Majorana fermion detected in a quantum layer cake

Topological insulator plus superconductor yields new evidence for unusual quasiparticle

Majorana fermions in a topological insulator

MAJORANAS IN MOTION  Majorana fermions (blue, red, and purple lines) travel through a topological insulator (horizontal bar) with a superconductor layered on top in this illustration of new experiments to detect the fermions. Green lines indicate electrons travelling on the edges of the topological insulator.

Beijing Sondii Technology Co Ltd.

Editor’s note: On November 17, 2022, Science retracted the study described in this article, in part because “an analysis of the raw and published data revealed serious irregularities and discrepancies,” writes H. Holden Thorp, the editor in chief of Science. “These issues have caused the editors at Science to lose all confidence in the conclusions of the paper.” Three of the authors of the original paper agree with the retraction, while 14 authors do not.

A particle that is its own antiparticle seems to have left its calling card within a solid material.

To observe the signature of that particle, a Majorana fermion, scientists coupled a thin film of a topological insulator — which conducts electricity on its edges but is insulating within — with a layer of a superconductor, in which electrons can flow without resistance. In this layer cake of materials, the researchers report in the July 21 Science, electrical conductivity varied in discrete jumps of the size expected for Majorana fermions.

“The experiment came out exactly in the way we predicted,” says theoretical physicist Shoucheng Zhang of Stanford University.

Italian theoretical physicist Ettore Majorana originally proposed in 1937 that these fermions could be a new type of fundamental particle. Instead of having oppositely charged antiparticles, the electrically neutral Majorana fermions would be antiparticles of themselves.

Scientists suspect that neutrinos — tiny neutral particles that swarm through the cosmos — might be Majorana fermions, but there’s no hard proof. Instead, the only Majorana fermions for which scientists have evidence are in the form of a “quasiparticle,” a disturbance within a material that behaves like a single particle, but actually is the result of the collective motion of many electrons. Rather than individually tracking the motions of each electron within a material, scientists think of the disturbance as its own particle, simplifying the math that explains how the material behaves.

Several previous experiments have found traces of Majorana fermions (SN: 11/15/14, p. 8), but the new result reveals a different side of the quirky quasiparticles. Unlike previously detected Majorana fermions, these are chiral, meaning that they travel along the edge of a 2-D layer of material in one direction, like cars circling a racetrack.

“Certainly as far as chiral Majorana fermions go, this is the only definitive evidence that has been reported,” says theoretical physicist Taylor Hughes of the University of Illinois at Urbana-Champaign, who was not involved with the research.

It’s also the first evidence of a Majorana fermion that moves around like a true particle, instead of remaining stuck in one place. Earlier evidence for the quasiparticles was found in one-dimensional nanowires, in which a Majorana fermion sat motionless at each end of the wire, like a particle bound to a particular spot within a material. 

Majorana fermions leave their mark in the material (made of chromium, bismuth, antimony and tellurium overlaid with superconducting niobium) by tweaking a phenomenon known as the quantum anomalous Hall effect. In certain magnetic materials, the electrical conductivity of a thin layer of material changes in steps as a small magnetic field is varied, increasing and decreasing in jumps of a certain size. The signature of the Majorana fermions is a conductivity jump of half the normal size. Since the Majorana fermion is its own antiparticle, says Zhang, “in a very rough sense, it is half of a usual particle,” resulting in half-sized jumps.

The detection of this signature is “really the only firm evidence of the presence of Majorana fermions,” says study coauthor Kang Wang, a condensed matter physicist at UCLA. He says previous hints of Majorana fermions could have been explained by other means.

“They have a nice, crisp result,” says condensed matter physicist Ali Yazdani of Princeton University, strongly suggesting the chiral Majorana fermions are there. But “there are things that need to be checked out,” such as whether the Majorana fermions really travel on the edge of the material as expected.

Majorana fermions may eventually find a purpose in quantum computers. Microsoft, for example, hopes to make topological quantum computers (SN: 7/8/17, p. 28) that would harness the particles’ unusual properties to stave off the scrambling of delicate quantum information.

Physics writer Emily Conover has a Ph.D. in physics from the University of Chicago. She is a two-time winner of the D.C. Science Writers’ Association Newsbrief award.

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