The case of the elusive Majorana: The so-called 'angel particle' is still a mystery

January 03, 2020

A 2017 report of the discovery of a particular kind of Majorana fermion--the chiral Majorana fermion, referred to as the "angel particle"--is likely a false alarm, according to new research. Majorana fermions are enigmatic particles that act as their own antiparticle and were first hypothesized to exist in 1937. They are of immense interest to physicists because their unique properties could allow them to be used in the construction of a topological quantum computer.

A team of physicists at Penn State and the University of Wurzburg in Germany led by Cui-Zu Chang, an assistant professor of physics at Penn State studied over three dozen devices similar to the one used to produce the angel particle in the 2017 report. They found that the feature that was claimed to be the manifestation of the angel particle was unlikely to be induced by the existence of the angel particle. A paper describing the research appears on January 3, 2020 in the journal Science.

"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," said 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."

In one such effort, particle physicists are using underground observatories that seek to prove whether the ghost-like particle known as the neutrino--a subatomic particle that rarely interacts with matter--might be a Majorana fermion. On a completely different front, condensed matter physicists are seeking to discover manifestations of Majorana physics in solid state devices that combine exotic quantum materials with superconductors. In such devices, electrons are theorized to dress themselves as Majorana fermions by stitching together a fabric constructed from core aspects of quantum mechanics, relativistic physics, and topology. This analogous version of Majorana fermions has particularly captured the attention of condensed matter physicists because it may provide a pathway for constructing a "topological quantum computer" whose qubits (quantum versions of binary 0s and 1s) are inherently protected from environmental decoherence--the loss of information that results when a quantum system is not perfectly isolated and a major hurdle in the development of quantum computers.

"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," said 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."

The team studied devices fashioned from a quantum material known as a "quantum anomalous Hall insulator" wherein the electrical current flows only at the edge. A recent study predicted that when the edge current is in clean contact with a superconductor, propagating chiral Majorana Fermions are created and the electrical conductance of the device should be "half-quantized" (a value of e2/2h where "e" is the electron charge and "h" is Planck constant), when subject to a precise magnetic field. The Penn State-Wurzburg team studied over three dozen devices with several different materials configurations and found that devices with a clean superconducting contact always show the half-quantized value regardless of magnetic field conditions. This occurs because the superconductor acts like an electrical short and is thus not indicative 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," said 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," said 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."

"This is an excellent illustration of how science should work," said Samarth. "Extraordinary claims of discovery need to be carefully examined and reproduced. 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."
-end-
In addition to Chang, Samarth, Chan, and Kayyalha, the research team includes Penn State faculty member Qi Li, and Wurzburg faculty members Laurens Molenkamp and Charles Gould. The project relied on materials synthesis carried out at Penn State's 2D Crystal Consortium user facility for synthesis of 2D quantum materials and was funded by the U.S. National Science Foundation, the Office of Naval Research, the U.S. Department of Energy, the Army Research Office, and the European Research Council.

Penn State

Related Physics Articles from Brightsurf:

Helium, a little atom for big physics
Helium is the simplest multi-body atom. Its energy levels can be calculated with extremely high precision only relying on a few fundamental physical constants and the quantum electrodynamics (QED) theory.

Hyperbolic metamaterials exhibit 2T physics
According to Igor Smolyaninov of the University of Maryland, ''One of the more unusual applications of metamaterials was a theoretical proposal to construct a physical system that would exhibit two-time physics behavior on small scales.''

Challenges and opportunities for women in physics
Women in the United States hold fewer than 25% of bachelor's degrees, 20% of doctoral degrees and 19% of faculty positions in physics.

Indeterminist physics for an open world
Classical physics is characterized by the equations describing the world.

Leptons help in tracking new physics
Electrons with 'colleagues' -- other leptons - are one of many products of collisions observed in the LHCb experiment at the Large Hadron Collider.

Has physics ever been deterministic?
Researchers from the Austrian Academy of Sciences, the University of Vienna and the University of Geneva, have proposed a new interpretation of classical physics without real numbers.

Twisted physics
A new study in the journal Nature shows that superconductivity in bilayer graphene can be turned on or off with a small voltage change, increasing its usefulness for electronic devices.

Physics vs. asthma
A research team from the MIPT Center for Molecular Mechanisms of Aging and Age-Related Diseases has collaborated with colleagues from the U.S., Canada, France, and Germany to determine the spatial structure of the CysLT1 receptor.

2D topological physics from shaking a 1D wire
Published in Physical Review X, this new study propose a realistic scheme to observe a 'cold-atomic quantum Hall effect.'

Helping physics teachers who don't know physics
A shortage of high school physics teachers has led to teachers with little-to-no training taking over physics classrooms, reports show.

Read More: Physics News and Physics Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.