This illustration shows how a quantum material's atomic structure has warped into a dramatic herringbone-like pattern. Researchers at SLAC and Stanford who created the material are just beginning to investigate how the material's properties can be influenced by an electric tug-of-war.
The SLAC National Accelerator Laboratory and Stanford University have developed a novel quantum material, the atomic structure of which has been completely altered into a herringbone pattern.
The distortions caused by this material, according to Woo Jin Kim, the study's lead researcher and a postdoctoral researcher at the Stanford Institute for Materials and Energy Sciences (SIMES) at SLAC, are "huge."
"This is a very important finding, and it's difficult to make any predictions about what might or might not arise from it, but the possibilities are limitless," said SLAC/Stanford Professor and SIMES Director Harold Hwang.
According to scientists, the new material appears to have intriguing magnetic, orbital, and charge order features.
In a paper published in the journal Nature, the research group décrit their findings.
Researchers modified the atomic structure of the material at right, which is composed of octahedral and tetrahedral layers and is now known as brownmillerite, in the same way Jenga players removed wooden blocks from a stack, and the resulted material, left, was dramatically distorted into a herringbone pattern due to a Jahn-Teller effect.
The herringbone-patterned material is the first demonstration of what is known as the Jahn-Teller effect in a layered material with a flat, planar lattice, similar to a high-rise structure with evenly spaced floors.
The JT effect addresses the problem an electron faces when it approaches an ion — anatom that lacks one or more electrons.
The electron will seek and occupy the lowest energy state in the atom's electron orbitals, just as a ball rolling along the ground will stop and settle in a low spot. Sometimes there are two vacancies with equally low energies.
Hwang said the JT effect dissected the surrounding atomic lattice in a way that left only one vacancy at the lowest energy level, solving the electron's problem.
In some instances, the entire crystal structure warps, thus the electron's dilemma is cooperatively resolved for all the ions.
In this case, exactly that was found.
A new quantum material that was created by an electronic tug-of-war between negatively charged cobalt ions and positively charged calcium ions is shown in this illustration, which warped the atomic lattice in a way that had never been seen before. Credit: Woo Jin Kim/SIMES
Hwang said the Jahn-Teller effect is believed to play a key role in the physics of a number of quantum materials.
Single molecules and 3D crystalline materials that contain ions arranged in octahedral or tetrahedral structures have already demonstrated the JT effect, causing scientists to speculate on what might happen in materials based on other elements or having a different structure.
A compound made of cobalt, calcium, and oxygen, called brownmillerite, was transformed into a layered material (CaCoO2) that allowed the JT effect to take hold. They did it using a chemical technique developed at SIMES a few years ago to make the first nickel oxide superconductor.
Kim synthesized a thin film of brownmillerite and chemically removed single layers of oxygen atoms from its lattice, much like players remove blocks from a Jenga tower. The lattice collapsed and settled into a flat, planar structure with alternating layers of positively charged cobalt ions — the JT ions —.
According to Kim, each cobalt ion tried to remove calcium ions from the layers above and below it.
"These lattice distortions were caused by a beautiful pattern of interplay between adjacent layers," he said. "And the lattice distortions in other materials are substantial, equal to 25% of the distance between ions in the lattice."
Hwang said the research team will be experimenting with X-ray equipment available at SLAC and elsewhere to modify the amount of electrons that are free to move around.
Byeong-Gwan Cho, Kyuho Lee, Motoki Osada, Anton V. Ievlev, Brian Moritz, Lena F. Kourkoutis, and Harold Y. Hwang, 22 February 2023, Nature. DOI: 10.1038/s41586-022-05681-2
This work was funded by the DOE Office of Science and the Gordon and Betty Moore Foundation's Emergent Phenomena in Quantum Systems Initiative.