Quantum materials provide the platforms on which the next generation of technologies will be built, with the potential to revolutionise computing, sensing, and the transmission of data and energy. Before this promise can be realised, however, we need to understand the physics of these materials and find new ways of controlling their properties.
This is both aided and hindered by a key feature of quantum materials: a strong coupling between what physicists call “degrees of freedom”. For example, structural degrees of freedom (the positions of the atoms in a crystalline lattice) can be coupled to magnetic degrees of freedom (the arrangement of quantum spins) such that changes in one induce changes in the other. This allows the magnetic properties to be controlled by distorting the lattice (and vice versa), but also makes it hard to disentangle what is driving quantum effects in the material.
A standard technique that physicists use to try to understand quantum materials is to see how they respond to changes in their environment, such as different temperatures or electric/magnetic fields. Recently, there has been much interest in using strain as a tuning parameter – literally stretching and squeezing the material. By applying precise strains along one direction, particular structural degrees of freedom can be isolated and their impact on the material properties investigated.
Now, researchers from UCL, ISIS Neutron and Muon Source, and Diamond Light Source have collaborated to combine strain tuning with neutron and X-ray scattering, allowing the structural and magnetic response of the materials to be measured simultaneously. They chose to study the material Ca3Ru2O7, which displays a complex phase transition at low temperature where all the spins simultaneously rotate by 90°.
Lead author Cameron Dashwood, a PhD student at UCL at the time of the study, explained:
Ca3Ru2O7 is a classic example of a quantum material where there are lots of things going on at the same time. The structural, electronic and magnetic degrees of freedom are intricately coupled together, so it is difficult to separate their influences.
The team found that they could trigger this spin reorientation just by straining the material, revealing the central role of the lattice. This insight was passed to their theoretician collaborators, who were then able to develop a microscopic model of the transition.
Adam Walker, co-lead author and fellow PhD student at UCL, continued:
The strain experiments provided a wealth of information to guide our theoretical work. It was really pleasing to see experiment and theory come together to provide new understanding of a complex problem.
For their experiments, the team used the WISH instrument at ISIS and beamline I16 at Diamond, designing custom strain setups at each to take full advantage of their particular strengths. This required significant collaboration with the instrument/beamline scientists, technicians and engineers at each facility. It represents the latest development in a long-standing partnership between UCL and the large-scale science facilities at Diamond and ISIS.
Dan Porter, beamline scientist on I16 at Diamond, noted:
This was a brilliant demonstration of the strengths of resonant X-ray scattering on I16. Using the new piezoelectric strain cells we were able to perform simultaneous X-ray diffraction, polarisation and spectroscopy measurements to disentangle the magnetic, electronic and structural properties, all automated at different temperatures and strains - some great work from the engineering and software teams ensured this was a success.
As well as shedding new light on the physics of Ca3Ru2O7, a key goal of the collaboration was that the new strain setups will be available for other groups to come and use for their own experiments.
Cameron said:
We put a lot of work into trying to make the equipment as easy as possible for people to use in the future. We fully integrated the new hardware and software into WISH and I16 so that measurements can be fully automated. Strain tuning is a technique with a lot of potential for new discoveries, so we are very excited to see what new research ideas it is used for.
To find out more about the I16 beamline or discuss potential applications, please contact Principal Beamline Scientist Alessandro Bombardi: [email protected].
Dashwood, C. D. et al. Strain control of a bandwidth-driven spin reorientation in Ca3Ru2O7. Nat. Commun. 14, 6197 (2023).
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