Thanks to the trapped electrons, the material that was supposed to be a conducting metal remains an insulator


July 13, 2023

(Nanowerk News) New research sheds light on the mechanism behind how this particular material changes from an electrically conducting metal to an electrical insulator. Researchers studying lanthanum strontium nickel oxide (La1.67Sr0.33NiO4) comes from the quantum material La2NiO4.

Quantum materials have unusual properties that result from how their electrons interact. Below the critical temperature, strontium doped materials are insulators. This is due to the separation of the inserted holes from the magnetic field, forming “lines”. As the temperature increases, these lines fluctuate and melt at 240K. At this temperature, researchers expect the material to become a conducting metal. Instead, it remains an insulating material.

Neutron scattering explains this interesting phenomenon. The results show that the material remains an insulator due to the vibrations of certain atoms that trap electrons thereby inhibiting electrical conduction. Neutron scattering intensity with momentum and energy at 240K for quantum matter Neutron scattering intensity with momentum and energy at 240K for quantum matter. The high-intensity red areas indicate normal atomic vibrations. The low intensity green areas (ovals) indicate the strong interaction between vibrations & electrons in the material. (Image: Dmitry Reznik)

This research has been published in Scientific research (The giant electron-phonon coupling of the respiratory field oxygen phonons in the dynamic line La phase1.67Sr0.33NiO4).

Quantum materials have properties that are not predicted by the parts that make up the material. For example, they can transition from metals to insulators or act as superconductors. They hold tremendous promise for applications in science and technology. This research describes the ability of electron-phonon interactions in metal-insulator transitions in one quantum material. The results will help validate theoretical models of materials having strongly interacting electrons. These theories will help scientists design new quantum materials for future technologies.

In metals, electrons can be thought of as free particles flying along trajectories imposed by the crystal structure. In recent decades, scientists have discovered new materials in which electrons strongly repel one another and reflect the vibrations of atoms in the host crystal. These materials exhibit unusual and technologically useful properties. These properties can include a dramatic decrease in electrical resistance in a magnetic field, surface-only conduction of electrons, and high-temperature superconductivity. Understanding these properties in different materials remains a major challenge for the scientific community.

This work uses a high-intensity neutron beam at the Spallation Neutron Source, a Department of Energy user facility at Oak Ridge National Laboratory (ORNL), to look deep into the La archetype quantum matter.2NiO4 where one-sixth of the lanthanum (La) atom is replaced by a strontium (Sr) atom (La1.67Sr0.33NiO4).

The team included researchers from the University of Colorado Boulder, ORNL, Brookhaven National Laboratory, and the RIKEN Center for Emergent Matter Science in Japan. These materials are isolated at low temperatures due to so-called “line” sequences resulting from complex interactions between electronic spins and holes caused by strontium doping. The doped material is expected to be a metal above 240K as the streaks melt.

However, the material remains insulating. The collaboration uncovered the strong friction between the holes and the vibrations of certain oxygen ions and found evidence of this interaction in other materials with a similar structure. Microscopic mechanisms could pave the way for the design of new materials with unusual properties useful for various quantum technologies.


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