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Home / Science / Crucial new aspect of charge density modulation uncovered in high-temperature superconductors

Crucial new aspect of charge density modulation uncovered in high-temperature superconductors



  New Aspect of Charge Density

Charge density waves are only the tip of the charge-modulation phenomenon iceberg: charge-density fluctuations are far more pervasive and can be crucial for deciphering the secrets of high-temperature superconductors. Credit: Yen Strandqvist / Chalmers University of Technology

Researchers from the Chalmers University of Technology and the Politecnico di Milano have identified a crucial new aspect of charge density modulation in high critical temperature cuprate superconductors. They have identified a new wave of electrons that could help uncover some of the secrets of superconducting materials. The results were published in the journal Science .

High critical temperature superconductors have a variable charge density, meaning that their electric charge is distributed unevenly. This is partly due to so-called "charge density waves" discovered a few years ago. However, it has only been observed sporadically that these exist under certain conditions. It was therefore not believed that they have any influence on the superconducting properties of the material.

What the researchers now discovered is an additional aspect of variable charge density, which they call "charge density variability." These were identified as additional collective charge modulation with a shorter correlation length. They are very penetrating, meaning that they are in a much wider temperature range, up to room temperature and beyond, and in varying degrees of oxygen doping compared to conventional charge density waves.

"These charge density variations could be a critical component of the highly unconventional room temperature properties of high temperature superconductors – a challenge that challenges our shared understanding of charge transport in metals," said Riccardo Arpaia, a postdoctoral researcher at the Department of Microtechnology and Nanosciences in Chalmers the research.

  Riccardo Arpaia, Chalmers University of Technology

Riccardo Arpaia, postdoctoral fellow at the Institute of Microtechnology and Nanosciences in Chalmers. Photo credits: Johan Bodell / Chalmers University of Technology

"One could say that the already well-known charge density waves were only the tip of the iceberg. The charge density fluctuations we've identified now are like the hidden mass of the iceberg, "says Riccardo Arpaia. "The discoveries were possible thanks to the main developments of synchrotron-based X-ray diffraction techniques and the quality of the samples we use."

The samples were produced at the Italian National Research Council in Naples and in Italy The working group of Chalmers under the direction of Professor Floriana Lombardi.

Another finding of the work investigates how the fluctuations of the charge density develop with the temperature of the material. While the previously known charge density waves change abruptly as soon as the critical temperature is reached – that is, depending on whether the material is in a superconducting state or not – the newly discovered charge density fluctuations are not influenced by the superconductivity. This indicates that the two characteristics are not in competition with each other. This finding may support the researchers' theory that charge density fluctuations are the key to explaining the secrets of these materials.

Because superconductors operate at such low temperatures, they must be cooled with liquid helium or liquid nitrogen, making them expensive and difficult to use outside certain commercial applications. However, if a superconductor could be brought closer to room temperature, it would have enormous potential. Therefore, there is great interest in better understanding the operation of this superconductor class.

Giacomo Ghiringhelli, professor of physics at the Politecnico di Milano, commented on the study as follows: "Since 2012, when charge density waves appeared in cuprates for the first time, their importance had not been denied – but their role remained unclear. The newly observed charge density fluctuations appear to be a very common property of these materials, meaning that they are likely to play a crucial role in transporting electric current to cuprates.

Read the article "Dynamic Charge Density Fluctuations that Traverse the Phase Diagram of a Cu-based High Temperature Superconductor" in the journal Science .

More information about superconductors

Superconductors are materials that, upon exposure to a certain temperature critical temperature "suddenly receive incredible new properties – especially that they can conduct electrical charge with a resistance of zero.

Most commercially used superconductors are referred to as the low critical temperature, which is typically less than about -240 degrees Celsius. High-critical superconductors, on the other hand, are those that have superconducting properties at a slightly higher temperature – albeit still hundreds of degrees below zero. The most common types are so-called cuprates, which are made from a mixture of copper and oxygen – exactly this class of superconductors studied by the researchers.

European Research Collaboration

Riccardo Arpaia, co -The chief author of the paper is a researcher at the Chalmers University of Technology, also part of the international post-doctoral program of the Swedish Research Council at the Politecnico di Milano in the group of Giacomo Ghiringhelli who designed the experiment.

] Chalmers and Politecnico di Milano are both members of the IDEA League, an alliance of five leading European technology universities dedicated to promoting and promoting European science and technology research through the exchange of academic resources and knowledge promote.

European Synchrotron Radiation Facility in Grenoble in collaboration with researchers from the Italian National Research Council (CNR) and the Sapienza University of Rome.

Resonant Inelastic X-Ray Scattering

Researchers identified charge density variations using a technique known as resonant inelastic X-ray scattering. RIXS is a spectroscopy technique in which photons (X-rays) are scattered from a material through interaction with electronic clouds.

RIXS, as its name implies, is a resonant technique because the energy of the incident photons coincides and therefore resonates with a specific electronic transition (the Cu L3 edge at 931 eV in the case presented in the paper). This strongly amplifies the signal. For this reason, RIXS is currently the best technique for detecting weak charge-density modulation with particularly short correlation lengths, even beyond the limits of neutron scattering and scanning tunneling microscopy.

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The Special The results presented in this work were made possible by the innovative "ERIXS" instrument, jointly realized by the ESRF and the Politecnico di Milano.

Reference: "Dynamic Variations in Charge Density Penetrate the Phase Diagram of a Cu-based High-Tc Superconductor" by R. Arpaia, S. Caprara, R. Fumagalli, G. De Vecchi, YY Peng, E. Andersson, D. Betto, GM De Luca, NB Brookes, F. Lombardi, M. Salluzzo, L. Braicovich, C. Di Castro, M. Grilli and G. Ghiringhelli, August 30, 2019, Science .
DOI: 10.1126 / science.aav1315


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