Researchers have found that charge density waves are just the tip of the iceberg of the charge modulation phenomenon; charge density fluctuations are much more pervasive, and may be crucial to unlocking the secrets of high temperature superconductors. Image: Yen Strandqvist/Chalmers University of Technology.Researchers from Chalmers University of Technology in Sweden and Politecnico di Milano in Italy have uncovered a crucial new feature of charge density modulations in cuprate superconductors. As they report in a paper in Science, they have identified a new electron wave that could help reveal some of the mysteries of superconducting materials.
Superconductors are materials that when exposed to a certain temperature known as the 'critical temperature' suddenly acquire incredible new properties – chiefly, the ability to conduct electrical charge with zero resistance. Most superconductors currently in commercial use have a low critical temperature – typically meaning below about -240°C. High critical temperature superconductors, meanwhile, are those that exhibit superconducting properties at a somewhat higher temperature – though still hundreds of degrees below zero. The most common type are known as 'cuprates', made from a mixture of copper and oxygen.
High critical temperature superconductors are known to have a variable charge density, meaning that their electrical charge is unevenly distributed. This partly results from what are known as 'charge density waves', which were discovered a few years ago. But these waves have only been observed to exist sporadically, under certain conditions. Therefore, they were not believed to be a contributing factor in the materials' superconducting properties.
But the researchers from Chalmers and Politecnico di Milano have now discovered an additional aspect to the variable charge density, which they term ‘charge density fluctuations’. These fluctuations are an additional charge modulation, collective and with a shorter correlation length. They are also very pervasive, meaning that compared to conventional charge density waves, they are present at a much greater range of temperatures – up to room temperature and beyond – and at different levels of oxygen doping.
The researchers identified these charge density fluctuations in cuprates by conducting inelastic X-ray scattering (RIXS), which is a spectroscopy technique where photons (X-ray radiation) get scattered from a material due to interaction with electronic clouds. They conducted RIXS at the European Synchrotron Radiation Facility in Grenoble, France, in collaboration with researchers from the Italian National Research Council (CNR) and the Sapienza University of Rome, Italy.
"These charge density fluctuations could be a crucial ingredient of the highly unconventional room temperature properties of high critical temperature superconductors – something which challenges our common understanding of the charge transport in metals," says Riccardo Arpaia, a postdoctoral researcher in the Department of Microtechnology and Nanoscience at Chalmers, who carried out the research.
"One could say the charge density waves, which were already very well known, were just the tip of the iceberg," he explains. "The charge density fluctuations which we have now identified are like the hidden bulk of the iceberg. The discoveries were possible thanks to the major developments of synchrotron-based x-ray scattering techniques, and to the quality of the samples we have used." The samples were fabricated at the Italian National Research Council in Napoli, and in the research group at Chalmers led by Floriana Lombardi.
The researchers also investigated how the charge density fluctuations evolve with the temperature of the material. Charge density waves change abruptly as soon as the critical temperature is reached – meaning, dependent on whether the material is in a superconducting state or not. In contrast, the newly discovered charge density fluctuations are unaffected by the superconductivity, indicating that the two characteristics are not 'in competition' with one another. This finding might strengthen the researchers' theory that the charge density fluctuations are key to explaining the mystery of these materials.
Because superconductors operate at such low temperatures, they require cooling from liquid helium or liquid nitrogen, making them expensive and difficult to use outside of certain commercial applications. But if a superconductor could be made to work closer to room temperature, it could have enormous potential. There is thus a lot of interest in improving our understanding of how this class of superconductors works.
"Since 2012, when charge density waves in cuprates were first observed, their importance had not been disputed – but their role had remained unclear," said Giacomo Ghiringhelli, professor of physics at Politecnico di Milano. "The newly observed charge density fluctuations appear to be a very general property of these materials, meaning they are likely playing a crucial role in the transport of electric current in cuprates."
This story is adapted from material from Chalmers University of Technology, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.