by Lori Tamura
Padraic Shafer, ALS staff scientist and leader of the ALS’s dichroism program, is the 2019 recipient of the David A. Shirley Award for Outstanding Scientific Achievement at the ALS. Members of the ALS Users’ Executive Committee selected Shafer “for unveiling the nature of chiral quantum materials through the innovative use of x-ray scattering at the Advanced Light Source,” and he will be giving a Shirley Award talk on the subject at next week’s User Meeting.
The award recognizes Shafer as a trailblazer at the ALS in the use of resonant soft x-ray diffraction (RSXD), a technique that has been described as “x-ray diffraction on steroids.”
“A large part of the success in this direction,” said Shafer, “has been applying polarization-dependent x-ray scattering to see chirality in new materials that have been under scrutiny lately, things like topologically protected magnetic structures, skyrmions, chiral domain walls, polar electric vortices, and so on.”
Most recently, Shafer’s RSXD expertise played a key role in discerning the chirality of electric-dipole skyrmions—the novel electric analogues of the more-familiar magnetic skyrmions. In that work, Shafer collaborated with many researchers, including Ramamoorthy Ramesh (faculty senior scientist in Berkeley Lab’s Materials Sciences Division) and Elke Arenholz (former dichroism program lead and deputy for Photon Science Operations at the ALS, currently associate director of CHESS, the Cornell High-Energy Synchrotron Source).
According to Shafer, “The project began at a lunchtime conversation where Ramesh said to Elke and me, ‘hey, we’ve just done some electron microscopy on this material, it looks like the electric dipoles are all curling up. Maybe you could tell us something more about what’s going on in there.’”
“When you use x-rays that are on-resonance or near-resonance of an electronic transition with RSXD,” said Shafer, “the atoms that scatter the x-rays have a strong directional dependence—a strong anisotropy—and you now become sensitive to the full three-dimensional symmetry of the material, which is necessary to look at these chiral structures.”
Other high-impact RSXD work published this year involved the discovery of a new form of charge ordering in a cuprate superconductor. This collaboration, led by Riccardo Comin of MIT, performed RSXD measurements at the ALS showing that, at low doping levels, the charge modulations exhibit a “glassy” behavior, with a tendency to periodic ordering but without any preference in orientation.
Such discoveries involving charge and spin—which define the fundamental properties of strongly correlated materials—are important steps toward solving many open questions of modern condensed matter physics. They’re also of practical interest because they are thought to represent a path toward implementing and controlling robust, high-speed, nonvolatile memory and low-power computing architectures in future microelectronic devices.
The RSXD endstation at ALS Beamline 4.0.2, which Shafer first began working on ten years ago as part of a Laboratory Directed Research and Development (LDRD) project under the guidance of Arenholz, is one of the few places in the world where this type of research can be performed. His efforts along these lines intensified around 2015 and led not only to the development of this advanced capability, but also to the creation of a vibrant community that counts on the ALS for their studies of ordering phenomena in a wide spectrum of materials.
The higher coherence and brightness of the Advanced Light Source Upgrade (ALS-U) Project will clearly benefit this diffraction-based program. Shafer envisions a nanoprobe capable of looking at heterogeneous structures across a sample. “We’ll be able to focus the beam potentially down to 50 nm,” he said, “and be able to scan around and see where there is one type of electronic order and where there is another.”
In the meantime, Shafer acknowledges that connecting with collaborators to produce the material samples and others to do the computational modeling is as important as the technique itself. “We try to attack problems from multiple dimensions and encourage our users to think the same way,” he said. “We have a piece of spectroscopy, a piece of x-ray scattering, a piece of x-ray imaging. Many of our users also go to neutron facilities, or use electron microscopy. One piece of the puzzle is rarely enough.”
The award will be presented to Shafer at the upcoming ALS User Meeting, where he will give the Shirley Award talk, “Chiral Nanostructures Singled Out by Resonant X-Ray Scattering.” The award is named after David Shirley, a Professor of Chemistry at UC Berkeley and Director of LBNL from 1980 to 1989, whose vision was instrumental in building the ALS.