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Emergent Symmetry: An Ideal On/Off Switch in Future Quantum Technologies

An uncommon occurrence identified by quantum scientists could be the key to developing a ‘perfect switch’ in quantum devices that can switch between being an insulator and a superconductor.

Image shows a representation of emergent symmetry, showing a perfectly symmetric water droplet emerging from a layering of snow. The ice crystals in the snow, by contrast, have a complex shape and therefore a lower symmetry than the water droplet. The purple color denotes the purple bronze material in which this phenomenon was discovered. Image Credit: University of Bristol

The study, led by the University of Bristol and published in Science, discovered that these two opposing electronic states occur within purple bronze, a one-dimensional metal made up of individual conducting chains of atoms.

Tiny changes in the material, such as those caused by a minor stimulus such as heat or light, can cause an immediate transition from an insulator with zero conductivity to a superconductor with unlimited conductivity and vice versa. This polarized adaptability, referred to as ‘emergent symmetry,’ has the potential to provide a perfect On/Off switch in future quantum technology breakthroughs.

It is a really exciting discovery which could provide a perfect switch for quantum devices of tomorrow. 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.

Nigel Hussey, Study Lead Author and Professor, Physics, University of Bristol

The resistance of purple bronze was heavily reliant on the direction in which the electrical current was introduced in the absence of a magnetic field. Its temperature dependency was likewise somewhat complex. The resistance is metallic at room temperature, but when the temperature drops, the resistance reverses, and the material seems to become an insulator. The resistance then plummets again at the lowest temperatures as it converts into a superconductor.

Despite this intricacy, the magnetoresistance was discovered to be extremely simple. It was practically the same regardless of which way the current or field was aligned, and it followed a perfect linear temperature dependence all the way from ambient temperature to the superconducting transition point.

Finding no coherent explanation for this puzzling behavior, the data lay dormant and published unpublished for the next seven years. 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,” Hussey added.

Prof Hussey was working at Radboud University in 2017 when he saw an advertisement for a session on purple bronze by physicist Dr Piotr Chudzinski. His interest was sparked because few researchers were spending a full session on this little-known subject at the time.

Hussey added, “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.

Prof Hussey, encouraged by this result, resurrected Xu and Wakeham’s magnetoresistance data and showed it to Dr Chudzinski. Chudzinski was interested in the data’s two central properties: linearity with temperature and the independence of current and field direction - as well as the fact that the material itself could display both insulating and superconducting behavior depending on how the material was developed.

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

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 further stated.

Symmetry breaking, the process where an electron system’s symmetry is lowered with cooling, is a phenomenon that physicists understand well. One example of this type 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 like when the ice cools down more, the intricate ice crystals “melt” back into a smooth, symmetrical droplet.

Imagine a magic trick where a dull, distorted figure transforms into a beautiful, perfectly symmetric sphere. 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.

Dr. Piotr Chudzinski, Research Fellow, Queen’s University Belfast

Another Ph.D. student at Radboud University, Maarten Berben, looked into 100 more individual crystals, some of which were insulating and others of which were superconducting, in an effort to determine whether the idea held water.

Hussey concluded, “After Maarten’s Herculean effort, the story was complete and the reason why different crystals exhibited such wildly different ground states became apparent. 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.

Journal Reference

Chudzinski, P., et al. (2023) Emergent symmetry in a low-dimensional superconductor on the edge of Mottness. Science. doi:10.1126/science.abp8948.


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