Schematic of the unit cell of SrTiO3. Illustration: Stefano Bonetti
Schematic of the unit cell of SrTiO3. Red is titanium, green is strontium and white is oxygen. The green and white cages indicate the equilibrium position of the unit cells, while the actual atomic position shows the displacement induced by a strong terahertz electric field. Illustration: Stefano Bonetti

The study is made by scientists at the SLAC National Accelerator Laboratory, the Max Planck Institute for the Structure and Dynamics of Matter, the Paul Scherrer Institute and Stockholm University.

“The same approach could in future also be used to control the properties of many other quantum materials and be instrumental to the implementation of a quantum computer”, says Stefano Bonetti, Associate Professor and Wallenberg Academy Fellow at the Department of Physics, Stockholm University, who is one of the senior authors of the article published in the scientific journal Nature Physics.

In this technically challenging study the team used ultrashort and intense x-ray bursts generated at the LCLS free electron laser at Stanford, to directly look at the atomic ‘dance’ triggered by intense and rapid electric fields.

Stefano Bonetti, Department of Physics. Photo: Magnus Bergström/The Wallenberg Foundations
Stefano Bonetti, Wallenberg Academy Fellow at the Department of Physics, Stockholm University. Photo: Magnus Bergström. The image has been provided by the Wallenberg Foundations.

“We can now show that with intense, short electric field bursts in the terahertz frequency, we can ‘shake’ atoms to such an extent that sizable electric polarization is created in the material for a short time, and that unexpected vibrations start to occur at a frequency higher than the one we used to start the motion”.

“Our results reveal a unique and not yet understood view of the physics of phonons, which are the quanta of atomic vibrations, in the same way photons are the quanta of light. It also indicates a new path towards accurately controlling atomic vibrations, and in turn the exotic properties of quantum materials, which are strongly affected by the positions and the motions of atoms that build them up”, says Stefano Bonetti.

He explains that the investigated quantum material, strontium titanate, with chemical formula SrTiO3, is a material where many exotic physical properties have been observed, which are believed to be closely connected to the presence of peculiar phonons.

“For instance, Alexander Balatsky at Nordita at Stockholm University has recently predicted that such phonons are associated with the onset of superconductivity. With their theory, Alexander Balatsky and co-workers predicted that by exerting strain this material, for example affecting how the atomic vibrations develop, the critical temperature for superconductivity could increase. This has now been observed experimentally and it is a major breakthrough in the field”, says Stefano Bonetti.

“It also shows the paradigm’s shift we are experiencing in physics. We now have both the theoretical and experimental tools, in particular x-ray free electron laser, to look at atoms at the temporal scales relevant for quantum mechanics. With new and better instruments being built all around the world, we may be on the verge of discovering secrets of nature that have puzzled us for decades.”

He adds that the greatest technological shifts that have changed society have always been sparked by fundamental discoveries.

“Even though we cannot be sure how and when this will happen again, we are without doubt standing in front of an unexplored territory where we will likely find surprises.”


The article “Terahertz-driven phonon upconversion in SrTiO3 ”  is published in the scientific journal Nature Physics.

The Swedish part of the project was financed with support from the Knut and Alice Wallenberg foundation.

“Femtosecond lasers spotlight new quantum materials” – article about Stefano Bonettis research at the homepage of Knut and Alice Wallenbergs Foundation.