Axial Phonons and Their Role in Magnetism

Licentiate thesis, 2025

Phonons are collective lattice excitations that can be understood as quantized modes
of vibration. In recent years, it has been shown that a particular kind of phonons,
namely axial phonons, or circularly polarized phonons, can induce magnetization of the
material. Excitingly, experimental evidence has shown that the arising magnetization
is unexpectedly large. Understanding this phenomenon has been an active topic of
research in recent years.
This thesis presents the contributions to this area based on two papers. The first paper
presents a microscopic theory for magnetization induced by axial phonons. This theory
is based on treating the interaction between the phonons and the electrons as a second
order perturbation of the system. The second paper reviews the recent progress of the
field. Here, the concept of axial phonons is introduced alongside with the phenomeno-
logical arguments for emergent magnetization. A summary of the experimental obser-
vations of the magnetization induced by axial phonons is also provided. Additionally,
an overview of recently proposed microscopic theories of the effect is presented. Fi-
nally, in an additional chapter the perspective of observing the effect of axial phonons
on other physical phenomena, such as the conventional and the anomalous Hall effect,
is discussed.