A New Way to Look at MITs

Certain tenets of materials are time-tested and well understood. Metals like silver are conductors that carry heat or electricity efficiently. Insulators like glass inhibit current. Yet not everything is that simple. Much more complex materials often have surprising properties deviating from this familiar territory, offering scientists a significantly wider landscape to explore.

In a recent Nature Communications paper, UT physicists and collaborators report on their investigation of two rare-earth nickelates. As a family, these materials have become important to scientists because they undergo a metal-to-insulator transition, or MIT, although how that happens is under debate. Assistant Physics Professor Steven Johnston is the corresponding author on the paper and explained the team’s findings, which offer a fresh perspective on the metallic state of rare-earth nickelates as well as new ideas about what drives the MIT.

“MITs are actually pretty uncommon,” Johnston explained. “You have to have a pretty serious change in the material, like reorganizing the electrons or atomic structure on large scales, in order to undergo one.” In the nickelates, the MIT is accompanied by a large scale structural change where alternating nickel-oxide octahedra (picture a diamond-like shape with eight faces) contract in a 3D checkerboard-like pattern.

He said the transition temperature for rare-earth nickelates is quite high—an advantage for fast switches and an open door to computing applications. Scientists can also tune the temperature where the transition occurs. While the traditional position has been that the natural repulsion between electrons drives the MIT, Johnston and his colleagues suggest an alternative. They propose that nickelates belong to a wider class of materials—negative charge transfer materials—which produces a strong coupling between the electrons and the atoms forming the crystal.

Coupling, Not Repulsion

Johnston and fellow scientists from UT, Oak Ridge National Laboratory, and the Instituto de Ciencia de Materiales de Madrid in Spain conducted neutron scattering and broadband dielectric spectroscopy (BDS) experiments to study the structure and carrier dynamics of LaNiO₃ and NdNiO₃. The former—consisting of lanthanum, nickel, and oxygen—doesn’t undergo a metal-to-insulator transition, while the latter (comprising neodymium, nickel, and oxygen) does.

Using the nanoscale ordered materials diffractometer (NOMAD) beamline at ORNL’s Spallation Neutron Source, they loaded samples into capillaries two millimeters in diameter and exposed them to the neutron beam for one hour per measurement. Bombarding a sample of neutrons allows scientists to see where atoms are in a material, and what they’re doing. Researchers converted the data into pair distribution functions (PDFs), which Johnston pointed out as a key element of the study. PDF analysis allows scientists to get a more precise picture of a material’s local structure; rather than looking at the crystal structure on the large scale, PDF tells you what neighboring atoms are doing in the material.

In both materials, they found that the nickel-oxide octahedra have local compressions in the metallic phase, indicating that the structural transition found across the MIT is to some extent “preformed.” However, in NdNiO₃ researchers also found evidence that these distortions “freeze” into a periodic structure as the insulator forms. This transition is like the water-ice transition.

The researchers conducted BDS measurements to get further insight. They saw that both materials were characterized by slow carrier dynamics, indicating that the electron mobility was severely impeded but became greater carrier mobility as they increased the temperature (a sign of “bad metal” behavior). This “puzzling observation,” as they described it, could be understood if the carriers are lattice polarons. A polaron is a system where an electron is introduced into the crystalline lattice structure of a material and induces polarization around itself. In some cases, the electron can get trapped inside these local distortions of the lattice, and the two are forced to move as one object. In these nickelate samples, Johnston explained that these bound distortions are present in the metallic phase, creating slow electrons. Overall, the results offer a new view of the metallic state of nickelates and an evolved understanding of the MIT in rare-earth nickelates, which may be analogous to other negative charge transfer materials like certain superconductors.

The findings were published in “Experimental evidence for bipolaron condensation as a mechanism for the metal-insulator transition in rare-earth nickelates” in the January 8 issue of Nature Communications.

Along with Johnston, the research team comprised Jacob Shamblin (a former postdoctoral research associate in UT Physics), Max Heres (a graduate student in UT Chemical and Biomolecular Engineering), Haidong Zhou (Assistant Professor, UT Physics), Joshua Sangoro (Assistant Professor, UT Chemical and Biomolecular Engineering), Maik Lang (Assistant Professor, UT Nuclear Engineering), Joerg Neuefeind (Instrument Scientist, ORNL Chemical and Engineering Materials Division), and J.A. Alonso (Senior Scientist, Instituto de Ciencia de Materiales de Madrid).