Shedding light on insulators: how light pulses unfreeze electrons

Metal oxides are abundant in nature and central to technologies such as photocatalysis and photovoltaics. Yet, many suffer from poor electrical conduction, caused by strong repulsion between electrons in neighboring metal atoms. Researchers at HZB and partner institutions have shown that light pulses can temporarily weaken these repulsive forces, lowering the energy required for electrons mobility, inducing a metal-like behavior. This discovery offers a new way to manipulate material properties with light, with high potential to more efficient light-based devices.

In most metal oxides, electrons 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 from HZB and partner institutions have now shown that ultrashort light pulses lasting just a few tens of femtoseconds can temporarily weaken these repulsive forces. For a brief moment, electrons are able to move at a lower energy cost, making the material 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 capture this effect on ultrafast timescales, the HZB team joined forces with several partners. The experiment took place at the LACUS in Lausanne (Switzerland), a centre specializing in ultrafast science, while the sample characterization, data analysis, and simulations were carried out using HZB infrastructure.

The team focused on nickel oxide (NiO), a charge-transfer insulator with an electronic structure similar to high-temperature superconductors. In NiO, they achieved unprecedented control: the reduction in electron repulsion scales linearly with light intensity, persists for hundreds of picoseconds, and relaxes back to equilibrium at the same pace regardless of excitation density. Altogether, these 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

• Max Planck Institute for the Structure and the Dynamics of Matter (Germany)
• Helmholtz Center for Materials and Energy (Germany)
• Elettra Synchrotron Trieste (Italy)
• Paul Scherrer Institute (Switzerland)
• University of Basel (Switzerland)
• University of California Davis (USA)
• Simons Foundation Flatiron Institute (USA)