The ‘Angel Particle’ is still missing

New research suggests that the discovery of a new particle — the Majorana fermion — was a false alarm, but the search for the ‘angel particle’ goes on.

Theory suggests that as neutrons (blue line) scatter off the graphene-like honeycomb material, they produce a magnetic Majorana fermion (green wave) that moves through the material disrupting or breaking apart magnetic interactions between ‘spinning’ electrons, but new research has failed to detect such particles. (ORNL/Jill Hemman)

The 2017 discovery of a particle that acts as its own anti-particle has been called into question in new research published by researchers from Penn State University. The team of physicists, headed-up by Cui-Zu Chang, an assistant professor of physics at Penn State, examined over three dozen devices similar to the one that produced Majorana fermion two years ago. They found that the phenomena observed in an earlier study were unlikely to have been caused by the presence of the fermion — nicknamed the ‘angel particle’ in reference to the 2000 Dan Brown novel ‘Angels and Demons.’

The Majorana fermion was of particular interest to researchers as such particles — first hypothesised in 1937 by Ettore Majorana — could act as their own antiparticle. This is in contrast to particles such as electrons and protons whose antiparticle counterparts are the positron and the antiproton, particles with equal but opposite charges. Majorana fermions are their own antiparticle and have no charge.

This quality would be of great use in the development of quantum computers. As the each Majorana fermion is essentially ‘half a subatomic particle’ a single qubit of information could be stored in two widely-separated fermions. Thus meaning that if some environmental interference should cause information loss in one, the other is unlikely to be perturbed and thus, the information is saved.

“When the Italian physicist Ettore Majorana predicted the possibility of a new fundamental particle which is its own antiparticle, little could he have envisioned the long-lasting implications of his imaginative idea,” explains Nitin Samarth, Downsbrough Department Head and professor of physics at Penn State. “Over 80 years after Majorana’s prediction, physicists continue to actively search for signatures of the still elusive ‘Majorana fermion’ in diverse corners of the universe.”

The search for chiral particles such as the Majorana fermion encompasses several different experimental methods, this includes efforts in underground observatories to determine if the neutrino — a virtually massless and chargeless particle that rarely interacts with matter — could possibly be a Majorana fermion. The Penn State team’s efforts, however, concentrated on the detection of the angel particle or its effects in solid-state devices that combine quantum materials with superconductors.

An exotic quantum state known as a “chiral Majorana fermion” is predicted in devices wherein a superconductor is affixed on top of a quantum anomalous Hall (QAH) insulator (left panel). Experiments performed at Penn State and the University of Würzburg in Germany show that the millimetre-size superconductor strip used in the proposed device geometry creates an electrical short, preventing the detection of chiral Majoranas (right panel). ( Cui-zu Chang, Penn State)

In such devices, it is hypothesised that electrons can ‘dress up’ as Majorana fermions in a cloak constructed from the rules of quantum mechanics, topology and relativity. These faux-Majorana fermions could also be protected from environmental decoherence in the same way outlined above for the real thing. Thus making them just as useful as the real thing in a topological quantum computer.

“An important first step toward this distant dream of creating a topological quantum computer is to demonstrate definitive experimental evidence for the existence of Majorana fermions in condensed matter,” says Chang. “Over the past seven or so years, several experiments have claimed to show such evidence, but the interpretation of these experiments is still debated.”

In order to potentially detect these angel particles, the team examined devices that are constructed from a quantum material referred to as a “quantum anomalous Hall insulator.” In such materials, electrical current flows only at its edge — and according to that 2017 research — this is when Majorana fermions are created. This also results in that useful ‘half-quantised’ state if a precise magnetic field is applied. But, what the Penn-State team discovered was this state was always demonstrated in devices with clean superconducting contact, regardless of the application of a magnetic field. The researchers believe that this is a result of the superconductor acts like an electrical short and therefore does not point to of the presence of the Majorana fermion.

“The fact that two laboratories — at Penn State and at Wurzburg — found completely consistent results using a wide variety of device configurations casts serious doubt on the validity of the theoretically proposed experimental geometry and questions the 2017 claim of observing the angel particle,” explains Moses Chan, Even Pugh Professor Emeritus of Physics at Penn State.

“I remain optimistic that the combination of quantum anomalous Hall insulators and superconductivity is an attractive scheme for realizing chiral Majoranas,” adds Morteza Kayyalha, a postdoctoral research associate at Penn State who carried out the device fabrication and measurements. “But our theorist colleagues need to rethink the device geometry.”

The team point out that the result is an excellent illustration of how science should work. The results of previous experiments are subject to replication and if further research fails to replicate the results of initial experiments it is a fair indication that a particular phenomenon is not present and that initial results were erroneous. As the famous physicist Richard Feynman explained, perfectly exemplifying the ruthless nature of the scientific process: “It doesn’t matter how beautiful your theory is, it doesn’t matter how smart you are. If it doesn’t agree with experiment, it’s wrong.”

“Extraordinary claims of discovery need to be carefully examined and reproduced,” elaborates Samarth. “All of our postdocs and students worked really hard to make sure they carried out very rigorous tests of the past claims. We are also making sure that all of our data and methods are shared transparently with the community so that our results can be critically evaluated by interested colleagues.”

The team’s research is published in the latest edition of the journal Science.