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NEWS

December 2021 ESRFnews

The ESRF and Italy s National Institute for Nuclear Physics (INFN) have renewed a collaboration agreement in the presence of Teresa Castaldo, Italy s ambassador to France, Geneviève Fioraso, the former French Minister for research and higher education, and Gabriele Fioni, the rector for research, innovation and higher education in Auvergne Rhône-Alpes, France.

The ESRF and the INFN have a long history of collaboration, dating back to the beginning of the ESRF. That collaboration has strengthened since 2011 with the INFN s help in the design and construction of the new ESRF EBS vacuum system.

The new agreement is expected to help advance the research and development of accelerator projects in Europe, with an exchange of expertise and a strengthening of technical assistance between the ESRF and the INFN (see Portrait, p25).

ESRF and INFN sign agreement

Straining to understand high- temperature superconductivity Mechanical strain opens a new window into the understanding of high-temperature superconductors, according to a study led by Chalmers University in Göteborg, Sweden, using the ESRF. The results of the study show that strain can be used to modify a superconductor s ground state and control so-called charge- density waves, which can upset the appearance of mysterious strange metal behaviour. High-temperature superconductors

made of doped copper oxides or cuprates have simultaneously tantalised and puzzled physicists since they were discovered in the mid-1980s. Over the years, the temperatures at which they become superconducting have got higher and higher, suggesting that it will be possible to design a cuprate that works in ambient conditions the key to futuristic applications such as super-fast electronics, levitating trains and lossless energy transmission. But theory has failed to make sense of their behaviour, and so is unable to point experimentalists in the right direction.

There are a couple of peculiarities with cuprates, however, that physicists see as potential routes to a better understanding. One is the existence of charge-density waves ripples of electric charge generated by patterns of electrons (or holes, their positively charged counterparts) in the material lattices. Starting in 2012, Giacomo Ghiringhelli at the Politecnico di Milano in Italy and colleagues have used resonant inelastic X-ray scattering (RIXS) at the ESRF to show that biaxial charge-density waves exist in almost all the known cuprate families. The other peculiarity of cuprates

is how their electrical resistance changes as they get hotter. In ordinary metallic conductors, resistance rises with temperature since several processes occur in these materials at the microscopic scale, involving the scattering of electrons: with the atomic lattice, with impurities and

with themselves in a complicated manner. In cuprates, by comparison, the resistance rises with temperature linearly until the material melts. The trait is known as strange-metal behaviour strange because it seems so simple. It is an open question whether

strange-metal behaviour and charge- density waves will ultimately expose the secret of high-temperature superconductivity. Now, however, it is clear that the two peculiarities themselves are linked. The Chalmers-led group which

includes researchers at the Politecnico di Milano and La Sapienza University of Rome in Italy, and the Brandenburg University of Technology in Germany imposed strain on ultra-thin films of a cuprate in the yttrium-barium-copper-oxide family by depositing them on a substrate with mismatching lattice spacing. In collaboration with Nick Brookes, scientist-in-charge of the ESRF s recently upgraded ID32 beamline, the researchers took RIXS data to show that the strain modifies the material ground state, and the charge-density wave was suppressed in one direction. Meanwhile, lab-based resistivity measurements at Chalmers showed that during this wave suppression, the strange-metal behaviour flourishes (Science373 1506). This was a very challenging

experiment, says Chalmers professor Floriana Lombardi. We couldn t have succeeded without the combined use of our lab measurements at Chalmers and the RIXS measurements at the ESRF. In particular, the high sensitivity available at the upgraded ID32 beamline was a crucial ingredient to measure charge-density waves in films just a few unit-cells thick. The researchers do not yet know

how the link between charge-density waves and strange metal behaviour could help the understanding of high-temperature superconductivity in cuprates but it is a new place to start looking for answers. For a phenomenon that has evaded theoretical understanding for so long, any lead is a good lead, says Chalmers co-author Riccardo Arpaia. And strain allows us to tune our studies, to explore the link in detail. It s almost like having a new set of materials.

The high sensitivity available at the upgraded ID32 beamline was a crucial ingredient

Users and staff alike are looking forward to the next ESRF User Meeting, from 7 to 9 February 2022. As for the last User Meeting, the event will be held online due to the limited physical social interactions that are possible under French government recommendations.

This was a heartbreaking decision but the current sanitary restrictions obliged us to choose the online option for our annual User Meeting, says Guillaume Morard, chair of the ESRF User Organisation Committee. A great online experience is better than a limited on-site meeting. This year s programme is very exciting, as it is focusing on topics of particularly great societal importance, such as climate change, gender equality and COVID-19 vaccines.

As well as the usual tutorials, plenaries, directors reports, posters and Young Scientist award, there will be three user-dedicated microsymposia on high-pressure science, ligand and fragment screening in drug design, and sustainable materials science (see Events, p26).

User Meeting stays digital