New many-body state in an iron crystal discovered
Researchers from Jülich, Poland and Japan have discovered and analyzed a new many-body state in an iron crystal. Its existence sheds new light on the physics of the interaction of conducting electrons and magnons which are excitations in magnetic systems. Magnons spread in a material as a result of a disturbance of the local magnetic order. The electron-magnon interaction is critical for fundamental physical properties, such as the temperature dependence of resistivity, but also offers prospects of faster and more energy-efficient computation. The JARA-FIT scientists Prof. Stefan Blügel and Prof. Claus Michael Schneider are significantly involved in the investigations.
Magnetic materials, known to mankind at least since the time of the Greek scholar Thales of Miletus, still hold surprises today. One of the modern sophisticated tools used to study magnetic materials is momentum- and spin-resolved photoelectron spectroscopy. In this type of experiment, light shines on the crystalline sample, resulting in the emission of electrons which are detected in the experimental apparatus. The measured electrons allow indirect conclusions to be drawn about events inside the sample, which helps physicists understand the origin of the properties of the examined material, such as why it is electrically conductive or magnetic.
In their recent paper, the researchers use this very method to reveal what is happening inside a magnetic iron crystal. They found that when an electron leaves the crystal, a hole is left behind, which experiences an unusual interaction with other electron-hole pairs that carry opposite spins. The high binding energy of the electrons with spin flip excitations surprised. This coupling is possible by so-called stoner excitations, which are more energetic than magnons.
The researchers were able to find this peculiar many-body state in an experiment on iron crystals exactly where theorists from the Peter Grünberg Institute – Quantum Theory of Materials (PGI-1) had previously predicted, employing a new computational method that uses no adjustable parameters. The scientists had developed this theory based on the many-body perturbation theory in order to describe the behaviour of electrons in ferromagnetic materials from first principles of quantum mechanics. The theory explicitly incorporates interactions between electrons and many-body spin-flip excitations and predicted that this coupling can lead to electronic band anomalies at surprisingly high energies. Exactly these anomalies have been confirmed by the experiments performed using a k-space microscope at the Elettra Synchrotron in Trieste, Italy, which the PGI-6 operates.