A research team at Penn State, partnering with the University of Würzburg in Germany, has flagged a 2017 discovery of Majorana fermion as a false alarm. 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 show that the millimeter-size superconductor strip used in the proposed device geometry creates an electrical short, preventing the detection of chiral Majoranas (right panel). Courtesy of Cui-Zu Chang, Penn State. Majorana fermions are enigmatic particles that act as their own antiparticle and were first hypothesized to exist in 1937. Physicists have maintained a consistent interest in the particle for its unique properties that are theorized to be capable of aiding in the construction of topological quantum computers. The research team, led by Penn State physics professor Cui-Zu Chang, studied over three dozen devices similar to the one used to produce the angel particle in the 2017 report. The researchers’ findings showed that the feature, which was claimed to be the manifestation of the so-called angel particle, was unlikely to be induced by the existence of said particle. “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, a 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 the particle is ongoing in a variety of sectors. One lead that particle physicists have been chasing is whether the subatomic particle known as a neutrino, described as a ghost-like particle that rarely interacts with matter, might be a Majorana fermion. Condensed matter physicists are looking into solid-state devices that combine exotic quantum materials with superconductors and whether they contain manifestations of Majorana physics. In solid-state devices, physicists have theorized that electrons will appear as Majorana fermions by stitching together a fabric constructed from core aspects of quantum mechanics, relativistic physics, and topology. “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,” Chang said. “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 research team studied devices fashioned from a quantum material known as a “quantum anomalous Hall insulator” where the flow of the electrical current remains 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. The electrical conductance of the device is predicted to then be “half-quantized” when subject to a precise magnetic field. Across the devices the researchers studied, they found that those with clean superconducting contacts always showed the half-quantized value regardless of magnetic field conditions. This is said to be due to the superconductor acting as an electric short, therefore not a sign of the Majorana fermion. “This is an excellent illustration of how science should work,” Samarth said. “Extraordinary claims of discovery need to be carefully examined and reproduced. 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.”