Understanding the world in its innermost heart, means not only knowing its fundamental building blocks; the interactions among these basic constituents are just as important. Christian Rüegg studies how the magnetic building blocks of matter work together on the fundamental atomic level and thus give rise to the electrical and magnetic properties of materials. Rüegg is a laboratory head at the Paul Scherrer Institute and at the same time a professor at the University of Geneva. For his future experiments, he has now won a highly prestigious European funding award: the ERC Consolidator Grant, given by the European Research Council. Over a five-year period, this grant will bring Rüegg a total of 2.4 million euros, or around 2.6 million Swiss francs. Rüegg’s research project,
Hyper Quantum Criticality,is essential for understanding quantum effects in magnets as well as for future technological applications of these effects, for example in quantum computers.
How the smallest building blocks of matter interact with one another – that is the research focus of physicist Christian Rüegg, a laboratory head at the Paul Scherrer Institute PSI and professor at the University of Geneva. Rüegg is an expert in the research area of quantum magnetism. That means he deals with magnets whose properties are determined by quantum mechanics. What may sound exotic to the public is at the same time of fundamental significance for science. So significant that Rüegg is now receiving a highly prestigious European funding award: the ERC Consolidator Grant, given by the European Research Council (ERC) of the European Union. Over a five-year period this grant will bring Rüegg a total of 2.4 million euros, or around 2.6 million Swiss francs. The contract was executed on 3 October, for a funding period beginning on 1 December 2016. The funding may be used for new research instruments and for scientific staff.
The interactions among magnetic atoms and the quantum effects they engender in magnets, which Rüegg studies in his experiments, are of great theoretical and practical interest:
We will need this knowledge to build functional quantum computers, Rüegg explains. Quantum computers could one day supersede today’s computer technology. They would exploit the laws of quantum mechanics, allowing new approaches to computation, search, and data storage while being in many respects considerably more efficient than previous computers.
Investigating interactions and quantum effects on the fundamental level
While many physicists study the basic building blocks of the universe – elementary particles such as atoms, electrons, or quarks – Christian Rüegg is also interested in the fundamental interactions between these constituents, because they too are decisive for the observable electrical and magnetic properties of matter. On this fundamental level, the effects of quantum mechanics play an important role. The questions that Rüegg pursues include, for example: What states can magnetic atoms, the magnetic building blocks of matter, take? How do the interactions between them change when conditions change – for example when the temperature drops, the pressure rises, or an external magnetic field exerts an influence? How can these states and interactions be controlled and, in the future, exploited?
Rüegg studies these relationships with the help of sophisticated experiments carried out among others at the PSI’s Swiss Spallation Neutron Source SINQ. In the laboratory he can directly investigate the states and interactions on the atomic scale – that is, on the level of the smallest units of matter.
Rüegg will devote part of the newly awarded ERC funding to setting up a new magnet system that can generate extremely strong magnetic fields and pressures at temperatures near absolute zero. In addition, he plans to hire a handful of early-career scientists to expand his existing research team at the PSI.
Since 2011, Rüegg is – once again – at the Paul Scherrer Institute PSI, where he is a researcher as well as head of the Laboratory for Neutron Scattering and Imaging. Also since 2011, he has been teaching as a professor at the University of Geneva. With this the Aargau native, who once attended the old canton school in Aarau, has come home. Prior to 2011, Rüegg spent six years as a postdoctoral researcher and associate professor at the London Centre for Nanotechnology, University and Imperial College London. Prior to that, he did his doctoral work at ETH Zurich and the PSI – at the very neutron laboratory he now heads. Before winning his Consolidator Grant, Rüegg had already received several notable scientific honors for his work on quantum magnetism, including the Lewy-Bertaut Prize, the Nicholas Kurti European Science Prize, a Royal Society University Research Fellowship in the UK, and the ABB Prize of the Swiss Physical Society.
Here at the PSI, a unique combination of research facilities and know-how is available to me, Rüegg says of his return to the PSI. Above all the neutron source SINQ and the upcoming free-electron X-ray laser SwissFEL are important for Rüegg’s research and his future projects at the PSI. Equally essential, however, are the experts that the PSI has at all of these research facilities, in many different scientific areas.
Scientific work is always teamwork, especially when very complex and novel instruments are used, Rüegg says.
Experiments with custom-made crystals
Specifically, Rüegg is studying the states and elementary interactions of magnetic atoms in particular crystals containing unusual chemical bonds. These crystals are custom-made in a way that, within their crystal lattice, two magnetic atoms are situated especially close to one another and thus will interact especially strongly.
These two magnetic atoms and their interactions – that is a fundamental unit of a quantum system that we can study very well experimentally, Rüegg explains.
We study them as being representative of all kinds of quantum systems and combine them into more complex structures, so-called many-particle quantum systems.
Rüegg’s crystals are produced in part right at the PSI or at the University of Bern. They are up to one cubic centimetre in size, because the individual signals that Rüegg is looking for are very weak. Only through the sum of enormously many atom pairs in a crystal does the signal become strong enough to be measured with a spectrometer. For the same reason, a high-performance neutron source and highly precise instruments are essential for Rüegg’s experiments. These are available to him at the SINQ.
With neutrons, we can obtain comprehensive information about these magnetic quantum systems, and thus about the interactions on the atomic scale. This information is very important in order to better describe, understand, and possibly optimise such systems for future applications, says Rüegg.
Studying temporal dynamics of quantum systems at the future SwissFEL
And Rüegg plans yet another significant extension of his methods with the ERC support: time-dependent measurements. These should be possible using X-rays and will be carried out at the Swiss Light Source SLS as well as at the upcoming free-electron X-ray laser SwissFEL at the PSI. Instead of taking
snapshots of physics in stabile states, as has been done up to now, scientists will for the first time have the potential to make
movies of quantum systems and their development over time.
Text: Paul Scherrer Institute/Laura Hennemann