Physicists observe competition between magnetic orders

They’re as thin as a hair, barely a hundred thousand times thinner – so-called two-dimensional materials, made up of a single layer of atoms, have been booming in research for years. They became known to a wider audience when two Russian-British scientists were awarded the Nobel Prize in Physics in 2010 for the discovery of graphene, a building block of graphite. The peculiarity of these materials is that they have new properties which can only be explained with the help of the laws of quantum mechanics and which may be relevant for improved technologies. Researchers at the University of Bonn (Germany) have now used ultra-cold atoms to gain new knowledge about previously unknown quantum phenomena. They discovered that the magnetic orders between two thin layers of coupled atoms compete with each other. The study was published in the journal Nature.

Quantum systems realize very unique states of matter from the world of nanostructures. They facilitate a wide variety of new technological applications, for example by contributing to secure data encryption, introducing ever smaller and faster technical devices and even enabling the development of a quantum computer. In the future, such a computer could solve problems that conventional computers cannot solve at all or only over a long period of time.

How unusual quantum phenomena occur is still far from fully understood. To shed light on this point, a team of physicists led by Professor Michael Köhl of the Matter and Light for Quantum Computing cluster of excellence at the University of Bonn uses so-called quantum simulators, which mimic the interaction of several quantum particles. – something that cannot be done with conventional methods. Even advanced computer models cannot calculate complex processes like magnetism and electricity down to the smallest detail.

Ultra-cold atoms simulate solids

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

In the quantum simulator, scientists have, for the first time, succeeded in measuring the magnetic correlations of exactly two coupled layers of a crystal lattice. “Thanks to the strength of this coupling, we were able to rotate the direction in which the magnetism is formed by 90 degrees – without changing the material in any other way”, explain the first authors Nicola Wurz and Marcell Gall, doctoral students of the group. research by Michael Köhl.

To study the distribution of atoms in the optical lattice, physicists used a high-resolution microscope with which they could measure the magnetic correlations between the different layers of the lattice. In this way, they studied the magnetic order, that is, the mutual alignment of atomic magnetic moments in the simulated solid state. They observed that the magnetic order between layers competed with the original order within a single layer, concluding that the more strongly the layers were coupled, the more correlations formed between the layers. At the same time, correlations within individual layers have been reduced.

The new results provide a better understanding of magnetism propagating in coupled layer systems at the microscopic level. In the future, the results will help to make predictions about material properties and realize new functionality of solids, among others. As, for example, high temperature superconductivity is closely related to magnetic couplings, the new discoveries could, in the long term, contribute to the development of new technologies based on such superconductors.

The Matter and Light cluster of excellence for quantum computing (ML4Q)

The Matter and Light pole of excellence for Quantum Computing (ML4Q) is a research cooperation between 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 goal of ML4Q is to develop new computing and network architectures using the principles of quantum mechanics. ML4Q builds on and extends complementary expertise in the three key research areas: solid-state physics, quantum optics and quantum information science.

The center of excellence is integrated into the transdisciplinary research area “Building Blocks of Matter and Fundamental Interactions” of the University of Bonn. In six different EMRs, scientists from a wide range of faculties and disciplines come together to work on relevant research topics for the future.