Discovery unravels emergent atomic vibrations in superlattices

Scientists and engineers collaborating across seven universities and two national laboratories have made a fundamental discovery about the atomic structure and vibrations in multilayer nanostructures, advancing the design of materials with unique infrared and thermal properties.

Their paper, Emergent Interface Vibrational Structure of Oxide Superlattices, was published January 26 in Nature.

Their discovery emerged from a long-standing collaboration of microscopy, spectroscopy and theory experts spanning fields from physics to engineering to materials science. Eric Hoglund, first author and postdoctoral researcher, and James Howe, Thomas Goodwin Digges Professor of Materials Science and Engineering, both at the University of Virginia School of Engineering and Applied Science, employed microscopy techniques to study the atomic structure and vibrations of perovskite oxides.

“You can tune desired properties such as magnetism, conductivity and heat transport or induce emergent phenomena by changing how different oxides couple to each other, how many times the oxides are layered and the thickness of each layer,” Hoglund said.

The precisely controlled samples of strontium titanate and calcium titanate, stacked repeatedly to form a superlattice, were grown by Ramesh Ramamoorthy, professor of physics and materials science and engineering, and his group at the University of California Berkeley and Lawrence Berkeley National Laboratory, and Roman Engel-Herbert, associate professor of materials science and engineering, physics and chemistry at Penn State University.

Hoglund and Jordan Hachtel, an R&D associate in the Center for Nanophase Materials Sciences at Oak Ridge National Laboratory, are the first researchers to use a new generation of scanning transmission electron microscopes to observe the atomic structure and vibrational aspects of the superlattices.

Using atomic-resolution imaging, Hoglund showed that when the interfaces of the superlattice become very close, the atomic arrangements characteristic of the layers cease to exist and the atom positions at the interfaces appear everywhere. Hachtel then used a novel technique called monochromated electron energy-loss spectroscopy, which combines high-energy resolution with high-spatial resolution for simultaneous imaging and spectroscopy, to unravel the consequence of the rearranging atoms.

“We were able to peer inside the superlattice to see vibrations in each layer and at each interface,” Hachtel said. “This allowed us to directly connect the motion of microscopic heat carrying vibrations to the atomic structure of the superlattice. Experiments like ours enable the rational design of infrared materials with tailored photonic and phononic properties.”

Sokrates T. Pantelides, University Distinguished Professor of Physics and Engineering, William A. & Nancy F. McMinn professor of physics, and professor of electrical engineering at Vanderbilt University, provided the theoretical underpinnings for detailed analysis of the experimental findings.

“Theory enables diverse observations to be combined into a coherent whole. In this case, the results enable the design of nanostructures with desired thermal and infrared properties,” Pantelides said.

Postdoctoral scholar De-Liang Bao and research assistant professor Andrew O’Hara in Pantelides’ group performed extensive quantum calculations that enabled the detailed analysis and identified the precise vibrations of each atom in the collective modes that Hoglund and Hachtel had observed.

The changing landscape of atomic vibrations affects infrared optical properties in the entire superlattice, as shown in experiments performed by graduate student Joseph Matson, a member of Vanderbilt University’s Nanophotonic Materials and Devices Lab, led by Joshua D. Caldwell, the Flowers Family Chancellor Faculty Fellow and associate professor of mechanical engineering and electrical engineering, and experiments conducted at Sandia National Laboratories by teammates in the Specere lab at Purdue University, led by Thomas Beechem, associate professor of mechanical engineering.

UVA’s ExSiTE research group, led by Patrick Hopkins, Whitney Stone Professor of Nuclear Engineering and professor of mechanical and aerospace engineering, provided the final proof. Senior scientist John Tomko and Ph.D. student Sara Makarem used ultrafast spectroscopy to demonstrate that interfaces in the superlattices control non-linear optical properties and the lifetime of thermal vibrations.

“I think this will enable advanced materials discovery,” Hopkins said. “Scientists and engineers working with other classes of materials may now look for similar properties in their own studies.”

The long-standing collaboration continues. Hoglund is in his second year as a postdoctoral researcher, working with both Howe and Hopkins. Together with Pantelides, Hachtel and Ramamoorthy, he expects they will have new and exciting atomic structure-vibration ideas to share in the near future.

Acknowledgment of Funders

A number of external funding organizations supported this research collaboration.

The Office of Naval Research supports the microscopy experiments conducted by Hoglund and members of Hopkins’ research group, through MURI program grant number N00014-18-1-2429. The Army Research Office also supports this work through grant number W911NF-21-1-0119. Makarem also acknowledges support from the NIH Biotechnology Training Program.

The U.S. Department of Energy’s Materials Science and Engineering Directorate, a part of Basic Energy Sciences in the Office of Science, supports theory work at Vanderbilt University under grant number DE-FG02-09ER46554, supplemented by the McMinn Endowment.

Electron energy loss spectroscopy experiments were conducted as part of a user proposal at the Center for Nanophase Materials Sciences, a DOE Office of Science User Facility, and used instrumentation within the Oak Ridge National Laboratory materials characterization core provided by UT-Battelle, LLC, under Contract No. DE-AC05- 00OR22725. 

Research conducted in Joshua D. Caldwell’s Nanophotonic Materials and Devices Lab was supported by the National Science Foundation, Division of Materials Research award number 1904793.

Calculations were performed at the National Energy Research Scientific Computing Center, a U.S. Department of Energy Office of Science user facility located at Lawrence Berkeley National Laboratory, operated under contract number DE-AC02-05CH11231.

The oxide heteroepitaxy synthesis work at the University of California Berkeley is supported by the quantum materials program of the DOE Office of Science, Basic Energy Sciences, under contract number DE-AC02-05CH11231. 

Ramamoorthy Ramesh grew superlattices with support from an Army Research Office multi-institutional research initiative under agreement W911NF-21-2-0162.

Jayakanth Ravichandran grew superlattices supported by two Army Research Office grants, W911NF-19-1-0137 and W911NF-21-1-0327.