The material purple bronze has a never-before-seen capacity to change between electrical states, becoming an insulator or a conductor depending on small shifts in physical conditions.
Purple bronze sounds like a contradiction in terms – isn't part of what defines the mix of copper and tin it's distinctly non-purple color? More confusingly still, the compound known as purple bronze contains none of the elements of the metal that defined an age. Its full name is lithium molybdenum purple bronze and its chemical formula is Li0.9Mo6O17. This combination of elements may not prove as important to the age of quantum data processing as the original bronze was 4,000 years ago, but it could still make new things possible.
Already interesting to solid-state scientists for the way its three-dimensional crystals behave like a one-dimensional metal, purple bronze is set to attract a lot more attention following the discovery of how easily its electrical properties can be changed.
Close to absolute zero, purple bronze is often a superconductor, applying no resistance at all to the flow of electricity. Although this is true of an expanding range of materials, purple bronze is distinctive in changing from superconductivity to being a Mott insulator with just a small change of temperature, or even light exposure (Mott insulators are materials that theory wrongly predicts will conduct electricity). Raise the temperature again to one at which humans are comfortable, and it turns into a regular conductor.
If the Internet is just a series of tubes, computers can be seen as little more than an array of switches. The viability of the much-heralded quantum computing revolution depends in large part on the capacity to flip many switches with ease, making purple bronze's qualities important.
“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,” said the University of Bristol's Professor Nigel Hussey in a statement.
Without a magnetic field purple bronze behaves like a diode, allowing current to flow one way, but not another. Then there is the unusual response to temperature described above. Magnetic fields tend to complicate things, but in this case the team found the opposite. In a moderately strong magnetic field purple bronze's conductivity becomes linearly related to temperature until superconductivity starts.
“Finding no coherent explanation for this puzzling behaviour, the data lay dormant and 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,” Hussey added.
By chance Hussey encountered Dr Piotr Chudzinski, who had a theory purple bronze's behavior could be attributed to “dark excitons”, objects that behave like particles, but are not in fact particles, and the way they interact with electrons. Hussey and Chudzinski devised experiments that confirmed this.
Now the collaboration has demonstrated the most remarkable feature of the material yet. Under the right conditions, there can be an effectively 50/50 chance that purple bronze is a superconductor or an insulator. With the two opposite states separated by such as thin barrier, it becomes very easy to flick from one to the other and back again.
When substances freeze, they lose their symmetry. Purple bronze does something similar, in its electrical behavior, but bizarrely regains that symmetry when conditions are cold enough.
“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.
The authors tested purple bronze's symmetry with three crystals of the material, two of which became superconducting at very low temperatures, and one that did not. Exploring the commonalities and differences of these samples helps reveal the causes of the strange behavior.
“Imagine a magic trick where a dull, distorted figure transforms into a beautiful, perfectly symmetric sphere,” Chudzinski said. “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.”
The study is published in Science.