Light tunes electronic properties of a metal oxide

Some materials behave like insulators, refusing to conduct electricity unless driven under the right conditions. Understanding and controlling this behaviour is vital for technologies ranging from neuromorphic computing to ultrafast switches. This is especially relevant in the case of metal oxides, which are cheap and abundant. One such material, nickel oxide (NiO), is a textbook example of a so-called charge-transfer insulator.

Most metal oxides suffer from poor electrical conductivity. Electrons therein behave like cars stuck in traffic: strong repulsive forces prevent them from moving into neighboring sites already occupied by other electrons, effectively freezing them in place. Materials governed by these repulsions (or correlations) conduct electricity poorly and underperform in, e.g. solar energy conversion.

Researchers led by Thomas C. Rossi and Majed Chergui at EPFL’s Lausanne Centre for Ultrafast Science (LACUS) have now shown that ultrafast optical pulses can precisely tune electron correlations in NiO. By shining light on the material, they were able to weaken these correlations in a controlled and reversible way, making NiO behave more like a metal. Unlike conventional methods that rely on temperature, pressure, or chemical changes to alter conduction, this approach uses light to achieve the same effect at ultrashort timescales.

To uncover this light-induced effect, the researchers used a pump-probe spectroscopy technique in the ultraviolet range. First, a short, intense laser pulse “photodopes” the material, injecting energy and exciting electrons. A second pulse follows shortly after to measure how the electronic structure of NiO responds.

The research team achieved unprecedented control: the reduction in electron repulsion can be tuned by varying the intensity of light, and it persists for hundreds of picoseconds. Altogether, these remarkable properties open exciting new perspectives for more efficient light-based devices, and next-generation technologies combining wide dynamic ranges of operation with ultrafast switching speeds.

Other contributors

• Helmholtz-Zentrum Berlin für Materialien und Energie GmbH
• Max-Planck Institute for the Structure and Dynamics of Matter, Hamburg
• Paul Scherrer Institute, Villigen
• EPFL Crystal Growth Facility
• University of California at Davis
• Simons Foundation Flatiron Institute, New York
• Elettra-Sincrotrone, Trieste