Charge carrier pairs in cuprate compounds shed light on high-temperature superconductivity
High-temperature superconductivity remains a mystery, but an international team at BESSY II has made a breakthrough. They've measured the energy of charge carrier pairs in undoped La2CuO4, revealing that interaction energies within the potentially superconducting copper oxide layers are significantly lower than those in the insulating lanthanum oxide layers. This discovery brings us closer to understanding high-temperature superconductivity and could have implications for other functional materials.
The research is published in the journal Nature Communications (https://www.nature.com/articles/s41467-025-65314-w).
Forty years ago, a new class of materials emerged: high-temperature superconductors. These materials conduct electricity without loss, not just at extreme cold temperatures, but also at higher temperatures, albeit still below room temperature. Such materials are already in use, but the phenomenon remains largely unexplained.
The key to their smooth glide through the crystal lattice under certain conditions lies in specific interactions between charge carriers. Now, an international team led by Professor Alexander Föhlisch at BESSY II has precisely measured the energy of charge carrier pairs on oxygen atoms in an experiment.
The samples, provided by the University of Rome, consisted of alternating layers of copper oxide and lanthanum oxide (La2CuO4). When doped with foreign atoms, this compound becomes superconducting below 40 Kelvin, with superconductivity occurring in the CuO layers while the LaO layers remain insulating. Missing electrons around oxygen atoms, known as oxygen holes, are believed to play a central role in superconductivity.
The measurements were conducted on undoped La2CuO4 at room temperature. The goal was to determine the strength of interactions between charge carriers in the two different oxide layers and how they differ.
The team used time-of-flight spectrometers with a unique configuration to detect electron pairs using Auger photoelectron coincidence spectroscopy. Special X-ray pulses (PPRE pulses) struck the sample at intervals of several hundred nanoseconds, allowing for precise measurement of the interaction processes that occur at millions of times faster rates.
The method proved effective, as it selectively observed the relevant copper oxide layer. The interaction energies were significantly lower in the copper oxide layer, central to superconductivity, compared to the insulating lanthanum oxide layers.
These findings contribute to our understanding of high-temperature superconductivity. Additionally, the measurement technique can provide insights into other functional materials, according to Alexander Föhlisch.
For more information, see Danilo Kühn et al, Direct observation of the on-site oxygen 2p two-hole Coulomb energy in La2CuO4, Nature Communications (2025). DOI: 10.1038/s41467-025-65314-w (https://dx.doi.org/10.1038/s41467-025-65314-w).
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