Purple bronze can switch on ’emergent symmetry’

Quantum scientists have discovered a rare phenomenon that could hold the key to creating a ‘perfect switch’ in quantum devices that flips between being an insulator and superconductor. A team led by researchers at the University of Bristol in the UK has found that these two opposing electronic states exist within purple bronze, a unique one-dimensional (1D) metal composed of individual conducting chains of lithium, molybdenum and oxygen atoms (Li_0.9Mo₆O₁₇). They report their findings in a paper in Science.

Tiny changes in this material, such as prompted by a small stimulus like heat or light, may trigger an instant transition from an insulating state with zero conductivity to a superconducting state with unlimited conductivity, and vice versa. This polarized versatility, known as ‘emergent symmetry’, has the potential to offer an ideal on/off switch for future quantum technologies.

“It’s a really exciting discovery which could provide a perfect switch for quantum devices of tomorrow,” said Nigel Hussey, professor of physics at the University of Bristol and lead author of the paper. “The remarkable journey started 13 years ago in my lab when two PhD students, Xiaofeng Xu and Nick Wakeham, measured the magnetoresistance – the change in resistance caused by a magnetic field – of purple bronze.”

In the absence of a magnetic field, the resistance of purple bronze is highly dependent on the direction in which the electrical current is applied. Its temperature dependence is also rather complicated. Around room temperature, its resistance is metallic, but this reverses as the temperature falls and the material appears to turn into an insulator. Then, at the lowest temperatures, the resistance plummets again as it transitions into a superconductor.

Despite this complexity, the researchers found that the magnetoresistance was extremely simple. It was essentially the same irrespective of the direction in which the current or field were aligned and followed a perfect linear temperature dependence all the way from room temperature down to the superconducting transition temperature.

“Finding no coherent explanation for this puzzling behavior, the data lay dormant and unpublished for the next seven years,” Hussey said. “A hiatus like this is unusual in quantum research, though the reason for it was not a lack of statistics.

“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.”

In 2017, Hussey was working at Radboud University in the Netherlands and saw an advert for a seminar by physicist Piotr Chudzinski on the subject of purple bronze. At the time, few researchers were devoting an entire seminar to this little-known material, so his interest was piqued.

“In the seminar, Chudzinski proposed that the resistive upturn may be caused by interference between the conduction electrons and elusive, composite particles known as ‘dark excitons’. We chatted after the seminar and together proposed an experiment to test his theory. Our subsequent measurements essentially confirmed it.”

Buoyed by this success, Hussey resurrected Xu and Wakeham’s magnetoresistance data and showed them to Chudzinski. The two central features of the data – the linearity with temperature and the independence to the orientation of the current and field – intrigued Chudzinski, as did the fact that the material itself could exhibit both insulating and superconducting behaviour depending on how it was grown.

Chudzinski wondered whether, rather than transforming completely into an insulator, the interaction between the charge carriers and excitons he’d suggested earlier could cause the former to gravitate towards the boundary between the insulating and superconducting states as the temperature is lowered. At the boundary itself, the probability of the system being an insulator or a superconductor is essentially the same.

“Such physical symmetry is an unusual state of affairs and to develop such symmetry in a metal as the temperature is lowered, hence the term ‘emergent symmetry’, would constitute a world-first,” Hussey said:

Physicists are well versed in the phenomenon of symmetry breaking – lowering the symmetry of an electron system upon cooling. The complex arrangement of water molecules in an ice crystal is an example of such broken symmetry. But the converse is an extremely rare, if not unique, occurrence. Returning to the water/ice analogy, it is as though upon cooling the ice further, the complexity of the ice crystals ‘melts’ once again into something as symmetric and smooth as the water droplet.

“Imagine a magic trick where a dull, distorted figure transforms into a beautiful, perfectly symmetric sphere,” said Chudzinski, now a research fellow at Queen’s University Belfast in the UK. “This is, in a nutshell, the essence of emergent symmetry. The figure in question is our material, purple bronze, while our magician is nature itself.”

To further test whether the theory held water, an additional 100 individual crystals, some insulating and others superconducting, were investigated by another PhD student, Maarten Berben, working at Radboud University.

“After Maarten’s Herculean effort, the story was complete and the reason why different crystals exhibited such wildly different ground states became apparent,” said Hussey. “Looking ahead, it might be possible to exploit this ‘edginess’ to create switches in quantum circuits, whereby tiny stimuli induce profound, orders-of-magnitude changes in the switch resistance.”

This story is adapted from material from the University of Bristol, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.