Mysterious ‘sudden death’ of quantum vortices in a superconductor stumps scientists
Physicists have observed the mysterious “sudden death” of quantum fluctuations inside a bizarre superconducting material.
The discovery, made in an atom-thin layer of the semimetallic compound tungsten ditelluride, requires a completely new theory to explain it.
Describing why this happens could reveal new insight into superconductors, materials in which electricity flows without resistance. Room-temperature superconductors are considered a “holy grail” of physics that could facilitate near-lossless energy transmission.
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“What we found, by directly looking at quantum fluctuations near the transition, was clear evidence of a new quantum phase transition that disobeys the standard theoretical descriptions known in the field,” Sanfeng Wu, an assistant professor of physics at Princeton University and a co-author of the Jan. 5 study in the journal Nature Physics, said in a statement. “Once we understand this phenomenon, we think there is a real possibility for an exciting, new theory to emerge.”
Phase transitions occur when the atomic arrangements of a material change — such as when a solid melts into a liquid or a liquid evaporates to become a gas. But they can also take place on the quantum level, causing the electrons inside a material to team up in so-called Cooper pairs and flow as a superfluid without any resistance.
This superconductor transition typically occurs only at temperatures close to absolute zero, but the discovery of superconductivity at higher temperatures has for decades tantalized scientists with the hope of reproducing the process at room temperature. But the search has been marred by experiments that haven’t been reproduced and even claims of data falsification.
To better understand when and how superconductivity emerges, the researchers behind the new study peeled a crystal of tungsten ditelluride down to a single-atom layer before supercooling it to a mere 50 milliKelvins (minus 273.10 degrees Celsius or minus 459.58 degrees Fahrenheit).
That left behind a strong insulator; its electrons were too hemmed in to conduct electricity with their flow. But when they added extra electrons and applied a voltage, they produced an extraordinary result: The material transformed into a superconductor.
“Just a tiny amount of gate voltage can change the material from an insulator to a superconductor,” lead author Tiancheng Song, a postdoctoral researcher in physics at Princeton, said in the statement. “This is really a remarkable effect.”
Materials flip phases by accumulating tiny fluctuations in their thermodynamic states. In 2D superconductors, these fluctuations occur thanks to quantum vortices, minuscule whirlpools of magnetic fields that, above a certain temperature and voltage, spread through a material and destroy its ability to superconduct.
This transition occurs at a threshold called the critical electron density — a point where the superconducting electrons in Cooper pairs have enough kinetic energy to break from their partners.
In the new study, the researchers measured the vortices produced by the applied voltage.
Past experiments suggested that these vortices disappear abruptly at high temperatures and magnetic fields (or once they have nudged the material out of the superconducting phase and into the resistive one).
But the experiment found the opposite, at higher temperatures and stronger magnetic fields, the vortices persisted long into the insulating phase of the material.
Additionally, when a superconducting material is cooled to near absolute zero and the electron density is tuned to the knife edge of the phase transition, it should reach a stage called a quantum critical point. With no temperature to drive the phase transition, the material should instead flip between insulating and superconducting phases according to the whims of random vortex fluctuations.
But when the experimenters cooled their material to near absolute zero, they observed that rather than weakly persisting just below the quantum critical point, the quantum whirlpools abruptly disappeared.
“We expected to see strong fluctuations persist below the critical electron density on the non-superconducting side,” Wu said. “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 this observation — the ‘sudden death’ of the fluctuations.”
“In other words, we’ve discovered a new type of quantum critical point, but we don’t understand it,” co-author Nai Phuan Ong, a professor of physics at Princeton, said in the statement.
The researchers said that to describe the shocking development they will need to develop a brand-new theory and test it in the lab. If they’re successful, they will have made a small but vital step in helping us understand the strange behavior of superconducting materials.
