"We can be proud that it worked out": BESSY and the Transregio-SFB on ultrafast spin dynamics

Collaborative Research projects as “Sonderforschungsbereiche” funded by Deutsche Forschungsgemeinschaft enable universities to build up their own research capacities. In the Transregio Sonderforschungsbereich 227 Ultrafast Spin Dynamic, the Freie Universität Berlin and the University in Halle-Wittenberg have also included HZB as a partner. The slicing facility of BESSY II plays a central role in this collaboration. With excellent results from the first phase, the Transregio-SFB 227 has completed its first interim review and is now preparing for the challenges ahead. A conversation with the two HZB physicists Niko Pontius and Christian Schüßler-Langeheine about the importance of such funding programmes for the research field.

The research topic is: ultrafast spin dynamics. Can you describe in simple words why it is important to study the dynamics of spins?

Christian Schüßler-Langeheine: All our technology, from semiconductor electronics to information technologies, is based on electric charges of electrons.  But electrons also have a spin, a magnetic moment. You can also use this spin to process or store data. And spins can be manipulated extremely quickly and with very little energy, so spin-based technologies are extremely efficient. We want to investigate and understand these processes.

Is this mainly about basic research?

SL: No, not only: the Transregio is very broadly positioned, we deal with absolute fundamental questions up to developments that are already very close to concrete components.

Niko Pontius: A concrete example from the Transregio SFB: Up to now, terahertz sources, i.e. sources for electromagnetic radiation in the terahertz range, have been very difficult to produce; actually, you need a particle accelerator like BESSY for that. But in one of the Transregio projects, we succeeded in turning a magnetic sandwich system into a terahertz source using a laser. This makes use of the fact that charges and spins are coupled.

The “Sonderforschungsbereiche” were actually set up primarily to strengthen excellent research at universities. How is the HZB involved in this SFB?

SL: There are a total of 17 +2 projects in this Transregional SFB, which are located at two universities in two cities: At the FU Berlin and at the University of Halle-Wittenberg. Clusters are forming locally around these two nuclei, and here in Berlin HZB is one of them, because with BESSY we can offer the perfect X-ray source for experimental investigations of ultrafast spin dynamics. Of the 17+2 projects, about 8 are directly related to the femtoslicing source.

The first phase of the SFB was meant for four years. Is the extension a surprise?

NP: The first review of an SFB is about the idea, and already with a very long time perspective of twelve years. In the first interim review, we had to deliver results, and the question was: Have you been able to implement the ideas and do you have a sustainable plan for the next eight years? In this respect, we can be very proud that it worked out.

Did you obtain surprising results already in the first phase of the SFB?

SL: Yes, for example at BESSY II. I have to elaborate a little on that. These time-resolved experiments usually work like this: you disturb the system with a laser pulse and see how the spins develop over time. Until now, it was thought that the laser pulse excited the electron system and heated it up - this took a few hundred femtoseconds until they cooled down again and only then did the magnetic dynamics start. But our experiments have now shown that the spin dynamics begin much earlier. Charges are already rearranged between different elements in the sample and spins are transferred. This was worked out in the first funding period and has strongly influenced the plan for the next funding periods.

Why is the slicing source at BESSY II so important for the investigations?

NP: We have a time resolution of 120 femtoseconds at the femtoslicing source, which is 400 times better than the time resolution without slicing (50 picoseconds). Only this high time resolution allows us to observe the dynamics right after excitation.

Are there other plus points for experiments at the slicing source?

NP: With the soft X-ray light from BESSY II, the magnetic properties in materials consisting of several elements can be separated in a very element-specific way. And we have a special technique, circular X-ray dichroism: this allows us to separate the spin part of the magnetic moment from the part that results from the circular motion of the electron. This results in a particularly detailed image.

Is that a unique selling point?

SL: For a long time, we were the only source worldwide that could offer the combination of circular polarisation and femtoslicing. In the meantime, there are other sources that build this.  But through many years of experience, we are leading the way and we keep improving the methods.

And what does the extension of the Transregio SFB mean for the HZB?

SL: It's a great recognition of our work. And a strong argument when we write proposals for collaborative research with our partners.

Congratulations! Are there now concrete projects to further expand the experimental possibilities?

SL: Yes. In order to investigate the electronic excitations in the very first phase directly after the laser pulse in more detail, we would like to set up a second branch at the femtoslicing beamline.

Thank you for the interview.