Patience is bitter, but its fruits are sweet

As a scientist, you have to be curious, objective, creative, precise, honest and most of all…patient. Patient for ideas to ripen, patient for solutions to pop up, patient for big breakthroughs to come. Scientists try to understand how the world works, but sometimes, it can easily take decades to understand only a small part of the puzzle. And it may often require an element of serendipity.

It took Nigel Hussey and his team 10 years to finally solve the mystery of the upturn of the electrical resistance at low temperatures in an exotic metal known as purple bronze, a puzzle that had foxed researchers for more than three decades. Purple bronze is itself a curious material. Metallic at high temperatures. Seemingly insulating at intermediate temperatures, then at the lowest temperature, a superconductor. Many proposals had been put forward to explain the crossover from a metal to an insulator below its crossover temperature but no consensus had been reached on its origin. But now, they believe they have the answer. Thanks to a chance encounter.

Purple bronze image (00000002)

Photographic image of purple bronze highlighting its beautiful purple sheen. This particular single crystal has been mounted for an X-ray experiment prior to its investigation in the high magnetic fields.

Mysterious symmetry axis

In 2009, Hussey and his team discovered a remarkable new phenomenon – the emergence of a new symmetry axis in the magnetoresistance of purple bronze that set in at precisely the same temperature at which the resistance passes through this minimum. Instead of aligning with the crystallographic axes, as seen in all other materials, the magnetoresistance was found to reach a maximum value whenever the magnetic field was directed at an angle that was aligned with a specific element of the crystalline structure. But what caused the emergence of this new real-space axis? No one really had a clue. And so the data remained unpublished.

Theory without proof

Flash forward to 2016. Hussey attended a seminar in Nijmegen given by Piotr Chudzinski, then a postdoc at the Institute of Theoretical Physics at Utrecht University. In his seminar, Chudzinski described a theory he had been developing over a number of years to explain the physics of purple bronze. The theory itself had not been well received by the community, as there was no experimental proof to back it up and Chudzinski was having troubles getting his paper published. At the end of the seminar, Hussey realized that his model might actually offer an explanation for the mysterious symmetry axis that had popped up in his results all those years ago. The couple discussed the issue at length and decided on a smoking-gun experiment to prove whether or not Chudzinski’s theory worked. Jianming Lu, Hussey’s new postdoc at the time, then conducted a new series of experiments in the high magnetic fields at HFML and the data matched Piotr’s hypothesis beautifully. Not only did the theory explain the emergence of the new symmetry axis, it is also shed light on the 30-year-old mystery of the resistivity upturn.

Purple bronze image

Series of field sweeps of the magnetoresistance of purple bronze at 1.2 K measured at various angles within the ac plane. As the field increases from zero, the resistance of the crystal initially grows, as found in other metals. Eventually, however, the resistance reaches a peak and starts to decrease towards its initial value. This reduction in the resistance is due to the melting of the excitons that had initially impeded the flow of charge along the chain direction (b-axis). At the ‘magic angle’ - labelled ϴCR - the peak is pushed towards the highest field studied, implying that at this orientation, the effect of the magnetic field on the excitons is the weakest. This is consistent with the theory developed by Dr. Piotr Chudzinski.

Worth the wait

Hussey is relieved that after so long the mystery is solved, and we are a bit closer to understanding the strange superconductivity that emerges out of the seemingly insulating state in purple bronze. “Suddenly, everything seemed to fit. This whole episode shows how science works sometimes. You can’t plan it. But it shows the benefit of talking to people about ideas or data, no matter how strange they may seem. Piotr and I will continue to collaborate on this fascinating topic. And hopefully one day, we will be able to unravel the mystery of superconductivity in purple bronze too. It might take another 10 years to crack the enigma. But as with this result today, it would definitely be worth the wait.”

Related publication

Emergence of a real-space symmetry axis in the magnetoresistance of the one-dimensional conductor Li0.9Mo6O17, Jianming Lu, Xiaofeng Xu, M. Greenblatt, R. Jin, P. Tinnemans, S. Licciardello,  M. R. van Delft,  J. Buhot,  P. Chudzinski and N. E. Hussey, Science Advances (2019)


Nigel Hussey