A step towards more sustainable superconductor applications

A recent study published in Nature Communications by teams from the Politecnico di Milano, Chalmers University of Technology in Göteborg, and Sapienza University of Rome has unveiled intriguing insights into high-critical-temperature copper-based superconductors. These materials exhibit unusual properties even above their critical temperature, behaving as ‘strange’ metals. Unlike normal metals, their electrical resistance varies with temperature in a unique manner.

The researchers have unearthed evidence of a quantum critical point associated with the ‘strange metal’ phase. Riccardo Arpaia, a researcher at Chalmers University's Department of Microtechnology and Nanoscience and the study's lead author, explained: “A quantum critical point identifies specific conditions where a material undergoes a sudden change in its properties due solely to quantum effects. Just like ice melts and becomes liquid at zero degrees Celsius due to microscopic temperature effects, cuprates turn into a 'strange' metal because of quantum charge fluctuations.”

This breakthrough stems from X-ray scattering experiments carried out at the European Synchrotron ESRF and the British synchrotron DLS. These experiments revealed charge density fluctuations in cuprates that impact their electrical resistance, contributing to their ‘strange’ nature. A key aspect of the research was determining the charge carrier density at which these fluctuations reach their minimum energy – the quantum critical point.

Giacomo Ghiringhelli, Professor at the Physics Department of Politecnico di Milano and the research coordinator, shared his enthusiasm: "This is the result of more than five years of work. We used a technique, called RIXS, largely developed by us at the Politecnico di Milano. Thanks to numerous measurement campaigns and to new data analysis methods, we were able to prove the existence of the quantum critical point. A better understanding of cuprates will guide the design of even better materials, with higher critical temperatures, and therefore easier to exploit in tomorrow's technologies."

Sergio Caprara and his colleagues at Sapienza University of Rome's Department of Physics, who proposed the theory attributing a pivotal role to charge fluctuations in cuprates, are also key contributors to this discovery. Caprara remarked: “This discovery represents an important advancement in understanding not only the anomalous properties of the metallic state of cuprates but also the still obscure mechanisms underlying high-temperature superconductivity.”

This discovery opens new horizons in the field of superconductivity, potentially leading to the development of materials that could revolutionise various technological applications, making them more sustainable and efficient.