Quantum Criticality in Spin Dimer System BaNd₂ZnS₅

We investigated the partial-disordered state of the Shastry-Sutherland system BaNd2ZnS5 utilizing the time-of-flight inelastic neutron scattering spectrometer HYSPEC at the Spallation Neutron Source (SNS) of the Oak Ridge National Laboratory (ORNL). Through modeling the field evolution of low-energy spin excitations up to 4T, we uncovered that the disordered states result from a delicate balance between the external magnetic field and antiferromagnetic (AF) inter-dimer Ising interactions. The intra-dimer interaction is predominantly Ising along the ordered moment direction, with a slight mixing of J++ and J--.<br/>Parametric measurements ranging from 0 to 4T of the order parameters of the ferromagnetic (FM) and antiferromagnetic (AFM) phases at 40mK were conducted using the DEMAND diffractometer at the High Flux Isotope Reactor of ORNL. The order parameter displays characteristics indicative of a quantum critical point and can be effectively modeled using semi-classical treatment based on SU(4) coherent states. Although the longitudinal mode can not be quantitatively characterized, its presence is essential to explain the observed data, suggesting that the puzzling metamagnetic transition originates from the ordered antiferromagnetic state transitioning to the disordered quantum paramagnetic state.