The TRR 227 kindly invites you to a special TRR 227 Seminar.

Speaker: Dr. Davide Bossini, Fachbereich Physik, Universität Konstanz

Title:

Dynamical renormalization of the spectrum of coherent magnons

Abstract

The collective dynamical response of a magnetic solid is encoded in the dispersion relation of magnons, which in antiferromagnets (AFs) possess frequency in THz regime. The optical activation of coherent magnons in AFs has been widely reported, even into nonlinear regimes of spin dynamics. The arbitrary optical control of the spectrum of the eigenmodes appearing in the dispersion relation is still lacking. In my talk I will discuss one strategy to achieve this goal. Our approach relies on a resonant optical drive of high-energy magnons, with wavevectors near the edges of the Brillouin zone. The transient spin dynamics reveals the activation and amplification of coherent low-energy zone-centre magnons, which are not directly driven. Strikingly, the spectrum (frequency, amplitude and lifetime) of these low-energy magnons is renormalized by the coupling with the photoinduced high-energy modes. Shifts of the magnonic eigenfrequencies up to 20% and amplifications up to a factor 5 of their ground-state values were observed. Interestingly, cooling down the material (hematite) to the AF collinear phase, the photoinduced spectrum renormalization is drastically different. We rationalise the observation in terms of a resonant scattering mechanism, in which zone-edge magnons couple nonlinearly to the zone-centre modes. Numerical simulations can reproduce the measured frequency renormalization. Our results present a milestone on the path towards an arbitrary tuning of the quasiparticles eigenfrequencies.