Energy of charge carrier pairs in cuprate compounds

High-temperature superconductivity is still not fully understood. Now, an international research team at BESSY II has measured the energy of charge carrier pairs in undoped La₂CuO₄. Their findings revealed that the interaction energies within the potentially superconducting copper oxide layers are significantly lower than those in the insulating lanthanum oxide layers. These results contribute to a better understanding of high-temperature superconductivity and could also be relevant for research into other functional materials.

Around 40 years ago, a new class of materials suddenly became famous: high-temperature superconductors. These materials can conduct electricity completely loss-free, not only at temperatures close to absolute zero (0 Kelvin or minus 273 degrees Celsius), but also at much higher temperatures, albeit still well below room temperature. Such materials are already being used in technology. However, the phenomenon is still not fully understood. Well established, however, is the role of specific interactions between the charge carriers which ensure that they glide smoothly through the crystal lattice under certain conditions.

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 from the University of Rome consisted of alternating layers of copper oxide and lanthanum oxide, with the molecular formula La₂CuO₄. When this compound is doped with foreign atoms, it can become superconducting below 40 Kelvin, with superconductivity occurring in the CuO layers while the LaO layers remain insulating. It is assumed that missing electrons around oxygen atoms, known as oxygen holes, play a central role in superconductivity. The measurements were carried out on undoped La₂CuO₄ at room temperature.

‘We wanted to find out how strong the interactions are between charge carriers in the two different oxide layers and how they differ,’ explains first author Dr Danilo Kühn, first author of the study, who conducted the measurements at BESSY II as part of the Uppsala-Berlin Joint Laboratory.

For the experiment, 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, leaving enough time to carefully measure the interaction processes that occur many millions of times faster.

‘Our method allowed is to analyse these interactions precisely because we selectively observe the relevant copper oxide layer,’ says Kühn. The interaction energies were significantly lower in the copper oxide layer— central to superconductivity — than in the insulating lanthanum oxide layers.

‘These results help us to better understand the mechanisms of high-temperature superconductivity,’ explains Alexander Föhlisch, and adds: ‘This measurement technique can also provide insights into other functional materials.’