics may still be revealed by appropriate experiments, such as measuring the nonlinear dynamics in a magnetic field along the charge density wave. The group aims to continue these experiments and hope their results will inspire others to develop new techniques for confirming the existence of axion counterparts in condensed matter.
This story is adapted from material from the American Institute of Physics, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.