A study of the antiferroquadrupolar ordering in HoB2C2 through inelastic neutron scattering

Inelastic neutron scattering measurements have been carried out to investigate the crystal field (CF) excitations of HoB2C2 and ErB2C2 in order to understand the mechanisms of the quadrupolar ordering (TQ) observed in HoB2C2 at 5K. The well-defined low-energy excitations observed at 20K undergo a dramatic broadening at TQ=5K but the linewidths narrow again down to 2K. In contrast, it is interesting to note that the high-energy response between 15 and 45meV remains well defined on being cooled through both the antiferromagnetic (AFM) and quadrupolar ordering transitions. This underlines the importance of the quadrupolar fluctuations on the low-energy crystal field excitations. This is further supported by the absence of such low-energy excitations in the non-antiferroquadrupolar (AFQ) compound ErB2C2.