Low-energy excitations, symmetry breaking and specific heat in YbBiPt

The heavy fermion compound YbBiPt has a very large linear coefficient of specific heat γ = 8 J mol^-1 K^-2 and this is understood, to first order, in terms of the observed low-energy neutron scattering response. However, at low temperatures, symmetry forbidden splittings at 1 and 2 meV, respectively are observed. These levels give good qualitative agreement with the measured specific heat, but poor quantitative agreement. Indeed, the specific heat drops more rapidly with temperature than can be accounted for assuming a temperature-independent density of states. We also present new low-temperature crystallographic data, which rule out any significant structural distortions.