by Ingrid Fadelli , Phys.org

Materials are known to interact with electromagnetic fields in different ways, which reflect their structures and underlying properties. The Lyddane-Sachs-Teller relation is a physics construct that describes the relationship between a material's static and dynamic dielectric constant (i.e., values indicating a system's behavior in the presence or absence of an external electric field, respectively) and the vibrational modes of the material's crystal lattice (i.e., resonance frequencies).

This construct, first introduced by physicists Lyddanne, Sachs and Teller in 1941, has since been widely used to conduct solid-state physics research and materials science studies. Ultimately, it has helped better explain and delineate the properties of various materials, which were then used to create new electronic devices.

Researchers at Lund University recently extended the Lyddane-Sachs-Teller relation to magnetism, showing that a similar relation links a material's static permeability (i.e., its non-oscillatory response to a magnetic field) to the frequencies at which it exhibits a magnetic resonance. Their paper, published in Physical Review Letters, opens new exciting possibilities for the study of magnetic materials.

"This study was inspired by my supervisor, Prof. Mathias Schubert, who previously explored how electric fields interact with phonons and suspected a similar relationship could exist for magnetic fields and materials," Viktor Rindert, first author of the paper, told Phys.org.

"The opportunity to investigate this came with our development of a terahertz ellipsometer capable of capturing the polarization response. With this tool, we rigorously tested whether such a relation exists, leading to the discovery of the Magnetic Lyddane-Sachs-Teller relation."

The Magnetic Lyddane-Sachs-Teller relation, the new relation unveiled by Rindert and his colleagues, is essentially a magnetic parallel of the construct devised by Lyddane, Sachs and Teller. Instead of applying the responses of materials to an external electric field, it links the static (DC) and dynamic (AC) responses of a material following its interaction with magnetic fields.

"Specifically, this relation connects the material's magnetic resonance frequencies to its static permeability," explained Rindert. "We validated this relation by measuring the magnetic resonance frequencies using our newly developed method, THz-EPR-GSE, and comparing our results to those obtained using SQUID magnetometry, a well-established and precise technique."

To demonstrate the existence of this relation, the researchers used an advanced optical technique developed in their lab, called THz-EPR-GSE, to measure the magnetic resonance frequencies of an iron-doped gallium nitride (GaN) semiconductor. Their measurements and analyses ultimately confirmed the existence of the predicted Magnetic Lyddane-Sachs-Teller relation.

The new relation uncovered by Rindert and his colleagues could be a valuable tool for gathering new insights into the magnetic excitations of semiconductors and other materials with magnetic properties. In the future, it could contribute to the advancement of various electronic devices and their underlying components.

"Our study provides a new fundamental relation in magneto-optics, particularly relevant for researchers working on antiferromagnetic and altermagnetic materials," added Rindert. "While the exact direction is still evolving, our immediate focus is on applying the THz-GSE-EPR technique to study paramagnetic point defects in ultrawide band gap semiconductors.

"This research is particularly relevant for power electronics applications, where such materials are critical in enhancing performance and efficiency."

More information: Viktor Rindert et al, Magnetic Lyddane-Sachs-Teller Relation, Physical Review Letters (2025). DOI: 10.1103/PhysRevLett.134.086703.