Physicists Probe Mystery of Majorana Fermion

Theoretical physicists at Dartmouth College in Hanover, New Hampshire, are on the track of an exotic, subatomic particle, whose existence was first predicted 75 years ago.

A schematic diagram shows Majorana particles as they might line up in two opposing magnetic fields, red, that interact with a superconducting topological insulator, blue (Neupert et al / The American Physical Society)

With headlines proclaiming the discovery of a particle consistent with the Higgs boson, particle physics has captured the imagination of the world. But this is only one puzzle seemingly solved in physics. The team of Dartmouth physicists delves into another enigmatic particle, predicted in 1937 by the brilliant Italian physicist Ettore Majorana.

Majorana is a mysterious particle that may exist on the boundary of matter and antimatter. Curiously, it is thought to be both a material particle and its own corresponding antiparticle. Matter and antimatter have long been a cause célèbre in both scientific and science fiction circles. When matter and antimatter collide, they typically disappear in a burst of energy – not so with the Majoranas, thought to be stable and robust.

By virtue of these attributes, the mysterious Majoranas may be instrumental in solving other mysteries, perhaps even redefining the nature of the Universe. Some astrophysicists suggest that Majorana particles comprise the elusive ‘dark matter’ thought to form more than 70 percent of the known Universe.

Despite intensive searches, no elementary particle has so far been found that is a Majorana particle. Over the last few years, however, condensed-matter physicists have realized that Majorana could collectively form as quasiparticles, built out of ordinary electrons in matter, under appropriate physical conditions.

Thanks to their reputed robustness, these Majorana quasiparticles are believed to be suitable as the building blocks of quantum computers. Though theoretical at the moment, quantum computers have the potential to be orders of magnitude more powerful than our current digital devices. As envisioned, they would have immense capacities to store information and the ability to solve important computational problems with unprecedented efficiency.

“The challenge we have is that we work with the microscopic, not something like stars and galaxies that you can see and relate to,” explained Lorenza Viola, a professor of physics at Dartmouth College, who co-authored a recent paper published in Physical Review Letters (arXiv.org version).

“I tell my students we are so big, this is so microscopic, and we just don’t have enough imagination to visualize things at this level. But if we could be tiny as an electron, we could understand the quantum world much better.”

In the paper, Prof Viola and her colleagues suggest a locale where Majoranas may be found. They propose a theoretical model to support Majorana quasiparticles forming a class of exotic materials known as topological superconductors.

The team describes the topological superconductors as having a ‘split personality.’ Their outside surfaces conduct electricity like a metal, but inside they are superconductors. The scientists hypothesize that Majoranas should occur only on the surface or on the interior-exterior interface.

Remarkably, unlike existing proposals, Prof Viola and colleagues’ proposal requires only conventional superconductivity in the bulk of the material, and no application of strong magnetic fields, preserving the important fundamental symmetry of ‘time reversal.’

The hunt for Majoranas is currently under way in laboratories across the world.  In a recent experiment conducted by Dutch researchers, a semiconductor nanowire covered with a superconducting film was cooled in a strong magnetic field and found to possibly support Majoranas, as signaled by a peak in the conductance at zero energy. As additional experimental evidence is collected and scrutinized, Prof Viola believes that s-wave topological superconductors will provide another rich arena in which to explore Majorana physics and further uncover new fundamental properties of topological quantum matter.

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Bibliographic information: Deng et al. 2012. Majorana Modes in Time-Reversal Invariant s-Wave Topological Superconductors. Phys. Rev. Lett. 108, 036803; doi: 10.1103/PhysRevLett.108.036803