Experiment Finds Evidence of Majorana Fermion

Experiment Finds Evidence of Majorana Fermion

Researchers at the University of California in collaboration with Stanford University have found firm evidence of the Majorana fermion, a long sought-after particle that acts as its own antiparticle.

Majorana fermions are named after Ettore Majorana, an Italian physicist who first proposed their existence in 1937. They are spin ½ particles, have no electric charge, and are their own antiparticles. No known elementary particles have been demonstrated to be Majorana fermions yet, although the neutrino is believed to be a potential candidate, and intense experimental efforts are under way to seek proof.

Over the past decade or so, condensed matter physicists and materials scientists have devoted considerable efforts into obtaining Majorana fermions from “quasiparticles”, a particle-like phenomenon that occurs in quantum materials when a collection of electrons behave in a specific coordinated way. Many schemes have been proposed, based on superconductors coupled with a special class of materials known as topological matter. Recently Professor Shoucheng Zhang’s group at Stanford proposed materials platforms and predicted direct transport signatures of the Majorana fermions. While the quasiparticles in the latest systems are not actual particles, they behave the same way and, in particular, follow the same mathematical equation Majorana used to describe a propagating particle.

“The race was on to realize Marjorana fermions in table-top experiments,” said Kai Liu, a study co-author and professor of physics at UC Davis.

To experimentally detect Majorana fermions, the research team created a “layer cake” of quantum materials from a superconductor and a magnetic topological insulator. A superconductor has zero electrical resistance, while a topological insulator conducts current only along its surface or edges but not through its middle. Coupling the two materials together created a topological superconductor, where electrons zip along two edges of the material’s surface without resistance. Normally, the two materials would interfere with each other, and the magnetic topological insulator was particularly challenging to make, so growing and integrating the materials was a critical step, Liu said.

A magnetic field applied to the “layer cake” enticed Majorana fermions to emerge as pairs of quasiparticles, which travel along the edges of the topological insulator. Under certain conditions, the hybrid structure splits each pair, allowing one member to go through and deflecting the other, leading to conductance half as high as that for electrons. These half-steps were the signal confirming the presence of mobile Marjorana fermions, referred to as “chiral”, because they move along a one-dimensional path in just one direction. While the experiment itself was extremely difficult, the signal it produced was clear and unambiguous, the researchers said. The results were published July 20 in Science.

Majorana fermions may become building blocks for quantum computers that would harness the particles’ unusual properties to stave off loss of quantum information. For example, a single qubit of information could be stored in two separate Majorana fermions, which are much less susceptible to environmental perturbations.

This experiment was led by UCLA Professor Kang Wang, in collaboration with Professor Jing Xia’s group at UC Irvine, Professor Kai Liu’s group at UC Davis, and Professor Shoucheng Zhang’s group at Stanford. The exotic materials were fabricated and integrated together by groups at UCLA and UC Davis, led by co-first authors Qing Lin He and Lei Pan at UCLA, and Edward Burks and Zhijie Chen at UC Davis. Transport studies were performed by groups at UCLA and UC Irvine, at low temperatures that captured the distinct signatures. The paper’s corresponding authors also include Xufeng Kou, who did the work at UCLA and is now a member of the faculty at Shanghai Tech University.

The study was supported in part by the SHINES Center, an Energy Frontier Research Center funded by the U.S. Department of Energy. Work at UC Davis was supported by the National Science Foundation.

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Adapted from news releases from UCLA and Stanford University.