The nature of excitations in a molecular Haldane system

It is well known that an S = 1 antiferromagnetic spin chain is predicted to have a quantum disordered ground state and a gap in its excitation spectrum. This so-called Haldane gap is topological in origin and contrasts dramatically with the case for S = 1/2, whose topology leads to a gapless spectrum. We have recently synthesised an intriguing new series based on S = 1 NiX2(3,5-lutidine)4 molecular complexes formed from quasi-one-dimensional columns of Ni-X...X-Ni. In an experiment at PSI in 2017 we showed that NiI2(3,5-lutidine)4 remains disordered down to 30 mK, suggesting that the system is a highly successful realization of a Haldane chain system. We now plan to investigate the order and dynamics of the system in high field as we close the spin gap and drive the system through a quantum critical point (QCP) in a new experiment, only possible using the HIFI spectrometer at ISIS.