New study validates 3D flat-band guided material discovery method

Rice University scientists have discovered a first-of-its-kind material, a 3D crystalline metal in which quantum correlations and the geometry of the crystal structure combine to counteract the movement of electrons and lock them in place.

The discovery is detailed in a study published in Natural physics. The article also describes the theoretical design principle and experimental methodology that guided the research team toward the material. One part copper, two parts vanadium and four parts sulfur, the alloy features a 3D pyrochlore lattice composed of corner-sharing tetrahedra.

“We’re looking for materials in which there are potentially new states of matter or new exotic features that haven’t been discovered,” said study co-corresponding author Ming Yi, an experimental physicist at Rice.

Quantum materials are a likely place to explore, especially if they host strong electronic interactions that give rise to quantum entanglement. Entanglement leads to strange electronic behaviors, including frustrating the movement of electrons to the point where they become stuck.

“This quantum interference effect is analogous to waves rippling across the surface of a pond and meeting each other head-on,” Yi said. “The collision creates a standing wave that does not move. In the case of geometrically frustrated lattice materials, it is the electronic wave functions that interfere destructively.”

The localization of electrons in metals and semimetals produces flat electronic bands, or flat bands. In recent years, physicists have discovered that the geometric arrangement of atoms in some 2D crystals, such as Kagome lattices, can also produce flat bands. The new study provides empirical evidence of the effect in a 3D material.

Using an experimental technique called angle-resolved photoemission spectroscopy, or ARPES, Yi and the study’s lead author, Jianwei Huang, a postdoctoral researcher in his lab, detailed the band structure of the copper- vanadium-sulfur and found that it harbors a flat band that is unique in several ways.

“It turns out that both types of physics are important in this material,” Yi said. “The geometric frustration aspect was there, as the theory predicted. The nice surprise was that there were also correlation effects that produced the flat band at the Fermi level, where it can actively participate in determining the physical properties.”

In solid matter, electrons occupy quantum states divided into bands. These electronic bands can be thought of as rungs on a ladder, and electrostatic repulsion limits the number of electrons that can occupy each rung. The Fermi level, an inherent property of materials and crucial for determining their band structure, refers to the energy level of the highest occupied position on the scale.

Qimiao Si, Rice theoretical physicist and co-corresponding author of the study, whose research group identified the copper-vanadium alloy and its pyrochlore crystal structure as a possible host for the combined frustration effects of geometry and strong electronic interactions, compared this discovery to the discovery of a new continent. .

“This is the very first work that actually shows not only this cooperation between geometric and interactional frustration, but also the next step, which is to bring the electrons into the same space, at the top of the (energy) scale, where “There is a maximum chance of their reorganization into new, interesting and potentially functional phases,” Si said.

He said the predictive methodology or design principle his research group used in the study could also prove useful to theorists who study quantum materials with other crystal lattice structures.

“Pyrochlor is not the only game in town,” Si said. “It is a new design principle that allows theorists to predictively identify materials in which flat bands appear due to strong electronic correlations.”

Yi explained that there is also plenty of room for further experimental exploration of pyrochlore crystals.

“This is just the tip of the iceberg,” she said. “It’s 3D, which is new, and given the number of surprising discoveries that have been made about Kagome’s networks, I imagine there could be equally, if not more, exciting discoveries to come. to do in pyrochlore materials.”

The research team included 10 Rice researchers from four laboratories. Physicist Pengcheng Dai’s research group produced the numerous samples needed for experimental verification, and Boris Yakobson’s research group in the Department of Materials Science and Nanoengineering performed first-principles calculations that quantified the effects of flat strip produced by geometric frustration.

ARPES experiments were conducted at Rice and at the Stanford Synchrotron Radiation Light Source at SLAC National Accelerator Laboratory in California and at the National Synchrotron Light Source II at Brookhaven National Laboratory in New York, and the team included collaborators from SLAC, Brookhaven and the University of Washington.