Zahid and team’s research opens up new possibilities in nanotechnology

A Bangladeshi physicist of Princeton University has led an international research team in discovering a novel quantum state of matter that can be 'tuned' at will - and it's 10 times more tunable than existing theories can explain. M Zahid Hasan and his team's discovery of this level of manipulability of quantum matter opens up enormous possibilities for next-generation nanotechnologies and quantum computing.

Hasan, the Eugene Higgins Professor of Physics, shot to fame in 2014 when he led a team of scientists discovering 'Weyl fermion', an elusive massless particle theorised 85 years ago. In pursuit to an alternative theory of gravity, Albert Einstein's colleague at Princeton, physicist Hermann Weyl, first predicted the particle back in 1929.

About the latest discovery, considered a potential gamechanger in quantum physics, Prof Zahid Hasan said, "We found a new control knob for the quantum topological world. We expect this is tip of the iceberg. There will be a new subfield of materials or physics grown out of this. ... This would be a fantastic playground for nanoscale engineering."

Hasan and his colleagues, whose research - "Giant and anisotropic spin-orbit tunability in a strongly correlated kagome magnet" - appears in the September 12 issue of Nature, are calling their discovery a "novel" quantum state of matter because it is not explained by existing theories of material properties.

Princeton University's Office of Communications' writer Liz Fuller-Wright wrote elaborately about Zahid Hasan and his team's discovery.

Hasan's interest in operating beyond the edges of known physics attracted Jia-Xin Yin, a postdoctoral research associate and one of three co-first-authors on the paper, to his lab. Yin said, when he talked to Professor Hasan, "He (Hasan) told me something very interesting. He's searching for new phases of matter. The question is undefined. What we need to do is search for the question rather than the answer."

The classical phases of matter - solids, liquids and gases - arise from interactions between atoms or molecules. In a quantum phase of matter, the interactions take place between electrons, and are much more complex, wrote Liz Fuller-Wright.

"This could indeed be evidence of a new quantum phase of matter - and that's, for me, exciting," said David Hsieh, a professor of physics at the California Institute of Technology and a 2009 Ph.D. graduate of Princeton, who was not involved in this research. "They've given a few clues that something interesting may be going on, but a lot of follow-up work needs to be done, not to mention some theoretical backing to see what really is causing what they're seeing."

Hasan has been working in the groundbreaking subfield of topological materials, an area of condensed matter physics, where his team discovered topological quantum magnets in 2012. In the current research, he and his colleagues "found a strange quantum effect on the new type of topological magnet that we can control at the quantum level," said Hasan, who was listed in the Thomson Reuters' World's Most Influential Scientific Minds-2014.

The key was looking not at individual particles but at the ways they interact with each other in the presence of a magnetic field. Some quantum particles, like humans, act differently alone than in a community, Hasan said. "You can study all the details of the fundamentals of the particles, but there's no way to predict the culture, or the art, or the society, that will emerge when you put them together and they start to interact strongly with each other," he said.

To study this quantum "culture," he and his colleagues arranged atoms on the surface of crystals in many different patterns and watched what happened. They used various materials prepared by collaborating groups in China, Taiwan and Princeton. One particular arrangement, a six-fold honeycomb shape called a "kagome lattice" for its resemblance to a Japanese basket-weaving pattern, led to something startling - but only when examined under a spectromicroscope in the presence of a strong magnetic field, equipment found in Hasan's Laboratory for Topological Quantum Matter and Advanced Spectroscopy, located in the basement of Princeton's Jadwin Hall.

All the known theories of physics predicted that the electrons would adhere to the six-fold underlying pattern, but instead, the electrons hovering above their atoms decided to march to their own drummer - in a straight line, with two-fold symmetry.

"The electrons decided to reorient themselves," Hasan said. "They ignored the lattice symmetry. They decided that to hop this way and that way, in one line, is easier than sideways. So this is the new frontier. ... Electrons can ignore the lattice and form their own society."

The researchers were shocked to discover this two-fold arrangement, said Songtian Sonia Zhang, a graduate student in Hasan's lab and another co-first-author on the paper. "We had expected to find something six-fold, as in other topological materials, but we found something completely unexpected," she said. "We kept investigating - Why is this happening? - and we found more unexpected things. It's interesting because the theorists didn't predict it at all. We just found something new."

"There are many things we can calculate based on the existing theory of quantum materials, but this paper (published in Nature) is exciting because it's showing an effect that was not known," he said. This has implications for nanotechnology research especially in developing sensors. At the scale of quantum technology, efforts to combine topology, magnetism and superconductivity have been stymied by the low effective g factors of the tiny materials.

"The fact that we found a material with such a large effective g factor, meaning that a modest magnetic field can bring a significant effect in the system - this is highly desirable," said Hasan. "This gigantic and tunable quantum effect opens up the possibilities for new types of quantum technologies and nanotechnologies."

The discovery was made using a two-story, multi-component instrument known as a scanning tunneling spectromicroscope, operating in conjunction with a rotatable vector magnetic field capability, in the sub-basement of Jadwin Hall. The spectromicroscope has a resolution less than half the size of an atom, allowing it to scan individual atoms and detect details of their electrons while measuring the electrons' energy and spin distribution.

"We're going down to 0.4 Kelvin. It's colder than intergalactic space, which is 2.7 Kelvin," said Hasan. "And not only that, the tube where the sample is - inside that tube we create a vacuum condition that's more than a trillion times thinner than Earth's upper atmosphere. It took about five years to achieve these finely tuned operating conditions of the multi-component instrument necessary for the current experiment," he said.

"All of us, when we do physics, we're looking to find how exactly things are working," said Sonia Zhang. "This discovery gives us more insight into that because it's so unexpected."

By finding a new type of quantum organization, Zhang and her colleagues are making "a direct contribution to advancing the knowledge frontier - and in this case, without any theoretical prediction," said Hasan. "Our experiments are advancing the knowledge frontier."

In an email conversation with this writer back in 2015, M Zahid Hasan was upbeat about his team's 2014 discovery of massless particle - Weyl fermion. Hasan hoped then that future electronics would be replaced by 'Weyltronics', a term coined by him.

"In Weyltronics, mass-less super-fast Weyl fermions will replace the slow moving electrons. A Weyl fermion is like half-of-an-electron. In early universe, two Weyl fermions combined to create one electron. Really cool stuff!" said the physics professor who hails from Gazipur of Bangladesh. "Although at this stage it is a physics discovery, in the longer run it can lead to some cool devices."

"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," said Hasan, also the Principal Investigator of the Princeton's Laboratory for Topological Quantum Matter and Spectroscopy. The Weyl fermion was discovered inside a synthetic metallic crystal called tantalum arsenide (TaAs). Hasan and his team bombarded TaAs crystal with a beam of photons (particles of light) to discover Weyl fermions. Tantalum's main use today is in tantalum capacitors in electronic equipment such as mobile phones, video game systems and computers. Highly corrosion-resistant hard, blue-grey, lustrous transition metal -- Tantalum -- is also used for medical implants and bone repairs.

Hasan explained electricity in these crystals could theoretically move 1,000 times faster than in conventional semiconductors, "and the crystals can be improved to do even better". The upshot could be faster electronics that consume less energy. "This is really great news for developing new forms of electronics and computing." The fact that Weyl fermions have no mass suggests that they could shuffle electric charge inside electronics far more quickly than electrons can, Hasan explained. Another potentially useful quality of Weyl fermions is that they cannot move backwards. Instead of bouncing away from obstacles, they zip through or go around roadblocks. In contrast, electrons can scatter backwards when they collide with obstructions, hindering the efficiency of their flow and generating heat, he adds. "Weyl fermions could be used to solve the traffic jams that you get with electrons in electronics -- they can move in a much more efficient, ordered way than electrons," said Hasan.

Among two sons and a daughter, Hasan is the eldest of Rahmat Ali and Nadira Begum couple. His father is a lawyer and a lawmaker and mother, housewife. M Zahid Hasan did his SSC from Dhanmondi Government Boys High School and HSC from Dhaka College with outstanding results. He studied at the University of Texas in Austin and got his PhD from Stanford University. He joined Princeton as a lecturer. Now, he is a professor of Physics with a specific interest in the field of Quantum Condensed Matter Physics at the university. His research work features in Physics Today, Nature News, Science News, New Scientist, Scientific American, and Physics Worlds.

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