Physicists observe the competition between magnetic orders

They are as thin as a hair, only a hundred thousand times thinner – so-called two-dimensional materials, consisting of a single layer of atoms, have been flourishing in research for years. They became known to a wider audience when two Russian-British scientists received the 2010 Nobel Prize in Physics for the discovery of graphene, a building block of graphite. The special feature of such materials is that they have new properties that can only be explained using the laws of quantum mechanics and that can be relevant to improved technologies. Researchers at the University of Bonn (Germany) have now used ultra-cold atoms to gain new insights into previously unknown quantum phenomena. They found that the magnetic orders between two linked thin-atom films compete with each other. The study was published in the journal Earth.

Quantum systems realize many unique states of matter that come from the world of nanostructures. This facilitates a wide range of new technological applications, e.g. Contributes to data encryption security, introduces ever smaller and faster technical devices, and even enables the development of a quantum computer. In the future, such a computer could solve problems that conventional computers could not solve at all or only over a long period of time.

How unusual quantum phenomena arise is far from fully understood. To shed light on this, a team of physicists led by Prof Michael Köhl at the Matter and Light for Quantum Computing Cluster of Excellence at the University of Bonn use so-called quantum simulators, which mimic the interaction of various quantum particles – something it cannot do with conventional methods are not done. Even modern computer models cannot calculate complex processes such as magnetism and electricity down to the finest detail.

Ultra-cold atoms simulate solids

The simulator used by the scientists consists of ultra-cold atoms – ultra-cold because their temperature is only a millionth degree above absolute zero. The atoms are cooled using lasers and magnetic fields. The atoms are located in optical gratings, that is, standing waves joined together by laser beams. In this way, the atoms simulate the behavior of electrons in a solid state. The experimental setup allows scientists to perform a wide range of experiments without external adjustments.

Within the quantum simulator, the scientists managed for the first time to measure the magnetic correlations of exactly two linked layers of a crystal lattice. “Through the strength of this coupling, we were able to rotate the direction in which magnetism forms by 90 degrees – without changing the material in any other way,” explain the first authors Nicola Wurz and Marcell Gall, doctoral students in Michael Köhl’s research group. .

To study the distribution of atoms in the optical grating, the physicists used a high-resolution microscope with which they could measure magnetic correlations between the individual grating layers. In this way, they investigated the magnetic order, that is, the mutual alignment of the atomic magnetic moments in the simulated solid state. They observed that the magnetic order between layers in a single layer competes with the original order, and concluded that the stronger layers are linked, the stronger correlations are formed between the layers. At the same time, correlations within individual layers were reduced.

The new results make it possible to better understand the magnetism that propagates in the coupled layer systems at the microscopic level. In the future, the findings should help to make predictions about material properties and to bring about, among other things, new functions of solids. For example, since high-conductivity superconductivity is closely linked to magnetic couplings, the new findings may contribute in the long run to the development of new technologies based on such superconductors.

The Matter and Light for Quantum Computing (ML4Q) Cluster of Excellence

The Matter and Light for Quantum Computing (ML4Q) Cluster of Excellence is a research collaboration by the universities of Cologne, Aachen and Bonn, as well as the Forschungszentrum Jülich. It is funded as part of the Excellence Strategy of the German federal and state governments. The purpose of ML4Q is to develop new computer and network architectures using the principles of quantum mechanics. ML4Q builds and expands complementary expertise in the three key research areas: solid state physics, quantum optics, and quantum information science.

The cluster of excellence is embedded in the transdisciplinary research area “Building Blocks of Matter and Fundamental Interactions” at the University of Bonn. In six different TRAs, scientists from a wide range of faculties and disciplines come together to work on future relevant research topics.