Disorder begins at the surface of quantum materials

In certain materials, the electrons arrange themselves in a well-defined, ordered pattern. This internal order can influence everything from how the material conducts electricity to how it responds to magnetic fields. One example is the layered manganese oxide and quantum material La_0.5Sr_1.5MnO₄, in which electrons of manganese atoms arrange themselves into a regular pattern – known as orbital ordering – leading to distinctive electronic and magnetic behaviour.

Researchers are increasingly interested in how light can be used to understand and control the orbital state of these materials. With the right kind of light pulse, it may be possible to switch or reshape their properties at incredible speeds. Therefore, understanding how these materials switch is an important step to making devices.

In many devices, surfaces of and interfaces between materials are known to play a major role in the device properties. Yet until now, it has not been possible to measure how quantum materials change at the surface when switched at high speeds by light. Previous studies have only captured the average response over the whole crystal. 

In this study, a team of scientists led by Aarhus University asked if the average response measured to date accurately captures the processes that occur at the surface, which will be relevant for any device. Remarkably, they found that they did not. 

To do this, they used an ultrafast X-ray laser to observe what happens when La_0.5Sr_1.5MnO₄ is exposed to laser light. Instead of a simple response, they found a complex process in which disorder begins at the surface and spreads unevenly through the material.
 

Shedding Light on Complex Behavior

To follow how the material changes after it is excited by light, the researchers used time-resolved X-ray surface diffraction at the Bernina experimental station of the SwissFEL. This advanced technique allowed them to record snapshots of how both the surface and bulk of La_0.5Sr_1.5MnO₄ reacted within femtoseconds after being hit by a laser pulse. By carefully adjusting scattering geometry, they could separate the signal coming from the surface from that of the deeper layers, giving a clear view of how order and disorder develop in different parts of the crystal.

The results showed that the surface region loses its orbital order much faster than the bulk. At the surface, this order quickly broke down through small, irregular movements of atoms, a process called local distortion. In contrast, the inner layers remained relatively stable for longer. 

These findings challenge earlier models of photoinduced phase transitions, which assumed a uniform transformation. They also point toward new strategies for using light to control materials in space and time, potentially allowing scientists to design devices where surface and bulk properties can be tuned separately for ultrafast electronics and quantum technologies.
 

Why does it matter?

Understanding how electrons and atoms respond locally is crucial for developing ways to control material properties on demand. This could lead to new strategies for ultrafast switching in electronic or magnetic devices, or even future quantum technologies where the speed and precision of control are key.

By showing that disorder and surface effects play a bigger role than previously thought, the study opens new questions about how to harness light to reshape complex materials in useful ways.