Research experience from California benefits Swiss X-ray free-electron laser SwissFEL

As far as Simon Gerber is concerned, X-ray free-electron lasers (XFELs) and their capacity to observe ultrafast processes are key to achieving new insights in many fields. One potential area of application is that of superconductivity, a phenomenon that has long fascinated the physicist and many of his colleagues: over 100 years ago the Dutchman Heike Kamerlingh Onnes discovered that many materials lose their electrical resistance at very low temperatures close to absolute zero at minus 273.15 degrees Celsius, allowing them to conduct electricity without loss of energy. However, in order to achieve this, these materials have to undergo a cooling process with liquid helium. If this effect would be achievable at higher temperatures—ideally, at room temperature—much energy could be saved.

Mysterious High-Temperature Superconductivity

When Georg Bednorz and Karl Alexander Müller discovered high-temperature superconductivity at IBM Rüschlikon, they were promptly awarded the Nobel Prize in the following year amid hopes that their discovery would soon translate into everyday use. Materials were found in which this effect took place at a higher so-called transition temperature beyond minus 170 degrees Celsius, more easily achievable by cooling with liquid nitrogen.

Since that time numerous physicists have dedicated themselves to finding an explanation for this phenomenon by trying to understand why the transition temperature in these materials is higher and which mechanisms contribute to superconductivity. They hope that the development of specific materials will help them find the key to raising transition temperatures even higher. Based on newly conceptualized experiments, Simon Gerber and his colleagues in Professor Zhi-Xun Shen's research group at the SLAC National Accelerator Laboratory and Stanford University have made important discoveries using the LCLS X-ray free-electron laser in California, on which they report in the current issue of the journal Science.

The mechanisms are clear as far as conventional superconductors are concerned: the reason why superconductivity occurs at the transition temperature lies in the interaction between the vibrations of the crystal lattice and the electron pairs that form in the superconducting state. For over thirty years, physicists have been engaged in controversial discussions on exactly how pronounced this coupling is in unconventional superconductors (including high-temperature superconductors) and the role it plays in the development of superconductivity in these materials. So far the problem had been that this coupling could not be directly and accurately measured, but only be calculated on the basis of theoretical assumptions and measured data.

X-ray Laser Shows Lattice Vibration

For the first time, a purely experimental approach has allowed us to determine the coupling in the unconventional superconductor iron selenide with great accuracy, says Simon Gerber. We were able to show that the coupling plays an essential role in iron selenide's material properties. In order to come to this conclusion, Gerber and his colleagues at SLAC and Stanford University combined data from two high-precision experiments. In both of these, the crystal lattice was excited by a laser pulse. One experiment then focused on analysing how lattice vibrations develop over time. The second experiment concentrated on the resulting electronic behaviour. The latter was made possible by the so-called photoemission method: this part of the experiment was led by Shuolong Yang, at that time doctoral candidate at SLAC and Stanford University, Patrick Kirchmann, SLAC researcher, as well as other colleagues.

Analysis of the lattice vibrations was carried out in a scattering experiment at the X-ray free-electron laser LCLS. It is only by using this ultrafast measurement method that we are able to establish with great accuracy how atoms move, emphasizes Gerber. Combining both experiments constituted a great challenge, because they cannot take place simultaneously, but need to be carried out in conditions that are as close as possible to being identical.

But it was worth the effort. Our approach has the advantage that we no longer have to rely on theoretical assumptions in order to calculate the strength of the coupling for specific electron paths. Our results originate purely from experiments and we can use them to compare various theoretical models with the aim of understanding the underlying physics. The researchers were also able to note that the degree of coupling between the atomic lattice and the electrons depends upon the electron state under observation. In addition, a correct theoretical description requires consideration of so-called many-body effects, which is a more complex approach than the conventional calculation methods in widespread use.

Returned from California to the PSI

Simon Gerber is convinced that this combined, purely experimental approach can contribute to shedding more light onto the longstanding controversy on understanding high-temperature superconductors. And there are further advantages. It's likely that similar coupling effects are also determining factors in other phenomena such as magnetism, which is why we want to continue developing this kind of combined experiments in the future. The physicist returned to the PSI over a year ago, motivated above all by the prospect of co-designing the new X-ray free-electron laser: We are building new instruments capable of carrying out specific experiments. Our input targets both, the instrumentation and the construction of the XFEL itself. It's exciting to see how far technology will allow us to go.

Beam line researcher Henrik Lemke, who left California's SLAC facility and came to the PSI two years ago, specifically to help develop SwissFEL, agrees. With five years of experience accumulated at SLAC, he will continue supporting researchers carrying out a range of experiments. We have the opportunity to learn from experience, focus on specific priorities and develop improvements.

Atomic Switches for Better Solar Cells and Data Storage

Lemke himself recently gained fascinating insights into molecular state switching on which he reported in May 2017 in the scientific journal Nature Communications. Lemke and colleagues from France and the US studied an iron compound, a larger molecule with an iron atom at its centre, itself surrounded, amongst others, by nitrogen atoms. This compound has already been examined using a variety of methods, so that many fundamental properties can be observed and compared with established results. The XFEL allows us to visualize changes in molecular structures that take place so quickly that that they were not previously perceptible with available structural measurement methods, explains Lemke. In theory, it is very difficult to calculate states of such short duration. Therefore it is important that we enable an experimental approach here.

One area in which rapid state switching plays a role is in Grätzel-solar cells, that can be produced at low cost. Here, electrons excited by sunlight leave their traditional location in a molecule and jump into a neighbouring nanoparticle, where they are able to contribute to the current flow. In the model molecule under observation, however, this process is overtaken by very rapid and effective state switching in the magnetic state, which might, in turn, be of interest for use as molecular storage with high storage density.

Magnetic State Switching Observed in Picoseconds

In the iron compound under consideration, researchers in Lemke's team were able for the first time to track the magnetisation process as a cascade and calculate why the spin state remains stable. The decisive parts of this process took place at incredible speed, in fractions of a picosecond—the millionth part of a millionth second. Researchers were able to observe how visible light travelling at a given wavelength excites an electron in the iron atom, so that it jumps to a neighbouring nitrogen atom in less than 1/40 of a picosecond. This new position enables a reorganisation of the electrons—the carriers of magnetism in the molecule. This in turn allows a net magnetisation to develop within one tenth of a picosecond. When electrons undergo such switching, the balance of the atoms within the molecule starts to shift and they begin to move in order to go back to an optimum position. Researchers were able to show that it appears as if the molecule is breathing before this movement, quickly stops and the spin transition stabilizes—an important precondition for data storage.

What we are seeing here is a race between two processes that provide us important information on the way they are connected to molecular structures. Such knowledge can help create molecules that will favour either one of these processes and enable technical applications, such as the conversion of light energy or a controlled molecular switch, emphasizes Lemke. SwissFEL enables us to observe processes that are ten times shorter than those previously observed, providing information on even faster, technologically relevant materials, in particular in the field of solids.

Simon Gerber adds: Both studies show that XFEL experiments are very powerful. However, since these facilities are rare, the time available for measurements is highly contested and very limited. SwissFEL will allow us to make further advances in this important research area.

Text: Uta Deffke

About PSI

The Paul Scherrer Institute PSI develops, builds and operates large, complex research facilities and makes them available to the national and international research community. The institute's own key research priorities are in the fields of matter and materials, energy and environment and human health. PSI is committed to the training of future generations. Therefore about one quarter of our staff are post-docs, post-graduates or apprentices. Altogether PSI employs 2100 people, thus being the largest research institute in Switzerland. The annual budget amounts to approximately CHF 380 million. PSI is part of the ETH Domain, with the other members being the two Swiss Federal Institutes of Technology, ETH Zurich and EPFL Lausanne, as well as Eawag (Swiss Federal Institute of Aquatic Science and Technology), Empa (Swiss Federal Laboratories for Materials Science and Technology) and WSL (Swiss Federal Institute for Forest, Snow and Landscape Research).

(Last updated in May 2017)