THEOS-MARVEL seminar: Thomas Olsen

Magnetic excitations from first and second principles. Classical vs. Quantum mechanical approaches

Prof. Thomas Olsen

Technical University of Denmark (DTU), Denmark

The Heisenberg model has proven highly successful in describing the fundamental low energy magnetic excitations in a wide range of materials. On the other hand, such excitations originate from electronic correlations and are notoriously difficult to describe by explicit first principles methods. Thus, the most common computational approach applies a Heisenberg description, with exchange parameters obtained from first principles – typically DFT. However, whether or not such mapping entails a classical or quantum mechanical Heisenberg model description has remained unclear. It is also not obvious to what extent such  a distinction matters at all. Here, we show that the distinction is in fact important. First it is shown that the classical approximations for the Heisenberg model is in general unreliable for thermodynamic properties. In particular, it leads to severe underestimation of critical temperatures. Second, we show that first principles evaluation of Heisenberg parameters typically introduces a systematic error if total energies are mapped to a classical model rather than a quantum one. We exemplify the approach by application to a variety of 2D magnetic materials.

About the speaker

Thomas Olsen received his PhD from the Technical University of Denmark (DTU) in 2010. After a postdoc in San Sebastián Spain, he was hired as assistant professor at DTU in 2015 and became associate professor in 2018 (current position). His career has been devoted to developing first principles methods for solid state physics using density functional theory and many-body perturbation theory. More recently, his main focus has been on first principles description of magnetic order and magnetic excitations. In particular, for two-dimensional materials.