Berkeley universities

Discovery reveals how atomic vibrations emerge

A hundred years of physics teach us that collective atomic vibrations, called phonons, can behave like particles or waves. When they hit an interface between two materials, they can bounce back like a tennis ball. If the materials are thin and repetitive, as in a superlattice, phonons can jump between successive materials.

There is now definitive experimental proof that at the nanoscale, the notion of multiple thin materials with distinct vibrations no longer holds. If the materials are thin, their atoms arrange identically, so that their vibrations are similar and present everywhere. Such structural and vibrational coherence opens up new avenues in material design, which will lead to more energy-efficient and low-power devices, new material solutions for recycling and converting waste heat into electricity, and to new ways of manipulating light with heat for advanced computing. 6G wireless communication.

The discovery emerged from a long-term collaboration of scientists and engineers from seven universities and two national laboratories of the US Department of Energy. Their paper, Emergent Interface Vibrational Structure of Oxide Superlattices, was published Jan. 26 in Nature.

Eric Hoglund, a postdoctoral fellow at the University of Virginia’s School of Engineering and Applied Sciences, took over the team. He got his doctorate. in Materials Science and Engineering from AVU in May 2020 in collaboration with James M. Howe, Thomas Goodwin Digges Professor of Materials Science and Engineering. After graduation, Hoglund continued to work as a postdoctoral researcher with support from Howe and patrick hopkinsProfessor Whitney Stone and Professor of Mechanical and Aerospace Engineering.

Hoglund’s success illustrates UVA’s purpose and potential Multifunctional Materials Integration Initiativewhich encourages close collaboration between different researchers from different disciplines to study the performance of materials, from atoms to applications.

“The ability to visualize atomic vibrations and link them to functional properties and new device designs, enabled by collaboration and co-consulting in materials science and mechanical engineering, advances MMI’s mission,” said Hopkins.

Hoglund used microscopy techniques to answer questions raised in experimental results Hopkins published in 2013, reporting on the thermal conductivity of superlattices, which Hoglund compares to a Lego building block.

“You can get the material properties you want by changing how different oxides couple, the number of oxide layers, and the thickness of each layer,” Hoglund said.

Hopkins expected the phonon to gain resistance as it passed through the lattice, dissipating thermal energy at each interface of the oxide layers. Instead, the thermal conductivity increased when the interfaces were very close to each other.

“This led us to believe that phonons can form a wave that exists in all later materials, also known as the coherent effect,” Hopkins said. “We found an explanation that matched the conductivity measurements, but we always thought this work was incomplete.”

“It turns out that when the interfaces get very close, the atomic arrangements unique to the material layer cease to exist,” Hoglund said. “The positions of atoms at interfaces and their vibrations exist everywhere. This explains why spaced interfaces at the nanometer scale produce unique properties, different from a linear mixture of adjacent materials.

Hoglund collaborated with Jordan Hachtel, an R&D associate at the Center for Nanophase Materials Sciences at Oak Ridge National Laboratory, to connect local atomic structure to vibrations using new generations of electron microscopes at UVA and Oak Ridge. Working with high spatial resolution spectroscopic data, they mapped interlayer vibrations across interfaces in a superlattice.

“This is the major breakthrough of the Nature journal,” Hopkins said. “We can see the position of atoms and their vibrations, this beautiful picture of a phonon wave based on a certain pattern or type of atomic structure.”

The collaborative march towards collective success

The highly collaborative effort began in 2018 when Hoglund shared research plans to characterize atomic vibrations at interfaces in perovskite oxides.

“I was going to Oak Ridge to work with Jordan for a week, so Jim and Patrick suggested I take the superlattice samples and see what we can see,” Hoglund recalls. “The experiments that Jordan and I did at Oak Ridge strengthened our confidence in using superlattices to measure vibration at the atomic or nanoscale.”

During one of his last trips to Tennessee, Hoglund met Joseph R. Matson, a Ph.D. student conducting related experiments at Vanderbilt University’s Nanophotonic Materials and Devices Laboratory headed by Joshua D. Caldwell, a member of the Chancellor’s Faculty of the Flowers family and associate professor of mechanical and electrical engineering. Using Vanderbilt’s instruments, they conducted Fourier-transform infrared spectroscopy experiments to probe optical vibrations throughout the superlattice. These well-established macroscopic measurements validated Hoglund’s new microscopy approach.

From these experiments, Hoglund deduced that the properties he was interested in – thermal transport and infrared response – arose from the influence of the interface on the well-ordered framework of oxygen atoms of the superlattice. Oxygen atoms arrange themselves in an eight-sided structure called octahedra, with a metal atom suspended inside. The interaction between oxygen and metal atoms causes octahedra to rotate through the structure of the material. The arrangements of oxygen and metal in this framework generate unique vibrations and give rise to the thermal and spectral properties of the material.

Back at UVA, Hoglund’s chance conversation with Jon Ihlefeld, an associate professor of materials science and engineering and of electrical and computer engineering, brought additional members and expertise to the effort. Ihlefeld mentioned that researchers affiliated with Sandia National Laboratories, Thomas Beechem, an associate professor of mechanical engineering at Purdue University, and Zachary T. Piontkowski, a senior member of Sandia’s technical staff, were also trying to explain the optical behavior of phonons and had also found the exact same oxide superlattices as the ideal material for their study.

Coincidentally, Hopkins had an ongoing research collaboration with Beechem, but with other materials systems. “Rather than compete, we agreed to work together and do something bigger than either of us,” Hoglund said.

Beechem’s involvement had an added benefit, bringing Penn State physicist and materials scientist Roman Engel-Herbert and his student Ryan C. Haisimaier into the partnership to develop sample materials for ongoing microscopy experiments at UVA. , Oak Ridge and Vanderbilt. So far, Ramamoorthy Ramesh, University of California, Berkeley, Professor of Physics and Materials Science and Engineering, and his Ph.D. students Ajay K. Yadav and Jayakanth Ravichandran were the team’s producers, providing samples to Hopkins’ ExSiTE research group.

“We realized we had all this really interesting experimental data linking vibrations at the atomic and macroscopic length scales, but all of our explanations were still somewhat guesswork that we couldn’t absolutely prove without theory,” Hoglund said.

Hachtel contacted a Vanderbilt colleague, Sokrates T. Pantelides, University Professor Emeritus of Physics and Engineering, William A. & Nancy F. McMinn Professor of Physics, and Professor of Electrical Engineering. Pantelides and members of his research group De-Liang Bao and Andrew O’Hara used density functional theory to simulate atomic vibrations in a virtual material with a superlattice structure.

Their theoretical and computational methods supported exactly the results produced by Hoglund and other experimenters on the team. The simulation also allowed the experimenters to understand how each atom in the superlattice vibrates with great precision and how this related to the structure.

At this point, the team had 17 authors: three microscopists, four optical spectroscopists, three computer scientists, five producers and two materials specialists. It was time, they thought, to share their findings with the wider scientific community.

An early reviewer of their manuscript advised the team to establish a more direct causal link between material structure and material properties. “We have measured interesting new phenomena establishing connections on multiple length scales that should affect material properties, but we had yet to convincingly demonstrate if and how known properties change,” Hoglund said.

Two graduate students at Hopkins’ EXSITE lab, lead scientist John Tomko and Ph.D. student Sara Makarem, helped provide the final proof. Tomko and Makarem probed superlattices using infrared lasers and demonstrated that the structure controls nonlinear optical properties and phonon lifetimes.

“When you send out a photon of one unit of energy, superlattices double that unit of energy,” Hopkins said. “John and Sara have built a new capability in our lab to measure this effect, which we express as the second harmonic generation efficiency of these superlattices.” Their contribution expands the capabilities of the ExSiTE laboratory to understand new light-phonon interactions.

“I think it will enable the discovery of advanced materials,” Hopkins said. “Scientists and engineers working with other classes of materials can now look for similar properties in their own studies. I expect that we will find that these phonon waves, this coherent effect, exist in many other materials. .

The long-standing collaboration continues. Hoglund is in his second year as a postdoctoral researcher, working with both Howe and Hopkins. Along with Pantelides, Hachtel, and Ramesh, he expects them to have some exciting new insights into atomic structure to share in the near future.