General ILL seminar, Organised by College 4

Magnetic molecules can host diverse fundamental quantum phenomena such as magnetic bistability, entanglement, tunnelling, and coherence of quantum superpositions. Consequently, molecules have the potential to be key ingredients for the second quantum revolution provided that we, on a fundamental level, understand them well enough to harness their quantum properties. Therefore, the scientific community has a responsibility to pursue fundamental knowledge about the quantum mechanics at play within molecules.

This thesis investigates a particular quantum phenomenon at play within molecules: delocalisation of valence electrons across several magnetic ions. Such delocalisation can give rise to so-called double exchange interactions, which contribute uniquely to molecular spin dynamics. Still, fundamental understanding of the phenomenon remains scarce, in large due to a lack of adequate experimental methods for probing this intrinsically complex phenomenon. To tackle this issue, this work sets out to establish inelastic neutron scattering (INS) as a key spectroscopic probe of double exchange spin dynamics.

To realise this goal, a combination of INS, high-frequency paramagnetic resonance, and far-infrared magnetospectroscopy was used to investigate the spin dynamics of increasingly complex valence delocalised molecules. These studies show that INS is highly sensitive to the unique spin dynamics that emerge in this class of molecules. To aid the interpretation of the INS data, a spin Hamiltonian modelling tool was developed that, for the first time, incorporates double exchange alongside traditional magnetic interactions. Simulations of momentum-resolved INS spectra furthermore showed that experiments performed on single crystals have the potential to quantify the degree of electron delocalisation based on experimental data. Thus, INS offers the possibility to obtain a complete description of double exchange spin dynamics.