Low moments in heavy-fermion systems

A theoretical and experimental search is underway for hidden order parameters in the phase of weakly broken symmetry that precedes superconductivity in several heavy-fermion intermetallics. The apparent order parameter in URu2Si2 breaks time reversal symmetry, as would magnetic dipole order, but with a moment that is too small to account for the large specific heat jump at the transition. Polarized neutron scattering and high-field experiments have eliminated theories proposing that quadrupoles or exotic triple-spin states are directly responsible for the new diffraction peaks of the ordered phase. For the dynamic behaviour it is suggested that, if the weak peaks were extrinsic, then quadrupoles could account for the soft spin-wave mode, the gap formation and longitudinal symmetry of the spin spectrum.