I will review Landau-Pomeranchuk instabilities and then show how a l =1, spin symmetric instability must lead to a state with an anisotropic gap in the quasi-particle spectra. The spin-antisymmetric instabilities on the other hand are cured by a state of “helicity order” in which the spin-quantization axis is fixed with respect to the momentum. Such a state is suggested for the hidden order parameter in URu2Si2. Various thermodynamic properties are calculated and some experiments for directly looking for the proposed order parameter are suggested.
University of California, Riverside