Condensed Matter Seminar: Shaken, not strained: How to control magnetic order with lattice vibrations

How do we control the ordering of spins in a magnetic material? We know that structure and symmetry of the crystal lattice are closely connected to magnetism and conventionally we try to adjust these factors in order to change the alignment of spins, for example by applying pressure or strain. This can only be done statically however, and many materials may break under the load necessary to achieve notable changes in the magnetic order. In this talk, I will show how coherent collective vibrations of atoms (optical phonons) provide an intriguing alternative. In recent years, the development of powerful terahertz sources has enabled resonant driving of optical phonons, yielding vibrational amplitudes so large that the dynamics are governed by nonlinear interactions between phonons and other degrees of freedom. I will examine the steps that led to the prediction of phono-magnetic effects, nonlinear phononic phenomena that can be regarded as lattice analogs of the opto-magnetic effects in nonlinear optics. I will show that optical phonons can not only be utilized to coherently control spin waves and induce spin polarization, but are able to create magnetic responses themselves in otherwise entirely nonmagnetic materials. Finally, I will discuss how this research can be incorporated into a bigger framework of dynamical quantum matter in the future, which focuses on non-equilibrium quantum phenomena in materials, including ultrafast phono-magnetism, nonlinear phononics, as well as novel light-matter states and cavity dynamics.