‘Sudden death’ of quantum fluctuations challenges current theories of superconductivity

Princeton physicists discovered an abrupt change in quantum behavior while experimenting with a thin three-atom insulator that can be easily transformed into a superconductor.

The research promises to improve our understanding of the quantum physics of solids in general and to propel the study of the quantum physics of condensed matter and superconductivity in potentially new directions. The results were published in the journal Natural physics in a paper titled “Unconventional superconducting quantum criticality in single-layer WTe₂“.

The researchers, led by Sanfeng Wu, assistant professor of physics at Princeton University, found that the sudden cessation (or “death”) of quantum mechanical fluctuations exhibits a series of unique quantum behaviors and properties that seem to escape to established theories. .

Fluctuations are temporary random changes in the thermodynamic state of a material about to undergo a phase transition. A familiar example of a phase transition is the melting of ice into water. The Princeton experiment studied the fluctuations that occur in a superconductor at temperatures near absolute zero.

“What we discovered, by directly examining quantum fluctuations near the transition, was clear evidence of a novel quantum phase transition that disobeys standard theoretical descriptions known in the field,” Wu said. Once we understand this phenomenon, we believe there is a real possibility that an exciting new theory will emerge.”

Quantum phases and superconductivity

In the physical world, phase transitions occur when a material such as a liquid, gas, or solid changes from one state or form to another. But phase transitions also occur at the quantum level. These occur at temperatures close to absolute zero (-273.15° Celsius) and involve the continuous adjustment of certain external parameters, such as pressure or magnetic field, without increasing the temperature.

Researchers are particularly interested in how quantum phase transitions occur in superconductors, materials that conduct electricity without resistance. Superconductors can speed up the information process and provide the basis for powerful magnets used in healthcare and transportation.

“How a superconducting phase can be transformed into another phase is a fascinating area of ​​study,” Wu said. “And we have been interested in this problem in atomically thin, clean, single-crystal materials for some time.”

Superconductivity occurs when electrons pair and flow in unison without resistance and without energy dissipation. Normally, electrons travel through circuits and wires irregularly, jostling each other in a way that ultimately proves inefficient and wastes energy. But in the superconducting state, the electrons act in concert in an energy-efficient manner.

Superconductivity has been known since 1911, although how and why it works remained largely a mystery until 1956, when quantum mechanics began to shed light on the phenomenon. But only in the last decade has superconductivity been studied in clean, atomically thin two-dimensional materials. Indeed, it was long believed that superconductivity was impossible in a two-dimensional world.

“This is because as you go down into lower dimensions, the fluctuations become so strong that they ‘kill’ any possibility of superconductivity,” said N. Phuan Ong, the Eugene Higgins Professor of Physics at the Princeton University and author of the article.

The main way fluctuations destroy two-dimensional superconductivity is through the spontaneous emergence of what is called a quantum vortex (plural: vortex).

Each vortex looks like a tiny whirlpool composed of a microscopic strand of magnetic field trapped in a swirling electronic current. When the sample is raised above a certain temperature, vortices appear spontaneously in pairs: vortex and anti-vortex. Their rapid movement destroys the superconducting state.

“A vortex is like a whirlpool,” Ong said. “These are quantum versions of the whirlpool observed when emptying a bathtub.”

Physicists now know that superconductivity in ultrathin films exists below a certain critical temperature known as the BKT transition, named after condensed matter physicists Vadim Berezinskii, John Kosterlitz and David Thouless. The latter two shared the Nobel Prize in physics in 2016 with Princeton physicist F. Duncan Haldane, professor of physics at Sherman Fairchild University.

The BKT theory is widely considered a successful description of how quantum vortices proliferate in two-dimensional superconductors and destroy superconductivity. The theory applies when the superconducting transition is induced by heating the sample.

Current experience

The question of how to destroy two-dimensional superconductivity without increasing the temperature is an active area of ​​research in the fields of superconductivity and phase transitions. At temperatures near absolute zero, a quantum transition is induced by quantum fluctuations. In this scenario, the transition is distinct from the temperature-induced BKT transition.

The researchers started with a massive crystal of tungsten ditelluride (WTe₂), which is classified as a layered semi-metal. The researchers began by converting tungsten ditelluride into a two-dimensional material by exfoliating or peeling off more and more of the material until they had a single, atom-thin layer.

At this level of fineness, the material behaves like a very strong insulator, meaning its electrons have limited movement and therefore cannot conduct electricity. Surprisingly, the researchers found that the material exhibits a host of new quantum behaviors, such as switching between insulating and superconducting phases. They were able to control this switching behavior by creating a device that functions like an “on/off” switch.

But that was only the first step. The researchers then subjected the material to two important conditions. The first thing they did was cool the tungsten ditelluride to exceptionally low temperatures, around 50 milliKelvin (mK).

Fifty milliKelvin corresponds to -273.10° Celsius (or -459.58° Fahrenheit), an incredibly low temperature at which the effects of quantum mechanics are dominant.

The researchers then converted the insulating material into a superconductor by introducing additional electrons into the material. It didn’t take much voltage to reach the superconducting state. “A tiny amount of gate voltage can transform the material from an insulator to a superconductor,” said Tiancheng Song, a physics postdoctoral researcher and lead author of the paper. “It’s truly a remarkable effect.”

The researchers found that they could precisely control the properties of superconductivity by adjusting the density of electrons in the material via the gate voltage. At critical electron density, quantum vortices rapidly proliferate and destroy superconductivity, thereby causing the quantum phase transition.

To detect the presence of these quantum vortices, the researchers created a tiny temperature gradient across the sample, making one side of the tungsten ditelluride slightly hotter than the other. “The eddies look for the colder edge,” Ong said. “In the temperature gradient, all the vortices in the sample drift toward the colder part. So you’ve created a river of vortices flowing from the warmer part to the colder part.”

The vortex flow generates a detectable voltage signal in a superconductor. This is due to an effect named after Nobel Prize-winning physicist Brian Josephson, whose theory predicts that whenever a vortex flow crosses a line drawn between two electrical contacts, they generate a small transverse voltage, which can be detected by a nano-volt. metre.

“We can verify that this is indeed the Josephson effect; if you reverse the magnetic field, the detected voltage reverses,” Ong said.

“This is a very specific signature of a vortex current,” Wu added. “Directly detecting these moving vortices gives us an experimental tool to measure quantum fluctuations in the sample, which would otherwise be difficult to achieve.”

Surprising quantum phenomena

Once the authors were able to measure these quantum fluctuations, they discovered a series of unexpected phenomena. The first surprise was the remarkable robustness of the vortices. The experiment demonstrated that these vortices persist at temperatures and magnetic fields much higher than expected. They survive temperatures and fields much higher than the superconducting phase, in the resistive phase of the material.

A second major surprise is that the vortex signal abruptly disappeared when the electron density was adjusted just below the critical value at which the quantum phase transition of the superconducting state occurs. At this critical value of electron density, which researchers call the quantum critical point (QCP) which represents a point at zero temperature in a phase diagram, quantum fluctuations determine the phase transition.

“We expected to see strong fluctuations persist below the critical electron density on the non-superconducting side, just like the strong fluctuations observed well above the BKT transition temperature,” Wu said.

“Yet what we discovered was that the vortex signals ‘suddenly’ disappear as soon as the critical electron density is crossed. And that was a shock. We can’t explain this observation at all – the ‘sudden death ‘ fluctuations.”

Ong added: “In other words, we have discovered a new type of quantum critical point, but we don’t understand it.”

In the field of condensed matter physics, there are currently two established theories explaining the phase transitions of a superconductor, the Ginzburg-Landau theory and the BKT theory. However, researchers have found that none of these theories explain the observed phenomena.

“We need a new theory to describe what’s happening in this case,” Wu said, “and that’s something we hope to address in future work, both theoretically and experimentally.”