Condensed Matter Seminar: Dynamical renormalization of a spin Hamiltonian via high-order nonlinear magnonics

In the absence of externally applied magnetic fields, the Hamiltonian of a magnetic material is determined by the exchange interaction, which is of electrostatic origin, and the spin-orbit coupling, whose magnitude depends on the atomic charge. Linear spin wave theory provides a representation of the entire spectrum of collective magnetic excitations, i.e. magnons, assuming the interactions to be constant and the number of magnons in the system negligible. However, the electric field of light is able to perturb electrostatic interactions, charge distributions and, at the same time, can create a magnon population. A fundamental open question therefore concerns the possibility to optically renormalise the magnon spectrum and the spin Hamiltonian. In my talk, I will show how we test this hypothesis by using femtosecond laser pulses to resonantly pump electric-dipole-active pairs of high- energy magnons near the edges of the Brillouin zone. The transient spin dynamics reveals the activation and a surprising amplification of coherent low-energy zone-centre magnons, which are not directly driven. Strikingly, the spectrum of these low-energy magnons differs from the one observed in thermal equilibrium, the latter being consistent with spin wave theory. The light-spin interaction thus results in a room-temperature renormalisation of the magnon spectra, as five-fold and three-fold increases of the amplitudes and 4% frequency shifts of their ground-state values were measured. We rationalise the observation in terms of a novel resonant scattering mechanism, in which zone-edge magnons couple nonlinearly to the zone-centre modes. In a quantum mechanical model, we analytically derive the corrections to the spectrum due to the photoinduced magnon population. Our results present a milestone on the path towards a coherent all- optical engineering of Hamiltonians in solids, as an arbitrary tuning of the quasiparticles eigenfrequencies would result in optically driving instabilities and phase transitions.