Controversial superconductor breakthrough disputed once again

May 14, 2021
The superconducting circuit used by Lancaster University scientists. The gap between the gates is only 100 nanometers, roughly the length of a virus. (Lancaster University)

The superconducting circuit used by Lancaster University scientists. The gap between the gates is only 100 nanometers, roughly the length of a virus. (Lancaster University)

A team of U.K. researchers has become the second group to declare that a potentially revolutionary behavior of superconducting materials found in earlier research was simply a case of leaking electrons, but the discoverers of the phenomenon insist the new research fails to explain away their findings.

Italian scientists recently reported that they could weaken the superconductivity of a material with a static electric field, a surprising ability that would allow for the construction of very useful superconductor devices.

But in a paper published May 12 in Nature Communications, the U.K. team tested a similar experimental setup and concluded that the reduction in superconductivity was caused not by the electric field effect but by a small stream of disruptive electrons.

At the core of the dispute are superconductors and how they react to outside electric influences. Often produced by ultracooling many metals and alloys, these materials have no electric resistance and can carry electric currents with practically zero energy loss, a very useful behavior that is already exploited in technologies such as MRI machines and particle accelerators.

The Italian researchers' claim is that superconductors can become weaker conductors when exposed to external electric fields, much like semiconductors do. By generating an electric field and imposing a "field effect," interactions with a semiconductor's electrons can adjust the material's conductivity, controlled easily with the voltage used to make the field — crucial in the operation of transistors and other semiconductor devices.

But given the very low resistance in superconductors and their dense packing of electrons, conventional physics says that electric fields cannot pierce deeply into a superconductor and meaningfully change its conductivity. Achieving the field effect in a superconductor would require enormous voltages, which would break down the circuits used in an experiment, said Yuri Pashkin, a professor and chair of experimental condensed-matter physics at Lancaster University and a corresponding author of the new study.

"It is well known that in metals and in superconductors, this effect should not exist," Pashkin said.

But in 2018 and 2019, a team of scientists affiliated with Italian university Scuola Normale Superiore reported a demonstration of the field effect in superconducting materials and nanodevices. 

In their first paper, published in Nature Nanotechnology, the researchers showed that low voltages of 40 volts or below could suppress the superconducting effects of metallic superconductors. Many of the same authors published three other papers in prestigious journals in the following year — Nano Letters, ACS Nano and Physical Review Applied — and expanded their investigation into the apparent breakthrough.

"These results could represent a groundbreaking asset for the realization of all-metallic superconducting field-effect electronics and leading-edge quantum information architectures," they wrote in the Nature Nanotechnology paper. 

The scientists' findings attracted efforts to replicate their findings and look for alternative explanations for the surprising field effect. 

"On one hand, it was a big surprise or even a shock for the physics community," Pashkin said. "But on the other hand, people just thought, 'OK, maybe they overlooked something.'"

Pashkin and his Lancaster co-authors conducted an experiment with a similar setup to the first Italian paper: a superconducting nanodevice called a Josephson junction undergoing a field created by two circuit gates. It was all connected to a high-frequency resonator that provided time measurements of higher fidelity than the earlier paper.

The study revealed changes in the resonator's noise properties and quality factor, leading the Lancaster scientists to suggest that heated electrons were hopping from the current being run to the superconductor, where the particles — not the field effect — disrupted the device's superconductivity.

"This was quite suspicious for us, to see that the noise increases and the quality factor decreases," Pashkin said. "By studying these two effects, we came to the conclusion that it was not the field effect but the leakage through the gate to the constriction at intense electric fields."

The Italian scientists ruled out electron field emission as a possible source of leakage, Pashkin said, because they believed the applied voltages were not high enough. But the professor's team found that the electric fields produced in the experiment were just right to cause electron emission and leakage.

The study is the third to explain away the electrostatic field effect, as papers published in January and February similarly concluded that the effect was caused by leaking electrons. 

"I think that already three experiments refuting this claim is already sufficient," Pashkin said, "sufficient for stopping this discussion and basically forgetting about this claim."

Although the controversy is "case closed," according to Pashkin, the scientists on the Italian team stand by their work and criticize the latest study for writing off the field effect with insufficient evidence.

Francesco Giazotto, a scientist at the NEST Laboratory and an author of three of the field-effect papers, said the new paper "pretends to provide an explanation" that is not supported by other experiment research. The ACS Nano paper from the Italy-based lab included a setup that avoided electron leakage and still demonstrated the conductivity-suppressing effect, according to Giazotto.

Giazotto also accused the new study of ignoring some technical details about how electrons are absorbed and emitted in the system, namely the asymmetry of the processes that would lead to a "totally different" response from a superconductor.

"These authors wanted to provide some simplistic interpretation of the effect that is, however, not correct and somewhat superficial," he said.

The explanation is more complicated than the U.K. scientists claimed, Giazotto said, and, "Much more investigation is required to settle the microscopic origin of the effect."

The study, "On the origin of the controversial electrostatic field effect in superconductors," published May 12 in Nature Communications, was authored by I. Golokolenov, Lancaster University, P. L. Kapitza Institute for Physical Problems of RAS and National Research University Higher School of Economics; and A. Guthrie, S. Kafanov, Y. Pashkin and V. Tsepelin, Lancaster University.

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