Research reveals rare metal could offer revolutionary switch for future quantum devices

A unique phenomena that quantum physicists have found may be the key to developing a "perfect switch" in quantum devices—one that allows them to transition between being an insulator and a superconductor.

The University of Bristol-led study, which was published in Science, discovered that purple bronze, a special one-dimensional metal made up of individual conducting chains of atoms, has these two opposing electronic states.

For example, minute alterations in the material may cause an instantaneous transition from an insulating state with zero conductivity to a superconductor with infinite conductivity, and vice versa, when triggered by minuscule stimuli like as light or heat. "Emergent symmetry," the term for this polarized adaptability, may prove to be an excellent On/Off switch in the future advances of quantum technology.

Lead author Nigel Hussey, a physics professor at the University of Bristol, stated, "This is a very interesting discovery that may offer the ideal switch for future quantum devices.

"The remarkable journey started 13 years ago in my lab when two Ph.D. students, Xiaofeng Xu and Nick Wakeham, measured the magnetoresistance—the change in resistance caused by a magnetic field—of purple bronze."

Purple bronze's resistance depended greatly on which way electrical current was introduced when there was no magnetic field. Its dependency on temperature was likewise rather intricate. The resistance is metallic at ambient temperature, but when the temperature drops, this is reversed, and the material seems to be becoming an insulator. Then, when it turns into a superconductor at the lowest temperatures, the resistance falls once further.

Surprisingly, the magnetoresistance was discovered to be quite simple despite its intricacy. It followed a perfect linear temperature dependency from room temperature to the superconducting transition temperature, and was virtually the same regardless of the direction in which the current or field was oriented.

Since there was no logical explanation for this perplexing behavior, the data became dormant and was not released for the following seven years. This kind of pause is uncommon in quantum research, but Prof. Hussey clarified that there was no statistical shortfall to account for it.

"Such simplicity in the magnetic response invariably belies a complex origin and as it turns out, its possible resolution would only come about through a chance encounter."

When Prof. Hussey was employed at Radboud University in 2017, he noticed an advertisement for a purple bronze seminar given by physicist Dr. Piotr Chudzinski. His attention was aroused since, at the time, not many scholars were spending a full seminar on this little-known subject.

Professor Hussey stated, "Chudzinski suggested in the lecture that the resistive upturn may be brought on by interference between the elusive dark excitons and conduction electrons. After the session, we had a conversation and came up with an experiment to validate his idea. Our further measurements effectively verified it."

Encouraged by this triumph, Professor Hussey brought back the magnetoresistance data from Xu and Wakeham and presented it to Dr. Chudzinski. Chudzinski was interested in the two main aspects of the data: the material's independence from the direction of current and field and its linearity with temperature. He was also intrigued by the material's potential to display both superconducting and insulating behavior, depending on the growth method.

Dr. Chudzinski questioned whether, as the temperature is lowered, the interaction between the excitons he'd introduced earlier and the charge carriers could cause the former to gravitate towards the boundary between the insulating and superconducting states rather than completely changing into an insulator. The likelihood of the system becoming an insulator or a superconductor is almost equal at the border itself.

According to Prof. Hussey, "Developing such symmetry in a metal as the temperature is lowered, hence the term 'emergent symmetry,' would constitute a world-first." Such physical symmetry is an exceptional condition of things.

Symmetry breaking, the process where an electron system's symmetry is lowered with cooling, is a phenomena that physicists understand well. One example of this kind of broken symmetry is the intricate arrangement of water molecules in an ice crystal. However, the opposite is a very uncommon, if not unique, situation. Going back to the water/ice comparison, it seems as though the intricacy of the ice crystals "melts" back into something as symmetrical and smooth as the water droplet as the ice is further cooled.

As a research fellow at Queen's University Belfast, Dr. Chudzinski stated, "Consider a magic act in which a beautiful, perfectly symmetric sphere appears out of a drab, misshapen image. This is the essence of emergent symmetry, to put it briefly. The figure in question is purple bronze, our material, and nature itself is our magician."

Another Ph.D. student at Radboud University, Maarten Berben, looked at 100 more individual crystals, some of which were superconducting and others of which were insulating, in order to see if the idea held water.

Prof. Hussey said, "Maarten's extraordinary work brought the narrative to a close and revealed the reason why many crystals had such drastically disparate ground states. In the future, this "edginess" may be used to build switches in quantum circuits where minute inputs cause significant, orders-of-magnitude shifts in the switch resistance."