Ultrafast laser spectroscopy observes changes in new materials

Scientists at the US Department of Energy’s Ames Laboratory are revealing the mysteries of new materials using ultrafast laser spectroscopy - a technique in which many quick images reveal subtle movements and changes inside the materials.

Physicist Jigang Wang and his colleagues recently used ultrafast laser spectroscopy to examine and explain the electronic properties of iron-based superconductors. Superconductors are materials which, when cooled below a certain temperature, display zero electrical resistance - a property that could someday make possible lossless electrical distribution. The mystery - and key to understanding more about superconductivity - lies in what goes on in the materials as they transition from normal to superconducting.

Ames Laboratory scientists use ultrafast laser spectroscopy to ‘see’ tiny actions in real time in materials. Credit: US Department of Energy Ames Laboratory.

“The stable states of materials aren’t quite as interesting as the crossover region when it comes to understanding materials’ mechanisms because everything is settled and there’s not a lot of action,” said Wang. “But in this crossover region to superconductivity, we can study the dynamics, see what goes where and when, and this information will tell us a lot about the interplay between superconductivity and magnetism.”

The challenge is that in the crossover region, all the different sets of materials properties that scientists examine, like its magnetic order and electronic order, are coupled; so when there’s a change to one set of properties, it changes all the others. The team thus set out to ‘see’ these tiny actions using ultrafast laser spectroscopy, described by Wang as “a new experimental tool to study dynamic, emergent behaviour in complex materials such as these iron-based superconductors”. 

In ultrafast laser spectroscopy, scientists apply a pulsed laser to a materials sample to excite particles within the sample. Some of the laser light is absorbed by the material, but the light that passes through the material can be used to take superfast ‘snapshots’ of what is going on in the material following the laser pulse and then replayed afterwards, like a stop-motion movie.

The technique is particularly well suited to understanding the crossover region of iron-arsenide-based superconductors materials because the laser excitation alters the material so that different properties of the material are distinguishable from each other in time, even the most subtle evolutions in the materials’ properties. Such material was developed for the investigation by Paul Canfield, an Ames Laboratory scientist who created and characterised a very high-quality single crystal.

Speaking of the team’s results, Wang said, “We answered the pressing question of whether an electronically driven nematic order exists as an independent phase in iron-based superconductors, as these materials go from a magnetic normal state to superconducting state. The answer is yes. This is important to our overall understanding of how superconductors emerge in this type of materials.”

The team’s results have been published in the journal Nature Communications.

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