Vijay Shenoy’s rashbon prophecy
On a cool, cloudy October day in Bangalore, Vijay Shenoy holds up a mug of coffee, traces his finger upwards from the bottom and expounds on a theory that could, as he puts it, “change civilization as we know it”, an idea as important as the discovery of fire.
A flautist and carnatic singer in his spare time, Shenoy, 42, describes the “absolutely fantastic things” that will happen if the coffee—not that it is a candidate for such experiments—were to be chilled to a fraction above absolute zero (-273 degrees Celsius or zero degree Kelvin), as cold as anything can get. This is the hidden, weird world of laboratory created superfluids, which loose viscosity, or become so thin that they can creep through molecular-sized openings. “If my coffee was a superfluid,” says Shenoy, “it would leak out, or, I would need to create a new ceramic to hold it in, and then the superfluid would climb the walls of my cup!”
Superfluids are not new.
Old videos show how liquid helium—ideal for superchilling because of its simple atomic structure—cooled to a fraction above absolute zero, creeps upwards and out of its container and coheres at the bottom into perfect drops, poetically called helium tears.
The principle behind a superfluid is much the same as Shenoy’s larger field of interest, superconductors, materials imbued with strange but spectacular capabilities. This is the arcane, confusing realm of quantum mechanics that Shenoy, an associate professor of physics at the Indian Institute of Science in Bangalore, and his students ruminate over.
In a normal metal, such as aluminium, fundamental particles called electrons flow through in the form of waves, easily diverted by randomly bouncing atoms. This is why wires resist the flow of electricity, leading to energy losses. In a superconductor, electrons clump together to form a collective wave that flows past those bouncy atoms, super-cooled into losing almost all energy and motion. This promises, for instance, electric wires with no transmission losses—as high as 50% in some Indian states—and infinitely faster, cheaper and smaller computers, the so-called quantum computers.
But that’s far in the future, when superconductors can work at room temperature and perhaps usher in a new industrial age. No one really understands the mechanics of zero resistance at such high temperatures. For a superconductor to realize its epoch-changing potential, it must ideally change state and work at room temperature, a Holy Grail that has not been realized since the theory was first put forth 113 years ago.
Part of the reason is that metals tend to be dense, complex associations of atoms. Instead, scientists have found it easier to work with gases such as helium—a loose and simple collection of atoms—that can be cooled to liquids. Somewhat analogous to water becoming ice, the change of state from a fluid to a teary superfluid is called a Bose-Einstein condensate, after the Bengali physicist Satyendranath Bose and Albert Einstein, who proposed it together in 1925.
To understand what Shenoy—a jovial man given to bursts of excitement over the intricacies of physics—does, you must first understand his tools, fermions and bosons, the building blocks of atoms and indeed of the universe. Only bosons, given their habit of sticking together, can create a Bose-Einstein condensate in what is called a quantum state, when they are supercooled to a billionths of a degree above absolute zero. Fermions are loners. They resist company. Their preference for solitude, for instance, shows up inside super-dense dwarf stars—collapsed leftovers of exploding red giant stars—do not crumple further because their fermions resist it.
Fermions and bosons are inherently different, atomic yin and yang, and this is why electrons—a type of fermions—are not easily persuaded to do tricks, such as the Bose-Einstein condensate.
Electrons must pair up to form bosonic objects, called Cooper pairs (predicted in the 1950s), if they are to transform into a state like the Bose-Einstein condensate, says Shenoy. Zero resistance in metals is akin to the zero viscosity of helium tears. “Can the same phenomena happen for electrons and other fermions?” asks Shenoy. “In essence, we would like to make fermions superfluidic, like the helium-4 (the helium used in superfluid experiments) bosons, but how do we get them to do this at ambient conditions in materials?”
This is one of the key questions in condensed matter physics, the discipline whose byproducts span the earliest transistor to the latest iPad Air (indeed, everything we use is condensed matter, from toothpaste to pencils). There are no easy answers because of the atomic complexities of such materials.
As the 21st century unfolds, a branch of physics called atomic-molecular-optics physics affords new opportunities to probe the vexing problems of condensed matter physics. This new direction, called cold-atom quantum emulation, uses atoms and light to simulate electrons in materials. The field of ultra-cold atoms really kicked off after 1997 with the development of cooling by laser (which led to the Nobel prize that year for its three inventors), allowing ultra-cold atoms to be held in traps created by magnetic fields and laser lattices. Ultra-cold atoms allow scientists unprecedented abilities to control interactions between smaller particles—and a means to disentangle the confusing web of relationships between large collections of subatomic particles. The lessons learnt from such studies could then be used to design wild and wonderful new materials, including those superconductors at room temperature.
The work of Shenoy and his group is currently restricted to their minds. It is there that they theorised the possibility of linking the anti-social fermions to one another, however weakly, and the resultant creation of a new particle called the rashbon. The rashbon, like a Cooper pair, is a boson and can become a condensate.
“It is a remarkably beautiful thing,” says Shenoy of the physics behind the rashbon, which he first wrote about and named in a 2011 paper and for which he has been awarded the Bhatnagar Prize this year, India’s national award for promising scientists under 45. A mechanical engineer from IIT Madras and PhD from Brown University, Shenoy discarded the world around for the world within.
In this world, he has mathematically prophesised that the rashbon—if it can be materialized—could spark a superfluid transition at a temperature several times warmer than in materials available today. “This provides a clue to generating higher temperature superconductors,” explains Shenoy.
However, the boson-like coupling of electrons is very tricky because certain varieties cannot exist in the same location, in the same state and at same time. The binding of fermions, Shenoy’s theory goes, is greatly enhanced by fiddling with their intrinsic properties, called spin-orbit coupling.
One of the most important phenomena in quantum physics, spin–orbit coupling describes how an electron’s orbit around the core, or nucleus, of an atom creates a magnetic field and affects the electron’s “spin”, a bizarre and somewhat oddly named physical quality because it isn’t exactly like the spinning of a top or a planet. The “spin” of subatomic particles carries a positive or negative sense and can be oriented up or down; it cannot be changed and varies with the kind of particle. There are additional laws, such as the one that says two “up” or “down” fermions cannot be at the same place. Spin-orbit coupling, as Shenoy’s group showed, can provide many more opportunities for fermions to join together and create rashbons.
Shenoy puts it simply. “If you tune spin-orbit coupling, you open a new range of possibilities, new directions for physics,” he says. But how do you manipulate something tinier than the tiniest thing, something that cannot be seen even by most powerful miscroscopes?
The answer is light. The temperature, density and strength of bonding electrons within an atom can be stage-managed with laser light.
Can such sub-atomic fiddling eventually be reproduced in to those yet-to-be-created new materials? “It’s a question of finding the right atomic species, a class of atoms that allows (the manipulation of) spin-orbit coupling,” says Shenoy, who, like others dabbling in quantum mechanics, has a lot of raw material to work with, a periodic table of the 103 elements known to science. None of this is easy.
How long, I ask Shenoy, will it take his rashbon prophecy to be confirmed in a laboratory, given that it took 70 years to prove what Bose and Einstein propounded. Shenoy laughs. “It won’t take quite that much time with cold-atom experiments; I hope it will happen in my lifetime,” he says. “We are at a stage when we know the fundamental laws of physics. The question now is: How do we get them to work for us?”
Samar Halarnkar is a Bangalore-based journalist. This is a fortnightly column that explores the cutting edge of science and technology. Comments are welcome at email@example.com.
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