Physicists observe competition between magnetic orders

System: A crystal lattice made of trapped light atoms in several two-layered sheets. Tomographic images show the density (rotational) in a single layer. They provide information about the magnetic arrangement of atoms. The image on the right shows the density of an average layer over twelve realizations (orange red). Credits: © Marcell Gall, Nicola Wurz et al. / Nature

Nature study: the research team from the University of Bonn gains knowledge of new quantum phenomena.

They are as thin as a hair, only a hundred thousand times thinner – the so-called two-dimensional material, consisting of a single layer of atoms, has flourished in research for years. They became known to a wider audience when two Russian-British scientists were awarded the 2010 Nobel Prize in Physics for the discovery of graphene, a graphite building block. A special feature of such materials is that they possess new properties that can only be explained with the help of the laws of quantum mechanics and that may be relevant to advanced technologies. Researchers at the University of Bonn have now used ultra-cold atoms to gain new insights into previously unknown quantum phenomena. They found that the magnetic sequences between two thin films joined together by atoms compete with each other. The study is published in the journal Nature.

Quantum systems realize unique states of matter originating from the world of nanostructures. They facilitate a wide variety of new technological applications, e.g. contributing to the secure encryption of data, introducing increasingly smaller technical devices and even enabling 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 for a long period of time.

How much unusual quantum phenomena arise is still 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 are using so-called quantum simulators, which mimic the interaction of several quantum particles – something that can not be done by conventional methods. Even the latest computer models cannot account for complex processes such as magnetism and electricity down to the last detail.

Ultra cold atoms simulate rigid bodies

The simulator used by scientists consists of ultracold – ultrasold atoms because their temperature is only one millionth of a degree above absolute zero. Atoms are cooled using lasers and magnetic fields. Atoms are located in optical networks, i.e. standing waves formed by the overlap of laser beams. In this way, atoms simulate the behavior of electrons in a solid state. Experimental installation allows scientists to perform a wide variety of experiments without external modifications.

Inside the quantum simulator, scientists have, for the first time, been able to measure the magnetic correlations of exactly two joined layers of a crystal lattice. “Through the force of this union, we were able to rotate the direction in which 90-degree magnetism is formed – without changing the material in any other way,” explain first authors Nicola Wurz and Marcell Gall, doctoral students in Michael’s research group. Köhl.

To study the distribution of atoms in the optical lattice, physicists used a high-resolution microscope with which they were able to measure magnetic correlations between individual lattice layers. In this way, they investigated the magnetic order, namely the reciprocal alignment of atomic magnetic moments in the simulated solid state. They observed that the magnetic order between the layers competed with the original order within a single layer, concluding that the stronger the layers joined, the stronger the correlations formed between the layers. At the same time, the correlations within the individual strata were reduced.

The new results make it possible to better understand the magnetism that propagates in cohesive layer systems at the microscopic level. In the future, the findings will help make predictions about the properties of the material and achieve new functionalities of rigid bodies, among others. Since, for example, high-temperature superconductivity is closely related to magnetic couplers, the new findings may, in the long run, contribute to the development of new technologies based on such superconductors.

Reference: “Competitive magnetic messages in a two-layer Hubbard model with ultra-cold atoms” by Marcell Gall, Nicola Wurz, Jens Samland, Chun Fai Chan and Michael Köhl, January 6, 2021. Nature.
DOI: 10.1038 / s41586-020-03058-x

Funding: The study was funded by the Bonn-Cologne School of Physics and Astronomy, a collaboration of the Universities of Bonn and Cologne, the Alexander von Humboldt Foundation, the TRR 185 Research Cooperation Center “OSCAR – Union Control of Atomic and Photon Quantum Matter fitted to tanks ”funded by the German Research Foundation, the Matter and Light for Quantum Computing Matter (ML4Q) Excellence Group and the Stiftung der Deutschen Wirtschaft.

Subject matter and light for quantum computing (ML4Q) Gathering of Excellence

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

The Excellence Group is embedded in the Trans-Disciplinary Research Area “Intertwining and Basic Interlocking Blocks” at the University of Bonn. In six different ARTs, scientists from a wide range of faculties and disciplines come together to work on research topics relevant to the future.

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