Tiny quantum electronic vortices in superconductors can circulate in ways not seen before

June 01, 2023

(Nanowerk News) Inside a superconductor, tiny tornadoes of electrons, known as quantum vortices, can occur which have important implications in superconducting applications such as quantum sensors. Now a new type of superconducting vortex has been discovered, an international research team reports (Science, “Superconducting vortices carry a temperature-dependent fraction of the quantum flux”).

Egor Babaev, professor at the KTH Royal Institute of Technology in Stockholm, said the research revised the prevailing understanding of how electronic flows can occur in superconductors, based on work on quantum vortices that was recognized in the 2003 Nobel Prize award.

Researchers at KTH, along with researchers from Stanford University, the TD Lee Institute in Shanghai and AIST in Tsukuba, found that the magnetic flux generated by vortices in superconductors can be divided into a wider range of values ​​than previously thought.

It represents new insights into the foundations of superconductivity, and is also potentially applicable in superconducting electronics. An artist’s depiction of quantum vortices. (Illustration: Greg Stewart, SLAC National Accelerator Laboratory)

Inside the quantum vortex

A vortex of magnetic flux occurs when an external magnetic field is applied to a superconductor. The magnetic field penetrates the superconductor in the form of tubes of quantized magnetic flux that form eddies. Babaev said that research initially suggested that quantum vortices pass through superconductors, each carrying one quantum of magnetic flux. But an arbitrary fraction of the quantum flux was not a possibility that entertained the former theory of superconductivity.

Using the Superconducting Quantum Interference Device (SQUID) at Stanford University, Babaev’s co-authors, research scientists Yusuke Iguchi and Professor Kathryn A. Moler, demonstrated at the microscopic level that quantum vortices can exist in a single electronic band. The team was able to create and move around these fractional quantum vortices, said Moler.

“Professor Babaev had been telling me for years that we could see something like this, but I didn’t believe it until Dr. Iguchi actually looked at it and did a number of detailed checks,” he said.

A ‘very unusual’ sight

The Stanford researchers found initial observations of this phenomenon “so unusual,” Iguchi said, that they repeated the experiment 75 times at various locations and temperatures.

The work confirms a prediction Babaev published 20 years ago, which stated that in certain types of crystals, one part of the electron population of a superconducting material can form clockwise-circulating eddies, while other electrons can simultaneously spin counter-clockwise eddies. “These combined quantum tornadoes can carry arbitrary fractions of the quantum flux,” he said.

“It revises our understanding of quantum vortices in superconductors,” he said.

Moler confirmed that conclusion. “I’ve seen vortices in new superconductors for more than 25 years, and I’ve never seen this before,” he said.

Babaev said that the power of the quantum vortex and the possibility of controlling it suggest that it could potentially be used as an information carrier in superconducting computers.

“The knowledge we have acquired, the spectacular method introduced by our colleague Dr. Iguchi and Professor Moler at Stanford, may in the long term have the potential to help certain platforms for quantum computation,” said Babaev.

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