“Purple Bronze Discovery” reveals the “perfect key” to future technology

Quantum scientists have discovered a phenomenon in purple bronze that could be key to developing the “perfect switch” in quantum devices that switch between being an insulator and a superconductor.

Research conducted by the University of Bristol and published in SciencesThese two opposing electronic states are found within purple bronze, a unique one-dimensional metal composed of chains of individual conducting atoms.

For example, small changes in a material, triggered by a small stimulus such as heat or light, may trigger an instantaneous transition from an insulating state with zero conductivity to a superconductor with unlimited conductivity, and vice versa. This polarization diversity, known as “emergent symmetry,” has the potential to provide a perfect on/off switch in future quantum technology developments.

The image shows a representation of emerging symmetry, showing a perfectly symmetrical drop of water emerging from a layer of ice. In contrast, ice crystals in snow have a complex shape and are therefore less symmetrical than a drop of water. The purple color indicates the purple-bronze material in which this phenomenon was discovered. Credit: University of Bristol

A 13-year journey

Lead author Nigel Hussey, professor of physics at the University of University of Bristol“It’s a really exciting discovery that could provide a perfect key to future quantum devices,” he said.

“The fascinating journey began 13 years ago in my laboratory when two PhD students, Xiaofeng Xu and Nick Wickham, 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 was highly dependent on the direction in which the electric current entered. Its temperature dependence was also complex. At room temperature, the resistivity is metallic, but as the temperature decreases, this reverses and the material appears to turn into an insulator. Then, at the lowest temperatures, the resistance decreases again as it turns into a superconductor. Despite this complexity, magnetoresistance is surprisingly simple. It was essentially the same regardless of the direction in which the current or field was aligned and followed a perfect linear temperature dependence all the way from room temperature down to the superconducting transition temperature.

“No coherent explanation could be found for this puzzling behavior, and the data remained dormant and unpublished for the next seven years. A gap like this is unusual in quantum research, although the reason for this was not a lack of statistics,” Professor Hussey explained.

“Such simplicity in magnetic response always belies a complex origin, and as it turns out, its potential solution will only come about through a chance encounter.”

A chance encounter leads to a breakthrough

In 2017, Professor Hussey was working at Radboud University and saw an advertisement for a seminar by physicist Dr. Piotr Chudzinski on the topic of purple bronze. At the time, few researchers would devote an entire symposium to this unknown substance, so his interest was piqued.

Professor Hussey said: “At the symposium, Chudzinski suggested that the high resistance might be caused by interference between conduction electrons and elusive composite particles known as ‘dark excitons’. We chatted after the symposium and together proposed an experiment to test his theory. Our subsequent measurements essentially confirmed this.”

Thanks to this success, Professor Hussey revived Shaw and Wakeham’s magnetoresistance data and presented them to Dr. Chudzinski. Two key features of the data — linearity with temperature and independence from current direction and field — intrigued Chudzinski, as did the fact that the same material can exhibit insulating and superconducting behavior depending on how the material grows.

Dr. Chudzinski wondered whether the interaction between charge carriers and excitons he presented earlier, rather than converting entirely to insulating, could cause the former to gravitate toward the boundary between insulating and superconducting states as the temperature decreases. At the same limits, the probability of a system being an insulator or a superconductor is essentially the same.

Professor Hussey said: “Such physical symmetry is an unusual case, and developing such symmetry in a metal as the temperature decreases, hence the term ‘emergent symmetry’, would be a world first.”

Physicists are well familiar with the phenomenon of symmetry breaking: the lowering of the symmetry of an electron system upon cooling. The complex arrangement of water molecules in an ice crystal is an example of this broken symmetry. But the opposite is an extremely rare, if not unique, occurrence. Returning to the water/ice analogy, it is as if after the ice is cooled further, the complexity of the ice crystals “melts” back into something consistent and smooth like a drop of water.

Emergent symmetry: a rare phenomenon

Dr Chudzinski, now a research fellow at Queen’s University Belfast, said: “Imagine a magic trick where a dull, distorted shape is transformed into a beautiful, perfectly symmetrical sphere. This, in short, is the essence of emerging symmetry. The person in question is our material, purple bronze, while our magician is nature itself.” .

To further test whether the theory contains water, an additional 100 individual crystals, some insulating and others superconducting, were examined by another PhD student, Martin Berbin, who works at Radboud University.

Professor Hussey added: “After Martin’s titanic efforts, the story is complete and the reason why different crystals appear to have such completely different ground states becomes clear. Looking to the future, it may be possible to exploit this ‘novelty’ to create switches in quantum circuits where small stimuli trigger Deep, large-magnitude changes in switching resistance.

Reference: “Emerging symmetry in a low-dimensional superconductor on the Mottness edge” by P. Chudzinski, M. Berben, Xiaofeng Xu, N. Wakeham, B. Bernáth, C. Duffy, R. D. H. Hinlopen, Yu-Te Hsu, S. Weidman, B. Tinnemans, Rongying Jin, M. Greenblatt, N. E. Hussey, November 16, 2023, Sciences.
doi: 10.1126/science.abp8948