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Eddies of quantum chaos spontaneously emerging in atomically thin layers of insulating material have stumped physicists, requiring revisions to models that could solve some pressing problems in a quest to understand superconductivity.
Experimental physicists from Princeton University in the US and the Japanese National Institute for Materials Science examined the spontaneous appearance of quantum fluctuations at a point of transition from electron traffic-jam to superconducting freeway cutting across a two-dimensional landscape.
"How a superconducting phase can be changed to another phase is an intriguing area of study," says Princeton physicist and senior author, Sanfeng Wu.
"And we have been interested in this problem in atomically thin, clean, and single crystalline materials for a while."
The electrons drifting through the copper wiring behind your drywall have a hard time moving from A to B. Switch on your television, and peak-hour madness unfolds in those wires, with electrons swerving and bumping, tooting their tiny electron horns and shaking their tiny electron fists as their tiny electron engines overheat.
Superconductivity is the dream. It's effortless motion from start to finish. No heat, no wasted energy. It's as efficient as efficient can be, perfect for generating powerful electromagnetic fields or high-speed computing that doesn't melt into a puddle.
Yet it's also not exactly an easy phase of conductivity to produce. It occurs when electrons lose their sense of individuality and fall into sync, forming what's known as Cooper pairs, capable of negotiating the atomic neighborhood with zen-like ease.
This demands a level of chill only achievable with some pretty impressive, heavy-duty equipment. Yet if researchers could understand precisely what triggers this quantum transition and the role temperature plays, they just might be able to make do with a little less cooling.
One area of research involves examining the quantum behavior of electrons trapped on what are effectively 2D surfaces. Deprived of the ability to move up and down, quantum phenomena make their transition into a superconductive state a lot more challenging.
"As you go to lower dimensions, fluctuations become so strong that they 'kill' any possibility of superconductivity," says Princeton physicist Nai Phuan Ong.
The primary killer of the electron's zen state is best described as a quantum vortex. Or as Ong describes it, "quantum versions of the eddy seen when you drain a bathtub."
According to what's known as the BKT transition, after Nobel laureates Vadim Berezinskii, John Kosterlitz, and David Thouless, these murderous whirlpools of doom vanish in 2D materials when the temperature sinks low enough.
Investigating this space of quantum tornadoes playing havoc with superconductive states, Wu and his team crafted a single layer of the semi-metal tungsten ditelluride, which at anything warmer than a whisker above absolute zero is an energy-stifling insulator.
Pumping in enough electrons, however, forces a current to flow in a superconducting manner.
Yet the researchers noticed something quite bizarre when the temperature plummeted. Add enough electrons, you get superconductivity. At a critical level of electron traffic, though, those party-pooping whirlwinds of quantum madness return, switching off the current.
Measuring the swirls revealed they weren't your average quantum vortices, remaining steady at higher temperatures and magnetic fields than theory dictates. When the number of electrons dips below a precise quantity, the vortices suddenly vanish.
"We expected to see strong fluctuations persist below the critical electron density on the non-superconducting side, just like the strong fluctuations seen well above the BKT transition temperature," says Wu.
"Yet, what we found was that the vortex signals 'suddenly' vanish the moment the critical electron density is crossed. And this was a shock. We can't explain at all this observation – the 'sudden death' of the fluctuations."
New models introduce the possibilities of new avenues of research that just might lead to new technology. Given the potential rewards of developing room-temperature superconductivity, it helps to have a good map of the weather on the quantum landscape.
This research was published in Nature Physics.
