The magnetic interaction strength depends on the geometry of the pathway between magnetic ions, a situation that can be utilized to manipulate these interactions both statically using an applied pressure and dynamically by exciting phonons with ultrafast light pulses. I focus on the dynamic manipulation of magnetic superexchange interactions in low-dimensional quantum spin systems which are characterized by the absence of long-ranged magnetic order down to the lowest temperatures. Hence, conventionally utilized magneto-optical phenomena like the Faraday effect or the magneto-optical Kerr effect which rely on the presence of an ordered magnetic moment cannot be employed to probe the magnetic states of these systems. The lack of magnetic order requires the exploration of other ultrafast pump-probe schemes that allow direct light-mediated access to the magnetic states of the system.
In this seminar I will discuss light-mediated pump-probe schemes to study low-dimensional quantum spin systems out of equilibrium. I will present my ongoing work on the spin-Peierls compound CuGeO3, which consists of quasi-1D antiferromagnetic S = ½ chains that are unstable towards a lattice distortion. This phenomenon, the spin-Peierls transition, promotes the formation of a gapped, non-magnetic, collective singlet ground state of spin dimers below a transition temperature TSP = 14 K. Upon excitation of phonons in the dimerized phase of this system we make two interesting observations. On long time scales (up to 1 µs) we observe extremely slow dynamics of the lattice. On short time scales we populate low-energy modes which could neither be identified as phonons nor as the known magnetic excitations of CuGeO3. Based on DFT calculations, X-ray scattering, and neutron scattering experiments, I will discuss potential magnetic and magnetoelastic origins of these modes.
Laboratory for Theoretical and Computational Physics