Shedding light on insulators: how light pulses unfreeze electrons
When nickel oxide (NiO) is excited with ultrashort UV light pulses, electron repulsions briefly become weaker, making the insulator behave more like a metal. To capture this effect, the researchers probed the material with multicolored UV pulses and measured their absorption and reflectivity. The results reveal unprecedented control over electron repulsions using light. © T. Rossi /HZB
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)
- Long-term stability for perovskite solar cells: a big step forwardPerovskite solar cells are inexpensive to produce and generate a high amount of electric power per surface area. However, they are not yet stable enough, losing efficiency more rapidly than the silicon market standard. Now, an international team led by Prof. Dr. Antonio Abate has dramatically increased their stability by applying a novel coating to the interface between the surface of the perovskite and the top contact layer. This has even boosted efficiency to almost 27%, which represents the state-of-the-art. After 1,200 hours of continuous operation under standard illumination, no decrease in efficiency was observed. The study involved research teams from China, Italy, Switzerland and Germany and has been published in Nature Photonics.
- Energy of charge carrier pairs in cuprate compoundsHigh-temperature superconductivity is still not fully understood. Now, an international research team at BESSY II has measured the energy of charge carrier pairs in undoped La₂CuO₄. Their findings revealed that the interaction energies within the potentially superconducting copper oxide layers are significantly lower than those in the insulating lanthanum oxide layers. These results contribute to a better understanding of high-temperature superconductivity and could also be relevant for research into other functional materials.
- Electrocatalysis with dual functionality – an overviewHybrid electrocatalysts can produce green hydrogen, for example, and valuable organic compounds simultaneously. This promises economically viable applications. However, the complex catalytic reactions involved in producing organic compounds are not yet fully understood. Modern X-ray methods at synchrotron sources such as BESSY II, enable catalyst materials and the reactions occurring on their surfaces to be analysed in real time, in situ and under real operating conditions. This provides insights that can be used for targeted optimisation. A team has now published an overview of the current state of knowledge in Nature Reviews Chemistry.