Magnetic field effects on the soft mode in a singlet ground-state dimer system: A neutron scattering study of ${\mathrm{Cs}}_3$${\mathrm{Cr}}_2$${\mathrm{Br}}_9$

Neutron scattering experiments in a magnetic field have been performed on the singlet ground-state dimer system ${\mathrm{Cs}}_3$ ${\mathrm{Cr}}_2$ ${\mathrm{Br}}_9$. At low fields the Zeeman splitting of the soft mode evolves in agreement with the isotropic random-phase approximation (RPA) model, with the notable absence of a quasielastic peak. At a temperature of 1.7 K the expected long-range magnetic order is not found at the predicted field of 2.8 T, indicating the shortcomings of the isotropic RPA model in the critical region. Magnetic intensity on the weak nuclear Bragg peak (1¯ 1¯ 4) indicates a probable ordering with a ferromagnetic component at 4.5 T and 1.7 K.