Topological Superconductors: Fertile Ground for Elusive Majorana Particle

Source: Sally Wood

A recent FLEET review has accelerated the search for Majorana fermions in iron-based superconductors. The elusive Majorana fermion, or ‘angel particle’ was proposed by Ettore Majorana in 1937, and simultaneously behaves like a particle and an antiparticle. Searching for new exotic matters or particles is an important job in the world of physics. The Majorana fermion surprisingly remains stable rather than being selfdestructive and promises information and communications with zero resistance. Together, this research addresses the rising energy of modern electronics, which already accounts for 8% of global electricity consumption. In addition, Majorana zero-energy modes in topological superconductors have made those exotic quantum materials the main candidate for realising topological quantum computing.

The Angel Particle

The fundamental particles such as electrons, protons, neutrons, quarks, and neutrinos each have their distinct antiparticles. Conventional fermion and anti-fermions constitute matter and antimatter and annihilate each other when combined. “The Majorana fermion is the only exception to this rule, a composite particle that is its own antiparticle,” said Professor Xiaolin Wang from the University of Wollongong (UOW), who is a corresponding author on this research. Despite the intensive searching for Majorana particles, the clue of its existence has been elusive for many decades. This occurs because the two conflicting properties render it neutral and its interactions with the environment are very weak. While existence of the Majorana particle is yet to be discovered, it may exist as a single-particle excitation in condensed-matter systems where band topology and superconductivity coexist. Dr Muhammad Nadeem is a FLEET postdoctoral researcher at UOW, who said there has been increased research in in these particles in recent years. “In the last two decades, Majorana particles have been reported in many superconductor heterostructures and have been demonstrated with strong potential in quantum computing applications.” A new type of material called ironbased topological superconductors was reported several years ago hosting Majorana particles without fabrication of heterostructures, which is significant for applications in real devices. “Our article reviews the most recent experimental achievements in these materials: how to obtain topological superconductor materials, experimental observation of the topological state, and detection of Majorana zero modes,” said the study’s first author and UOW PhD candidate Lina Sang. In these systems, quasiparticles may impersonate a particular type of Majorana fermion such as ‘chiral’ Majorana fermion. This moves along a one-dimensional path and Majorana ‘zero mode,’ which remains bounded in a zero-dimensional space. When condensed-matter systems, which host Majorana fermions, are experimentally it helps researchers to steer the engineering of low-energy technologies.

The Genius Who Disappeared

Ettore Majorana was an Italian man described as “the greatest theoretical physicist of our time” by the father of the nuclear age, Enrico Fermi. It is believed Majorana was the first scientist to propose the notion of neutrons but failed to publish his findings. He was highly sought after on the international stage, including offers to work at the Universities of Cambridge and Yale. However, he refused both opportunities and rarely showed up to his University of Naples teaching role. He was known as a scientific trailblazer who would retreat and silence himself when he was urged to publish his breakthroughs.

First author FLEET PhD student Lina Sang in the labs at the Institute for Superconducting and Electronic Materials (ISEM), University of Wollongong.

Scanning tunnelling microscopy (STM) images: edge of FeSe/STO, with inset atomicresolution STM showing the topmost Se atom arrangement and crystal orientation.

Corresponding author ISEM Director Prof Xiaolin Wang (University of Wollongong).