Will Weyl fermion be the holy grail of 21st century physics?

Quamrul Haider
Published : 3 August 2015, 09:24 AM
Updated : 3 August 2015, 09:24 AM

Recently, three independent groups of physicists, one led by a Bangladeshi‒American, Professor Zahid Hasan of Princeton University, found evidence of the existence of quasiparticles called Weyl fermions. A quasiparticle is not an elementary particle like an electron, proton, or a neutron that exists in nature. It is a disturbance in a medium that behaves like a particle.

The discovery has aroused the interest about this massless object. What is Weyl fermion? How was its existence predicted? Why all the hype? What are its implications and uses?

In 1928, P. A. M. Dirac proposed a relativistic formulation of quantum mechanics for the electron, a member of the fermion family, from which spin emerges as a natural consequence. A fermion is an elementary particle that has an odd half-integer spin (1/2, 3/2, etc.).The relativistic formulation was necessary because the orbital speed of electrons circling around the nucleus is comparable to the speed of light. His theory also predicted the existence of the electron's antiparticle — the positron — which was discovered in 1932.

In 1929, Hermann Weyl, a German mathematician,was the first person to investigate the consequences of zero mass in the Dirac equations for electrons. In doing so, he found that the Dirac equations were reduced to equations for a new type of massless particle travelling at the speed of light. These particles were given the name Weyl fermions.

In 1930, when Wolfgang Pauli proposed the existence of neutrinos and antineutrinos to explain beta-minus decay, a radioactive process in which a neutron in an atom's nucleus turns into a proton, an electron (beta-minus) and an antineutrino, physicists thought that Weyl fermions could be these particles. However, Pauli rejected the notion on the ground that Weyl's theory violates the preservation of parity, an unheard-of idea back then when parity conservation was a sacred cow.

Parity is an operator which transforms a phenomenon into its mirror image, and parity conservation means nature doesn't care whether we use the original state or the image to study a phenomenon. Besides, in 1998 it was found that neutrinos and antineutrinos travel slower than the speed of light, and hence they are not massless.

Weyl's theory was resurrected in 1957, after T. D. Lee, C. N. Yang and Abdus Salam showed that the much cherished parity conservation is violated in beta-minus decay. Since then, Weyl fermion became a subject of theoretical interest only, and no serious effort was made to hunt it down.

At the beginning of this century, Weyl fermions once again drew the attention of physicists engaged in research involving materials such as graphene and topological insulators that exhibit properties described by the Dirac equations without the mass term. Graphene, a wonder material considered to be the "crowning achievement" of 21st century physics, is a single layer of carbon atoms that are bonded together in repeating pattern of hexagons. Topological insulator is a material whose interior is insulator but outer surface is conducting.

The discovery of Weyl fermion is expected to open up new horizons in the fields of condensed matter physics and materials science. They will become a vital part of research in which graphene is used. Since its discovery in 2003, graphene has attracted the attention of scientists conducting research in the areas of bioengineering, optoelectronics,composite materials, ultrafiltration, touchscreens, photovoltaic cells, energy storage, ballistic armor and much more.

The massless nature of Weyl fermions could be exploited to make electric charges move through a conductor much faster than normal electrons, thereby pushing electronics to new heights. Hopefully, these fermions could pave the way towards building more powerful quantum computers too. Weyl fermions could also be used to transport electric charge through transmission lines a long distance with minimum loss of energy. This would overcome the problem of energy loss due to heating currently faced by the power plants.

So, will Weyl fermion become the holy grail of the 21st century physics? The answer will depend on the impact Weyl fermion will have all across the research frontier, from graphene to "hot" superconductors to quantum computers to topological insulators and beyond.

Dr. Quamrul Haider is a Professor of Physics at Fordham University, Bronx, New York.