A rare phenomenon discovered by quantum scientists may hold the key to creating a ‘perfect switch’ in quantum devices that can switch between being an insulator and a superconductor. The study, led by the University of Bristol and published in Science, discovered that these two opposing electronic states exist 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 small stimulus such as heat or light, can cause an instant transition from an insulator with zero conductivity to a superconductor with unlimited conductivity, and vice versa. This polarized versatility, referred to as ’emergent symmetry,’ has the potential to provide an ideal On/Off switch in future quantum technology developments.
Lead author Nigel Hussey, Professor of Physics at the University of Bristol, said: “It’s 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 Ph.D. students, Xiaofeng Xu, and Nick Wakeham, measured the magnetoresistance – the change in resistance caused by a magnetic field – of purple bronze.”
Finding no coherent explanation for this puzzling behavior, the data lay dormant and published unpublished for the next seven years.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.
Prof Hussey
The resistance of purple bronze was highly dependent on the direction in which the electrical current was introduced in the absence of a magnetic field. Its temperature dependence was also somewhat complex. The resistance is metallic at room temperature, but as the temperature drops, the resistance reverses and the material appears to become an insulator. The resistance then plummets again at the lowest temperatures as it transitions into a superconductor. Despite this complexity, magnetoresistance was discovered to be extremely simple. It was essentially the same regardless of which way the current or field was aligned, and it followed a perfect linear temperature dependence all the way from room temperature to the superconducting transition temperature.
“Finding no coherent explanation for this puzzling behavior, the data lay dormant and published unpublished for the next seven years.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,” Prof Hussey explained.
“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, Prof Hussey was working at Radboud University and saw advertised a seminar by physicist Dr 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 conduction electrons and elusive, composite particles known as ‘dark excitons,'” Prof Hussey said. After the seminar, we talked and proposed an experiment to test his theory. The following measurements essentially confirmed it.”
Prof Hussey, encouraged by this success, resurrected Xu and Wakeham’s magnetoresistance data and showed it to Dr Chudzinski. Chudzinski was intrigued by the data’s two central features: linearity with temperature and independence from current and field orientation. He was also intrigued by the fact that the material itself could exhibit both insulating and superconducting behavior depending on how the material was grown.
Dr Chudzinski wondered whether rather than transforming completely into an insulator, the interaction between the charge carriers and the excitons he’d introduced 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.
Prof. Hussey went on to say: “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.”
Physicists are well-versed in the phenomenon of symmetry breaking, which occurs when an electron system’s symmetry is reduced due to cooling. An ice crystal’s complex arrangement of water molecules is an example of broken symmetry. The inverse, on the other hand, is an extremely rare, if not unique, occurrence. Returning to the water/ice analogy, it appears that as the ice cools further, the complexity of the ice crystals ‘melts’ into something as symmetric and smooth as a water droplet.
Dr Chudzinski, now a Research Fellow at Queen’s University Belfast, said: “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.”
Another PhD student at Radboud University, Maarten Berben, investigated an additional 100 individual crystals, some insulating and others superconducting, to see if the theory held water.
“After Maarten’s Herculean effort, the story was complete, and the reason why different crystals exhibited such wildly different ground states became apparent,” Prof Hussey added. In the future, this ‘edginess’ could be used to create switches in quantum circuits where tiny stimuli induce profound, orders-of-magnitude changes in switch resistance.”
