In a recent breakthrough detailed in Nature Communications, a collaborative effort among researchers from Politecnico di Milano, Chalmers University of Technology in Göteborg, and Sapienza University of Rome sheds new light on a perplexing aspect of copper-based superconductors with high critical temperatures. These materials exhibit unique behavior, acting as “strange” metals even when temperatures surpass the critical threshold. In this state, their electrical resistance undergoes temperature-dependent changes distinct from conventional metals.
The study points towards the presence of a quantum critical point associated with the “strange metal” phase. This finding marks a significant stride in superconductivity research, holding promise for sustainable technologies and contributing to a more environmentally friendly future.
Riccardo Arpaia, a researcher at the Department of Microtechnology and Nanoscience at Chalmers and the lead author of the study, explains, “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 0°C due to microscopic temperature effects, cuprates turn into a ‘strange' metal because of quantum charge fluctuations.”
The research relies on X-ray scattering experiments conducted at the European Synchrotron ESRF and the British synchrotron DLS. These experiments unveil charge density fluctuations impacting the electrical resistance of cuprates in a manner that renders them “strange.” Systematic measurement of how the energy of these fluctuations varies allowed the identification of the charge carrier density at which this energy reaches its minimum: the quantum critical point.
“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 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,” adds Giacomo Ghiringhelli, Professor at the Physics Department of the Politecnico di Milano and coordinator of the research.
Sergio Caprara, alongside colleagues at the Department of Physics of Sapienza University of Rome, formulated the theory assigning a crucial role to charge fluctuations in cuprates. He remarks, “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.”
Source: Polytechnic University of Milan