New insights into the metal-insulator transition in quantum materials

New research has provided insights into the mechanism behind the transformation of a special material from an electrically conducting metal to an electric insulator. The study focused on lanthanum strontium nickel oxide (La1.67Sr0.33NiO4), which is derived from a quantum material called La2NiO4. Quantum materials exhibit unconventional properties resulting from the interactions among their electrons. Below a critical temperature, the strontium-doped material behaves as an insulator due to the formation of “stripes” that separate the introduced holes from the magnetic regions. When the temperature rises, these stripes become unstable and dissolve at 240K. It was expected that the material would transition to being a conducting metal at this temperature, but surprisingly, it remained an insulator. Neutron scattering techniques have shed light on this fascinating phenomenon, revealing that specific atomic vibrations trap electrons and hinder electrical conduction.

Quantum materials possess properties that cannot be predicted based solely on their constituent elements. They can undergo transitions from metals to insulators or exhibit superconductivity. These materials hold significant potential for applications in science and technology. The recent research demonstrates the tunability of electron-phonon interaction in a quantum material, shedding light on the metal-insulator transition. The findings will contribute to validating theoretical models of materials with strongly interacting electrons, ultimately aiding scientists in designing novel quantum materials for future technologies.

Traditionally, electrons in metals are considered free particles moving along defined trajectories dictated by the crystal structure. However, in recent years, researchers have discovered materials where electrons strongly repel each other and interact with atomic vibrations within the crystal lattice. These materials display extraordinary and technologically valuable properties, such as a substantial decrease in electrical resistance in magnetic fields, surface-limited electron conduction, and high-temperature superconductivity. Understanding the properties exhibited by diverse materials of this nature remains a significant challenge for the scientific community.

The study employed high-intensity neutron beams at the Spallation Neutron Source, an Oak Ridge National Laboratory (ORNL) facility supported by the Department of Energy, to investigate La2NiO4—a prototypical quantum material. Specifically, the researchers examined La1.67Sr0.33NiO4, in which a portion of the lanthanum (La) atoms are replaced by strontium (Sr) atoms. These materials exhibit insulating behavior at low temperatures due to the formation of “stripe” patterns resulting from the intricate interplay between electronic spins and the introduced holes due to strontium doping. It was anticipated that the doped material would become metallic once the stripes melted above 240K. However, contrary to expectations, the material remained an insulator. The collaborative effort uncovered a significant interaction between the holes and specific vibrations of oxygen ions, providing evidence of this phenomenon in other materials with a similar structure. This microscopic mechanism could potentially facilitate the design of new materials with unconventional properties beneficial for various quantum technologies.

Source: US Department of Energy

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