August 19, 2015

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Princeton University Physics Professor M. Zahid Hasan, born and brought up in Bangladesh, electrified the world of physics last month by discovering the elusive massless quasi particle, Weyl fermion, predicted 85 years earlier. By its ability to act as high mobility particle that can behave as matter and antimatter inside a crystal, the use of the particle can help advance electronics and computing by making these energy-efficient and faster.

Physicist and mathematician Hermann Weyl predicted the existence of Weyl fermions in 1929. In a recent paper published in Nature Communications, Professor Zahid Hasan and his team of international collaborators theorised the existence of Weyl fermions as quasi particles in a tantalum arsenide type of crystals. Through high precision experiments at Princeton University laboratories, they evaluated several crystal structures before finding the asymmetric crystal tantalum arsenide.

Cooling the crystal to near absolute zero in a two-storied scanning tunnelling spectromicroscope, they ascertained if the crystal matched the theoretical specifications for hosting a Weyl fermion. Those crystals that passed the test were taken to Lawrence Berkeley National Laboratory in California and subjected to high energy accelerator-based photon beams. Weyl fermions’ existence was confirmed by studying the shape, size and direction of the photon beams after they had traversed the crystals.

The findings were published in Science on July 16. Negatively charged electrons drive modern electronics. Massless Weyl fermions can potentially provide more stable, efficient and speedier transport of particles than electrons once materials are further perfected for device fabrication. The Weyl fermions come in both varieties – right-handed and left-handed- meaning they spin in the same direction of motion, or its opposite. “The physics of the Weyl fermion are so strange, there could be many things that arise from this particle that we’re just not capable of imagining now,” says Professor Hasan.

Professor Hasan emphasised that unlike many other exotic particles which appear fleetingly in the aftermath of particle collisions, Weyl fermions, discovered inside the synthetic tantalum arsenide crystal, can be controlled for utilisation in devices.

“For a physicist, the Weyl fermions are most notable for behaving like a composite of monopole and antimonopole-like particles when inside a crystal,” Professor Hasan added.

“This means that Weyl particles that have opposite magnetic-like charges can nonetheless move independently of one another with a high degree of mobility,” the Professor elaborated.

Professor Zahid Hasan with other Princeton scientists who worked on the experiment in which the quantum Hall effect was seen in a material without the presence of an applied magnetic field. Photo: Brian Wilson

While electrons are lost when they collide with an obstruction, Hasan et al found that Weyl fermions can be used to create massless electrons that move very quickly without backscattering, which hinders efficiency and generates heat in modern electronics.  “Weyl electrons simply move through and around roadblocks giving them ultrahigh mobility,” Professor Hasan explained.

Professor Zahid Hasan is the latest in the line of world famous physicists hailing from the subcontinent. Bangali mathematical physicist Satyendra Nath Bose (1894-1974), who spent 25 years at Dhaka University’s Physics Department (1921-45), was the first. In 1924, Bose sent a paper to Albert Einstein arguing that the Maxwell-Boltzmann distribution did not apply to subatomic particles, and proposed a different statistic. 

Einstein agreed, translated Bose’s paper, “Planck’s Law and Hypothesis of Light Quanta” into German, and published it in Zeitschrift für Physik with Bose as the sole author, launching “Bose-Einstein statistics.” To honour Bose, Nobel Prize-winning physicist Paul Dirac named the particles obeying Bose-Einstein statistics “bosons” (such as, Higgs boson).  Dirac also coined the term “fermions” (such as Weyl fermions) for the particles obeying Fermi-Dirac statistics, to honour Enrico Fermi. Fermions obey Pauli exclusion principle.

Bose inspired Pakistan’s Professor Mohammad Abdus Salam (1926-96), a theoretical physicist, who shared the Nobel Prize in 1979 for his work on electroweak unification.  The first Indian scientist to win the Nobel Prize was Chandrasekhara Venkata (CV) Raman (1888-1970) in 1930.  Just as Hermann Weyl had predicted the fermions named after him and Professor Zahid Hasan discovered it, Adolf Smekal had theoretically predicted the inelastic scattering of photons in 1923, before Raman experimentally discovered it. These days, Raman spectroscopy is considered a part of Chemistry.  CV Raman’s nephew, astrophysicist Subrahmanyam Chandrasekhar (1910-1995), University of Chicago, also won the Nobel Prize in 1983.

Mohammad Zahid Hasan (Taposh) is the eldest of the three children of Mr. Mohammad Rahman Ali and Mrs. Nadira Ali Talukdar.  Zahid’s mother encouraged him to study science. In 1986, at 16, Zahid published his first book on science, Aesho Dhumketur Rajjae (Come to the World of Comets: An Astrophysics Primer), unwittingly foreshadowing his own meteoric rise.  Zahid stood second in the combined merit list of Dhaka Board, from Dhanmondi Government Boys School in the SSC examination of 1986, and first in HSC from Dhaka College in 1988.  

After attending a few days of class at Dhaka University’s Physics department, Zahid left for the University of Texas at Austin on a scholarship, to study physics. At Austin, Zahid came under the influence of (took classes from) Professor Steven Weinberg, who had shared the 1979 Nobel Prize with Professor Mohammad Abdus Salam.

Zahid completed his masters and doctorate at Stanford University. Not only did Zahid collaborate with many distinguished physicists, his own renown began spreading far and wide. In 2002, Princeton University invited him to present a seminar on his research, and offered him a faculty position afterwards. Zahid became a full professor in 2011.

If one were to list in this write up the innumerable awards this young physicist has won in America and abroad, that would exhaust the space available here. Suffice to say Professor Zahid Hasan is a most sought after speaker and delivers seminars worldwide to expound on the fields of research he excels in, including “topological insulators” which can potentially be the basis for a new kind of quantum information and computing.

It has been my privilege to know Zahid ever since he came to Princeton in 2002. I have never met a person so brilliant and famous, yet so self-effacing and humble. Zahid’s wife Sarah is an equally brilliant MIT-educated engineer, who works for Microsoft. They have two adorable children: Arik Ibrahim Hasan (13) and Sarina Maryam Hasan (11).

Currently, Zahid is the toast of Princeton University’s Physics Department, which boasts of 17 former and current Nobel Prize winners. Yet, the way Zahid carries himself, one would never know! Professor Zahid Hasan is the pride and inspiration of his community – Bangladeshis, Muslims and South Asians in America.



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