Physicists Observe Competition Between Magnetic Orders

Nature study: research team from the University of Bonn gains insights into novel quantum states

They are as thin as a hair, only a hundred thousand times thinner - so-called two-dimensional materials, consisting of only one layer of atoms, are booming in research. They became known to a wider audience in 2010 when two Russian-British scientists received the Nobel Prize in Physics for the discovery of graphene, a building block of graphite. What is special about such materials is that they have novel properties that can only be explained with the help of quantum mechanical rules and that can be relevant for improved technologies. Researchers at the University of Bonn have now obtained new insights into previously unknown quantum states with the help of ultra-cold atoms. Their discovery: The magnetic orders between two coupled thin layers of atoms compete with each other.

Quantum systems realize their very own states of matter, which arise from the world of the smallest particles and structures. They can enable many new technological applications, for example contribute to the secure encryption of data, bring about ever smaller and faster technical devices or even enable the development of a quantum computer. Such a system could solve problems in the future that classic computers only master with great effort or not at all.

How the unusual quantum states arise is by no means fully understood. In order to shed light on the darkness, the physicists led by Prof. Dr. Michael Köhl at the Cluster of Excellence “Matter and Light for Quantum Computing” at the University of Bonn uses so-called quantum simulators. These simulate the interaction of many quantum particles, which is not possible with conventional methods. Because: Even the most modern computer models cannot calculate complex processes such as magnetism and electricity down to the last detail.

Ultra-cold atoms simulate solids
The simulator used by the Bonn scientists consists of ultra-cold atoms - ultra-cold, as their temperature is only a millionth of a degree above absolute zero. The atoms are cooled down with the help of lasers and magnetic fields. The atoms are located in optical lattices, i.e. standing waves that are formed by superimposing laser beams. The atoms simulate the behavior of electrons in a solid. The experimental setup enables the scientists to carry out a variety of experiments without external modifications.

Within the quantum simulator, the scientists succeeded for the first time in measuring the magnetic correlations of exactly two coupled levels of a crystal lattice. “The strength of this coupling enabled us to turn the direction in which magnetism develops by 90 degrees - without changing the material in any other way,” explain first authors Nicola Wurz and Marcell Gall, doctoral students in Prof. Michael Köhl's research group.

To study the distribution of atoms in the optical lattice, the physicists used a high-resolution microscope with which they could measure the magnetic relationships between the individual lattice layers. In this way they investigated the magnetic order, i.e. the mutual alignment of the atomic magnetic moments in the simulated solid. Their observation: The magnetic order between the layers competed with the original order within a single layer. This means: the more the levels were linked, the stronger the correlations between the levels. At the same time, there were fewer correlations within a single level.

The new results make it possible to better understand the magnetism that spreads in the coupled layer systems on a microscopic level. Among other things, the findings should help to make predictions about material properties and to achieve new functionalities of solids in the future. Since, for example, high-temperature superconductivity is closely linked to magnetic couplings, the new findings could in the long term contribute to the development of new technologies based on such superconductors.

Advancement:
The study received financial support from the Bonn-Cologne Graduate School of Physics and Astronomy, a cooperation between the Universities of Bonn and Cologne, the Alexander von Humboldt Foundation, the Collaborative Research Center TRR 185 "OSCAR - control of atomic and photonic quantum matter through." tailor-made coupling to reservoirs ”, the cluster of excellence“ Matter and Light for Quantum Computing (ML4Q) ”and the Foundation of German Business.

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

The Cluster of Excellence Matter and Light for Quantum Information (ML4Q) is a research association of the Universities of Cologne, Bonn and Aachen as well as the Research Center Jülich. It is funded within the framework of the excellence strategy of the federal and state governments. The aim of ML4Q is to create new computer and network architectures that are based on the principles of quantum mechanics. ML4Q bundles the unique expertise of the partners involved in three key disciplines of physics: solid-state research, quantum optics and quantum information.

The cluster of excellence is embedded in the transdisciplinary research area (TRA) “Building blocks of matter and fundamental interactions” at the University of Bonn. Scientists from a wide variety of faculties and disciplines come together in six different TRAs to work together on future-oriented research topics.