Trapped Electrons Cause Unexpected Insulating Behavior in Material Originally Expected to Conduct Electricity

A groundbreaking study has uncovered the mysterious mechanism behind the transformation of a unique material from an electrically conducting metal to an insulator. Researchers focused on lanthanum strontium nickel oxide (La_1.67Sr_0.33NiO₄), derived from the quantum material La2NiO4. Quantum materials possess extraordinary properties resulting from the interactions of their electrons. At temperatures below a critical threshold, the strontium-doped material behaves as an insulator, due to the formation of “stripes” that separate introduced holes from the magnetic regions. However, as the temperature increases, these stripes fluctuate and melt at 240K. Contrary to expectations, the material remains an insulator instead of becoming a conducting metal. Neutron scattering technology has provided insights into this fascinating phenomenon, revealing that specific atomic vibrations trap electrons and impede electrical conduction.

Quantum materials possess properties that defy predictions based on their constituent elements. They can transition from being metals to insulators or exhibit superconductivity. These materials hold tremendous potential for scientific and technological applications. The latest research explores the ability to manipulate electron-phonon interaction during the metal-insulator transition in a quantum material. The findings will help validate theoretical models of materials with highly interactive electrons, ultimately aiding scientists in designing new quantum materials for future technologies.

In conventional metals, electrons behave as free particles following predefined paths determined by the crystal structure. However, in recent years, scientists have discovered new materials where electrons strongly repel each other and bounce off atomic vibrations within the crystal lattice. These materials exhibit extraordinary properties that have practical implications. Examples include a significant decrease in electrical resistance in the presence of magnetic fields, surface-only electron conduction, and superconductivity at high temperatures. Understanding the underlying mechanisms responsible for these properties across various materials remains a major challenge in the scientific community.

The research involved the use of high-intensity neutron beams at the Spallation Neutron Source, an Oak Ridge National Laboratory (ORNL) facility supported by the Department of Energy. The team included researchers from the University of Colorado Boulder, ORNL, Brookhaven National Laboratory, and the RIKEN Center for Emergent Matter Science in Japan. The study focused on the archetype quantum material La₂NiO₄, in which a portion of the lanthanum (La) atoms were replaced with strontium (Sr) atoms to create La_1.67Sr_0.33NiO₄. At low temperatures, these materials are insulators due to the “stripe” order that arises from the complex interplay between electronic spins and the holes introduced by strontium doping. The doped material was expected to transition into a metal above 240K when the stripes melted. However, the material remained an insulator. The collaborative effort uncovered a strong interaction between the holes and specific vibrations of oxygen ions, providing evidence of this phenomenon in other structurally similar materials. This microscopic mechanism could unlock the potential for developing new materials with extraordinary properties applicable to various quantum technologies.